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How do ATP and NADPH from the light reactions power carbon fixation?
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photosynthesis
Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
In the Calvin-Benson cycle, ATP and NADPH act as the essential fuel for converting inorganic carbon into organic sugars. Produced during the light reactions, these molecules drive the cycle in the stroma of the chloroplast through two primary functions:12
- Energy Provision: ATP provides the necessary chemical energy to power the endergonic steps of the cycle, including the regeneration of the five-carbon compound RuBP, which allows the cycle to continue.12
- Reducing Power: NADPH provides the high-energy electrons (reducing equivalents) required to reduce the three-carbon compounds formed after carbon dioxide fixation into glyceraldehyde-3-phosphate (Gal3P), a high-energy sugar precursor.12
For every molecule of carbon dioxide fixed, the cycle requires three molecules of ATP and two molecules of NADPH. By donating these electrons and energy, ATP and NADPH enable the transformation of simple carbon dioxide into the carbohydrates that sustain the plant.12