C3 plants: fixing directly into the Calvin cycle

C₃ plantsfix carbon dioxide directly into the Calvin cycle as the three-carbon compound glycerate 3-phosphate (GP). Common and widely distributed, they include some of our most important crop plants such as wheat, soya beans, and rice. C₃ plants function efficiently in temperature conditions. However, they suffer two major disadvantages in hot, dry environments.

First, to obtain sufficient carbon dioxide, C₃ plants must open their stomata (small pores in their leaves). Unfortunately, when stomata are open, they not only allow carbon dioxide to enter the plant, but also allow water to escape. So in hot dry conditions C₃ plants have to either cease photosynthesising or run the risk of wilting and dying.

The second disadvantage relates to the ability of ribulose biphosphate carboxylase (ribosco) to combine with oxygen. Ribosco is the enzyme that catalyses carbon dioxide fixation. On a hot, sunny day carbon dioxide concentrations around photosynthesising cells decrease, because a large proportion of the carbon dioxide is being used up on photosynthesis. In these conditions, ribosco combines with oxygen rather than carbon dioxide in a process called photorespiration. The process results in the loss of fixed carbon dioxide from the plant, reducing photosynthetic efficiently and plant growth. Unlike photosynthesis, photorespiration does not produce sugar molecules; and unlike respiration, it yields no ATP. As much as half of the carbon dioxide fixed in the Calvin cycle may be released by photorespiration. Therefore, in hot, arid conditions, or in conditions where carbon dioxide levels are low, C₃ plants do not grow well.

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