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How do plants use carbon dioxide during photosynthesis?

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A labeled diagram of the periodic table entry for carbon, showing its atomic number 6, symbol C, electron configuration, atomic weight range, solid physical state, hexagonal crystal structure, and weakly acidic oxide properties. Color coding and icons indicate it is a nonmetal, solid at room temperature, with hexagonal structure and weak acidity. — carbon Carbon and its properties.

Plants use carbon dioxide ( $C O_{2}$ ) during the "dark reactions" of photosynthesis, a process known as carbon fixation. In this stage, plants utilize the chemical energy (ATP and NADPH) generated during the light-harvesting reactions to convert inorganic carbon into organic compounds, such as sugars.12

The primary mechanism for this conversion is the Calvin-Benson cycle, where the enzyme Rubisco catalyzes the addition of $C O_{2}$ to a five-carbon sugar called ribulose 1,5-bisphosphate (RuBP). This reaction produces molecules that are eventually synthesized into glucose.31

Plants have evolved different strategies to manage this process:
* C3 plants: Fix $C O_{2}$ directly into RuBP using Rubisco.4
* C4 plants: Use an additional, spatially separated pathway to concentrate $C O_{2}$ before it enters the Calvin-Benson cycle, which helps minimize photorespiration in warm or dry environments.45
* CAM plants: A variant of the C4 pathway where plants fix $C O_{2}$ at night to prevent water loss during the day.4