ALL OF IGCSE EDEXCEL BIOLOGY (A*-U) – GCSE Biology Revision – SCIENCE WITH HAZEL


Hi, guys. My tripod seems jammed, so I’m just going to film it like this. Luckily, I don’t need to say very much. It’s just to introduce my latest upload, which is IGCSE EDEXCEL BIOLOGY all-in-one video. It’s about two hours long. The timings are all below in the description box if you want to skip through and look at other topics. But yeah, I hope you find it really helpful. Just so you know it’s for people sitting the A*-U, so old spec and the last exams which will be sat in 2018, so people sitting their GCSEs in 2018, this video is for you. As usual, I put preferred answers all the way through. You might know already, but I have a PDF, which I can email to you for three pounds. Details on how to get hold of one are in the description box. But yeah, I hope you find it helpful. If you did, please like this video and don’t forget to sub because it takes me ages to do. Anyway, enjoy. Let’s quickly recap animal and plant cells so you know where you’re starting from and none of the organelles – so that’s the little cell structures – sound unfamiliar to you. Remember, animal and plant cells are really similar. The only difference really is that animal cells are shaped like a fried egg They only have a cell membrane surrounding them, and the nucleus is right in the middle. Remember, plant cells, they have the addition of a rectangular cell wall which encases the rest of the cell. They have a vacuole, which is another rectangle, in the middle. And they have chloroplasts. Remember, those are green, and that’s where photosynthesis – that number one plant process – takes place. (no audio) We’re going to talk about fungi. Remember, fungi are things like mushrooms and toadstools and yeast, and they’re responsible for some pathological diseases, such as athlete’s foot. Don’t worry about that. Let’s just look at the structure. These are going to be very similar to plant cells. So you’re going to have a cell wall, but unlike a plant’s cell wall which is made out of cellulose, a type of sugar, fungi cell walls are made out of chitin. Then you’re going to have a cell membrane, again like a plant cell, with a vacuole in the middle. Then you have a nucleus, also found in one corner, with cytoplasm. So as I’ve said previously, remember, fungi structure is very similar to a plant cell. So if you’re struggling in the exam, can’t remember, try and use your plant cell as your guide. Just remember, don’t include any chloroplasts because that won’t get you any marks. Okay. Moving on. Viruses. Now, viruses, there’s a lot of confusion as to whether they’re living, and that’s because they’re so tiny, they can’t reproduce themselves; they have to lock into another organism. So that makes them really easy to learn the structure of. All you need to know is that there’s a protein coat that can be in a variety of shapes and sizes which surrounds the bit of DNA or RNA which makes up the genetic information inside the virus, and that’s it. There is no distinct nucleus inside a virus. Finally, bacteria: responsible for some of the most horrendous diseases out there, things like tuberculosis, they’re bacterial diseases; cholera is another one . This time, you’ve got more of a distinct cell. You have a cell wall. You have a little tail, and we call that a flagella, and that helps the bacteria move to where it wants to go. Sometimes, you’ll see diagrams with a capsule around the edge, or a slimy coat. That’s just an additional structure. Don’t worry too much about that. The crucial thing about a bacterial cell is that there’s no distinct nucleus. What you have instead is a nucleoid, and then you’ll see some tiny other structures, and these are plasmids, and these are also genetic information. You have a cell membrane, as usual. You have cytoplasm. So just remember those roles based on what you know about animal and plant cells. Let’s start with some animal cells. We’re going to start with red blood cells. Remember, their job is to transport – Why are you growling and covering me in hair? Remember, their job is to transport oxygen around the body, so they’re packed full with a pigment called haemoglobin. Make sure you mention that because you’ll get a whole mark for saying the word haemoglobin. So haemoglobin is the pigment inside red blood cells which binds to oxygen. They have a very distinct shape, which is that they are biconcave disc-shaped, which means they go in both sides, and what that allows is to have a larger surface area to volume ratio so they can transport more oxygen. They have no nucleus, so there’s more space for transportation of oxygen. Is there anything else I wanted to say? Nope, that’s everything I wanted to say there. Now, ciliated epithelial cells or cilia: remember, we find those lining the windpipe – the trachea – and they have special hairs on them And what they do is they waft, and what that does is they waft mucus containing lots of bacteria out of the lungs, out of the trachea, into the mouth, where it can be swallowed and destroyed by stomach acid. So, cilia are pretty specialized. Sperm cells now. I like sperm cells because they’re really easy to remember. First of all, remember that they have a very characteristic shape. They have a whiplash tail which helps them to swim. They have a central portion which is packed full of mitochondria to provide energy to enable them to swim. They have a head containing genetic information. And lastly, they have some enzymes in their head which help break down the outer casing on the egg when they need to penetrate it. Those enzymes are found in a special structure called the acrosome – another very good specialist word. Now we’re moving on to specialist plant cells. So we’re going to start with the xylem. Remember, xylem transport water from the roots up the stem to the leaves, where it’s needed in photosynthesis. Xylem have very special structure. First of all, they’re dead, and that means that they’re hollow because they’ve lost their cell contents ages and ages ago. That hollowness means that, obviously, there’s lots of space for water to be transported. They have special walls made out of lignin, and that lignin is a very strong substance which will help keep the xylem open. Now, coupled with xylem, you find that there are phloem. Remember, phloem transport sugars around the plant – so from the leaves where the sugar’s made, up and down the plant for storage. Now, you find that the ends of the phloem cells break down and form sieve plates, and those sieve plates allow the food to move easily in the water in which its carried. Because phloem cells need lots of energy, they’re coupled with companion cells. Now, companion cells are full of mitochondria, which provide the phloem with energy. Let’s take a regular plant cell now. So we call this the palisade layer if we’re looking at the structure of the leaf, or the palisade mesophyll. Obviously, they contain lots of chloroplasts. Why? Because it’s full of a pigment called chlorophyll, which absorbs light in photosynthesis. So that’s really, really key. You do find that these plant cells – or these palisade cells – are located in the top of the plant in order to capture as much sunlight energy as possible. And because, as with all plant cells, they have a large permanent vacuole – and that helps keep the cells rigid. Lastly, we’re going to look at root hair cells. So if you’re asked to draw a root hair cell, basically draw a plant cell but with a big, elongated area, which is the root hair, and obviously don’t include chloroplasts because it’s a root. It’s underground; there’s no sun going to reach there, so it would look really stupid if you added chloroplasts. And just draw all the other structures as usual: your cell membrane, your cytoplasm, your cell wall, et cetera. And just point out that that root hair gives a very large surface area to allow all that water to be absorbed. Remember, it has a large permanent vacuole, which helps speed up the water movement by osmosis. And again, they have lots of mitochondria. Why? Because they need lots of energy to actively transport minerals from the soil into the root hair. Our nose and our mouths are connected to our windpipe, which we call the trachea, and then that splits off into two branches called bronchi. Then those branches become even more split and even smaller, and we call those the bronchioles. And lastly we end in air sacs, which we call alveoli, and that’s where gas exchange actually takes place. That’s where oxygen leaves the alveoli and moves into the capillaries, and carbon dioxide leaves the capillaries and moves into the alveoli. And I’m just going to tell you now that that occurs by diffusion. So if we take oxygen – remember, diffusion is the net movement of gas from an area of high concentration to low concentration. So that would be oxygen moving from inside the alveoli, through its walls into the capillaries, and that is diffusion. Remember that the alveoli are adapted to maximise amount of gas exchange that can take place. They’re very thin – one cell thick – so there’s a short diffusion distance. They’re moist. They have a very large surface area. And is there one more thing? Yes. They have a plentiful supply of blood capillaries. So those are four marks for you right there. However, let’s now talk about how we actually get air into our lungs, so a breath in. And this is the same sort of language you’re going to need to use every single time one of these questions comes up. So, breathing in. Now, it’s up to you if you want to talk about the internal and external intercostal muscles. I’m gonna mention that for completeness’ sake, but feel free to miss out this first step when you’re describing the processes of breathing in and breathing out. But when you take a breath in, effectively your internal intercostal muscles relax, your external intercostal muscles contract, and what that does is it forces your ribcage up and outwards. And that’s the first mark I would really specify. I wouldn’t talk about the muscle bit. It’s quite hard. Just talk about the ribcage moving upwards and outwards. At the same time, your diaphragm contracts, and what it does is it flattens, and effectively that creates much larger volume inside your chest, or in your thorax, if you’re feeling fancy. Because you’ve increased the volume inside here, effectively, the pressure has decreased, so the pressure outside – the atmospheric pressure – is much higher, so air is automatically sucked into your lungs. (no audio) If we look at the opposite argument, when we breathe out, you can just say exactly the opposite. So your ribcage is going to move down and inwards. Your diaphragm will relax, which will mean that it will move up. That creates a much smaller volume inside your thorax, causing the pressure to be raised inside your thorax, and that means that, therefore, the pressure inside your chest is higher compared with the atmospheric air surrounding, so air is forced out of your lungs. (no audio) Right, so, when you’re smoking … There are so many chemicals inside cigarette smoke – thousands and thousands. I don’t know the exact number. But unfortunately, lots of these are pretty dangerous, and they’re chemicals which we call carcinogens. Carcinogen is simply a cancer-causing agent. And there are lots of substances that you will have heard of inside cigarette smoke, such as: tar, which is a black, sticky substance which clogs up in your lungs; nicotine, this is the addictive drug which is why smokers find it so hard to give up this habit; and carbon monoxide. I think I’ll talk about carbon monoxide first of all. Remember, when you breathe in, your oxygen is transported in your red blood cells and carried around your body in your blood, and the red blood cells are full of a pigment called haemoglobin, which is where the oxygen binds to. However, what happens when you smoke is carbon monoxide has a higher affinity, so it binds better to haemoglobin than oxygen does, so what happens instead is: the red blood cells start carrying carbon monoxide around the body rather than oxygen, so you have less oxygen in your body, which is obviously a bad thing. What knock-on effect this has is that it means that you have higher blood pressure and your heart has to pump harder, and the reason why is because it needs to pump harder in order to deliver more oxygen around the body than it would normally, and that’s because some of those red blood cells are transporting carbon monoxide. When carbon monoxide binds with haemoglobin, you form something called carboxyhaemoglobin, and that’s a specialist word, especially for people doing IGCSE. Nicotine makes your blood more sticky, so it’s more viscous, and again your heart needs to pump harder, and that increases your blood pressure just to get your blood circulating around your body properly. Let’s talk about the cilia now. Now, the cilia are specialized cells, which line your airways, and they’re called ciliated cells, but they have special hairs called cilia, and what they do is they waft. And they waft anything that you accidentally swallow down into your trachea. They waft that out, and they waft it into your mouth, and then you swallow it so that it can be destroyed by your hydrochloric acid in your stomach. So they’re obviously super important because they’re moving all this nastiness that you breathe in out of your body so they doesn’t coat your lovely, pink lungs. The problem is with smoking, is you paralyse your cilia, so they no longer work. So what happens is any bacteria has reached your airways gets trapped rather than removed, and that leads to infection. When there’s an infection, what happens is white blood cells invade to try and destroy it. Your cells are destroyed, generally. And the problem is they don’t get replaced with the original cell; they get replaced with scar tissue. So your whole airway is a bit screwed up really, and you end up with a smoker’s cough as well because all that mucus and things, they’re all staying there in your airways, and you can feel it in your lungs. So what you want to do is you want to cough super hard to remove it all from your body, and that’s what a smoker’s cough is, which is why it sounds pretty horrible when you hear people doing it. It’s like – I’m not going to do the noise; it’s going to sound really bad on this, so just take my word for it. Smoker’s cough is due to the build-up of mucus in the airways. Now we’re going to talk about lung cancer. Now, that’s all those carcinogens I was talking about – those chemicals inside cigarette smoke. They end up in the cells of your lungs. Some of them will cause them to rapidly reproduce, which is what cancer is, and you’ll end up with a tumor growth that will cause massive obstructions, and if it spreads, that’s what can actually lead to death. So that’s lung cancer sorted. Very, very horrible disease. Another nasty disease we talk about is emphysema. Now, remember the alveoli are little air sacs in the base of your lungs, and that’s where gas exchange takes place. Now, they have a super large surface area in order to make sure as much gas exchange takes place per second as possible. However, when you smoke, what you do is you break down the walls between one alveolus to the next, and what that does is it increases the size of each sac, and that has the knock-on effect of decreasing the surface area available for gas exchange. I might draw you diagram and try and show you this. But the point is, if you have a large compartment, you’re going to be having far less gas exchange than if you had small compartments, and therefore, emphysema leads to a massive shortness of breath. And you see people attached to oxygen tanks because they can’t breathe enough oxygen. (no audio) So, let’s start with aerobic respiration: aerobic meaning using lots of oxygen, as opposed to anaerobic respiration, which means respiration without oxygen. So, let’s first of all, take the equation for aerobic respiration. So, we’re going to be taking oxygen into our bodies. So it’s oxygen + glucose, which we’ve