Photosynthesis is a biological process where plants, algae, and some bacteria convert light energy into chemical energy. This involves transforming carbon dioxide and water into glucose, a sugar used for energy and growth, while releasing oxygen. Carbon dioxide is an essential component that plants absorb from their environment. Its availability directly impacts the efficiency and output of photosynthesis.
The Fundamental Role of Carbon Dioxide
Carbon dioxide plays a key role in the light-independent reactions of photosynthesis, known as the Calvin cycle. During this cycle, carbon atoms from CO2 molecules are fixed into organic compounds. This process begins when the enzyme RuBisCO catalyzes the reaction between carbon dioxide and a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This step forms an unstable six-carbon compound that splits into two molecules of 3-phosphoglyceric acid (3-PGA).
The Calvin cycle uses energy from ATP and electrons from NADPH, produced during the light-dependent reactions, to convert 3-PGA molecules into glyceraldehyde-3-phosphate (G3P). Some G3P molecules synthesize glucose and other carbohydrates for plant energy and structure. The remaining G3P molecules are recycled to regenerate RuBP, allowing the cycle to continue capturing carbon dioxide. RuBisCO is the most abundant enzyme on Earth due to its role in carbon fixation.
Photosynthesis Response to Changing Carbon Dioxide Concentrations
The rate of photosynthesis is affected by carbon dioxide concentration. When CO2 levels are low, carbon dioxide becomes a limiting factor, even if light and water are plentiful. In sealed greenhouses, CO2 levels can drop to 200 parts per million (ppm) during the day due to plant uptake, reducing photosynthetic rates. Plants require a minimum CO2 concentration, around 150-200 ppm, to sustain photosynthesis.
As carbon dioxide concentrations increase from low levels, the rate of photosynthesis rises. This increase continues until a saturation point is reached, where other factors like light intensity, temperature, or enzyme availability limit the process. For most crops, this saturation point occurs at CO2 concentrations between 1,000 to 1,300 ppm under ideal conditions. Beyond this range, further CO2 increases may not boost the photosynthetic rate, following diminishing returns.
At very high CO2 levels, especially with high oxygen or temperatures, photorespiration can occur. This process involves RuBisCO, the carbon-fixing enzyme, mistakenly binding with oxygen instead of carbon dioxide. This reaction consumes energy and releases CO2, reducing photosynthesis efficiency in C3 plants.
Real-World Examples of Carbon Dioxide Influence
Atmospheric carbon dioxide levels influence global plant growth, known as the CO2 fertilization effect. Between 1982 and 2020, global plant photosynthesis increased by 12 percent, tracking the 17 percent rise in atmospheric CO2. This effect leads to increased biomass, with above-ground plant growth increasing by 21 percent and below-ground growth by 28 percent. Crops like wheat, rice, and soybeans can experience yield increases of 12 to 14 percent under elevated CO2.
Greenhouse environments utilize CO2 enrichment to optimize crop yield. Growers increase CO2 concentrations, typically to around 1,000 ppm, to maximize plant photosynthetic potential. This controlled elevation can increase photosynthesis efficiency by up to 30 percent, leading to higher yields, faster growth, and earlier flowering.
Different plant types respond uniquely to CO2 concentrations. C3 plants, which constitute about 90 percent of land plants, show a greater increase in growth with elevated CO2. C4 plants, such as corn and sugarcane, concentrate CO2 around the RuBisCO enzyme, making them more efficient at lower atmospheric CO2 levels and less responsive to further increases. While C3 plants benefit from increased CO2, C4 plants do not show as pronounced an increase in growth or yield.