The Calvin Cycle

  • 23.1 The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water

  • 23.2 The Calvin Cycle Is Regulated by the Environment

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Rain forests are primary sites of carbon fixation. Approximately 50% of terrestrial carbon fixation takes place in these forests. The logging of rain forests and the subsequent loss of carbon fixation account in part for the increase in carbon dioxide in the atmosphere.

The light reactions transform light energy into ATP and biosynthetic reducing power, NADPH. The second part of photosynthesis uses these raw materials to reduce carbon atoms from their fully oxidized state as carbon dioxide to the more reduced state as a hexose. These reactions are called the dark reactions or the light-independent reactions because light is not directly needed. The dark reactions are also called either the Calvin–Benson cycle, after Melvin Calvin and Andrew Benson, the biochemists who elucidated the pathway, or simply the Calvin cycle. The source of the carbon atoms in the Calvin cycle is the simple molecule carbon dioxide. The Calvin cycle brings into living systems the carbon atoms that will become biochemical constituents of all organisms.

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