Types of Photosynthesis for AP Biology
Practice problems for these concepts can be found at: Photosynthesis Review Questions for AP Biology
Plants do not always live under ideal photosynthetic conditions. Some plants must make changes to the system in order to successfully use light and produce energy. Plants contain a structure called a stomata, which consists of pores through which oxygen exits and carbon dioxide enters the leaf to be used in photosynthesis. Transpiration is the natural process by which plants lose water by evaporation from their leaves. When the temperature is very high, plants have to worry about excess transpiration. This is a potential problem for plants because they need the water to continue the process of photosynthesis. To combat this evaporation problem, plants must close their stomata to conserve water. But this solution leads to two different problems: (1) how will they bring in the CO2 required for photosynthesis? and (2) what will the plants do with the excess O2 that builds up when the stomata are closed?
When plants close their stomata to protect against water loss, they experience a shortage of CO2, and the oxygen produced from the light reactions is unable to leave the plant. This excess oxygen competes with the carbon dioxide and attaches to RuBP in a reaction called photorespiration. This results in the formation of one molecule of PGA and one molecule of phosphoglycolate. This is not an ideal reaction because the sugar formed in photosynthesis comes from the PGA, not phosphoglycolate. As a result, plants that experience photorespiration have a lowered capacity for growth. Photorespiration tends to occur on hot, dry days when the stomata of the plant are closed.
A group of plants called C4 plants combat photorespiration by altering the first step of their Calvin cycle. Normally, carbon fixation produces two 3-carbon molecules. In C4 plants, the carbon fixation step produces a 4-carbon molecule called oxaloacetate. This molecule is converted into malate and sent from the mesophyll cells to the bundle sheath cells, where the CO2 is used to build sugar. The mesophyll is the tissue of the interior of the leaf, and mesophyll cells are cells that contain bunches of chloroplasts. Bundle sheath cells are cells that are tightly wrapped around the veins of a leaf. They are the site for the Calvin cycle in C4 plants.
What is the difference between C3 plants and C4 plants? One difference is that C4 plants have two different types of photosynthetic cells: (1) tightly packed bundle sheath cells, which surround the vein of the leaf, and (2) mesophyll cells. Another difference involves the first product of carbon fixation. For C3 plants, it is PGA, for C4 plants, it is oxaloacetate. C4 plants are able to successfully perform photosynthesis in these hot areas because of the presence of an enzyme called PEP (phosphoenolpyruvate) carboxylase. This enzyme really wants to bind to CO2 and is not tricked by the devious oxygen into using it instead of the necessary CO2. PEP carboxylase prefers to pair up with CO2 rather than O2, and this cuts down on photorespiration for C4 plants. The conversion of PEP to oxaloacetate occurs in the mesophyll cells; then, after being converted into malate, PEP is shipped to the bundle sheath cells. These cells contain the enzymes of photo synthesis, including our good pal rubisco. The malate releases the CO2, which is then used by rubisco to perform the reactions of photosynthesis. This process counters the problem of photorespiration because the shuttling of CO2 from the mesophyll cells to the bundle sheath cells keeps the CO2 concentration high enough so that it is not beat out by oxygen for rubisco's love and attention.