The Calvin Cycle in Photosynthesis

Overview

• Second set of reactions in photosynthesis.
• Also known as light-independent or dark reactions.
• Occurs after light-dependent reactions.
• Takes place in the stroma, the fluid surrounding thylakoids.
• Requires ATP and NADPH, produced during light reactions.

Key Components

• ATP: Provides energy for the Calvin Cycle.
• NADPH: Supplies electrons and hydrogens necessary for sugar production.
• CO2: Used to build sugars.

Phases of the Calvin Cycle

Phase 1: Carbon Fixation

• Enzyme RuBisCO adds CO2 to 5-carbon compound RuBP.
• Formation of a 6-carbon compound that splits into two molecules of 3-phosphoglycerate.
• Carbon fixation involves three CO2 molecules.

Phase 2: Reduction

• ATP provides energy to form a reactive intermediate compound.
• NADPH reduces the intermediate to produce Glyceraldehyde 3-phosphate (G3P).
• G3P: Major product, used to eventually form sugars.
• Net gain of one G3P for every three CO2 molecules.

Phase 3: Regeneration

• One G3P exits the cycle; five G3Ps are used to regenerate RuBP.
• 15 carbon atoms (5 G3Ps with 3 carbons each) regenerate 3 RuBP molecules.
• ATP provides the energy needed for regeneration.

Cycle Repetition and Glucose Formation

• To form glucose, the cycle must run twice (6 CO2 to produce 1 glucose molecule).
• Two G3P molecules are needed to synthesize one glucose.

Importance

• Glucose and other organic compounds produced support life.
• Plants and algae use photosynthesis to grow.

Interactive Learning

• Engage in "Photosynthesis Interactive" to understand the Calvin Cycle in depth.

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