Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy into chemical energy, primarily in the form of sugars. This conversion is the basis of nearly all life on Earth, providing the energy source and oxygen necessary for most organisms to survive. Carbon dioxide ($\text{CO}_2$) is the raw material for building the plant’s structure. The availability and concentration of $\text{CO}_2$ directly influence the speed and efficiency of photosynthesis.
$\text{CO}_2$ as a Fundamental Ingredient
Carbon dioxide supplies the carbon atoms that form the backbone of sugar molecules through a process known as carbon fixation, which takes place inside the plant’s chloroplasts. The overall chemical reaction uses carbon dioxide and water as inputs, with light energy driving their conversion into glucose (a sugar) and oxygen gas as outputs. The balanced chemical equation is $6\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$.
$\text{CO}_2$ enters the plant leaf from the atmosphere through microscopic pores called stomata. These openings, controlled by guard cells, regulate gas exchange, allowing $\text{CO}_2$ to diffuse into the leaf’s interior where the photosynthetic machinery is located. Regulating the stomata is a constant trade-off for the plant, balancing the need to take in enough $\text{CO}_2$ for sugar production against the need to minimize water loss through transpiration.
The Limiting Factor: How $\text{CO}_2$ Controls Photosynthesis Rate
Under normal atmospheric conditions, the concentration of carbon dioxide often acts as a limiting factor, meaning the rate of photosynthesis is directly constrained by how quickly the plant can acquire and utilize $\text{CO}_2$. The central enzyme responsible for carbon fixation in most plants is Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO. RuBisCO initiates the Calvin-Benson cycle by binding $\text{CO}_2$ to a five-carbon molecule, a step that is fundamental to sugar synthesis.
A significant inefficiency arises because RuBisCO is not perfectly specific; it can bind to oxygen ($\text{O}_2$) as well as $\text{CO}_2$. When $\text{CO}_2$ levels are low relative to $\text{O}_2$, the enzyme binds oxygen, initiating a wasteful process called photorespiration. Photorespiration consumes energy and releases previously fixed carbon back into the atmosphere as $\text{CO}_2$, significantly reducing the plant’s overall photosynthetic efficiency.
The rate of this wasteful oxygenation reaction increases significantly at higher temperatures. For $\text{C}_3$ plants, which account for about 95% of species, this makes them particularly sensitive to ambient $\text{CO}_2$ concentrations. Increasing the internal $\text{CO}_2$ concentration helps RuBisCO favor the productive carbon-fixing reaction over the wasteful oxygenation reaction.
Saturation and Enhancement: The $\text{CO}_2$ Fertilization Effect
When $\text{CO}_2$ concentration is increased above the current atmospheric level, which is over 400 parts per million (ppm), the rate of photosynthesis in $\text{C}_3$ plants often increases substantially. This phenomenon is known as the $\text{CO}_2$ fertilization effect. This enhancement occurs because the higher availability of $\text{CO}_2$ allows RuBisCO to operate more frequently in its preferred, carbon-fixing mode. This effect is utilized commercially in controlled environments like greenhouses, where growers intentionally enrich the air to boost crop growth and yield.
The photosynthetic rate does not continue to increase indefinitely as $\text{CO}_2$ levels rise. Eventually, the process reaches a $\text{CO}_2$ saturation point, typically around 700 to 1,000 ppm for many $\text{C}_3$ species. At this point, adding more $\text{CO}_2$ yields no further benefit because other components of the photosynthetic machinery, such as light energy, become the new limiting factors. Higher $\text{CO}_2$ levels can also cause the plant to partially close its stomata, which conserves water and increases water-use efficiency.
Beyond $\text{CO}_2$: The Role of Other Environmental Factors
The extent to which $\text{CO}_2$ affects photosynthesis is highly dependent on other environmental variables, particularly light intensity and temperature. If light levels are too low, the plant cannot produce the energy needed to run the Calvin-Benson cycle, rendering any increase in $\text{CO}_2$ concentration ineffective. Similarly, if the temperature is suboptimal, the enhanced $\text{CO}_2$ supply will not translate into a higher photosynthetic rate.
Plant species also exhibit different sensitivities to $\text{CO}_2$ based on their evolutionary adaptations. $\text{C}_4$ plants have evolved a mechanism to chemically concentrate $\text{CO}_2$ around the RuBisCO enzyme. This concentrating mechanism effectively eliminates photorespiration, making $\text{C}_4$ plants significantly more efficient than $\text{C}_3$ plants in hot, bright environments. Consequently, $\text{C}_4$ plants are far less limited by atmospheric $\text{CO}_2$ and show a much smaller increase in photosynthetic rate when $\text{CO}_2$ is raised above current ambient levels.