C3 Carbon Fixation

C3 carbon fixation is the most common pathway by which plants turn carbon dioxide into sugar. The first stable product is a three-carbon molecule, 3-phosphoglycerate, which gives the pathway its name, and it is used by roughly 85 percent of plant species, including wheat, rice, soybean and most trees.

In the Calvin cycle, the enzyme Rubisco attaches CO₂ to a five-carbon sugar, ribulose bisphosphate, producing two molecules of 3-phosphoglycerate. These are converted, using ATP and NADPH made in the light reactions of photosynthesis, into sugars. The entire process takes place in the mesophyll cells of the leaf, in the stroma of the chloroplast.

Rubisco can also react with oxygen, which starts the wasteful process of photorespiration. This becomes more frequent in hot, dry conditions, when plants close their stomata to save water and the CO₂ inside the leaf falls, so that a quarter or more of fixed carbon can be lost at high temperature.

Plants in hot environments often use C4 carbon fixation, which concentrates CO₂ around Rubisco, or CAM photosynthesis. C3 plants benefit more from extra atmospheric CO₂, an effect explained in the CO₂ fertilization effect. The pathway is a key part of the carbon cycle, and it dominates the growth of forests and most crops.