A brief overview of C4 photosynthesis
The fundamentals of C4 photosynthesis are shown in a simplified form in the figure below. As you will recall, the photosynthesis processes of C4 plants are divided between mesophyll and bundle sheath cells. Two steps of C4 photosynthesis that occur in the mesophyll cells are the light-dependent reactions and a preliminary fixation of CO2 into a molecule called malate.
CO2 is released from malate in the bundle sheath cells, where it is fixed again by Rubisco and the Calvin-Benson cycle. The PEP is then recycled back to the mesophyll cells, and the carbohydrate products of photosynthesis are distributed through the plant.
How does this process help to explain the lower light compensation point and higher light saturation point of C4 plants? Decarboxylation of malate (release of the CO2) creates a higher concentration of CO2 in the bundle sheath cells than that found in photosynthetic cells of C3 plants. CO2 enrichment allows C4 plants to sustain higher rates of photosynthesis. Furthermore, because the concentration of CO2 relative to O2 in bundle sheath cells is higher, rates of photorespiration in C4 plants is lower than in C3 plants. In other words, the rate of "dark respiration" is lower, and the plant has a lower light compensation point.
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