GENBIO - CALVIN CYCLE

DEFINITIONS

Calvin Cycle

The second stage of photosynthesis where carbon dioxide is converted into sugar molecules using the chemical energy supplied by ATP and NADPH.

Alternative names of the Calvin Cycle

Light-independent reactions, dark reactions, or reductive phosphate cycle.

Reason the Calvin Cycle is called light-independent

The reactions do not directly require light but rely on ATP and NADPH produced during the light reactions.

Location of the Calvin Cycle

The stroma of the chloroplast.

Scientists who discovered the Calvin Cycle

Melvin Calvin, Andrew Benson, and James Bassham.

Method used to discover the Calvin Cycle

The use of radioactive carbon-14 to trace the path of carbon atoms during carbon fixation.

Organism used in Calvin Cycle experiments

Unicellular algae called Chlorella.

Purpose of stopping photosynthesis in experiments

Alcohol was used to stop photosynthesis at different intervals to identify intermediate compounds.

Stage of photosynthesis represented by the Calvin Cycle

The second stage of photosynthesis following the light reactions.

Main function of the Calvin Cycle

To synthesize sugar from carbon dioxide using ATP and NADPH.

Direct product of the Calvin Cycle

Glyceraldehyde-3-phosphate (G3P).

Carbon content of G3P

A three-carbon sugar molecule.

Relationship between G3P and glucose

Two G3P molecules combine to form one glucose molecule.

Number of Calvin Cycle turns needed to produce one G3P

Three turns.

Number of CO₂ molecules required to produce one G3P

Three molecules of carbon dioxide.

Three phases of the Calvin Cycle

Carbon fixation, reduction, and regeneration.

Carbon fixation (carboxylation)

The incorporation of inorganic carbon dioxide into an organic molecule.

Key enzyme in carbon fixation

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).

Reduction phase of the Calvin Cycle

ATP and NADPH are used to reduce molecules and produce G3P.

Role of ATP in the reduction phase

Supplies energy needed to convert intermediates into G3P.

Role of NADPH in the reduction phase

Provides high-energy electrons and hydrogen ions to reduce compounds.

Regeneration phase of the Calvin Cycle

The reformation of ribulose-1,5-bisphosphate (RuBP) so the cycle can continue.

Molecule regenerated in the Calvin Cycle

Ribulose-1,5-bisphosphate (RuBP).

Importance of RuBP regeneration

Allows continuous fixation of carbon dioxide.

ATP required to produce one G3P

Nine molecules of ATP.

NADPH required to produce one G3P

Six molecules of NADPH.

Requirements to produce one glucose molecule

Six CO₂, eighteen ATP, and twelve NADPH.

C3 plants

Plants that use the Calvin Cycle directly and produce a three-carbon compound as the first stable product.

First stable product in C3 plants

3-phosphoglycerate (3-PGA).

Examples of C3 plants

Rice, wheat, oats, and soybeans.

C4 plants

Plants with adaptations that reduce photorespiration and conserve water.

Examples of C4 plants

Corn, sugarcane, and sorghum.

Photorespiration

A process where RuBisCO binds oxygen instead of carbon dioxide.

Cause of photorespiration

Low carbon dioxide levels and high oxygen levels inside the leaf.

Effect of closed stomata

Prevents CO₂ from entering and O₂ from leaving the leaf.

Result of photorespiration

Produces a two-carbon compound and yields no sugar.

ATP production in photorespiration

No ATP is produced.

Difference between Calvin Cycle and photorespiration

The Calvin Cycle produces sugar, while photorespiration produces no sugar and wastes energy.