Other Metabolic Processes

Photorespiration involves the intake of O2 and release of CO2, consuming energy in the process. Adaptations to combat the energy loss of photorespiration include the C4 and CAM pathways.
In photosynthetic organisms, photosynthesis is carried out regularly and serves as the main pathway for fixing atmospheric carbon and generating sugars. However, certain environmental conditions can trigger other metabolic pathways. For example, photosynthesis entails a great deal of water loss. However, many plants thrive in arid climates, where water loss from normal photosynthesis would be lethal for the plant. Examples include cacti and sugarcane. Furthermore, all plants undergo some amount of respiration (taking in oxygen, O2, and releasing carbon dioxide, CO2). When the update of O2 and release of CO2 is driven by light, it is known as photorespiration, which consumes energy and decreases the output from photosynthesis. This process in plants differs greatly from cellular respiration, the process by which organisms break down molecules for energy.


In photorespiration, rubisco adds O2 to RuBP, consuming energy and generating CO2.
In photosynthesis, rubisco adds a molecule of CO2 to RuBP. In photorespiration, instead of adding CO2, rubisco adds a molecule of O2 to RuBP. The product of this reaction is rearranged, and CO2 is released. While this process is similar to cellular respiration, in photorespiration, energy is consumed rather than released. It is still unclear why photorespiration happens at all. What is certain is that a low concentration of atmospheric CO2 can lead to increased photorespiration. Photorespiration adds CO2 to the atmosphere, thus increasing the concentration of atmospheric CO2 and allowing the Calvin cycle to proceed. A plant that undergoes these typical metabolic pathways is known as a C3 plant because the pathways fix carbon into three-carbon molecules. However, as much as 25% of reactions in C3 plants catalyzed by rubisco are photorespiration reactions rather than photosynthetic reactions.

C4 Photosynthesis

C4 plants fix carbon into four-carbon compounds before passing the carbon into the Calvin cycle.

In hot, dry climates, the loss of energy caused by photorespiration is too great for plants to survive. Some plants have thus evolved alternative ways of fixing carbon. These pathways are C4 photosynthesis and crassulacean acid metabolism (CAM).

C3 plants are so named because they fix carbon into three-carbon molecules. A C4 plant uses an alternate metabolic pathway in which carbon is fixed into four-carbon molecules. The anatomy of these plants is different from that of C3 plants in a way that aids in the carbon fixation process. They contain two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells. A bundle-sheath cell is the cell in which the Calvin cycle takes place in C4 plants around the veins of the leaves. The bundle-sheath cells are tightly packed into sheaths. The more loosely packed mesophyll cells lie between the bundle-sheath cells and the leaf surface, never more than two or three cells away from the bundle-sheath cells. This arrangement allows easy transfer of molecules from the initial carbon fixation pathway that occurs in the mesophyll cells into the Calvin cycle that occurs in the bundle-sheath cells.
In C4 plants (those that precede the Calvin cycle with a process that fixes carbon into four-carbon molecules), carbon fixation takes place in the mesophyll cells, which lie near the tightly packed bundle-sheath cells that surround the vascular tissue. Carbon fixation is the conversion of inorganic carbon (i.e., carbon dioxide) to organic compounds. The proximity of the mesophyll cells to the bundle-sheath cells allows for easy transfer of molecules from the carbon fixation pathway to the Calvin cycle.
Carbon fixation begins in the mesophyll cells with an enzyme known as PEP carboxylase that catalyzes the carbon fixation. It adds a molecule of CO2 to phosphoenolpyruvate (PEP) in C4 photosynthesis. The product is a four-carbon molecule called oxaloacetate. Unlike rubisco, PEP carboxylase has no affinity for O2, so photorespiration cannot take place in these cells. In hot, dry climates, where stomata are often partially closed, this avoidance of photorespiration and its resulting energy loss is vital. After oxaloacetate is formed in the mesophyll cells, it may be converted into other four-carbon molecules, such as malate. The malate then moves across the bundle-sheath cells through structures called plasmodesmata. A plasmodesma (plural, plasmodesmata) is a small channel between mesophyll cells and bundle-sheath cells through which molecules pass between carbon fixation in the mesophyll cells and the Calvin cycle in the bundle-sheath cells. These channels extend through the cell wall of a plant cell and directly connect the cytoplasm of adjacent plant cells. In the bundle-sheath cells, the four-carbon molecule releases a molecule of CO2, which then enters the Calvin cycle. The molecule that is produced after CO2 is passed on to the Calvin cycle in C4 photosynthesis is pyruvate, which regenerates PEP. The pyruvate is transported through the plasmodesmata back into the mesophyll cells. There, ATP is used to regenerate PEP, which can then be used again to fix CO2.
In the C4 pathway, carbon is fixed in the mesophyll cells by PEP carboxylase with phosphoenolpyruvate (PEP), forming oxaloacetate, a four-carbon molecule. Oxaloacetate may then be rearranged into another four-carbon molecule (malate shown here), which is transported through the plasmodesmata (small channels between mesophyll cells and bundle-sheath cells) into the bundle-sheath cell. There, a CO2 molecule splits off and enters the Calvin cycle. The remaining three-carbon molecule, pyruvate, is transported back into the mesophyll cell. ATP is used to regenerate PEP, and the cycle begins again.


CAM plants fix carbon into organic acids at night and carry out photosynthesis during the day using the carbon in these acids.
Succulents (such as cacti and aloes) and some other water-storing plants have evolved a different mechanism to survive hot, dry climates. A CAM plant uses an alternate metabolic pathway in which carbon is fixed into organic acids at night and passed on to the Calvin cycle during the day. Unlike C3 and C4 plants, which have their stomata open during the day, CAM plants open their stomata at night, when the cooler temperatures lead to less evaporation of water through the leaves. This means the light reactions occurring in C3 plants cannot precede the Calvin cycle. Instead, during the night while the stomata are open, CAM plants fix carbon into organic acids (organic meaning they contain carbon). CAM plants then store these acids in vacuoles (large structures in plant cells) until the day, when the light reactions needed to drive photosynthesis can occur. The organic acids can then supply CO2 to the Calvin cycle. This functions similarly to the C3 cycle, in which carbon is fixed in a separate pathway before being passed into the Calvin cycle. However, in C3 plants, the pathways are separated physically, whereas in CAM plants, they are separated by time.
C4 plants separate carbon fixation and the Calvin cycle by carrying out the pathways in different places. CAM plants separate them by carrying out the pathways at different times of day. These adaptations allow C4 and CAM plants to survive in environments where C3 plants cannot.