Carbon 3 plants
Definition (Encyclopedia of Global Warming)
During photosynthesis, plants absorb carbon dioxide (CO2) from the atmosphere and convert 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. The process of converting CO2 to organic molecules is called carbon fixation. The oldest and most common carbon fixation pathway is the carbon 3 pathway. Most trees and agricultural crops, including rice, wheat, soybeans, potatoes, and vegetables, use carbon 3 metabolism. In the carbon 3 pathway, the first organic molecule formed from CO2, phosphogylcerate (PGA), contains three carbon atoms. Essential to this process is a five-carbon molecule called ribulose bisphosphate (RuBP) that has an affinity for CO2. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) binds water and a single CO2 molecule to RuBP and immediately splits the six-carbon intermediate molecule into two PGA molecules, one of which includes the original CO2 molecule from the atmosphere.
(The entire section is 152 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
Although RuBisCO has an affinity for CO2, it also binds molecular oxygen (O2), depending on the relative ratio of the concentrations of CO2 to O2. When CO2 levels are low, O2 begins to out-compete CO2 for the binding site on RuBisCO, and the rate of photosynthetic carbon fixation decreases. In fact, as more O2 is taken up by RuBisCO, a photosynthesizing plant begins to produce CO2 through a process called photorespiration. This competition between CO2 and O2 is temperature dependent. At contemporary levels of atmospheric CO2, about 380 parts per million, temperatures above 25° Celsius are unfavorable for carbon 3 plants, because CO2 levels are suboptimal.
Some greenhouse growers speed up production of many crops by adding supplemental CO2. This suggests that the rising levels of CO2 associated with global warming should stimulate carbon 3 photosynthesis in most plants, which should help reduce CO2 levels. However, it may not be so simple. For atmospheric gases to get into plants, they must diffuse through microscopic openings in the surfaces of leaves and green stems called stomata. Warming causes plants to lose water vapor through their stomata, a process called transpiration, 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 contained in plant tissues’ intercellular...
(The entire section is 233 words.)