The Importance of the C4 Metabolic Pathway
As emphasized before, the largest reason why this metabolic pathway is more efficient than the
C3 metabolic pathways in hotter and drier conditions is the carbon fixation methods. In
the light dependent reactions photosynthesis can only occur with water as an input because it
supplies electrons. To obtain those electrons, the water is hydrolyzed, or split into 2 protons and
a half of a molecule of atmospheric oxygen. In a C3 plant, this oxygen can react with the
RuBP in the RuBP carboxylase enzyme, initiating photorespiration that severely reduces the
efficiency of photosynthesis. The C4 path puts the initial carbon fixation process in a
cell that has access to this oxygen. These mesophyll cells are in a layer that surrounds the
bundle-sheath cells, where the Calvin Cycle occurs. Because of this structural feature, the
bundle-sheath cells cannot receive any oxygen that the light dependent reactions produce.
However, these advantages are only visible in certain conditions. At regular temperatures —
around 20°C — and non extreme light intensities, the photosynthetic efficiencies from
both the C3 and C4 pathways are about the same. But at hot temperatures, the
efficiency gap is much more visible, with the C4 plants being more efficient.