Carbon 4 plants
Definition (Encyclopedia of Global Warming)
During photosynthesis, plants absorb carbon dioxide (CO2) from the atmosphere and convert, or fix, it into a variety of organic molecules that can be used to provide energy to cells or to provide the raw materials to build more cells and tissues. Carbon 4 fixation evolved over the past 30 million years, but it spread widely in tropical grasslands and savannas only about 5 million years ago as a result of its ability to concentrate CO2 in photosynthetic plant tissues in a cooling climate without undergoing photorespiration. Carbon 4 plants have come to account for about 30 percent of worldwide photosynthesis, and they include important crops such as maize, sugarcane, millet, and sorghum. The carbon 4 pathway is particularly important in hot, dry climates, because this pathway is much more efficient than the carbon 3 pathway at fixing low levels of CO2 without competition from oxygen.
Carbon 4 plants often have a tight bundle sheath of carbon 3 cells around the vascular bundles of leaves. This physically separates the cells where “normal” carbon 3 photosynthesis occurs from the leaf mesophyll cells where atmospheric CO2 is fixed. In carbon 4 mesophyll cells, the enzyme Phosphoenolpyruvate carboxylase (PEP carboxylase) binds CO2 to a three-carbon PEP molecule to form a four-carbon organic acid, malic acid. The malate diffuses from the mesophyll cells where it is produced into the bundle sheath cells where CO2 is released to be...
(The entire section is 228 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
Carbon 4 evolved and spread rapidly in response to low levels of atmospheric CO2 during past geological periods, because PEP carboxylase is much more efficient than is ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in binding CO2, and it is not affected by oxygen levels. With rising CO2 levels, it might seem that the advantage of carbon 4 over carbon 3 photosynthesis would decrease. However, hot, dry conditions strongly favor the carbon 4 pathway. Atmospheric gases must diffuse through microscopic openings in the surface of leaves and green stems called stomata. Warming causes plants to lose water vapor through their stomata, which leads to wilting and even death. Plants respond to water loss by closing their stomata, thus reducing gas exchange with the atmosphere. If the stomata remain closed for very long, CO2 soon becomes depleted inside the plant tissues, and this favors carbon 4. Hot temperatures and dry conditions have a much greater influence on photosynthesis than do ambient CO2 levels. As a result, with continued global warming, plants with carbon 4 metabolism will be increasingly important in agriculture and carbon 4 plants will become even more widespread in the environment.
(The entire section is 183 words.)