eaten in our food, an arrow, and then you’re going to produce carbon dioxide, which we’ll breathe back out, lots of water, which you’re breathing out as water vapour, and finally energy, which is the whole reason we’re doing this. So, like I said, it occurs in the mitochondria of cells, and it’s our way of releasing lots of energy. Now, what do we need to use that energy for? Well, we need to use it to build up smaller molecules into large ones – so, for example, building amino acids into proteins. We need it to maintain our body temperatures because, as mammals, we know that we are warm-blooded. And lastly, we need energy to help our muscles contract. There’s lots of other things we need energy for, but those are the three main ones. However, sometimes, we can’t get enough oxygen into our bodies to allow us to aerobically respire, and this time, we’ll choose to do anaerobic respiration. Some such example of this could be when you’re sprinting, like the 100-metre race at school. You just can’t breathe fast enough. The advantage of this is that you can use it when there’s not enough oxygen around, but the problem is that it releases far less energy in comparison to aerobic respiration. And you may have seen also that glucose, rather than being broken down immediately into carbon dioxide and water, is instead broken down into lactic acid. Now, the lactic acid is toxic, and it leads to that horrible, stitchy, crampy feeling, so it leads to muscle fatigue and cramping, and that’s kind of problematic. So what we need to do is take in even more oxygen into our body post run, post race, in order to break down that lactic acid into carbon dioxide and water. And that extra amount of oxygen that we need is something we call the oxygen debt, and all it does is it means that it’s allowing way more oxygen into our bodies to allow that lactic acid to be broken down so it stops causing fatigue. So we’re going to start at the mouth. First of all, there’s two types of digestion: physical and chemical. So, physical digestion is literally just breaking large chunks of food into smaller chunks, but you’re not actually changing the structure. Chemical is completely different. You’re breaking down a large piece of food, but that’ll actually become a new molecule, and really what we’re talking about there is enzymes. So, in the mouth, you have physical digestion because you obviously have your teeth, and they’re going to be crunching down, so your molars will be grinding up the food, your incisors will be cutting down the food. However, at this point, we get an enzyme released called amylase, and that gets released by the salivary glands, and what that’s doing is it’s breaking down starch into sugars. So, from the mouth, we pass into the oesophagus -the food pipe – and at this point, peristalsis takes place. And peristalsis is literally the muscular contraction that occurs due to the circular muscles. And what that does is it forces the food – the bolus, so that’s the lump of food – down your esophagus, into your stomach. So, it reaches the stomach, and we know that the stomach is full of an acid – hydrochloric acid, which has a low pH. And what that’s there for is to digest and break down bacteria, to basically prevent us getting ill. At this point, we have another enzyme released, and that’s protease, and as the name – Gosh, cat hair’s going everywhere. Can’t breathe. You stop being like that. Anyway, so, protease, as the name suggests, breaks down proteins into amino acids. So, there’s our large molecule proteins found in meats and pulses, and we’re going to be making amino acids, which are small, absorbable – is that word? – molecule. At this point, our pH is going to have drastically decreased, so when the stomach releases the food into the small intestine, the food is going to be very, very acidic and its pH will be around three. Now, this is a problem because at this point, we’re having a load more enzymes being released. There’s going to be more protease, more amylase, but also the addition of lipase, and they’re going to come from small intestine and the pancreas. Now, this is problematic – the low pH – because what will happen is these enzymes will denature, which means that they’ll stop working, and that’s because the pH is so far from their optimum pH, which will be around 7. So at this point, we have another very important substance released, which is called bile. So, what bile does is it neutralises the stomach acid and therefore increases the pH to around seven so these other enzymes can work properly. Bile is made by the liver. It’s very important that you remember that. It’s stored in the gallbladder. And then it’s released into the small intestine. So, try remember those three stages. As I’ve already mentioned, its first job is neutralization of stomach acid, but it has a second very important role, which is emulsification. Now, don’t let that, like, worry you. All that emulsification means is it means breaking down large droplets into small droplets. The reason being is that by creating smaller droplets, you are increasing the surface area, and at this point, what we’re really doing is we’re breaking down fat into smaller droplets to create a larger surface area for the enzyme lipase to work on. Remember, lipase breaks down fats into fatty acids and glycerol, and it’s a pretty rubbish enzyme, which is why bile is important because it increases the surface area Cat! Oh, so gross. Oh! Why? Do you stop molting ever? Right. So, we talked about bile. We talked about our enzymes. And at this point, basically, our food is pretty much all broken down, and we’re moving into the small intestine, and we’re going to get absorption of the digested food through the walls of the small intestine so it can be transported around in the blood. The walls of the small intestine are very specially adapted for their function, and they have these things called villi, which are like projections, and what they do is they increase the surface area for absorption to make sure it happens much faster. And on top of each villi, there are microvilli, and they increase the surface area even further to make sure as much food as possible is absorbed through the walls of the small intestine. So we’ve dealt with our food. All that’s left is the undigested food, so the fiber, which is why it’s super important that you eat things like Weetabix© because that will keep everything moving through your digestive system properly and stop you getting constipation. And then we get into the large intestine, and all that really does is absorb water and make sure that the end product is nice and – not nice and compact but that it isn’t like diarrhoea, basically. Gosh, this is a horrible topic. So water is reabsorbed in the large intestine, and then eventually, all we have left is the faeces, which is stored in the rectum and released by the anus. (no audio) Let’s start by our key definition of enzymes. Right, so an enzyme is a biological catalyst. That means that it speeds up the rate of a reaction without being used up. In terms of how an enzyme works, remember that an enzyme – you might have seen a picture. It’s like, you know, Pac-Man©. And it has, like, a mouth, and we call that the active site. And what you have is you have a substance which fits into that specific active site on the enzyme. And what you find is that this substrate – we call that substance – breaks apart and produces a product. When the two are combined, what we have here is an enzyme substrate complex. As with all science, it’s really important you use crucial keywords, things like specific, optimum, active site. Even if you’re not sure of your answer in the exam, just literally vomit out all of these words onto your paper, and I promise you’ll pick up some excess marks somewhere along the way, and it will really help bump up your grade. I think we need to talk about the effect of the conditions on enzyme-controlled reactions because enzymes are very, very fussy little things, and they really don’t work very well if you don’t treat them right. So the first condition we can look at is temperature, and with all reactions, if you have a very low temperature, you tend to find that the enzyme-controlled reaction will occur very slowly, and that’s just due to collision theory. Because if you have your enzyme and you have your substrate, and they’re just moving around really slowly, very lethargic, very low temperatures, you’ll find that they don’t really come together very often. So our active site and our substrate doesn’t come together, and you end up with a very, very slow rate of reaction. What happens then is, as you increase the temperature, obviously these enzymes and the substrates will be moving around far more quickly, and you’ll see a really nice increase in the rate of activity in the enzyme-controlled reaction, and then right at the top – I’ll find a graph to show you – what you see is there’s an optimum temperature, and that really means – make sure you use the word optimum in the exam – it means the temperature at which the enzyme works best – so pop down optimum temperature. You can normally read it off the graph, and it’s right at the top. And then what you see is that after you’ve exceeded that optimum temperature … the rate of reaction literally drops off, and the word we used to describe that is denaturation. So the enzyme becomes denatured, and what that means is that the active site has changed shape, and therefore, the substrate can no longer fit in. Because remember, it’s like a jigsaw puzzle. They’ve got very specific shapes, and if the substrate can no longer fit in, then basically your enzyme is screwed. Make sure you never describe the enzyme as dying. You won’t get any marks for that. The word you need is denatured. And then finally, the second thing we need to talk about is pH. Again, enzymes have an optimum pH. So, for example, in the body that will just be about seven because pH seven is neutral. However, certain other enzymes will prefer a far more acidic pH. So protease, for example, which is found in the stomach: the stomach is full of hydrochloric acid; it has a pH of around three. Therefore, protease’s optimum pH will be around three. As soon as you decrease it to two or increase it to four, you’ll end up with your enzyme rate of reaction dropping off, and therefore, you get a very distinctive curve, which looks like this. So, we’re going to leap straight in with testing for glucose. Now, you want to use Benedict’s reagent here. And you’re going to heat your sample with some Benedict’s reagent in a water bath. If glucose is present, you will see a brick red colour appear. If it’s not present, then you’ll just see the blue, which is the Benedict’s reagent. And that is testing for glucose. Testing for starch is simplicity itself. You simply add a drop of iodine. Now, iodine will turn blue-black in the presence of starch. It will stay, like, a yucky brown color if there’s no starch there. To test for proteins, you’re going to add Biuret reagent. I don’t know if I’m pronouncing it right, but I’ll spell it right now. And what will happen if you’ve got protein is you will see a lovely mauve / purple colour. Where are you rushing off to, Lyra? And then lastly, testing for fats, you’re going to use the emulsion test. So first of all, start by adding some ethanol to your sample – which is a type of alcohol. Then you’ll add some water. If fat is present, you will see a milky white emulsion, or suspension, and it will just go cloudy, effectively. We’re going to use the same thing in order to help you remember these definitions because there’s a hierarchy. They all build on each other. So remember, organelles are the small structures found inside cells. So, an example could be mitochondria; it could be nucleus; it could be chloroplasts. Now, a cell, in that case, therefore, is a group of organelles working together to perform the same function, and you’re going to use those words to surround each of your answers. Now if that’s not making sense, I’ll move on to the definition of a tissue, and you’re going to say it’s a group of cells working together to perform the same function. And then if you move on to an organ, well, it’s a group of tissues working together to perform the same function. And then lastly, you’ve got an organ system, which, as you might expect, is a group of organs working together to perform the same function. First of all, when you’re talking about the heart, you need to know about the structure of the heart, you need to know the names of the vessels feeding in and out. And I’m going to help you with that. First rule is to remember that arteries carry blood away from the heart always. So artery, away. So that’s a way to remember it. And veins carry blood towards the heart. Now, the heart has four chambers that you need to know. Those are the atrium – left and right – and the ventricles – left and right again. So I’ve got a diagram here which is really simplified, but I promise that it does provide the information you need to know. Now, I find it easiest learning it like a box because I think some of the heart diagrams out there are very complicated. But I will show you one later on so you know that I’m not lying to you. Right. So, if you draw a box and we divide it into four, make sure the top two portions are smaller than the bottom two because the atrium – the atria, which is plural – are smaller than the ventricles, so if you divide your heart into a box, the top two compartments are the atria – the left and right. Make sure you get the left and right sides the right way around. Underneath, you have the much larger compartments of the heart, so that’s why we’ve drawn our boxes bigger, and this is the left and right ventricle. Okay, so, we’re going to draw two vessels coming out of the ventricles because blood enters via the atrium, flows through the atrium into the ventricles, and flows out of the ventricles via two vessels. What did I say earlier? The name of those vessels is the arteries, and we just need to assign their correct scientific name. So blood that enters via the left atrium into the left ventricle will leave the heart in the biggest artery, the biggest blood vessel in the body, and that’s the aorta So make sure you get the spelling of that right. And if we go to the other side, blood which enters via the right atrium feeds into the right ventricle. That will leave via the pulmonary artery. And now I’m just going to talk a bit about naming. The word pulmonary means to do with the lungs, so that tells you that the pulmonary artery is feeding blood to the lungs. Because it’s the circulatory system, it means it’s a complete cycle, so it doesn’t really matter where you start, as long as you kind of end up back where you started from. So I’m going to start, for some reason, at the lungs. So what happens in the lungs? Right. Well,the lungs are the organ where gas exchange takes place and oxygen enters our bloodstream. So what’s gonna happen: Oxygen is going to enter the blood by the capillaries and the alveoli in the lungs. It’s gonna enter the red blood cells, so it’s going to be carried in the pulmonary vein. Why? Because pulmonary means to do with the lungs; vein because we’re bringing blood back to the heart. So it’s gonna be entering, and it’s going to be entering on the left-hand side of the heart. Just a quick aside point: the two sides the heart are split into oxygenated and deoxygenated side, so, as that sounds, it means that the heart is split into a side which contains oxygen and a side which doesn’t. So the left-hand side always contains oxygen, and that’s why I’ve drawn the vessels red, meaning that it’s full of oxygenated blood. So, our oxygenated blood enters the heart via the pulmonary vein into the left atrium. From there, it flows into the left ventricle, and then it flows out of the heart via the aorta, where it goes around the rest of the body in order to deliver oxygen to cells which are respiring, as they need it for aerobic respiration. So you can imagine that the blood then becomes deoxygenated because all the