Nadph

pyruvate malate phosphate dehydrogenase dikinase

Malate

Plasmodesmata

Plasma membranes Pyruvate Malate

Plasma membranes Pyruvate Malate

NADP+

Ribulose 3-Phosphoglycerate

1,5-bisphosphate

NADP+

Ribulose 3-Phosphoglycerate

1,5-bisphosphate

Bundle-sheath cell

Triose phosphates

The pathway of CO2 assimilation has a greater energy cost in C4 plants than in C3 plants. For each molecule of CO2 assimilated in the C4 pathway, a molecule of PEP must be regenerated at the expense of two high-energy phosphate groups of ATP. Thus C4 plants need five ATP molecules to assimilate one molecule of CO2, whereas C3 plants need only three (nine per triose phosphate). As the temperature increases (and the affinity of rubisco for CO2 decreases, as noted above), a point is reached (at about 28 to 30 °C) at which the gain in efficiency from the elimination of photorespiration more than compensates for this energetic cost. C4 plants (crabgrass, for example) outgrow most C3 plants during the summer, as any experienced gardener can attest.

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