Carbon 3 plants
Carbon 3 plants, also known as C3 plants, are a category of plants that utilize the carbon 3 pathway for photosynthesis, the process by which they convert carbon dioxide (CO₂) into organic molecules for energy and growth. This pathway is the most common form of photosynthesis, employed by a wide range of species, including many trees and key agricultural crops such as rice, wheat, and soybeans. The initial product of carbon fixation in these plants is phosphoglycerate (PGA), a three-carbon molecule formed through the interaction of CO₂ with ribulose bisphosphate (RuBP) facilitated by the enzyme RuBisCO.
However, the efficiency of carbon fixation in C3 plants can be adversely affected by high oxygen levels, which can lead to competition for binding sites on RuBisCO, resulting in a process known as photorespiration. This challenge is compounded by temperature, as higher temperatures can create suboptimal conditions for carbon fixation due to reduced CO₂ availability. While rising atmospheric CO₂ levels associated with climate change may theoretically boost C3 plant productivity, factors such as increased transpiration and stomatal closure can complicate this relationship. Understanding the dynamics of C3 plants is crucial for assessing their role in ecosystems and their potential response to climate warming.
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Carbon 3 plants
Definition
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 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.
Significance for Climate Change
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 the production of many crops by adding supplemental CO2. This suggests that the rising levels of CO2 associated with global warming should stimulate carbon 3 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 spaces soon is depleted enough that photorespiration begins to outpace photosynthesis.
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