Transcript Chapter 8

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 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 fivecarbon molecule that combines with
carbon dioxide.
2. Reducing PGA
• a. Six-carbon molecule immediately
breaks down, forms two PGA (3phosphoglycerate[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-3phosphate) 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