Photosynthesis

Page
9

Fig. 10-18-2

Ribulose bisphosphate

(RuBP)

3-Phosphoglycerate

Short-lived

intermediate

Phase 1: Carbon fixation

(Entering one

at a time)

Rubisco

Input

CO2

P

3

6

3

3

P

P

P

P

ATP

6

6 ADP

P

P

6

1,3-Bisphosphoglycerate

6

P

P

6

6

6 NADP+

NADPH

i

Phase 2:

Reduction

Glyceraldehyde-3-phosphate

(G3P)

1

P

Output

G3P

(a sugar)

Glucose and

other organic

compounds

Calvin

Cycle

Slide 72

Fig. 10-18-3

Ribulose bisphosphate

(RuBP)

3-Phosphoglycerate

Short-lived

intermediate

Phase 1: Carbon fixation

(Entering one

at a time)

Rubisco

Input

CO2

P

3

6

3

3

P

P

P

P

ATP

6

6 ADP

P

P

6

1,3-Bisphosphoglycerate

6

P

P

6

6

6 NADP+

NADPH

i

Phase 2:

Reduction

Glyceraldehyde-3-phosphate

(G3P)

1

P

Output

G3P

(a sugar)

Glucose and

other organic

compounds

Calvin

Cycle

3

3 ADP

ATP

5

P

Phase 3:

Regeneration of

the CO2 acceptor

(RuBP)

G3P

Slide 73

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis

The closing of stomata reduces access to CO2 and causes O2 to build up

These conditions favor a seemingly wasteful process called photorespiration

Slide 74

Photorespiration: An Evolutionary Relic?

In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle

Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

Slide 75

Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

Slide 76

C4 Plants