cells have taken the oxygen. That means that we need to pick up more oxygen. However, this blood can’t just return to the lungs by itself. It needs to go via the heart in order to create that pump. So, we have our deoxygenated blood returning to the heart, and this time, remember, it’s going to enter via vein, and we’re going to call this vein the vena cava, which is the main vein of the body. So it goes via the vena cava into the right atrium. Flows from the right atrium into the right ventricle, and then it’s going to leave the heart via an artery as usual, and this time, we’re going to call it the pulmonary artery. Why? Because it’s to do with the lungs because this blood will be returning to the lungs. (no audio) Actual liquid portion of your blood is called plasma, and it’s like a yellow liquid, and that carries dissolved things like sugar and urea and carbon dioxide back to the lungs, where it is removed from your body. We’ve mentioned red blood cells. Obviously, there are also white blood cells, and they play a really important role in your immune response – so basically, protecting you from pathogens and microorganisms. So, remember, they do that by ingesting pathogens or by producing antibodies which can destroy them. Lastly, you have platelets, and these are small fragments of cells, and what their role is is that they clot the blood at the site of a wound. So if you cut yourself, it won’t just bleed indefinitely because your platelets will come along and they will seal in that blood. (no audio) Now, remember, when we’re talking about the immune system, we’re talking about – remember what happens when you have bacteria, and they enter your bodies, and they make you ill. And obviously you get sick, but for most people you don’t stay sick for, like, years and years and years. So we’re talking about how our body actually fights against those bacteria, and that is your immune system, and it’s specifically your white blood cells, and they have some fancy names. So, for example, if a cold virus enters your body, what happens is white blood cells called lymphocytes come along, and they recognise specific molecules on those cold viruses, and we call those molecules antigens. So a very specific lymphocyte will come along, and it will release antibodies, which will go and attack that cold virus, and it will help destroy it, and that’s how our immune system actually works. Now, the way vaccinations work: remember, that’s what happens if you’re going to somewhere tropical on holiday or as a kid, you’ll have probably had a lot of vaccinations, maybe against HPV, meningitis, that sort of thing. And the way vaccination works is it kind of speeds up your immune system. You have an injection, and what they’re injecting is a weakened or dead form of the pathogen. Now, remember, the pathogen is the disease-causing organism. So in this case, it’s the flu virus; it could be the polio virus; it can be many different types of pathogen. But what a vaccination is is it’s a dead form – dead or weakened form of that pathogen. Why does it need to be dead or weakened? Well, obviously, you don’t want it to enter your body and make you ill because that will make that vaccination completely pointless. So, they inject it into your body. Very specific lymphocytes will come along. They’ll recognise the antigen on that pathogen, and they’ll start releasing antibodies. And some of those lymphocytes then turn into memory cells. And that’s a really clever, cool thing because a memory cell literally memorises that particular pathogen – so whether it’s the flu virus or the polio virus – so that if you happen to come in contact at some other point in your life with that polio, with that flu virus, your body already has memory cells which are waiting, and the moment that that polio enters your body, you’ve got massive numbers of memory cells that will start booting out lots and lots of antibodies. Those antibodies will go and attack and actually stop you getting ill in the first place. So vaccinations are really, really important. Herd immunity is just a term describing when a large number of people get vaccinated against something. Because something like measles virus will need a large number of people to infect, so it will infect a person who will then go and infect another person that will go and infect another person, and that’s how the measles virus is spread. Now, obviously, if you go and vaccinate loads of people against the measles virus, they are going to not be enough people for the measles virus to spread properly between, and that’s how you actually get herd immunity against something like measles. Basically means when a large number of people are vaccinated against a specific disease. Now, the types of vaccinations you might have heard of are things like MMR, which is measles, mumps, and rubella, and they may ask you that. And I’ve already mentioned you may be vaccinated against something like polio. If you’ve been on holiday, it might be tetanus or cholera, typhoid – all pretty nasty diseases that need vaccinating against. The nervous system is the way in which we pick up the changes in our surroundings. And the nervous system brings about our response. So, in order to understand what’s going on, first of all, we need to understand a few key words. So, there are special cells which transmit the information, and we call those information nerve impulses, and the cells are called neurons. If we have lots and lots of these neurons together, then we have nerves. So, to begin with, I’m just going to talk you through the sense organs because these are the things which pick up the change in our environment. So, crucially we have our eyes, which are sensitive to light. We have our ears, which are sensitive to sound and balance, and that’s what keeps us upright. Our nose and tongue are sensitive to chemicals that we can smell and taste in our food. And our skin is sensitive to many things including pressure, temperature, and pain, and that’s what keeps us safe. So we register that something is painful, so we withdraw our hand or whatever it is away from the thing which is causing pain, so it is really important that our nervous system is working really well. So, I’m gonna start with talking through the pathway involved in a response. We, first of all, start with our stimulus, and that’s really the thing that’s causing us to know that something has changed in our environment. So the stimulus could be seeing something, it could be touching something hot, it could be pressure – anything, really. So in this situation, I’m going to say that I’ve – I don’t know – that I’ve touched a warm cup of tea – not really hot, just warm. So the stimulus is the heat from the tea. Then, the next step is the receptor, and these are the receptors in my skin in my hand which tell me that actually, yeah, I felt something. It’s temperature. It’s warm. So the receptor receives that information about the warmth, and it sends along the first neuron, and because this neuron is involved in the sensing of the stimulus, we call it the sensory neuron. That travels along, and it reaches the CNS – the central nervous system. And this bit is key in sorting out our response. So, the central nervous system consists of the spinal cord and the brain. And the brain is going to decide what it wants to do about it. So in this case, I’m going to decide that I want to pick up my tea. Then, the information – the impulse – passes from the central nervous system down the motor neuron, which is a second type of neuron, and it passes along to the effector. Now, the effector is ordinarily a muscle or a gland. So if the effector is a muscle, it will respond by contracting. So, in this case, I will literally contract my muscles to pick up my tea. Or if it’s a gland, then it will respond by secreting a substance or a hormone. Right. So, we have gone from our motor neuron to our effector, and then that has literally brought about our response. So if we’re talking about the steps involved in this whole process, we can start with our stimulus, then list our receptor, the sensory neuron, the central nervous system, the motor neuron, and the effector, and then the response. Now, there’s a second type of response you need to know about, and that’s the reflex arc – the reflex response – and this is an involuntary thing. (no audio) I’m going to go through the diagram of the eye, starting from the very front. And the very first layer, you have the cornea, which is kind of like a protective coat on the eye which is see-through to allow light into the eye. And the cornea is part of the eye which is responsible for bending the light as it enters the eye, and we call that bending of light refraction. Remember, the pupil’s just a hole. It looks black, but it’s just a hole. It’s not really a structure, and that’s surrounded by the iris, which is different colours. Mine’s brown. Accommodation is simply the fine focusing of the eye, and that’s when the lens changes shape to allow the light rays to converge on the retina in the correct place. So in the situation where you’re looking at an object really close to, what will happen is your ciliary muscles contract, the suspensory ligaments become slack, and that enables your lens to become short and fat so that the light rays which come into the eye, they’re bent very strongly by the lens. Let’s take another situation. So when you’re looking at an object really far away, the amount of bending that needs to take place to those light rays is far less than if you’re looking an object really close to. So in that situation, you find that your ciliary muscles relax, your suspensory ligaments become very tight – they become taut – and that pulls your lens really long and thin. And because it’s thinner, it means that the light rays will be bent less. I hope that makes sense to you. And why are we trying to bend those rays? Well, it’s so that the light rays come together. They converge on your retina. And the retina is at the back of your eye, where light rays are focused onto. And what the retina does is it contains light receptor cells which are sensitive to light, and they basically change the information that they receive into an electrical signal which your brain can then interpret. And where the optic nerve leaves the retina, you’ll find that there’s a blind spot. (no audio) Today’s video is on homeostasis, and that is maintaining a constant internal environment within our bodies. I’m going to be looking at water level, blood sugar levels, and temperature. And I’m going to start by looking at water levels. Now, we need to keep our water level – our water content of our blood within a safe range, and we do that by monitoring the amount of water present. And what monitors that is the osmoreceptors, which are present in the hypothalamus, which you can find in your brain. So, what happens is the blood flows through the hypothalamus, and the hypothalamus decides if there’s too little or too much water, and depending on that, a different response will be brought about. Let’s take the first situation, which is when there’s too little water. So in this case, we want to produce less urine so that we can maintain more of that water, keep it back into our bodies and keep our blood water levels up. And my Fitbit is vibrating; that feels really strange. So what happens in this case is the osmoreceptors will detect that there is too little water in our blood, will send a signal to the pituitary gland to release more ADH. Now, remember ADH is a hormone that stands for anti-diuretic hormone. A diuretic is something which makes you wee, something like coffee or tea. So an antidiuretic hormone will be, like, effectively anti-weeing hormone, so it will actually help you conserve more water. So it might help you if you can actually remember it like that. So this excess of ADH travels in our blood to our kidney, where it acts on the collecting duct. So what happens is the ADH acts on the collecting duct, and it makes the walls more permeable. So what that means is that more of the water flowing through the collecting duct is reabsorbed back into the blood, thereby increasing our water levels. This means that there’s less water left over to travel to our bladder, and therefore, as a result, we’ll produce less urine. So our urine in this case will be more concentrated, it will be more yellow, it will be smellier, and it will be lower in volume. Let’s take the opposite scenario. If we’ve drunk too much water, our osmoreceptors will decide that there is too much water in our blood, so it will send a signal to the pituitary gland to release less ADH. That means that less ADH acts on the collecting duct, less water is reabsorbed into the blood, and therefore, more water is available to form urine. So our urine in this case will be higher in volume, it will be less concentrated, and it will be less yellow, and it will be less smelly altogether, so more pleasant I would say. (no audio) Right. Whistled through that topic. Now we’re going to look at blood sugar levels. It’s really important that we maintain our blood sugar levels within a safe range. After we’ve eaten, what happens is we need to lower our blood sugar levels, so insulin will be released from the pancreas. Now, what insulin does is it causes glucose to be converted into a storage compound which we call glycogen. That glycogen is stored in the liver, thereby removing the excess glucose from our blood. However, if we haven’t eaten for a while or we’ve done a lot of exercise, we’ll find that our blood sugar levels will decrease rapidly. So we need to up them. So in this circumstance, the pancreas releases a second hormone, this time called glucagon – not to be confused with the storage compound of glycogen – and that glucagon causes the glycogen in the liver to be converted back into glucose, thereby increasing our blood sugar levels. (no audio) Third topic: we’re talking about temperature. There are thermoreceptors in our brain, i.e. receptors which are sensitive to temperature. And they’re present again in the hypothalamus. And they will decide if our temperature is either too high or too low. Remember, we need to keep within the perfect range because of our enzymes because if it’s too high, the enzymes denature, they don’t work anymore, or if it’s too low, the enzymes don’t work fast enough to catalyze our metabolic reactions. So if we are too cold, what will happen is we will shiver, and that contraction of our muscles to cause us to shiver generates heat energy. Second of all, our vessels, rather than vasodilating will vasoconstrict. That means they narrow, which pulls them away from the surface of our skin and means that less heat is radiated. And then finally, the hairs will stand up on our arms. Remember, air is a very good insulator. And what that will do is it will trap a layer of insulating air close to our skin, and it will mean that heat is lost less rapidly by conduction. If we are too hot, what will happen this time is the opposite: we’ll find that the hairs lay down on our arms because we don’t need that effect so strongly. We don’t need to trap insulating air because actually we want to encourage heat loss. The vessels in our faces, they will vasodilate. That means they widen. That means the blood flows closer to our skin, so therefore, the heat can be radiated off our face more quickly, and you’ll see us all going pink after we’ve exercised, for example. And then lastly, we sweat. Now, sweating is a great way of reducing our temperatures because what happens is that sweat needs to be removed from the surface of our skin through evaporation. The evaporation of the sweat requires energy, and that takes heat energy away from us, again lowering our temperatures. (no audio) So, let’s start and think about what our kidneys are like. First of all, they’re two fist-sized organs, and they sit in the base of your back around here. They’re super important because they’re involved in creating urine, and they do a hell of a lot of excreting of substances which, if they were allowed to burn up in our bodies, would be