The C3 and C4 photosynthetic pathways are two ways that different plants fix carbon. Both share a great deal in common, but the differences are more important than the similarities. In particular, C4 plants are better-suited for growth at high temperatures and in drier conditions, where the mechanism they employ improves their efficiency.
In C3 plants, CO2 is fixed with the aid of Earth's most abundant enzyme, rubisco, which attaches a molecule of CO2 to ribulose 1,5-bisphosphate. The newly formed six-carbon sugar is an unstable intermediate and will rapidly break apart into two molecules of 3-phosphoglycerate. Neither these steps nor the remaining steps in the reductive pentose phosphate cycle differ from those that occur in C4 plants; the C4 plants, however, have another cycle piggybacked onto this first one.
In C4 plants, carbon fixation begins with dissolved CO2 in the form of the bicarbonate ion, HCO3-. Bicarbonate is combined with phosphoenolpyruvate or PEP by an enzyme called PEP carboxylase. The product of this reaction is a molecule called oxaloacetate. Depending on the species, oxaloacetate may be either converted to an amino acid called aspartate or to malate. Converting it to malate requires reducing power in the form of nicotinamide adenine dinucleotide phosphate (NADPH); converting it to aspartate requires another amino acid. Either way, the CO2 has now been stored in a form that can be exported for the next step.
Malate or aspartate is exported from the cells where it was produced via channels called plasmodesmata. It passes into another set of cells called bundle-sheath cells. If the plant uses malate, an enzyme called malic enzyme converts the malate into pyruvate and CO2, reducing a molecule of NADP+ in the process. If the plant uses aspartate instead, the aspartate will first be converted to oxaloacetate then to malate, then malic enzyme or PEP carboxylase will handle the rest. Either way, the end products here are CO2 and pyruvate. The CO2 can then be fixed by rubisco, just as in the C3 plant, and the remaining steps are the same.
Why do C4 plants have another set of steps piggybacked onto the pathway found in C3 plants? The answer lies in the nature of the rubisco enzyme. Rubisco can bind not only CO2 but oxygen as well, and when it does so, it incorporates the oxygen into ribulose 1,5-bisphosphate to make 3-phosphoglycerate and a useless molecule of 2-phosphoglycerate. Reclaiming the latter takes energy, and this wasteful process of photorespiration impairs the plant's efficiency. By delivering the CO2 from an outer layer of cells to the bundle sheath cells, C4 plants help to maximise CO2 concentration in the right place and prevent photorespiration. The PEP carboxylase that figures so prominently in the C4 pathway has a very low affinity for oxygen and thus uses only the bicarbonate as a substrate.
It's important to bear in mind that C4 plants aren't necessarily more efficient. Regenerating the PEP used in the C4 pathway requires expenditure of chemical energy in the form of ATP. Consequently, while C4 plants have an efficiency gain because they experience less photorespiration, they have an efficiency loss when compared to C3 plants. As the temperature rises, however, photorespiration becomes an increasing problem for C3 plants, and eventually the losses to photorespiration are so great that the C4 plant surpasses its C3 rival's efficiency. This point generally takes place between 28 and 30 degrees Celsius. In warmer climates and sunny summers, C4 plants have the edge.
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