In the most frequently observed route, oxaloacetic acid is next reduced to malic acid in a reaction that uses nadp as an elec­tron donor:

The malic acid produced apparently delivers CO₂ to the Calvin cycle. In the vicinity of the Calvin cycle, malic acid is oxidized by the enzyme malic add dehydrogenase, which decar-boxylates the acid at the same time:

This step replaces the NADP_red used earlier in the Cj pathway. The CCh released enters the Calvin cycle by the regular route by combination with RuDP, and the Calvin cycle turns as usual. The other product of this reaction, pyruvic acid, is phosphorylated at the expense of ATP to replace the phos-phoenol pyruvic acid used in the first step of the C₄ cycle.

At first glance the C₄ pathway appears to be a "futile" cycle that results only in the net breakdown of ATP. However, it evidently increases the availability of CO₂ to the Calvin cycle by compensating for what appears to be an evolutionary im-

perfection in the activity of RuDP carboxylase, the enzyme catalyzing the first uptake of CO₂ in the Calvin cycle. Recently, oxygen has been shown to compete effectively with CO₂ for the active site on RuDP carboxylase, diverting the enzyme from its central role in the Calvin cycle to activity as an oxygenase (see Bahr and Jensen, 1974). The products of the oxygenation of RuDP by the enzyme enter the process of photorespiration in peroxisomes and are lost to the Calvin cycle (see Supplement 7-3). Plants with the C₄ cycle, such as grasses, are able to get around this deficiency in RuDP car­boxylase through the following compensating mechanism.

In grasses, the C₄ cycle occurs in the chloroplasts of mesophyll cells that lie close to the external surface of the plants (Fig. 8-25). Because the cells are near the plant surface, they contain oxygen in relatively high concentration. How­ever, these cells lack RuDP carboxylase, effectively preventing diversion of carbohydrate intermediates to the photorespira­tion pathway through activity of the enzyme as an oxygenase. Instead, CO₂ is taken up in the C₄ cycle, leading to an exten­sive pool of four-carbon acids. Malic acid from these cells dif­fuses to deeper tissues of the plant to a cell type called bundle sheath cells. In these cells, malic acid is oxidized, releasing CO₂ in quantity. The chloroplasts of these cells contain RuDP car­boxylase in normal amounts. Since oxygen is present in re­duced concentration in the deeper layers, the RuDP car-

boxylase effectively carries out its Calvin cycle function of CO₂ fixation, without diversion to its alternate oxygenase role. The result is much greater efficiency in the activity of RuDP car-boxylase in CO₂ fixation and, consequently, much greater ef­ficiency in photosynthesis. Under optimum conditions, the plants with the C₄ cycle carry out photosynthesis at about twice the rate of plants without it. Interestingly, in the bundle sheath cells of some plants with the C₄ pathway, thylakoids occur in chloroplasts in single, extended form as well as in grana stacks. The relationship of this unusual thylakoid ar­rangement to the pattern of photosynthesis in the bundle sheath cells is unknown.

Plants with the C₄ cycle are found primarily in tropical and subtropical regions, particularly in more arid habitats. Water loss is reduced in C₄ plants through a side effect on the stomata, the minute openings in leaves that admit CO₂ and allow water vapor to escape. The greater efficiency in CO₂ up­take reduces the size of these openings and the amount of H₂O lost by transpiration through them. As a result, C₄ plants are about twice as economical in water use as other plants. Among the plants utilizing the Q pathway are the important crop grasses corn, sugar cane, and sorghum.