pretty poisonous and toxic. So they’re major excretory organs. So what does the word excretion mean? This is one of those definitions you just need to learn. So, excretion is the removal of waste products of metabolism from the body. So, metabolism is all the chemical processes which occur in the body, and therefore waste products are produced, and we need to remove those, and that’s what excretion is: the removal of waste products of metabolism. So there are loads of different things we need to excrete, things like urea. Now, urea is the breakdown product from excess proteins in our diet because we’re not allowed to store excess protein, so what happens is it gets broken down into a substance called urea, which we then have to excrete using our kidneys. We also produce a lot of heat. We produce water. We produce lots of salts. We produce carbon dioxide. Obviously, carbon dioxide is lost through our lungs. Excess salts are lost through our skin. So we’re taking stuff into our body, and then we’re removing all the things which could become toxic. There is one thing you do need to know, though, and that’s the definition of egestion. Don’t get that confused with excretion. It’s quite a common problem. Egestion is not excretion. Egestion is simply the removal of faeces from the anus. I know that’s really awkward. You need to write it like that to make sure you get the marks in the exam. So, pooing is different from excreting. Just to be super horrible about the whole thing. Just a quick overview of the kidneys. So there are two kidneys. They receive blood via the renal artery. Now, renal is just the word we used to associate with the kidney. So renal artery, renal vein: we’re talking about the kidney. So the renal artery provides the kidney with blood. The kidney filters the blood, removes all the poisons, all the excess salts, et cetera, and then it produces urine. And so it has two real functions: one’s filtering and the other one is creating urine because we need to make sure the water content of our blood remains constant so that it’s not too watery or it’s not too concentrated. Let’s start by looking at the anatomy of the kidney. So the outermost edge is called the cortex. And we will talk about the nephron in way more detail, but this is where you’ll find most of the nephrons. Then beneath that is the medulla – the middle part – and that’s where the loop of Henle hangs out. Then you have the pelvis, and that’s the white structure that comes out of the kidney, and that’s where the collecting ducts run down into. And then finally you have the ureter, and that’s simply a tube which transports urine that’s made by the kidney to the bladder, where it’s stored. Then it passes along the urethra to leave the body. So let us talk about the nephron in a huge amount of detail. So these nephrons are teeny, tiny structures, and they’re the ones which carry out filtering. So, there are, like, over a million nephrons in the kidneys. And what happens is blood comes along the renal artery, and it enters what’s called the afferent arteriole, and that’s just a really wide part, and then it enters a pillory structure called the glomerulus, and then it feeds back out, and we have an efferent arteriole, and the crucial thing is is that the afferent – the arteriole coming in – is wider than the afferent arteriole – the arteriole coming out – and what that does is it creates pressure, and it creates pressure between the glomerulus and the Bowman’s capsule, which is the structure which forms the first part of the nephron that surrounds this blood capillary, the glomerulus. So what you find is ultrafiltration takes place. And all that means is that substances are forced out of the blood into the kidney, and the sort of substances we’re talking about are things like glucose, salts – both sodium chloride and potassium chloride – urea, bile salts, and amino acids. Crucially, proteins are not allowed to enter the Bowman’s capsule, and that’s because the basement membrane is too small to allow those proteins to squeeze through. So you really shouldn’t find protein in your urine, and if you do, that’s kind of a sign that something wrong, and you could have diabetes or something. But anyway, so autofiltration occurs, which is filtering under pressure. Therefore, glucose, some amino acids, salts, things like that, urea are allowed to enter the Bowman’s capsule. Then you enter the next part of the nephron, which is called the proximal convoluted tubule. Proximal, first. You can say first convoluted tubule if you prefer. And then convoluted meaning twisted. So we can see that this part of the nephron is very twisted. The following part is the loop of Henle, or the loop of Henle, depending on how your teacher pronounces it, followed by the distal convoluted tubule – the second convoluted tubule is an easier way of saying that – and then finally the collecting duct, where the urine is actually produced. Just so you know, selective reabsorption is a process whereby certain substances are reabsorbed back into the blood, i.e. selective reabsorption. So this happens in the nephron, and it happens in the convoluted tubules, and it’s important that we reabsorb all the good stuff – so things like glucose and amino acids and some salts – and that we allow certain other products to just carry on into our urine because they’re the toxic ones. They’re the ones we want to lose. So that’s things like urea. Now, remember, urea is produced by the breakdown of proteins because we can’t store excess protein in the body, so we have to break it down into urea so we can lose it in our urine. Now, your definition for diffusion is that it’s the net movement of particles from an area of high concentration to an area of low concentration. This happens very readily. Things change when you’re talking about active transport. This is the total opposite in that this time, the net movement of particles is from an area of low concentration to an area of high concentration, and this requires a lot of energy. I’m going to be covering osmosis, and I’m going to be talking about the effect that placing various types of cell in different solutions has on them, and I’m going to give you an overview of the definition and talk about keywords like plasmolysis, crenated, and all these sorts of things which sound hard, but I promise that they’re not. So osmosis, crucially you need to know that it’s to do with transport in water. So our definition is: it’s the net movement of water particles from an area of high concentration to an area of low concentration through a partially permeable membrane. Make sure you have partially permeable membrane in there because there will be a mark available for that. Going to talk now about the effect of placing plant cells in different concentrations of solution. Now, remember that a plant cell differs from an animal cell in that it has a cellulose cell wall, a vacuole, and chloroplasts. So what happens if you place the plant cell in a very salty solution is you’ll find that there’s more water – so higher water concentration, higher water potential – within the plant cell, and that there’s a lower water concentration, lower water potential surrounding it; therefore, water, based on our osmosis definition, will move from the inside of the plant cell to the outside. This will cause the whole cell to shrink, so you’ll get this very distinct shrunken appearance. And we call this plasmolysis because what happens is the cell membrane pulls away from the cell wall, and you end up with this characteristic wave line. So obviously, this isn’t a great situation for our plant cells to be in. If we turn this around and we place our plant cell instead in pure water – so that’s with a very high water potential, a very high water concentration – there’s obviously going to be more water outside of a plant cell compared with inside. So water is going to enter the plant cell this time by osmosis, and you end up with the plant swelling up and becoming turgid, which is where the cell wall kind of swells out and it looks quite balloon-like. This is a great situation for our plant to be in because when the cells are turgid, it means that the stem can support itself, for example, and the plant doesn’t wilt. The reason why this plant cell doesn’t explode is because of the existence of the cell wall, which helps support the cell and prevent the pressure of the water inside breaking the cell. So it’s very important for plants. However, if you have a solution instead now that the plant is in that’s kind of the same as what’s inside the plant itself, you just end up with a very regular-looking cell. It won’t be particularly swollen, but it won’t be shriveled either, and that’s because the movement in and out of the cell will be the same. Let’s take animal cells now. I’m going to talk about a red blood cell, for example. So, we’re going to place our red blood cell in a salty solution. Because there’s more water inside the red blood cell – similar to the plant cell – water will leave the cell and move outside, and the little cell would become all shriveled up again, and we call that crenated or crenellation. So our red blood cell isn’t very happy at all. Now, if we put our red blood cell in a pure water situation, you find that water comes flooding in. Why? Because there’s much higher water potential, much higher water concentration outside of the cell. But the problem is this time that the water floods in, but because there’s no cell wall, there’s nothing to stop the ever-expanding red blood cell from bursting, so effectively it pops, which is a horrendous situation for our cell to be in because it’s been destroyed. We call this process whereby it bursts haemolysis. And it’s a very dangerous situation for your body to be in. Make sure you’ve got the diagrams of each straight in your mind. And for me the easiest one is the female one, so I’m gonna start there. So, you’ve got a couple of ovaries. Remember that they produce eggs and that they’re also responsible for manufacturing that hormone called oestrogen. Remember that oestrogen, first of all, builds up the lining of the uterus. Secondly, it inhibits the production of FSH. And thirdly, it causes the secondary sexual characteristics that you see in females, such as hip widening, menstruation, pubic hair growing, et cetera, et cetera. The ovaries connect to the uterus through tubes, and those tubes are either called fallopian tubes or the oviduct, depending on what you want to call them, and it’s in the oviduct that fertilization occurs, and that’s when the sperm hits the egg, so the two gametes combine, and that is fertilization, and that occurs in the entrance of the oviduct. At that point of fertilization, a zygote is formed, and that’s the first cell that’s made, and it implants itself in the wall of the uterus, and it will undergo divisions by mitosis in order to create an embryo, which will then develop into the fetus, and later you’ll have a baby, which you’ll give birth to. So that’s where the baby plugs into – into the wall of the uterus. However, when the woman gives birth, what happens then is the cervix will open, and the baby will have to pass down the vagina and out. It will be in the real world. Let’s look at the male one now. Slightly more complicated. So, you’ve got a common tube this time, and that is called urethra, and it passes along the shaft of the penis, and what that does is it allows semen and urine out of the body. And connected to the urine, you will see the bladder, which is obviously where urine is stored after it’s been produced by the kidney. So, the urine passes out of the body via the urethra. You’ve got several other glands and things – things like the prostate gland, seminal vesicles, and what they do is they add fluid to the semen. Then you need to actually produce the semen. And the main part of the semen is the sperm. And remember it’s the testicles which will make the sperm, and the testicles sit in a sack of skin called a scrotum. The reason why they sit in that scrotum rather than being more internally found is because the testicles need to be at a lower temperature in order to develop the sperm properly, and so that’s why the testicles effectively hang outside of the body, so that they can produce the sperm and it can mature properly. Now that sperm, once it’s made, will pass along that sperm duct – or you can call it the vas deferens. Then it will meet the urethra, where it will pass out of the head of the penis, into the vagina. And then if the woman has an egg released at the same time and all the conditions are right, then a baby will develop, and it will be born nine months later. It’s quite a hard topic to explain. I hope I managed to find some diagrams to help you with this. Don’t forget that the testicles also produce testosterone, which is a hormone responsible for secondary male characteristics such as voice deepening – so the breaking of a voice – pubic hair growth, broadening shoulders, that sort of thing in guys, so be aware of those roles. We need to know about the menstrual cycle. Being a cycle, it means it starts and finishes. And it happens every single month for around 28 days – is an average length of cycle, but that can obviously vary from woman to woman. Right. So, we’re going to start by naming some hormones because that’s always a good place to start, and I’m going to start with FSH. So FSH is follicle stimulating hormone. Now, these follicles are basically the eggs inside the ovaries. And it’s kind of like before they’re fully mature. And so by definition, follicle stimulating hormone kind of means it’s the hormone which causes the follicles – the eggs – to mature. So if you’re asked in the exam the role of the FSH, I would say that it causes the eggs in the ovary to mature. FSH’s second job is to stimulate the ovary to produce oestrogen, and that’s another very important hormone. But first of all I’m going to talk about lutenising hormone, LH. Now, lutenising hormone is a very important hormone because what its role is is to cause ovulation, and ovulation is the exact point at which the egg is released from the ovary. So if they ask you what ovulation is, you say the release of an egg from the ovary. What is the role of LH? To cause ovulation. So at that point, the egg pops out, and it will travel down the fallopian tube. Next up, we need to talk about oestrogen. As I said before, oestrogen is produced by the ovary. Now, oestrogen is really important because it causes the uterus lining to build up, and it’s important that that uterus lining becomes very thick in order to support the egg which, if it gets fertilized, will implant itself. Progesterone is essential because it maintains the wall of the uterus. And without progesterone, what would happen is the uterus lining will flake away, and that would be a period. So it’s really important that at all stages during pregnancy that progesterone levels remain really high. It’s worth noting that oestrogen also inhibits FSH production. That’s really important. Why? Because if you’re building up the uterus lining in order for a zygote to implant, the last thing you want is more eggs in the ovary maturing. So, remember, we have our green plant, and it needs to absorb carbon dioxide and water. And the water, remember, comes in at the roots. And two products produced are oxygen and also glucose, which is what the plant’s actually after. If they ask you for the balanced symbol equation, remember, it’s sixes, sixes, sixes. So, for example, we need 6CO2s plus 6H2Os – which remember, that’s six waters – and that goes to C6H12O6, which is the formula of glucose, plus 6O2. Remember that photosynthesis is an endothermic reaction, which means it takes in energy. Now, if they ask you about how the leaf is adapted for photosynthesis, you’re going to mention the following things: first of all, it’s broad and flat, which means it has a lot of surface area to attract that sunlight, to absorb that sunlight onto the leaf; it’s thin, and that means that diffusion distances – so things which are diffusing are things like the gases – that