Photosynthesis

Page
10

C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

This step requires the enzyme PEP carboxylase

PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Slide 77

Fig. 10-19

C4 leaf anatomy

Mesophyll cell

Photosynthetic

cells of C4

plant leaf

Bundle-

sheath

cell

Vein

(vascular tissue)

Stoma

The C4 pathway

Mesophyll

cell

CO2

PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)

ADP

ATP

Pyruvate (3C)

CO2

Bundle-

sheath

cell

Calvin

Cycle

Sugar

Vascular

tissue

Slide 78

Fig. 10-19a

Stoma

C4 leaf anatomy

Photosynthetic

cells of C4

plant leaf

Vein

(vascular tissue)

Bundle-

sheath

cell

Mesophyll cell

Slide 79

Fig. 10-19b

Sugar

CO2

Bundle-

sheath

cell

ATP

ADP

Oxaloacetate (4C)

PEP (3C)

PEP carboxylase

Malate (4C)

Mesophyll

cell

CO2

Calvin

Cycle

Pyruvate (3C)

Vascular

tissue

The C4

pathway

Slide 80

CAM Plants

Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

CAM plants open their stomata at night, incorporating CO2 into organic acids

Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

Slide 81

Fig. 10-20

CO2

Sugarcane

Mesophyll

cell

CO2

C4

Bundle-

sheath

cell

Organic acids

release CO2 to

Calvin cycle

CO2 incorporated

into four-carbon

organic acids

(carbon fixation)

Pineapple

Night

Day

CAM

Sugar

Sugar

Calvin

Cycle

Calvin

Cycle

Organic acid

Organic acid

(a) Spatial separation of steps

(b) Temporal separation of steps

CO2

CO2

1

2

Slide 82

The Importance of Photosynthesis: A Review

The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits