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C4 pathway


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A pathway of carbon fixation, found most commonly in low-latitude plants, including many grasses, that are adapted to high temperatures and high light intensity. The first product formed as a result of the carboxylation by CO2 of an acceptor molecule, phosphoenolpyruvate (PEP), is the four-carbon compound oxalo-acetate (OAA); hence the name C4. OAA is converted to another four-carbon compound that leaves the mesophyll cells just below the leaf cells containing chlorophyll, passes through spaces between the cells (called plasmodesmata), and enters bundle-sheath cells that are tightly packed around the leaf veins. There the compound gives up its CO2, which enters the light-independent stage of photosynthesis (see dark reactions). PEP carboxylase, the enzyme catalysing carboxylation, has no affinity for oxygen. Consequently C4 plants suffer less than C3 plants from losses due to photorespiration. Nevertheless, C4 plants outcompete C3 plants only under high light and high temperature conditions. Their efficiency is lower than C3 plants in high latitudes. Zea mays (maize or corn) and Saccharum officinarum (sugar cane) are C4 plants.

Subjects: Plant Sciences and Forestry — Ecology and Conservation.


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