In C3 plants, an eznyme called rubisco plays a key role in photosynthesis. This enzyme tacks molecules of carbon dioxide onto a five-carbon sugar, starting the first step in the Calvin cycle, which converts carbon dioxide to sugar. These six-carbon molecules almost immediately break up into three-carbon molecules; hence the name C3, for three-carbon. Unfortunately, however, rubisco can also bind oxygen instead of carbon dioxide, causing a process called photorespiration. When photorespiration occurs, the resulting two-carbon compound is exported from the chloroplast and broken down; this process consumes energy and makes photosynthesis less efficient for the plant.
In C4 plants, two different kinds of cells are involved in photosynthesis. The first group, bundle-sheath cells, form sheaths around the leaf veins, while the other group, the mesophyll cells, arrange themselves around the bundle-sheath layer. CO2 is captured in the mesophyll cells, where an enzyme called PEP carboxylase adds the CO2 to a compound called phosphoenolpyruvate (PEP) to make a four-carbon product. This four-carbon product is exported to the bundle-sheath cells, where it breaks down into CO2; the rubisco enzyme then grabs this CO2 and feeds it into the Calvin cycle. Unlike rubisco, PEP carboxylase has little or no affinity for oxygen, so this two-step process helps to minimize the extent of photorespiration by boosting CO2 concentrations in the bundle-sheath cells, where the Calvin cycle takes place.
The underside of a plant's leaf is studded with microscopic pores called stomata; the plant uses these tiny holes to "breathe." On hot, dry days, however, plants need to close their stomata partially or even completely to avoid losing too much water. In a C3 plant, as oxygen concentration builds up inside the leaf, the rate of photorespiration increases. C4 plants, by contrast, are better able to minimize photorespiration, so they're better-suited to survival in hot, sun-baked conditions.
C3 plants are by far more common than their C4 competitors. Only 3 percent of flowering plants are C4; corn, sugarcane and sorghum are some of the most notable examples. Despite their smaller numbers, C4 plants account for as much as 25 percent of total photosynthetic activity on land. Scientists have tried to modify C3 plants to minimize photorespiration without much success. According to "Biology," it's possible that photorespiration plays a protective role by eliminating damaging byproducts of other reactions; if so, even though it's inefficient, photorespiration might confer other advantages.