is a nice, short distance, so they’ll diffuse nice and quickly; you’ve got air spaces within the leaf, which will enable those gases to diffuse again. Remember, the palisade layer has lots of chloroplasts, and those chloroplasts contain chlorophyll, which trap that sunlight. And chloroplast is actually where photosynthesis takes place. Within the leaf, you’ve got a vein, and the vein brings up water from the roots and actually supplies the leaf with all its water needs. You’ve got stomata and guard cells, lastly. Remember that they’re super important. The stomata is a pore inside the leaf which actually allows carbon dioxide into the leaf or water to leave. And those guard cells surrounding the stomata actually control whether the stomata is open or not. So obviously, if it’s open, CO2 can get in, and if it’s closed, it cannot. (no audio) Now we’re moving on to the limiting factors which affect photosynthesis. So there’s three ones you need to remember. That is: carbon dioxide, temperature, and light intensity. All three of these things can limit the amount of photosynthesis can take place. Let’s start with the most simple one of those: temperature. Obviously, if you have low temperature, then those reactants involved will be moving slowly. It’s a bit like the effect of temperature on enzymes. So, the particles don’t come together fast enough, and therefore the whole reaction slows down. So low temperatures will lead to low rates of photosynthesis. Obviously, at very high temperatures, you’ll see that the enzymes involved in photosynthesis will be denatured, so you don’t want to raise that too high. In terms of carbon dioxide, obviously, low amounts of carbon dioxide will limit photosynthesis because actually it will slow down how much photosynthesis can actually take place because without CO2, no photosynthesis. Same with light. Light powers the whole process. If you’ve got low light levels, then you’re going to see low rates of photosynthesis. And remember that all of these three things interact with each other. So you can have the most amount of CO2 ever, but if you have a low temperature or a low light level, you’re not going to see that much photosynthesis. You could have the most amount of light ever, but if you’ve got low CO2 levels, again, we’re limiting the amount of photosynthesis. Just going to quickly touch on experiments involving photosynthesis, and they tend to use something called elodea, which is pondweed, to measure the rates of photosynthesis. And the way if they’ll do this, remember – hopefully you’ve been shown this at school – is they’ll get a piece of that pondweed, that elodea, and because it’s pondweed, it lives in water, so you pop it into a beaker full of water. Because it’s photosynthesizing, we’ve already said that oxygen will be given off, and you can actually see those bubbles being released by the elodea. So sensibly, you can actually count the number of bubbles that appear within a certain time frame – so something like 60 seconds is good here. So if you count the number of bubbles produced in 60 seconds, you can kind of get an estimation of how quickly photosynthesis is taking place. And if you alter the conditions that the elodea is experiencing, then you can actually alter the rate of photosynthesis in that way. So one popular way is to alter the light intensity. So you may or may not have got a lamp, popped it at 10 centimetres away from that pond weed, counted the number of bubbles. 20 centimetres away, counted the bubbles. 30 centimetres away, counted the bubbles. So in terms of your independent variable, remember, that’s the variable you’re changing. So in this case, it would be the light intensity, i.e. the distance the lamp is away from that elodea. The dependent variable is what you’re measuring. So you’re measuring the number of bubbles in 60 seconds. And the control variable is everything you need to control to keep it a fair test, to keep it the same. So obviously, you need the same species of pond weed, so elodea in this case. You need to have the same length of pond weed because obviously a longer pond will photosynthesize more. You need to have the temperature the same, the pH of the water the same, and technically the concentration of carbon dioxide the same, although that will be quite hard to control. So let’s dive straight in. Okay. Hopefully my diagram will have flashed up, and you’ll see lots of different layers. We’re going to start at the topmost layer. This is known as the waxy cuticle, and really that protects the leaf from the sunlight. Under that, it’s a bit like a sandwich. We have an upper epidermis. Then we get into the really important part, and that’s the palisade layer, or the palisade mesophyll. Look at those cells. They’ll very closely resemble what you’ve learnt to be your regular plant cell, and that’s because they are. And the crucial thing about these cells is they’re packed full of chloroplasts. And remember, chloroplasts are full of chlorophyll, and that’s where photosynthesis actually takes place. Underneath, you have the spongy mesophyll, and surrounding that are air spaces, and that’s really important because it allows our gases – oxygen and carbon dioxide – to circulate nicely. Some diagrams will include a vein, and all you need worry about is that’s where water enters the leaf. And remember, that’s entered via the roots in the xylem: X Y L E M. Okay. Moving on. We have a lower epidermis. Remember that sandwich I talked about? And then we get to the really important part, and that’s the bottommost layer. And you will see a stoma and two guard cells. The guard cells control whether the stoma is open or closed because depending on the surrounding conditions, the plant will choose to close its stoma or open it. I’m just going to talk a little bit about transpiration right now. So remember, transpiration is the evaporation of water from the surface of the leaf, and that occurs through small pores called stoma or stomata. You will find that they are increased transpiration rates depending on certain conditions. The amount of the water that leaves the leaf is based on the amount of water surrounding the leaf because it’s all to do with the diffusion gradient. So obviously, water is moving from high concentration to low concentration. First of all, if it’s dry, that means that there’s very few water molecules outside the leaf because the air is dry, which means there’s lots more water inside the plant, and therefore, based on what you know about diffusion, the movement of water will occur very quickly. So water will be leaving the stoma, and you’ll have high transpiration rates. Equally, if it’s wet, if it’s humid, there’s a lot of moisture in the air, a lot of water molecules, so the difference between the amount of water inside and outside the leaf will be very little, so our gradient will be small, and therefore, transpiration won’t be happening very much. Next, if it’s windy, all these water molecules will have lots of energy, and they’ll be moving away from the leaf very quickly, and so you’ll find that as the water diffuses out of the leaf, it gets blown away quickly, and again that means there’s not very much water surrounding the leaf, so transpiration rates fall behind when it’s windy. And equally, they’ll be much lower when it’s still air because the water particles, the water molecules won’t be being blown away. Next up, if it’s sunny, you’ll find that transpiration rates are really high. Why is that? It’s quite a complicated answer. If it’s sunny, it means that lots of photosynthesis is taking place. What does photosynthesis require? It requires sunlight, but it also requires CO2. CO2 enters the leaf by the stoma, so in order to photosynthesize, the plant needs to open its stoma in order to allow that CO2 in. If you open the stoma, it means water will automatically start to leave. At nighttime, you find that the stoma tend to close because there’s no sunlight, there’s no need to have it open, and no need for CO2 to enter, so you will find that less transpiration happens then. At high temperatures, you find the transpiration rates are increased, and that’s just because the kinetic energy of the water molecules has increased. And like the windiness thing, it means that they’re just moving out of the stoma, out of the leaf, much faster. And if you’re in doubt with this and you can’t remember anything I’ve said or anything you’ve learned in your lessons but you’re in the exam, just think about the conditions which would help which would help washing to dry, and you’ll certainly pick up some marks. Now, the last thing I wanted to talk to you about was the potometer. A potometer is used to measure the rate of transpiration. Here’s a really terrible diagram of a potometer. So what’s happening in this diagram is that water is leaving the leaves by transpiration and therefore, in order to replace that water, you find that the stem draws in more water from the tube it’s attached to, which is attached to a reservoir. And there’s a little scale that you can use to measure how far the water has moved and how much water’s been absorbed by the plant, effectively. So you just read it off of the capillary tube, and that will tell you how much water has been absorbed. We assume that the rate of water uptake and the rate of transpiration are identical. Now, what sorts of things could you do to the leafy shoot in order to measure the rates of transpiration under different conditions? Well, you could put a bag over the plant in order to increase the humidity and hopefully see a decrease in the rates of transpiration and the rate of water uptake. You could use a hairdryer to emulate windy conditions, which would blow water away from the surface of the leaf. You could shine a light on the leaf. You could increase the temperature. So there’s lots of things. However, in the exam, they like asking you about the setup of the equipment and any flaws and any ways you can fix them and how you can minimize error. So first of all, you want to avoid getting water on the leaves, so you might need to dry them if you accidentally splash them, and that’s in order to prevent humid conditions being inadvertently given to your experiment when you’re not actually interested in that. Second of all, you need to cut the shoot underwater, and that’s to stop air getting inside the stem and preventing water movement because you’ll have an airlock. You also want to cut the shoot at an angle to make sure it has a really good fitting into the rubber tubing. It’s important to use Vaseline® to seal the joints in order to stop air getting in places where it ought not to be. Finally, you use the capillary tube to magnify the amount of water uptake so you can actually see what’s going on. So, let’s first of all remind ourselves what the plant organs are. These are things like roots, stem, leaves, and flowers. Now, plants need to be sensitive. They need to respond to various stimuli, and we call plants’ response to stimuli a tropism. So there’s your definition: a tropism is a plant’s response to a directional stimulus. And what that really means is that plants need to respond to the sunlight. Why? Because the stem needs to grow upwards in order to support the leaves so they’re closer to the Sun so that they can carry out photosynthesis more effectively. What about the roots? Well, the roots need to grow away from the Sun, they need to grow downwards. Why? Because they need to anchor the plant and stop it blowing away. And they also need to be found in soil so that they can absorb water and mineral salts, which is important for their growth and photosynthesis, of course. So it’s very important that when you plant a seed, that when it grows, the shoot grows upwards and the roots grow downwards, and that’s where tropism comes in. Now, there are several different tropisms, things like phototropism – photo- meaning to do with light. So that’s a plant’s response to light. Things like the stem will show positive phototropism, i.e. they’ll grow upwards towards the light, and some roots will show negative phototropism. They’ll grow downwards, away from light. A second type of tropism you need to know about is called gravitotropism or geotropism. Those are two names for the same things, and as the name suggests, that’s a plant response to gravity. So things like roots will have a positive geotropism, whereas shoots will show a negative one. And then finally, the last well-known one is hydrotropism – hydro- meaning to do with water. So therefore again some roots will demonstrate positive hydrotropism, whereas shoots will display negative. But this really does vary between plants, so some will just not really be affected by either gravity or the light. But on the whole, that’s the summary, and that’s the overview you need to know about. Right. So how do plants actually respond to these various tropisms? Well, it’s through plant hormones, and it’s a specific family of hormones called auxins, and what they do is, depending on their distribution in the plant – will cause cells to grow really quickly or really slowly. So, for example, auxins, when they’re more concentrated in the stem, what you’ll find is they’ll distribute themselves on one side of the stem, and what they do is they cause the cells to grow faster, so you end up with the one side where the auxins are collected, the cells grow longer and faster, and that means that the other side can’t keep up so you get this automatic bending, and that’s how you see plants that can bend towards the sunlight. It’s because of the auxins and their uneven distribution. So they’ll collect on the side away from the light. They’ll cause those cells to grow rapidly and grow longer than the cells closer to the light. And then that will cause the whole stem to bend towards the light. I hope that makes a lot of sense to you. I’m going to start by labelling this picture of a plant. I think I’ve meant it to look like a tulip, but I don’t really know. So let’s go. First of all, the obvious things. These are the petals. And then we’ll look at the male parts of the plant first of all. So up here, we have the anther. And that’s where the pollen’s found. And then that is supported by the filaments … which are the stalks on which the anthers rest. And together they form the male parts of the plant, and we call it the stamen. And the way you can remember that is it’s men, which is male, so that’s the male part of the plant. Let’s look at the female parts of the plant now. This is the stigma. And remember, that’s where the pollen grain lands. And underneath, it is supported by the style. And under that we have the ovule, containing the eggs which are the ova, found inside the ovaries. So the female parts of the plant collectively is called the carpel. And that consists of the stigma, the ovary, and the style. So remember, pollination is just exactly the point when the pollen lands on the stigma, and that’s all you need to write if they ask you. Remember, the pollen is the male gamete, and a male gamete is a male sex cell. So in mammals that would be sperm, but in plants we call it pollen. And then for both mammals and plants, the female gamete is called ova or egg. Finally, don’t forget to label this, and these are the sepals, and they’re kind of like leafy bits which are found at the bottom of the ovary. So let’s just talk a little bit more about pollination. Remember, if you need to define pollination, you need to talk about the fact that it’s when pollen lands on the stigma. So here’s our pollen grain. And remember what happens at this point is a pollen tube grows … causing the pollen to be transported down the style into the ovary, where it can finally meet the ova. And it’s that moment when the pollen binds with the ova that fertilization occurs. Once fertilization has taken place, a number of changes take place inside the ovule and the ovary. So remember the ovary is just the room which contains all the ova, and each ova has a little surrounding coat which is called the ovule, and what happens here is the ovule itself develops into a seed, whereas the