C4 photosynthesis

Technical articles

Photosynthesis

Schematic of C3 CAM and C4 photosynthesis

C4 photosynthesis is a more advanced and efficient type of photosynthesis than C3 photosynthesis, especially under conditions of high light intensity, high temperatures, and low water availability. It is named so because the first product of carbon fixation is a four-carbon compound, oxaloacetate.

Differences with C3 photosynthesis

Photosynthesis is a chemical reaction that uses light and an "electron donor" to convert CO2 into sugar. The electron donor can be iron, nitrites, sulfur hydroxide, or arsenic. It is generally water. It is found in algae, plants, and some bacteria (cyanobacteria). Water-based photosynthesis, that of plants, is called "oxygenic photosynthesis." It breaks down water and CO2 to produce sugar, water, and oxygen:

6CO2 + 24H2O + light ➡ C6H12O8 (glucose) + 12O2 + 12H2O

For the majority of plants (those called C3), this reaction is associated with an energy- and water-costly activity called photorespiration. This strategy allows only moderate biomass production but is well adapted to variable climatic conditions. It is generally considered that there is a thermal optimum of 25°C. This type of photosynthesis allows capturing 1 gram of carbon per 400 g of water.

Another strategy (C4) avoids photorespiration. The photosynthesis process is carried out in two distinct cells. The thermal optimum shifts to 35°C and the plant uses only 250 g of water to fix 1 g of carbon. This is the strategy of tropical plants such as maize, sorghum, sugarcane, and millet.

From a technical point of view, the difference between C4 and C3 photosynthesis lies in the spatial separation of the initial CO2 fixation steps and the Calvin-Benson cycle:

1. In C4 plants, the initial CO2 fixation occurs in mesophyll cells, where CO2 is converted into oxaloacetate (a 4-carbon compound) by the enzyme PEP carboxylase.
2. Oxaloacetate is then converted into malate or aspartate, which are transported to the bundle sheath cells.
3. In these cells, CO2 is released and enters the Calvin-Benson cycle, as in C3 photosynthesis.

C4 Photosynthesis

Advantages of C4 photosynthesis

• Better water use efficiency
• Reduction of photorespiration
• Better adaptation to hot and dry climates
• Faster growth under optimal conditions

C4 plants

About 3% of plants use C4 photosynthesis, including important crops such as maize, sugarcane, millet, sorghum, moha, and switchgrass.

C3 Photosynthesis

C3 photosynthesis is the most common type of photosynthesis, used by about 85% of plants. It is called C3 because the first stable product of carbon fixation is a three-carbon compound, 3-phosphoglycerate.

However, there are significant photosynthetic differences even among C3 species for various reasons:

• size, density, and regulation of stomata
• photosystem efficiency (not exactly the same proteins)
• chlorophyll/chloroplast concentration
• metabolic efficiency
• leaf and branch architecture...
• some C3 plants have fewer stomata than others and close their stomata faster in response to stress...

Main characteristics

• CO2 fixation and the Calvin-Benson cycle occur in the same cells.
• The key enzyme is RuBisCO, which fixes CO2 directly into the Calvin-Benson cycle.
• More efficient under moderate temperature and sufficient humidity conditions.

C3 plants

The majority of plants, including rice, wheat, vegetables, and trees, use C3 photosynthesis.

Limitations

• Less efficient than C4 photosynthesis under high light and high temperature conditions
• More sensitive to photorespiration, which reduces photosynthetic efficiency

CAM Photosynthesis

C3 and C4 plants must be able to evapotranspire while photosynthesizing. If it is too hot, the plant can either close its stomata to preserve water and cease all metabolic activity, or continue photosynthesis at the risk of water stress.

Only CAM plants, essentially succulents (cacti), can manage this situation. Like C4 plants, these plants perform photosynthesis in two phases. At night, they carry out gas exchange, then in the morning, after soaking up dew, they close their stomata and finish metabolizing the CO2 absorbed during the night without losing a drop of water. Their optimum is 35°C during the day and 15°C at night, as gas exchange can only occur with some warmth. But they require only 50 g of water to capture 1 g of CO2.

The difference between each plant is not so much the amount of CO2 captured, but rather the potential biomass produced based on the available water and given temperature context.