The Calvin Cycle: How Plants Turn CO₂ into Sugars through Photosynthesis

simplified diagram of the Calvin cycle in photosynthesis — Circular diagram illustrating the three phases of the Calvin cycle in the chloroplast.

The calvin cycle is the second major phase of photosynthesis. It takes place in the stroma of chloroplasts and does not require light directly. Instead, it uses the ATP and NADPH produced during the light reactions to convert carbon dioxide into glucose—a vital source of energy for the plant and the entire ecosystem.

🌀 Diagram: Calvin Cycle Steps

Illustration of the Calvin cycle, showing carbon fixation, reduction, and RuBP regeneration in the chloroplast. copywrite : https://cdn.kastatic.org/ka-perseus-images/4c9fbc7e4f158fd4bf3e1114e9a7ebe47d08f020.png

🧬 Step 1: Carbon Fixation

Carbon enters the cycle as CO₂ and is attached to a five-carbon compound called RuBP (ribulose-1,5-bisphosphate). This carboxylation reaction is catalyzed by Rubisco, forming an unstable 6-carbon compound that rapidly splits into two 3-carbon molecules of 3-PGA (3-phosphoglycerate).

Rubisco is one of the most abundant enzymes on Earth and is highly regulated. Its activity is enhanced by a slightly alkaline pH (~8.0) and high Mg²⁺ concentrations—conditions maintained in the stroma during light reactions.

⚗️ Step 2: Reduction

Each 3-PGA is phosphorylated by ATP and then reduced by NADPH to form G3P (glyceraldehyde-3-phosphate). Out of every six G3P molecules produced, only one exits the cycle to contribute to sugar synthesis.

♻️ Step 3: Regeneration of RuBP

The remaining five G3P molecules undergo a series of complex rearrangements to regenerate three molecules of RuBP. This step consumes additional ATP and allows the cycle to continue processing more CO₂.

🌱 Metabolic Fates of G3P

G3P is a versatile metabolic intermediate. It can:

Remain in the chloroplast to be converted into starch, stored as energy reserve
Exit the chloroplast to the cytosol, where it is used to synthesize sucrose for transport via the phloem to sink tissues like roots or fruits

⚖️ ATP & NADPH Imbalance and Cyclic Photophosphorylation

The Calvin cycle consumes 3 ATP and 2 NADPH per CO₂ fixed. Since the light reactions produce equal amounts of both, an imbalance arises. To compensate, plants engage in cyclic photophosphorylation, which produces ATP without generating NADPH or O₂.

💡 Conclusion

The calvin cycle is more than a sugar-making pathway—it’s a tightly regulated, ATP-intensive process that connects light energy with the chemical energy needs of the plant. With its elegant use of CO₂, water, and solar power, it sustains not only plants but also the animals and humans that rely on them.