ovary – so the larger place where everything is found – will later develop into a fruit. Now we need to look at the main differences between insect-pollinated plants and wind-pollinated plants. Starting with the insect-pollinated one, remember, first of all, they have nectaries, and these are stores of sugar. Because remember, the insect isn’t just coming to pollinate the plants because it wants to, because it’s feeling nice. The reason why it does it is it’s literally happening by accident. The insect’s actually interested in the nectar, and it accidentally scrapes along the anthers and picks up pollen, which is exactly what the plant’s after. So you do need to mention, first of all, it has a nectary, whereas the wind-pollinated one doesn’t. Next, let’s look at the straightforward things, which are the petals. So in insect-pollinated plants, the petals are much bigger than those of the wind-pollinated ones, and they’re also far more brightly coloured compared with the wind-pollinated ones. In terms of attracting insects, we have a very strong scent with the insect-pollinated one and pretty much no scent with the wind-pollinated one. Let’s talk about the position of the anthers and the filaments now. As you can see, in the insect one, you find them inside, enclosed in the petals. So, effectively, when the insect goes to get its nectar, it scrapes along the pollen, and it is automatically picking it up, whereas you don’t want that – you don’t want those enclosed anthers and filaments in wind-pollinated plants because, effectively, you wouldn’t actually get any pollination occurring because they’d never be able to blow away the pollen. So you want them like you see here on the right-hand side, where everything’s super exposed, so that when the wind blows, all that pollen will go flying across the air and hopefully land on another plant. Lastly, let’s much mention the stigma. So in insect-pollinated plants, the stigma is super sticky so that the pollen grains attach nice and easily from insects, whereas wind-pollinated ones have very feathery stigmas, and that’s so that they can catch pollen grains blowing in the wind. (no audio) I’m now going to talk about a food chain. Remember, a food chain is just a way of showing what eats what. We call each stage of the food chain a trophic level, and it’s just literally what I just said. A trophic level is a stage of a food chain. Food chains always start with producers, and these tend to be green plants. The reason it starts with a producer is because a producer absorbs energy from the Sun. So yes, food chains begin with producers. What eats a producer? Well, it’s the consumer. Because it’s the first consumer, we call it the primary consumer. Next up, we have the secondary consumer, which eats the primary consumer, and then we have the tertiary consumer, which eats the secondary consumer. Let’s again touch on a few other key words: herbivore, carnivore, and omnivore. Remember, herbivore just eats vegetation, clearly. A carnivore eats meat. And an omnivore, like us and pigs, they eat both mixture of vegetation and animal matter. You will tend to find that the last organism in the food chain, which will tend to be the tertiary consumer, we call that the Topcon. Well, the reason being is that very rarely does it get eaten by something else. So something like a polar bear never has to worry about being eaten because it is the top carnivore in an Arctic food chain. Sometimes when we’re feeling jazzy, we like to arrange food chains in terms of pyramid of numbers, and pyramids of numbers are just a way of showing the exact numbers of each organism at each level of the food chain. So you’ll find that the bottom tends to be the plant, so it could be the number of oak trees, the number of leaves, the number of grass blades, for example. And then the level above that will be something like a rabbit, which feeds on the grass. And then the level above that could be a fox, which eats the rabbit. And the thing is, we can’t guarantee that the pyramid of numbers will indeed be pyramidal in shape, and that’s because sometimes the individual numbers of the vegetation aren’t always bigger than the rabbits, for example. And I’m actually going to explain what I mean by that. The point is, if you had an oak tree as the start of your food chain, there’s only one oak tree, so actually the base of your pyramid would be really narrow in comparison to the number of, let’s say, caterpillars feeding on it. So a pyramid of numbers isn’t actually a very good way of representing a food chain because you end up with some really strange-shaped pyramids. Therefore, we choose to use a pyramid of biomass. A pyramid of biomass is the total mass of organisms at each trophic level, and it’s irrespective of numbers, and it gives us a much better shape to our pyramid. A question they always ask is: Why doesn’t all the energy inside the grass become part of the rabbit? So say grass is our producer, rabbit is our primary consumer. The reason being is this: because there’s a huge amount of loss – energy loss – between the grass and the rabbit, and it’s actually approximately 90%. And the reason being is that, first of all, the rabbit doesn’t eat all of the grass. For example, it may not eat its roots. Second point is that the grass isn’t entirely digestible, so some of it will pass straight through rabbit and become its faeces. And third of all, a lot of that energy will be used by the rabbit to move and also to keep it warm, and for respiration, et cetera, so you can’t guarantee that all that grass actually transfers itself into the mass of the rabbit. So yeah, 90% of energy is lost at each level of the food chain in movement; in maintaining the body temperature of the animal; and also in excretary losses, like producing urine and urea; or egestion, such as the production of faeces. So yeah, huge amounts of energy losses. The next thing I just wanted to touch on is if you have any predator-prey cycle questions, remember that the prey numbers will always peak before the predator numbers, and that’s because there’s a time delay, as the predator numbers can only peak when there’s enough prey to feed on. And then as soon as the prey numbers drop, then you will see a secondary drop in the predator numbers, and that’s because there’s less food for them to feed on. (no audio) So let’s talk about the water cycle. Now, it’s a cycle, so it doesn’t really have a start and an end, but you just need to pick anywhere and keep going with it. I think the easiest thing is to start with evaporation. So water evaporates from the surface of our oceans, our seas, our streams, and it ends up in the air . Now it cools and condenses, and in this way, we’re forming clouds. Now, we know what comes out of clouds; that’s rain, and the fancy word for rain is precipitation. So water falls out of our clouds as precipitation and ends up on the ground, where there are lots of plants and trees willing to absorb that water through their roots. How does water end up back in the atmosphere? Well, remember these plants transpire, which means they lose water from the surface of their leaves, and also all living organisms respire. So the water’s entered the atmosphere again, and then the continual cycle of evaporation, condensation, precipitation, transpiration, and respiration continues. Just remember an odd word: percolation, which is when water runs off the ground through gaps in the rocks. Let’s start with where carbon is, and carbon is found as carbon dioxide in the air. Now, which organisms use carbon dioxide? Well, that would be green plants that photosynthesize. So your first point will be that plants take in carbon dioxide for photosynthesis. Then your second point is that they use the carbon to make sugars. But the point is the plants take in carbon to help them grow. So then what happens is the plants respire, as do all living organisms, and remember that the byproduct of respiration is carbon dioxide, so that’s the first bit of CO2 being released back into the atmosphere. What else do plants do? Well, they die. And what causes the rotting of the plants? Well, it’s microorganisms. So the microorganisms feed on those plants, and guess what. They respire, so there’s more CO2 returning to the atmosphere. Next up, the plants might not die straight away. They may be eaten by animals. So the animals will munch on that plant, the carbon in the plant will now become part of the animal’s body, and then guess what. The animals respire, so they release CO2 back into the atmosphere. At some point, the animals will die. And then we know what happens here. We know that the microorganisms will feed on those bodies, the carbon will become part of the microorganism, and then the microorganism respires, again releasing CO2. So, as you can see, the start of the cycle starts with CO2 being absorbed in photosynthesis. And then basically, you just need to talk through all the different ways in which CO2 is released back into the atmosphere, and that’s obviously going to be by respiration. Now, the nitrogen cycle is not as hard as it seems originally. You need to just start at one point and then build on from there. And the point I always start at is the fact that there are nitrates in the soil, and these are our nitrogen-containing compounds. So what will happen is the plants will absorb those nitrates through their roots, and they’ll use them in their growth, and they’ll use them to create amino acids and vitamins and things like that. So the nitrogen is now very much part of the plant. At this point, an animal may come along and eat that plant; it feeds on it. And therefore, that nitrogen that’s part of the plant becomes part of the animal’s body. At some point, both the plants and the animals will die, and then decomposers will take those animal bodies, the plant bodies, and they will convert them to ammonia. And then you need to know a very crucial name of a bacteria here, which is nitrifying bacteria, which will convert ammonia first to nitrites and then to nitrates, which will be returned to the soil. So nitrifying bacteria are definitely very good things. Unfortunately, there are things called denitrifying bacteria, which exist in the soil, and what they do is they convert the nitrates into nitrogen gas in the air, which is really frustrating for the farmers because obviously they want as many nitrates as possible in their soil to help a plant to grow properly, so if you’ve got these annoying denitrifying bacteria doing the opposite job, that’s quite frustrating. However, luckily, there are other good bacteria which exist, and we call these nitrogen-fixing bacteria, and you can find these on special plants called legumes, and these are the peas and the broad beans, et cetera. And what they have is they have special root nodules, which are kind of like bulbous projections that stick out of the root, and they contain these nitrogen-fixing bacteria. And what these do is they convert nitrogen in the air into the plant’s very own supply of nitrates, which is fantastic because it helps the plant grow really quickly. They also add an excess to the soil. So basically, farmers will choose to crop rotate, which means plant various different crops over the different years, and they’ll choose to plant legumes and peas and beans every four years or so because what that will do is it will add a huge injection of nitrates to the soil and help all the plants that grow there afterwards grow better. So these are amazing things. Lastly, just remember things like lightning. Lightning adds nitrates directly to the soil. And remember the Haber process, which I’ve done a video on in chemistry, to do with equilibria. Remember the Haber process – and that’s the manufacture of ammonia – is the primary way in which we can actually make nitrogen-containing fertilizers. But, yeah, that’s the whole overview. You just start with your nitrates in the soil being absorbed into the plant, being eaten by other animals, being decomposed. Then the bacteria cause the ammonia to be converted to nitrates, which go back into the soil. And then add a few extra details too: the Haber process, nitrogen-fixing bacteria, and lightning, and you’re good to go with that six-mark question. It’s really not so difficult. So, I’m going to continue by talking about human impact on the environment, and we’re going to start by looking at eutrophication. Remember eutrophication is the process whereby rivers become devoid of all aquatic life, and that’s due to fertilizers and sewage being washed into the rivers. If that’s not making a lot of sense to you, I’m going to go through all the steps now. So step one sewage or excess fertilizers are washed into rivers. These fertilizers and sewage cause the rapid growth of algae, which is like a green plant. The algae dies due to competition for light because literally there’s just not enough light for all those algae trying to photosynthesize, so they die. The dead algae provide food for microorganisms. The microorganisms grow in number. And because they’re respiring aerobically, they use up all the oxygen in the river, and then before you know it, there’s no oxygen for the fish, so they all die. And I’ll probably add a summary right now so you can write it down. Next up, we need to look at acid rain. So remember, first of all, we need to look at the formation of acid rain and then the effects it has on the environment. So acid rain is caused by one of two things. First of all, the nitrogen and the oxygen in car engines reacts at the super high temperatures, forming nitrous oxides or nitrogen oxides, and they get released into the air. They react with water – obviously from the rain from the clouds – and that forms nitric acid. And so it’s a weak acid, and it will fall onto the ground, and it is acid rain. The second way in which we make acid rain is through sulphur impurities in fuels. So when those fuels are burnt, that sulphur reacts with the oxygen, forming sulphur dioxide. Again, that sulphur dioxide gas can react with water in the atmosphere forming sulfuric acid, which falls, again causing acid rain. Don’t get too stressed, by the way. This acid is very weak compared with the acid you get in your chemistry lessons. The point is, over time, it can have quite adverse, detrimental effects on the environment. And now we’re going to look at those. So firstly, acid rain damages trees. It also damages limestone buildings. I know that’s not an effect on the environment, but it’s worth mentioning. Make sure you mention in there limestone. And lastly, it gets into lakes and rivers and makes it too acidic for the poor aquatic animals to survive. Now I’m going to look at deforestation. Remember, deforestation just means the cutting down of trees. Why do people do it? It’s so that they can get land for farming. It’s so that they can actually harvest those hardwoods to make furniture out of. However, it can have many, many effects on the environment, and we need to look at those now. So first of all, deforestation means that those trees are cut down, which means they’re no longer alive, so they can no longer photosynthesize, which means they can no longer absorb carbon dioxide. That’s a bad thing because we want to remove that carbon dioxide from the atmosphere so that we don’t have global warming, so we don’t enhance the greenhouse effect. The second problem with cutting down trees is – the thing about trees is they have a huge amount of carbon locked up in their trunks, and the moment you cut them down, it releases all that carbon dioxide Into the atmosphere, so you have, like, a double whammy effect, so it’s like doubling the amount of carbon dioxide released, leading to global warming. I will discuss all the global warming issues very shortly. Next up, if you cut down trees, then you’ll no longer have roots in the soil, so that topsoil which before was anchored down using the roots, effectively what happens is, when it rains the topsoil gets washed away, which leaves the ground very infertile. And also, when that top soil washes away, it washes away also all the