Alternate Mechanisms
In hot arid climates, plants are faced with the problem of dehydration. In order to conserve water, plants can close down their stomata, which are microscopic pores in the leaves that allow gases and water to enter and exit. Thereby less water evaporates and leaves the plant. However, it also means less carbon dioxide can enter and oxygen concentrations build up in the plant. Under such conditions photorespiration may occur.
In photorespiration, rubisco joins oxygen, not carbon dioxide, to RuBP. This results in a two-carbon compound, which is later modified to carbon dioxide. This process is wasteful because it does not produce any ATP or sugars. In fact, it consumes these energy resources. Plants that are most susceptible to photorespiration are C3 plants, which fix carbon initially to form a three-carbon compound (3-PGA in the Calvin Cycle). This is because rubisco directly binds the substrate (carbon dioxide or oxygen) to RuBP without checking which it is first. Examples of C3 plants include rice, soybeans, and wheat.
There are two main ways to reduce photorespiration - C4 photosynthesis and CAM.
C4 photosynthesis: Leaves of C4 plants are unique in their anatomy. There are two different types of photosynthetic cells, mesophyll and bundle-sheath cells. The former of which is closer to the surface of the leaf, so carbon dioxide enters the mesophyll cells first. Here it gets joined to PEP (a 3-carbon compound) through the enzyme PEP carboxylase to form oxaloacetate (4 carbons, hence the name C4 plants). It then gets converted into malate, another 4-carbon compound, which enters the the bundle-sheath cells. Now the malate gets split into carbon dioxide and pyruvate. The carbon dioxide enters the Calvin cycle in the bundle-sheath cells, while the pyruvate goes back into the mesophyll cells to be converted into PEP, which accepts carbon dioxide to regenerate oxaloacetate.
Since the enzyme PEP carboxylase has no affinity for oxygen, it guarantees that carbon dioxide will be added to form oxaloacetate. However, the conversion of pyruvate to PEP requires ATP input, which slightly reduces the efficiency of photosynthesis. Examples of C4 plants are sugarcanes and corn.
In photorespiration, rubisco joins oxygen, not carbon dioxide, to RuBP. This results in a two-carbon compound, which is later modified to carbon dioxide. This process is wasteful because it does not produce any ATP or sugars. In fact, it consumes these energy resources. Plants that are most susceptible to photorespiration are C3 plants, which fix carbon initially to form a three-carbon compound (3-PGA in the Calvin Cycle). This is because rubisco directly binds the substrate (carbon dioxide or oxygen) to RuBP without checking which it is first. Examples of C3 plants include rice, soybeans, and wheat.
There are two main ways to reduce photorespiration - C4 photosynthesis and CAM.
C4 photosynthesis: Leaves of C4 plants are unique in their anatomy. There are two different types of photosynthetic cells, mesophyll and bundle-sheath cells. The former of which is closer to the surface of the leaf, so carbon dioxide enters the mesophyll cells first. Here it gets joined to PEP (a 3-carbon compound) through the enzyme PEP carboxylase to form oxaloacetate (4 carbons, hence the name C4 plants). It then gets converted into malate, another 4-carbon compound, which enters the the bundle-sheath cells. Now the malate gets split into carbon dioxide and pyruvate. The carbon dioxide enters the Calvin cycle in the bundle-sheath cells, while the pyruvate goes back into the mesophyll cells to be converted into PEP, which accepts carbon dioxide to regenerate oxaloacetate.
Since the enzyme PEP carboxylase has no affinity for oxygen, it guarantees that carbon dioxide will be added to form oxaloacetate. However, the conversion of pyruvate to PEP requires ATP input, which slightly reduces the efficiency of photosynthesis. Examples of C4 plants are sugarcanes and corn.
CAM: CAM stands for crassulacean acid metabolism. In this process, the plant's stomata close during the day to avoid water loss and open up during the (relatively cooler) night to take up carbon dioxide. The carbon dioxide is converted to organic acids and stored in the mesophyll cells until the morning. During the day, the light reactions produce ATP and NADPH, and the carbon dioxide stored in the organic acids are then released for use in the Calvin cycle.
Examples of CAM plants include pineapples and cacti.
Finally, note that in the C4 mechanism, the initial steps of carbon fixation are spatially separated, whereas in the CAM mechanism, they are temporally separated.
Examples of CAM plants include pineapples and cacti.
Finally, note that in the C4 mechanism, the initial steps of carbon fixation are spatially separated, whereas in the CAM mechanism, they are temporally separated.