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. 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.