The Calvin Cycle

Atmospheric carbon dioxide diffuses into the stroma of the chloroplast, where it is fixed to ribulose bisphosphate (RuBP), a five-carbon acceptor sugar. The enzyme RuBisCO catalyzes this carbon fixation reaction, attaching one carbon atom from CO₂ to RuBP. The resulting six-carbon molecule is unstable and immediately splits into two three-carbon molecules of 3-phosphoglyceric acid (3-PGA).

ATP provides energy to convert 3-PGA into glyceraldehyde-3-phosphate (G3P) and to regenerate RuBP acceptors. NADPH adds hydrogen atoms to 3-PGA during the reduction phase. Six turns of the Calvin Cycle require 18 ATP molecules and 12 NADPH molecules to generate one glucose molecule while rebuilding RuBP to continue the cycle.

Glyceraldehyde-3-phosphate (G3P) is a three-carbon sugar produced when 3-PGA is reduced by NADPH and phosphorylated by ATP. One G3P molecule exits the cycle after every six CO₂ fixations. Two G3P molecules combine to build glucose with six carbon atoms, while remaining G3P molecules regenerate RuBP through additional ATP-dependent reactions.

RuBP regeneration allows the cycle to continue fixing atmospheric CO₂. After carbon fixation produces 3-PGA and G3P, the remaining G3P molecules must undergo a series of reactions using additional ATP to rebuild RuBP acceptors. Without regeneration, the cycle cannot restart, and carbon fixation would halt.

Six turns of the Calvin Cycle are required to fix six carbon dioxide molecules from the atmosphere. This process uses 18 ATP and 12 NADPH to generate one molecule of glucose with six carbon atoms. The cycle must turn six times because each turn fixes only one CO₂, and glucose contains six carbons.

The Calvin Cycle consists of carbon fixation, reduction, and regeneration phases. Carbon fixation attaches CO₂ to RuBP, forming 3-PGA. Reduction converts 3-PGA to G3P using ATP and NADPH. Regeneration uses remaining G3P and additional ATP to rebuild RuBP, allowing the cycle to continue fixing CO₂.

The Calvin Cycle depends on ATP and NADPH produced by light-dependent reactions occurring in the thylakoid membranes. These energy carriers move to the stroma where the Calvin Cycle uses them to reduce atmospheric CO₂ into carbohydrates. ADP and NADP⁺ produced by the cycle return to the thylakoids to be recycled in light reactions, supporting photosynthesis production of glucose in plants.