Photosynthesis Has Two Sets of Reactions

• Light-dependent reactions takes place in the thylakoids; cannot take place unless light is present, traps energy.

• Light-independent reactions take place in the stroma; can occur in either the light or the dark, fixes carbon.

7.3. Solar Energy Is Captured

Photosynthesis Light Dependent

Carbohydrate Is Synthesizedby the Calvin Cycle

in the Stroma

A. Light-Independent Reactions

• 1. The second stage of photosynthesis; light is not directly required.

• 2. Require CO2, which enters through leaf and NADPH and ATP, which have been produced by light-dependent reactions.

C. The Calvin Cycle Has Three Stages

C14 Labeled

Photosynthesis Light Independent

1. Fixing Carbon Dioxide

• a. CO2 fixation is the attachment of CO2 to an organic compound.

• b. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide.

2. Reducing PGA

• a. Six-carbon molecule immediately breaks down, forms two PGA (3-phosphoglycerate[C3]) molecules.

2. Reducing PGA

• b. Each of two PGA molecules undergoes reduction to PGAL in two steps.

• c. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce PGA to PGAL.

3. Regenerating RuBP

• a. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP (ribulose bisphosphate).

3. Regenerating RuBP

• b. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate RuBP.

• c. First molecule identified by Calvin was PGA [C3], a three-carbon product;

Calvin cycle is also known as C3 cycle.

B. The Importance of PGAL

• 1. PGAL (glyceraldehyde-3-phosphate) is product of Calvin cycle; is converted to many organic molecules.

Photosynthesis Takes Other Routes;

C3 and C4 Plants

• 1. In C3 plants, Calvin cycle fixes CO2

directly; first molecule following CO2 fixation is PGA, a C3 molecule.

• 2. C4 leaves fix CO2 by forming a C4

molecule prior to the involvement of the Calvin cycle.

C3 and C4 Plants

• C3 Plants– Wheat– Rice– Oats

• C4 Plants– Corn– Sugarcane– Bermuda grass

Photorespiration

• a. In hot weather, stomates close to save water; CO2 concentration decreases in leaves; O2 increases.

• b. In C3 plants, O2 competes with CO2, resulting in production of only one molecule of PGA.

• c. Called "photorespiration" since oxygen is taken up and CO2 is produced; produces only one PGA.

C4 Plants Flourish When It Is Hot and Dry

• Photorespiration does not occur in C4 leaves even when stomates are closed.

• C4 plants have advantage over C3 plants: in hot and dry weather, photorespiration does not occur (e.g., bluegrass dominates lawns in early summer, crabgrass takes over in hot midsummer).

The End.

Practice Quiz Chapter 10