The Calvin Cycle in Photosynthesis

Introduction

• The Calvin cycle, also known as the light-independent reactions, is the second stage of photosynthesis.
• Carbon atoms in organisms originate from carbon dioxide (CO2) molecules in the air.
• Through the Calvin cycle, carbon from CO2 is fixed into sugars.

Overview of the Calvin Cycle

• CO2 enters the leaf through stomata and diffuses into the stroma of chloroplasts.
• The Calvin cycle synthesizes sugars without direct light, using ATP and NADPH from the light-dependent reactions.
• Occurs in the stroma of chloroplasts.

Reactions of the Calvin Cycle

The Calvin cycle consists of three main stages: carbon fixation, reduction, and regeneration.

1. Carbon Fixation

• CO2 combines with ribulose-1,5-bisphosphate (RuBP) to form a six-carbon compound.
• This compound splits into two molecules of 3-phosphoglyceric acid (3-PGA), catalyzed by rubisco.
• Diagram: Illustrates the reaction catalyzed by rubisco showing molecular structures.

2. Reduction

• ATP and NADPH convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
• ATP provides a phosphate group, and NADPH donates electrons.
• Diagram: Shows the reduction stage reactions and molecular structures.

3. Regeneration

• Some G3P molecules exit the cycle to contribute to glucose formation.
• Others are recycled to regenerate RuBP using ATP.
• Diagram: Shows the recycling of G3P to regenerate RuBP.

Summary of Calvin Cycle Reactants and Products

• Three turns of the cycle produce one G3P molecule, which contributes to forming glucose.
• Carbon: 3 CO2 results in 6 G3P, with 1 leaving the cycle and 5 regenerating RuBP.
• ATP: 9 ATP molecules are converted to 9 ADP.
• NADPH: 6 NADPH molecules are converted to 6 NADP+.
• Six cycle turns (6 CO2, 18 ATP, 12 NADPH) produce one glucose molecule.

References

• The information is adapted from articles by OpenStax and other biology textbooks.
• Key Source: "The Calvin Cycle" by OpenStax College, Concepts of Biology.