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How do C3 plants fix carbon dioxide?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

C3 plants fix carbon dioxide directly into the five-carbon compound ribulose 1,5-bisphosphate (RuBP) using the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). This carboxylation reaction produces an unstable six-carbon intermediate that immediately breaks down into two molecules of the three-carbon acid 3-phosphoglycerate (PGA). Because this first stable product contains three carbon atoms, these plants are designated as C3.12

This process occurs within the stroma of the chloroplast as part of the Calvin-Benson cycle, which consists of three stages: carboxylation, reduction, and regeneration. To drive this cycle, the plant utilizes ATP and NADPH generated during the light reactions of photosynthesis.3

While efficient in cooler climates, C3 plants are subject to photorespiration, a process where Rubisco acts as an oxygenase rather than a carboxylase, particularly at higher temperatures or when carbon dioxide levels are low, which can reduce overall photosynthetic efficiency.1