C4 pathway (Hatch–Slack pathway) The metabolic pathway followed in the light-independent phase of photosynthesis by tropical plants, such as sugar cane and maize, and by plants that live in arid environments; these plants are known as C₄ plants. The initial fixation of atmospheric carbon dioxide occurs with phosphoenolpyruvate (PEP), through the mediation of the enzyme PEP carboxylase, to form the four-carbon compound oxaloacetic acid (see illustration; compare C3 pathway). Oxaloacetic acid is then converted into malic acid; this breaks down to give carbon dioxide, which passes to the Calvin cycle, and pyruvic acid, which generates more PEP, thereby ensuring a constant supply of this compound for further carbon dioxide fixation. The C₄ pathway allows photosynthesis to occur at very low concentrations of carbon dioxide as PEP carboxylase has an extremely high affinity for carbon dioxide. This pathway also works well at high temperatures and light intensity, enabling efficient photosynthesis in tropical plants. In addition, the malic acid formed can be stored before being used, to be later broken down to carbon dioxide when required in the Calvin cycle. This is important in desert plants, which need to close their stomata during the day to reduce water loss (see crassulacean acid metabolism). The anatomy of the C₄ plant leaf is adapted to suit this metabolic pathway (see bundle sheath cells).

C4 pathway A pathway of carbon fixation, found most commonly in tropical plants that have high rates of growth and photosynthesis and are adapted to high temperatures, strong light, low carbon dioxide levels, and low water supply. Its importance lies mainly in its reduction of photorespiration. The first product formed as a result of the carboxylation by CO₂ of an acceptor molecule, phosphoenolpyruvate (PEP), is the four-carbon oxalo-acetate (OAA). Eventually the CO₂ is handed on to, and refixed by, the Calvin cycle. Because of their higher affinity for carbon dioxide, C₄ plants tend to grow more rapidly than C₃ plants and produce greater yields. In temperate regions C₄ plants may out-compete C₃ plants to become pests (e.g. Spartina cordgrass).

C4 pathway A pathway of carbon fixation, found most commonly in tropical plants that have high rates of growth and photosynthesis and that are adapted to high temperatures, strong light, low carbon dioxide levels, and low water supply. Its importance lies mainly in its reduction of photorespiration. The first product formed as a result of the carboxylation by CO₂ of an acceptor molecule, phospho-enolpyruvate (PEP), is the four-carbon oxalo-acetate (OAA). Eventually the CO₂ is handed on to, and refixed by, the Calvin cycle.

C3 pathway The metabolic pathway followed in the light-independent phase of photosynthesis by most plants of temperate regions, in which the first product is the three-carbon compound glycerate 3-phosphate. This is formed when carbon dioxide combines with ribulose bisphosphate in the first reaction of the Calvin cycle. Plants that follow this pathway are referred to as C₃ plants. Compare C4 pathway.

C3 pathway The most common pathway of carbon fixation in plants. See also PHOTOSYNTHESIS and PHOTORESPIRATION.