Essay by ilnet2000High School, 12th grade November 2005

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D. Modes of Photosynthesis

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.

3. CAM plants fix CO2 by forming C4 molecule at night when stomates can open without loss of water.

4. C4 Photosynthesis

a. In a C4 plant, mesophyll cells contain well-formed chloroplasts arranged in parallel layers.

b. In C4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts.

c. In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells.

d. C3 plants use RuBP carboxylase, Rubisco, to fix CO2 to RuBP in mesophyll; first

detected molecule is PGA.

e. C4 plants use the enzyme PEP carboxylase (PETCase) to fix CO2 to PEP (phosphoenolpyruvate);

end product to oxaloacetate (a C4 molecule).

f. In C4 plants, CO2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped

into the bundle-sheath cells; here CO2 enters Calvin cycle.

g. In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2-3 times that of C4 plants.

5. Photorespiration

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

b. In C3 plants, O2 competes with CO2 for the active site of RuBP carboxylase, 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.

d. Photorespiration does not occur in C4 leaves even when stomates are closed because CO2 is delivered

to Calvin cycle in bundle sheath cells.

e. C4 plants have advantage over C3 plants: in hot and dry weather, photorespiration does not occur

(e.g., bluegrass dominates...