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Plants differ in how they fix CO₂, and can be distinguished as C₃ or C₄ plants, based on whether the first product of CO₂ fixation is a three- or four-carbon molecule. In C₃ plants such as roses, wheat, and rice, the first product is the three-carbon molecule 3PG—as we have just described for the Calvin cycle. In these plants the cells of the mesophyll, which makes up the main body of the leaf, are full of chloroplasts containing rubisco (Figure 10.15A). On a hot day, these leaves close their stomata to conserve water, and as a result, rubisco acts as an oxygenase as well as a carboxylase, and photorespiration occurs.

C₄ plants, which include corn, sugarcane, and tropical grasses, make the four-carbon molecule oxaloacetate as the first product of CO₂ fixation (Figure 10.15B). On a hot day, they partially close their stomata to conserve water, but their rate of photosynthesis does not fall. What do they do differently?

C₄ plants have evolved a mechanism that increases the concentration of CO₂ around the rubisco enzyme while at the same time isolating the rubisco from atmospheric O₂. Thus in these plants the carboxylase reaction is favored over the oxygenase reaction; the Calvin cycle operates, but photorespiration does not occur. This mechanism involves the initial fixation of CO₂ in the mesophyll cells and then the transfer of the fixed carbon (as a four-carbon molecule) to the bundle sheath cells, where the fixed CO₂ is released for use in the Calvin cycle (Figure 10.16). The bundle sheath cells are located in the interior of the leaf where less atmospheric O₂ can reach them than reaches cells near the surface of the leaf.

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The first enzyme in this C₄ carbon fixation process, called PEP carboxylase, is present in the cytosols of mesophyll cells near the leaf’s surface. This enzyme combines CO₂ with a three-carbon acceptor compound, phosphoenolpyruvate (PEP), to produce the four-carbon fixation product, oxaloacetate. PEP carboxylase has two advantages over rubisco:

So even on a hot day when the stomata are partially closed and the ratio of O₂ to CO₂ rises, PEP carboxylase just keeps on fixing CO₂.

Oxaloacetate is converted to malate, which diffuses out of the mesophyll cells and into the bundle sheath cells (see Figure 10.15B). (Some C₄ plants convert the oxaloacetate to aspartate instead of malate, but we will only discuss the malate pathway here.) The bundle sheath cells contain modified chloroplasts that are designed to concentrate CO₂ around the rubisco. There, the four-carbon malate loses one carbon (is decarboxylated), forming CO₂ and pyruvate. The latter moves back to the mesophyll cells where the three-carbon acceptor compound, PEP, is regenerated at the expense of ATP. So the “expenditure” of ATP in the mesophyll cell “pumps up” the CO₂ concentration around rubisco in the bundle sheath cell, ensuring that rubisco will function as a carboxylase and begin the Calvin cycle.

Under relatively cool or cloudy conditions, C₃ plants have an advantage over C₄ plants in that they don’t expend energy to “pump up” the concentration of CO₂ near rubisco. But this advantage begins to be outweighed under conditions that favor photorespiration, such as warmer seasons and climates. Under these conditions C₄ plants have the advantage, especially if there is ample light to supply the extra ATP required for C₄ photosynthesis. For example, Kentucky bluegrass is a C₃ plant that thrives on lawns in April and May. But in the heat of summer it does not do as well, and Bermuda grass, a C₄ plant, takes over the lawn. The same is true on a global scale for crops: C₃ plants such as soybean, wheat, and barley have been adapted for human food production in temperate climates, whereas C₄ plants such as corn and sugarcane originated and are still grown in the tropics.

THE EVOLUTION OF CO₂ FIXATION PATHWAYS C₃ plants are more ancient than C₄ plants. Whereas C₃ photosynthesis appears to have begun about 2.5 billion years ago, C₄ plants appeared about 12 million years ago. A possible factor in the emergence of the C₄ pathway is the decline in atmospheric CO₂. When dinosaurs dominated Earth 100 million years ago, the concentration of CO₂ in the atmosphere was four times what it is now. As CO₂ levels declined thereafter, the C₄ plants would have gained an advantage over their C₃ counterparts in high-temperature, high-light environments.

As described in the opening of this chapter, atmospheric CO₂ levels have been increasing over the past 200 years. Currently, the level of CO₂ is not enough for maximal CO₂ fixation by rubisco, so photorespiration occurs, reducing the growth rates of C₃ plants. Under hot conditions, C₄ plants are favored. But if CO₂ levels in the atmosphere continue to rise, the reverse will occur and C₃ plants will have a comparative advantage. The overall growth rates of crops such as rice and wheat should increase. This may or may not translate into more food, given that other effects of the human-spurred CO₂ increase (such as global climate change) will also alter Earth’s ecosystems.