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 Synthesized by the Calvin Cycle in the Stroma

A. Light-Independent Reactions 1.The second stage of photosynthesis; light is not directly required. 2.Require CO 2, 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. CO 2 fixation is the attachment of CO 2 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[C 3 ]) 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 [C 3 ], a three-carbon product; Calvin cycle is also known as C 3 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 C 3 plants, Calvin cycle fixes CO 2 directly; first molecule following CO 2 fixation is PGA, a C 3 molecule. 2. C 4 leaves fix CO 2 by forming a C 4 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; CO 2 concentration decreases in leaves; O 2 increases. b.In C 3 plants, O 2 competes with CO 2, resulting in production of only one molecule of PGA. c.Called "photorespiration" since oxygen is taken up and CO 2 is produced; produces only one PGA.

C 4 Plants Flourish When It Is Hot and Dry Photorespiration does not occur in C 4 leaves even when stomates are closed. C 4 plants have advantage over C 3 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