nutrients, and again, you might end up with eutrophication issues in surrounding streams and rivers. So deforestation has huge, huge knock-on effects on the environment. It’s not a good thing. Let’s look at methane. Remember, guys, that methane is released by cows farting. I know that’s disgusting, but literally, it is. Like, a huge amount of methane is produced by cows farting. You might want to write it a bit more nicely in your exam, the fact that it’s – their digestive processes cause the release of methane gas. The second way in which humans contribute to methane is through farming rice because, effectively, the rice grows, and it releases a huge amount of methane as it grows in paddy fields. So you want to talk about cows farting and the rice paddy fields as being major sources of methane. The reason why methane is such a terrible thing is because, again, it’s a greenhouse gas, and I think, because I keep talking about greenhouse gases, global warming, I think it’s about time I actually told you what happens there. So with all this carbon dioxide, methane, water vapor – these are all examples of greenhouse gases – what happens is, there’s a layer of gas – so the greenhouse gases around the Earth – and what happens is the Sun rays come down, and some of them bounce back out into space and some stay in our atmosphere. Now, the ones which stay in our atmosphere heat up our planet, and that’s an important process because it keeps us at the right temperature. However, if you add more greenhouse gases to that layer, you’re going to trap more of the sunlight energy, which causes an increase in the temperature, and that is what global warming is. In terms of the effect global warming has is that – the major issue is that it melts polar ice caps. That means, like, the massive icebergs that sit in the Arctic and the Antarctic. And if you melt a huge amount of ice, it’s going to cause the sea levels to rise, and then when the sea levels rise, you’re going to flood low-lying land, which leads to a loss of habitat for animals. It leads to loss of biodiversity – so that just means the variation, the huge variety of species that exist on our planet. There’s other things which you need to mention. It causes a change in the migration patterns of birds because some birds will fly from one part of the Earth to the other in winter and summer, and when global warming happens, they get confused, so they go to the wrong place. It will lead to a change in the distribution of animals – so literally where they live – because obviously, those animals that live on low-lying land will no longer be able to live there. And in general, it will lead to a change in the Earth’s climate. So it will rain more, rain less, be hotter in winter, cooler in summer, that sort of thing. So global warming has massive, massive issues. You may be asked a six-mark question on this, so I’m going to write a little summary of all the effects of global warming so you can write them down for yourselves. A random thing you do need to know about is peat bog destruction. Now, peat bogs, they’re, like, these areas of, like, marshland, and they’re full of peat, which is like this brown, muddy substance. And the thing about peat is that it’s very acidic, and it has very low oxygen levels. It provides a very unique environment. And so not much can survive. So plants don’t break down fully. But the good thing about peat bogs is they hold an enormous amount of carbon. They’re massive carbon stores. So again, that’s great because we don’t want to enhance the greenhouse effect. However, the issue is people are destroying the peat bogs because peat is a wonderful fertilizer like compost for the garden, so gardeners like to buy it to sprinkle on the ground because it will mean that their roses grow better. So with that destruction, you get a huge release of carbon dioxide, again leading to global warming and everything that I just mentioned. The issue is, peat bogs take thousands of years to, like, develop, and so we can’t replace them as quickly as we’re destroying them. So yeah, peat bog destruction is not a good thing. So what you will need to know is the good things and the bad things about fish farming and also how you would do it. So what you would do firstly is you would feed the fish on a high-protein diet. That’s always worth a mark. You would need to control any disease, so you may need to add antibiotics. And the reason why disease will be more prevalent is just because the fish are so close together. The water needs to be very carefully controlled in terms of the temperature, oxygen availability, and the build-up of waste because that could lead to disease. There should be pesticides added to the water in order to kill any parasites. And you need to make sure that no predators can actually reach the fish and kill them and eat them that way. In terms of disadvantages, well, of course, you’ve got issues with antibiotics being fed to the fish. And actually what that may do is it will end up in the food chain, and there’s a lot of questions over – question marks over whether humans want to be consuming these things The other thing is, is that the fish have to be fed little pellets which are made of other fish, and it’s actually estimated that more wild fish get killed as a result of trying to feed the farmed fish compared with if we didn’t have any farm fish, which is obviously ridiculous. Now, remember, proteins are biological molecules, and they’re made up of long chains of amino acids, which fall together to form protein. Protein are very useful molecules. We find them, obviously, in enzymes. Keratin, collagen – these are structural proteins. So proteins are very much a part of our lives, and they’re very important, and we need to know how to build them, and it’s quite a complicated process. Now, in terms of controlling what proteins get made, you need to dictate the order of the amino acids in order to produce the correct protein, and this is a very complicated process involving genes. It’s important that you know the definition of a gene, which is that it is a section of DNA which codes for a particular protein. Now, DNA stands for deoxyribose nucleic acid, and it is the basic building blocks. DNA dictates what our personalities are like, what we look like, our characteristics, so it’s a very important molecule. And you need to know that it is arranged in a double helix, which effectively means it’s like a ladder that twists on itself … hence the helix. The DNA, it’s found inside chromosomes, and chromosomes are these structures that we find in the nucleus of the cell because remember, the nucleus contains the genetic information. So we have our nucleus, which contains chromosomes. Remember, it contains 23 pairs in most body cells, the exception here are our sex cells the gametes, so the sperm and eggs. And we have our chromosomes, and inside we find our DNA. What we need to do in order to produce proteins is we need to copy that DNA. We need to bring it to the ribosome inside the cell, and we need to carry out a process called translation, which will actually cause the amino acids to be built up in their correct sequence. But I digress. That’s a bit further on in the video. If we look at our DNA molecule, you’ll remember that there are four bases: A, C, T, and G. So that’s adenine, guanine, thymine, and cytosine. And so – and what they do is they pair up – complementary base pairs is what we call it. So if you have an adenine, it always goes with thymine, and if you have a cytosine, then it always goes with guanine. And the way to remember which one pairs with which is just remember that the straight letters, the A and the T, go together, and the curved letters, the C and the G, go together. Now, so we’ve got the middle part, so these bases are the rungs of our DNA molecule, and you find that the – what are they called? – the longer parts, the ladder parts – oh, that’s horrible English. But they’re made out of a sugar called ribose and a phosphate, and it goes sugar, phosphate, sugar, phosphate, sugar, phosphate, and those are the side parts of the ladder. And then the rungs are these base pairs that we’re talking about – the A, the C, the T, and the G. Now, these base pairs are held together by hydrogen bonds. In order to replicate, we need to undo those base pairs, so we need to pull apart the ladder. And there’s an enzyme which does this, and it will pull the ladder apart, and what it will do is expose both of the base pairs. Then what happens is we’re going to produce a complementary strand of one of those base pairs. So mRNA leaves the nucleus via the nuclear pore, and it goes and attaches to a ribosome. It sits on the ribosome with all of its bases exposed, and then what happens is a second type of RNA called tRNA comes along, and what it does is it brings along the amino acid which corresponds to the correct sequence of mRNA bases, so you’ll have three bases because three bases is what each amino acid is dictated by, and then depending on that sequence found, you’ll bring a different amino acid along. Before you know it, you’ve got a chain of amino acids building up, each joined by a peptide bond, and that’s basically how our proteins begin their lives – although they’re not alive. But that’s basically what happens: you’ll have your chain of amino acids which can then fold and become a polypeptide and a protein. We need to divide our cells and for lots of reasons. So for one reason – one reason is for growth; the second reason is for repair. Sometimes you damage your skin, and it is important – for example, if you cut yourself – that the cell that replaces the damaged skin cell is genetically identical, it’s exactly the same because if it’s not, that might lead to some uncontrolled cell division, which might lead to cancer in a tumor, so it’s not what you want. You want your original cell to be replaced with the exact same cell, and we call that a type of cell division mitosis because what happens is the parent cell copies itself and produces two genetically identical daughter cells, and these are clones. So remember, a clone is a genetically identical cell or individual. Okay, so we are copying our cells, and therefore we only need one parent cell, and we produce two daughter cells, and they are genetically identical, and this is an example of asexual reproduction. It’s very straightforward. It happens very quickly, and it only requires one parent. So there’s some relative advantages of mitosis. Meiosis is different. The crucial thing about meiosis is it creates genetic variation. And how does it do that? Well, what it’s doing is, you copy the chromosomes, but before they divide, they swap over portions of each chromosome to create genetic variety, and then the cells divide twice, producing four cells, each with half the number of chromosomes. This is what we use for sexual reproduction, and it creates genetic variation, which is very important from an evolution / natural selection point of view, and it is used to produce gametes, such as sperm and eggs. (no audio) Remember with genes, we have the word alleles. An allele is a different form of the same gene. So to put that into context, it’s something like your eye colour. The alleles for eye colour could be blue, could be green, could be grey, could be brown, okay? However, if you look around in the everyday population, you’ll find that most people out there really have brown-coloured eyes, and that’s because the allele for brown eye colour is dominant. Right. To introduce another tricky word, we’re going to talk about the word phenotype. Now, the phenotype is the physical appearance of the gene. So for example, in my case, it would be brown eyes. However, I don’t know if I have two of the same alleles or two different alleles, and that’s because brown is dominant. So what that means is that my genotype – and what the genotype is, that’s the genes you have inside you, so the actual genes that you contain – my alleles could be big B little b (Bb) and I’d still have brown eyes, even though I have one allele for brown eyes and one allele for blue eyes. Or it could be two big Bs (BB), and then that would mean that I was homozygous because what homozygous means is that you have two copies of the same allele. So that could be Big B Big B (BB), brown eyes. However, if I’m heterozygous, that would mean I’d have one copy of brown, one copy of blue, so that’s big B little b (Bb). However, I would be heterozygous, but I still be brown-eyed because brown is dominant over blue. Sorry that does sound quite tricky. I’m thinking that as I’m saying it. Now, where does the big B and the little b come from? We just use letters to assign alleles, basically, so we know what we’re talking about. So if you’re talking about an allele, you’re going to give a little letter. Now, you can pick whichever letters you want, but people tend to pick letters where there’s a real differentiation, a real difference between the capitalization and the small lettering, like Bb. But now we’re going to have a look at actually drawing Punnett squares. (reading visual aid) Okay. So there’s a really simple way of doing this, and it’s really important that you follow the same steps, and then the answer will just plop out on your lap. And I promise, it’s really easy. So first of all, write two headings: mother – can you read that? – and father. Okay, I write phenotype here on the left-hand side. It helps me as well with the understanding. So what’s the phenotype? Right. That’s what the mother and father actually have in terms of eye colour. So I’m going to write blue … and father, brown. Right. Genotype. I’m going to pick the letter B because it makes sense. Both of the alleles begin with B. So the genotype for the mother. Now, I know automatically that it’s going to be small b, small b (bb), and that’s because blue is recessive, which means that you need two of the same alleles in order for the feature to be exhibited in the person. So … Oh, I can hear the cat meowing. Can you hear the cat meowing? Anyway, but the fact that it’s homozygous also tells me that the letters will be the same, so I’m going to write bb. And there’s the mother’s genotype. Right. The father. Because they’re brown-eyed, remember because they’re brown they could be Big B, little b (Bb) or two Big Bs (BB). However, heterozygous tells me that the alleles are different, so it’s going to be big B, little b (Bb). Right. Next one is going to be gametes. Right. A gamete is a sex cell, so that’s going to be the egg in the mother’s case and the sperm in the father’s case. Now, remember … So first of all, remember that a clone is a genetically identical individual that has been produced asexually from one parent. The first way in which we clone animals is using embryo cloning. So in this situation, you get, for example, your prize bull, your prize cow. You mate them, and then obviously the sperm from the bull will fertilize the egg from the cow, and it will develop into first a zygote and then an embryo. Then what the farmer does is that he washes out the uterus of the cow to remove the embryo from the cow, and at this point the embryo is so early on in its life that actually all the cells are unspecialized. They’re undifferentiated, and what that means is that the cells can develop pretty much into anything at that point. So if you break up that early embryo into lots of different parts, you can then transplant the mini embryos into the uterus of what we call surrogate cows. So these are, effectively, incubating machines where the calf will grow for nine months, but they won’t actually be genetically related to the calf in question. Finally, adult cell cloning. This is slightly more complicated, but in this situation what you do is you get a body cell – so, like, a skin cell, a muscle cell, any kind of cell – a body cell, though, that has a full number of chromosomes – from whichever animal you’re trying to clone. You remove that body cell, and you take the nucleus out of the body cell, and that is what we call enucleating the cell. So remove that nucleus from the cell. Then