Photosynthesis: Comparing C3, C4 and CAM

Photosynthesis is the process by which plants capture the energy of the sun to make carbohydrates. But there are minor variations in how plants do this, depending on the environment in which the plant lives. In this tutorial, we’ll look at the differences between the three types of photosynthesis: C3, C4 and CAM. Most plants – around 85% or so- carry out C3 photosynthesis. As we will see, the name C3 has to do with the type of carbon molecule produced during the process. C3 photosynthesis occurs in the mesophyll cells of the C3 plant, which are located just under the surface of the leaves and other tissues. Let’s take a quick look at how photosynthesis occurs in these cells. During C3 photosynthesis, CO2 is taken up and transported directly to the Calvin Cycle, where it is fixed into a 3 carbon molecule. Thus the name C3. If the stomata are open, CO2 moves in, and oxygen moves out of the cell. But under hot or dry conditions when the stomata are closed, oxygen accumulates which inhibits the amount of C3 that can be made. Some plants have evolved an adaptation that allows them to be successful in hot, dry conditions. These plants carry out C4 photosynthesis, instead of C3 photosynthesis. During C4 photosynthesis, CO2 moves into the mesophyll cell, and is immediately fixed into a 4 carbon molecule. Thus the name C4 photosynthesis. But the anatomy of a C4 plant is different from that of a C3 plant. A somewhat simplified version of this process is shown here. In a C4 leaf, chloroplasts are located in mesophyll cells, but they are also located in bundle sheaf cells which surround the leaf vein. CO2 moves from the mesophyll cells, into the bundle sheaf cells, where the Calvin Cycle occurs. This shields the plant from the oxygen build-up that would otherwise occur when the stomata are closed during dry conditions. A second way of addressing the problem of oxygen is to change the timing of photosynthesis. This is called CAM photosynthesis, and in may ways it is similar to C4 photosynthesis except the timing of the process changes. During CAM photosynthesis, CO2 is taken up during the night when the stomata are open, and is fixed into a 4 carbon molecule. It is then stored until daylight as an intermediate molecule, which gives the CAM cycle its name. This adaptation allows photosynthesis to occur in drier climates than in both C3 and C4 photosynthesis. A great example of a CAM plant is the cactus. Let’s do a quick review. During C3 photosynthesis, CO2 is taken up and transported directly to the Calvin cycle. The C4 pathway alters the location of photosynthesis, and the CAM pathway changes the timing. Both of these adaptations allow some plants to survive in environments that are less than ideal for the typical C3 plant.

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