In photosynthetic organisms, photosynthesis 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. Many plants thrive in arid climates where water loss from normal photosynthesis would be lethal for their cells. Examples include cacti and sugarcane. Furthermore, all plants undergo some amount of respiration (taking in oxygen, O2, and releasing carbon dioxide, CO2). Driven by the lack of sufficient CO2 and excess O2 during daylight, photorespiration occurs. Photorespiration is the uptake of O2 and release of CO2, 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, instead of adding CO2, rubisco adds a molecule of O2 to RuBP. The product of this reaction is 3-phosphoglycolate instead of 3-phosphoglycerate, and CO2 is released. While this process is similar to cellular respiration, in photorespiration, energy is consumed rather than released. While the causes of photorespiration are not fully understood, 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. A C3 plant performs regular photosynthesis 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 typical C3 reactions.
In hot, dry climates, the loss of energy caused by photorespiration can be too great for plants to survive. Some plants have thus evolved alternative ways of fixing carbon. Carbon fixation is the conversion of inorganic carbon (for example, carbon dioxide) to organic compounds. These pathways are C4 photosynthesis and crassulacean acid metabolism (CAM). A C3 plant is a plant that undergoes the typical metabolic pathways that fix carbon into three-carbon molecules. A C4 plant uses an alternate metabolic pathway in which carbon is initially 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 a cell in which the Calvin cycle takes place in C4 plants around the veins of the leaves. This is unlike C3 plants, in which both the light-dependent and Calvin cycle take place in the mesophylls. 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.Carbon fixation begins in the mesophyll cells with an enzyme known as PEP carboxylase,which 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 (to reduce water loss), this nonutilization of photorespiration and its resulting energy loss are 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.
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. The "CAM" stands for crassulacean acid metabolism, which was first described from succulent plants in the family Crassulaceae. 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 of typical C3 photosynthesis 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. 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.