we need to get an egg cell from a surrogate or just any other cow, and what we do is we remove that nucleus too, and we discard it, and we place the nucleus from the body cell inside the egg cell … because the egg cell is going to just contain this information. And we use an electric shock to cause them to fuse together. At that point, we just need to implant that new egg cell into the uterus of the surrogate, so that gets implanted into the uterus, and then it undergoes mitosis to divide to create the embryo, which will then develop into the new calf. (no audio) Right. Lastly, let’s look at the cactus. Now, the cactus obviously, again, lives in the hot environment. It stores water in its stem. It has spines rather than leaves, and what that does is it prevents water loss by transpiration. They have shallow, extensive roots. I hope that makes sense to you. Basically, their roots spread out a very long way – metres and metres away from the actual plant. And what that means is if it rains, then what can happen is that the plant can absorb as much of that water, even if it’s really far from them. Darwin was a scientist who was alive in the 19th century, and he went off on this voyage across the world on a ship called HMS Beagle, and he went to the Galapagos Islands, and he found many, many different species. He found lots and lots of different fossils. And he looked at those and he thought, “Hold on. I don’t think religion can explain everything we see here.” So what he did was he then developed the theory of evolution and the theory of natural selection. Now, don’t get them confused. The theory of evolution states that all species alive today and many more millions which have become extinct over the years originated from small life forms, which later evolved and became the more complex life forms we see on today’s planet. Natural selection is his mechanism for describing that evolutionary change. And this is now the really important bit, and every single question I’ve seen on natural selection goes like this. So the crucial point is that there is variation within a species. That means that I’m different to my family. It means that you’re different to your sister unless, obviously, you’re an identical twin. And what that means is that some individuals in that species are better suited to the environment compared with others. And they always like to use things like swordfish or elephants to describe this – or giraffes. So let’s take the giraffe, for example. So some giraffes back in time had longer necks, which meant that they were able to reach leaves higher up the tree. Now, all those giraffes that didn’t have the long necks died, so it meant that those with the longer necks were more likely to survive and breed successfully, thereby passing their genes on to their offspring, so therefore, their offspring had longer necks too. And if you were to provide a four-mark answer on that, you would literally just say there was variation within a species due to mutation. Those with the better adaption, such as having a longer neck, are more likely to survive and reproduce. And then lastly, you need to say that they pass those genes on to their offspring. Now, at the same time as Darwin, there was a man called Lamarck who was around, and he had a slightly different idea, which we now discount, and he thought that if you used a particular feature many times over your lifetime, then it was passed on to your offspring. So, for example, with the swordfish, he thought that if you carry on using your longer sword, then it meant that the offspring would also have longer swords. But they soon realized that wasn’t the case because, for example – I think there were experiments where they chopped off the mice’s tail, and according to Lamarck, then the baby shouldn’t have had a tail, either, but actually, they were born with tails, so that kinda disproved his theory. Now let’s get back to Darwin. This was a very controversial thing that he was saying because it disagreed with the widespread belief that God created the world. So you need to be able to say why wasn’t Darwin’s theory accepted immediately, and the first reason is because there was insufficient evidence. People just didn’t know enough. Secondly, they had no idea of the mechanism of inheritance. Things like genes weren’t known about then. So although he had this idea, he couldn’t really explain how it came about. And thirdly, the widespread notion was actually that it was religion and God that was responsible for the origins of the Earth. So his arguments very much disagreed with what was the common thought. It’s more of a common-sense topic, but I do know that some of you struggle with it, so I don’t want to ignore it. Right. Adaptation. That is to do with organisms and them having characteristics which mean that they are better suited for a particular environment. I’m going to take it from an animal that lives in a cold environment, an animal that lives in a hot environment – and we’re also going to have a look at plants. So let’s just dive straight in to looking at things like polar bears and arctic foxes. Right. What you will find is, first of all, they have white fur. Why? For camouflage, so they blend into the surroundings. They have – something like a polar bear, I’m gonna actually use that. They have large feet. Why? To stop themselves sinking into the snow. Because if you have larger feet, then you have a larger area to spread their weight over. It’s the same reason why skis work – is because you just don’t sink in as much. Right. What else do they have? They have they have small ears. Why? Because they need a small surface area here because they want to minimise their heat loss to the surroundings, as obviously, it’s very cold and they want to keep warm. They’ll have a thick layer of fat for insulation. They’ll have thick fur for the same reason: to conserve heat. If they ask you stuff like, oh, adaptations for catching prey, then you need to talk about sharp teeth for tearing into flesh, long legs so they can run fast. But they don’t usually ask that; it’s more of adaptations to their environment. If we take an animal in a hot country, something like a camel that lives in a desert, again, they have large feet so that they don’t sink into the sand. They have large ears to increase the surface area to maximise the amount of heat that they can lose because obviously they don’t want to over heat under the hot desert sun. You’ll find that they have long eyelashes, which will prevent sun getting into their eyes. They have thin fur, thin layer of fat to minimize the amount of heat that they maintain. Now we’re going to talk about selective breeding. Selective breeding is all about humans breeding plants and animals with desired characteristics. Such characteristics could include a high crop yield if you’re talking about a vegetable. It could include lots of milk if you’re talking about a dairy cow. If you’re talking about a beef cow, it could be calves that have a massive amount of muscle so that there’s plenty of food to be got off them. It could even be domestic dogs being bred with certain characteristics, such as a docile nature if they’re in your house because obviously you don’t want an aggressive dog in your house. Now, if you were going to selectively breed a cow that had a lot of muscle on it, you’d obviously pick a bull that had a lot of muscle. You’d pick a female cow that had a lot of muscle. You’d interbreed them. You’d expect their child, their baby cow, their calf – there’s the word – to have very similar characteristics. So, yes, it’s all about picking parent plants, parent animals with the sort of characteristics that you’re after. So, microorganisms. Remember, these are very, very tiny organisms, and there’s several different groups of them. First of all, you’ve got the Protista – I hope I’m saying that right. Protists. And these are single-cell organisms, and they include things like amoeba. We’ve got fungi, and the ones which you’ll be familiar with are things like yeast which we use to make our bread rise or we can use in fermentation to make alcohol. We have bacteria. This isn’t to do with biotechnology, but they may ask you in the exam for some examples of bacteria, so name anything like tuberculosis or salmonella or E. coli. If they ask you about viruses, you can give examples such as HIV and flu. Hey, cat. (cat meowing) Hi. Do you know that I don’t like this topic? Have you come to say hi? What do you think? There’s the camera. Boring face. Pop yourself on here for me. Oh, where was I? So, key examples of viruses are HIV and the flu virus. Yeah, and that’s really all I wanted to talk about. Now, we can modify the genes of these different organisms in order to get them to do things that we want them to do. So in the case of genetic engineering of insulin production, what you’re doing is you’re taking a human insulin gene, inserting it into a bacterial plasmid and basically getting it to do what you want it to do. Now, the thing about most of these microorganisms is that they work anaerobically, which means they don’t require oxygen to respire, and then based on that, they’ll produce different things. So if we add yeast, for example, to sugar, what you can produce is alcohol and CO2. Now, carbon dioxide is used to make bread rise, and the alcohol is, obviously, used to help manufacture our favorite spirits and wines and beers. Now, what you find is most of these microorganisms are grown in fermenters. Where did the cat go? And they may ask you a few questions about the fermenters. So, for example, if the bacteria do respire aerobically, they may ask you what the purpose of the air inlet is, and you would say to allow oxygen in, to allow the bacteria to respire aerobically. They may ask you what the role of the cooling jacket is, and that’s in order to maintain a perfect optimum temperature because remember, the enzymes will denature inside these microorganisms if they’re allowed to get too hot, and if it gets too cold, then you may find that they don’t actually work effectively enough. They’ll also have to control the pH. Why? Because again, if the enzymes reach a pH which is not their optimum, they may denature, which will alter their active site. So just be clear on the different conditions. Am I steaming up? Oh, my goodness. It’s so hot today. I mean, it’s what? It’s May, and it’s, like – it feels like it’s 30 or 40 degrees. I know you probably think I’m crazy, but it feels very, very warm. So let’s talk about beer making. Now, unlike wine, beer is made from barley. Now, barley does not contain sugars as its carbohydrate. Grapes do contain sugar. So therefore, that sugar can directly be used by the yeast in order to ferment and produce alcohol. However, we need some extra steps involving barley because its carbohydrate is starch. So first of all, we get some barley seeds. They need to germinate because they’ve been dormant for a while. So you need to add water to them to allow them to germinate. At this point, we’re not interested in them anymore, so we actually kill them by heating them, but what the heat does is it doesn’t actually denature the enzymes, so the enzymes are still good to go. We then add the enzyme amylase. Remember, amylase breaks down starch. So that amylase will break down the starch into maltose or glucose, depending on which one you’re happier with. Then we add something called hops. And what they do is they give the beer the very distinctive taste, and what we need to do at that point is add yeast because that’s our crucial microorganism which will actually produce the alcohol. So what the yeast does is it feeds upon the sugar and converts it into alcohol and CO2 – carbon dioxide. Lastly, we centrifuge the beer, which means we spin it. We filter it to remove any impurities that we don’t want. And we pasteurise it, which means we heat it a little bit to remove microorganisms. This is hard. What a complicated topic. (no audio) In the manufacture of yoghurt, we are not using yeast anymore. We are using lactic acid bacteria because what we’re doing is we’re converting milk into that cell or thicker thing we know as yoghurt. So first of all, what you need to do is you need to grab the milk and you need to pasteurise it, and what that means is heat it so that it kills any bacteria which are present. So what they do is they heat it to 85 degrees for approximately 30 minutes in order to kill any pre-existing microbes. After that, we homogenise the milk, and what that means is it means distributing the fat droplets in the milk equally and evenly throughout the mixture. Then we cool the milk down to about 45 degrees, and at this point we can add the bacterial culture, which will actually cause the milk to convert itself into yoghurt. 45 degrees: we incubate that milk, and what will happen in that case is the bacteria will digest some of the milk proteins, and it will also convert the lactose, which is the sugar found in milk, into lactic acid, giving it that distinctive sour taste. Lastly, we just cool that yoghurt down further, to about five degrees, and then at this point, we can start adding any of the flavourings which make yoghurt so delicious, unless you prefer natural, and then in that case, it is done. (no audio) So what is genetic engineering? Well, it’s when you alter the genes of an organism. And why do we want to do that? Well, it’s so that we can produce huge amounts of a required substance. So, the example we always look at is insulin. And remember that insulin is a hormone. It’s released from the pancreas. And what it does is it lowers blood sugar levels. For people who can’t produce insulin, they have type 1 diabetes, and they can get really ill, indeed, so it’s really important that they can inject insulin into their bodies. Now, back in the day, they used to use insulin from dogs and pigs and things, but this obviously wasn’t ideal. And if they ask you, in the exam, a question why, just say things like it wasn’t an exact match between the pig insulin and the human, so the human’s immune system would try to reject the pig insulin. And you could also talk about it from an ethics point of view. Obviously, there’s a lot of people out there don’t like the idea of pigs being specifically bred to be killed for insulin. So, many ethical issues. However, let’s now look at the detail involved in genetic engineering. So what we do, we tend to do exactly the same steps. We get our bacterium. Remember that bacteria doesn’t have a distinct nucleus, but it has little loops of genetic information, which we call plasmids. Plasmid is cut open using restriction enzymes. The gene is also cut out of the human using restriction enzymes. And then you use a different enzyme to stick them back together. This is the ligase enzyme. So that sticks the plasmid together with the insulin gene. And remember those sticky ends which attach both, but again, that is a lot of information. Then you now need to pop this recombinant plasmid back into a bacterium, and it will just grow naturally, and it will produce a huge amount of insulin. Now, remember another keyword, which is vector. Now, this is just a vehicle used to carry another organism’s genetic information. So in this case, it is the plasmid. So, yeah, lots of detail. The other sorts of things they might ask you – they might dip into enzymes here, and they’ll ask you: Why is it important that you keep the fermenter at a correct temperature, that it doesn’t get too high? Just say because if it gets too high, the enzymes denature and the substrate no longer fits the active site. The same with pH. If they say why is it bad to let acidity build up inside the fermenter, just say because outside of the optimum pH, the enzymes will denature blah, blah, blah, blah, blah. They may ask what gas is allowed in, and you would say oxygen to allow the bacteria to aerobically respire. See, I’m really thinking hard of all these questions that I’ve seen in the past to see if I can remember them, to give you top tips now, but don’t worry … (music)

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