CO2-fixing enzyme

Rubisco (RuBP carboxylase/oxygenase)

PEP carboxylase and rubisco

First product of CO2 fixation

3PG (3-carbon compound)

Oxaloacetate (4-carbon compound)

Affinity of carboxylase for CO2



Photosynthetic cells of leaf


Mesophyll + bundle sheath

Classes of chloroplasts



So even on a hot day when the stomata are closed, the CO2 concentration in the leaf is low, and the O2 concentration is high, PEP carboxylase just keeps on fixing CO2.

Oxaloacetate diffuses out of the mesophyll cells and through plasmodesmata into the bundle sheath cells, located in the interior of the leaf. The chloroplasts in bundle sheath cells contain abundant rubisco. There, the four-carbon oxaloacetate loses one carbon, forming CO2 and regenerating the three-carbon acceptor compound, PEP, in the mesophyll cells. Thus, the role of PEP is to bind CO2 from the air in the leaf and carry it to the bundle sheath cells, where it is "dropped off" at ru-bisco. This process essentially pumps up the CO2 concentration around rubisco, so that it acts as a carboxylase and begins the Calvin-Benson cycle.

Kentucky bluegrass, a C3 plant, thrives on lawns in April and May. But in the heat of summer, it does not do as well, and crabgrass, a C4 plant, takes over the lawn. The same is true on a global scale for crops: C3 plants, such as soybeans, rice, wheat, and barley, have been adapted for human food production in temperate climates, while C4 plants, such as corn and sugarcane, originated and are grown in the tropics. Table 8.1 compares C3 and C4 photosynthesis.

C3 plants are certainly more ancient than C4 plants. While C3 photosynthesis appears to have begun about 3.5 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 ruled 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 more efficient C4 plants would have had an advantage over their C3 counterparts.

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Essentials of Human Physiology

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