C4 photosynthesis

In C4 plants, the acids produced during carbon fixation diffuse to specialized cells surrounding a structure called the bundle sheath. There, deeper in the leaf, the four-carbon acids are broken down to a three-carbon acid and CO2. C4 photosynthesis also benefits plants by reducing, or even eliminating, photorespiration. Although RUBISCO is a carboxylase (it binds CO2 to RuBP), it can also serve as an oxygenase, binding O2 to RuBP. When this happens, RuBP is partly broken down and a CO2 molecule is released from the plant, a process known as photorespiration. Needless to say, release of CO2 from a plant is counter to the needs of photosynthesis, resulting in reduced energetic efficiency and increased water loss associated with needing to keep stomata open longer. One of the major factors influencing the rate of photorespiration is the concentration of O2 relative to that of CO2, with increased photorespiration in a relatively oxygen-rich and carbon dioxide-poor atmosphere. If RUBISCO is both an oxygenase and a carboxylase, why don't C4 plants, which still contain RUBISCO, photorespire? Quite simply, their unique morphology segregates RUBISCO into the bundle sheath cells. By doing this, plants put this enzyme in an area of high CO2 concentration (remember, it is transported out of the mesophyll cells), and low O2 concentrations. Why are the bundle sheath cells of lower O2 concentration than the mesophyll? In large part because the O2 generated by the light reactions of photosynthesis occurs in the mesophyll and not the bundle sheath cells. The impacts of atmospheric oxygen and carbon dioxide levels will have strong selective forces on plant photosynthesis and evolution, something we will discuss more in the Ecology In Action section of this chapter. It is interesting to notice that C4 plants appear to have evolved primarily during periods of low atmospheric CO2 (Sage 2004).