9.6: The Calvin Cycle

Overview

Oxygenic photosynthesis plays a central role in the global carbon and oxygen cycles. The carbohydrates produced support nearly all food webs, while the oxygen by‑product enables aerobic life.

Light‑dependent and light‑independent reactions

Photosynthesis occurs in two main stages, each in a different part of the chloroplast: light‑dependent reactions and light‑independent reactions, also called the Calvin‑Benson cycle or simply the Calvin cycle.

Light‑dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy, which is used to split water molecules, releasing O₂ and generating ATP and NADPH. These energy carriers then move to the stroma, where the Calvin‑Benson cycle occurs.

The Calvin‑Benson cycle uses ATP and NADPH to reduce atmospheric CO₂ into carbohydrates. ADP and NADP⁺, produced as by‑products, return to the thylakoids to be recycled in the light reactions.

Calvin‑Benson Cycle

The Calvin‑Benson cycle consists of three phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor (RuBP).

• Carbon fixation: CO₂ enters the leaf through stomata. In the stroma, the enzyme RuBisCO attaches each CO₂ molecule to ribulose‑1,5‑bisphosphate (RuBP), forming an unstable six‑carbon intermediate that immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA).
• Reduction: Each 3‑PGA molecule is phosphorylated by ATP to form 1,3‑bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde‑3‑phosphate (G3P), a three‑carbon sugar. For every six CO₂ molecules fixed, two G3P molecules exit the cycle. These G3P molecules help form glucose and other organic compounds.
• Regeneration: The remaining G3P molecules undergo a series of reactions that regenerate RuBP, using additional ATP. This regeneration step allows the cycle to continue fixing CO₂.