Section 2

The Calvin Cycle

• The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

• The Calvin cycle produces a compound called G3P. Most G3P molecules are converted into RuBP to keep the Calvin cycle operating. However, some G3P molecules are used to make other organic compounds, including amino acids, lipids, and carbohydrates.

• Plants that fix carbon using only the Calvin cycle are known as C3 plants. Some plants that evolved in hot, dry climates fix carbon through alternative pathways—the C4 and CAM pathways. These plants carry out carbon fixation and the Calvin cycle either in different cells or at different times.

• Photosynthesis occurs in two stages. In the light reactions, energy is absorbed from sunlight and converted into chemical energy; in the Calvin cycle, carbon dioxide and chemical energy are used to form organic compounds.

• The rate of photosynthesis increases and then reaches a plateau as light intensity or CO2 concentration increases. Below a certain temperature, the rate of photosynthesis increases as temperature increases. Above that temperature, the rate of photosynthesis decreases as temperature increases.

Vocabulary

Calvin cycle (p. 120) carbon fixation (p. 120)

CAM pathway (p. 122)

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