Question

Would you agree that a C₃ plant has to expend more energy per molecule of CO₂ converted to carbohydrate than a C₄ plant? Why or why not?

Answer 1

Yes the C₃ plant has expand more amount of energy to fix CO2 than a C₄ plant because during photosynthesis C3 plants fixes carbon via the Calvin Cycle. During the one-step process, the enzyme RuBisCO causes an oxidation reaction in which some of the energy used in photosynthesis is lost in a process known as photorespiration. The result is about a 25% reduction in the amount of carbon that is fixed by the plant and released back into the atmosphere as carbon dioxide. Where as the C4 plants has one step in the pathway before the Calvin Cycle that reduces the loss of amount of carbon that is lost in overall process. The carbon dioxide that is taken in by the plant is moved to bundle sheath cells by the malic acid or aspartic acid molecules. The oxygen content inside bundle sheath cells is very low, so the RuBisCO enzymes are less likely to catalyze oxidation reactions (required less energy) and waste carbon molecules. The malate and aspartate molecules release the carbon dioxide in the chloroplasts of the bundle sheath cells and the Calvin Cycle begins.