C4 Plants

• Most organisms that carry out aerobic respiration feed pyruvate into the mitochondria, where it will be oxidized by the pyruvate dehydrogenase complex and the citric acid cycle. If oxygen is absent or present at low concentrations, these organisms may convert it to ethanol or lactate instead. A large class of plants called C4 plants, however, have another, more interesting use for pyruvate: They use it as part of a pathway that fixes carbon dioxide from the air.
  
  

PEP Carboxylase

• This pathway begins in so-called bundle-sheath cells, which export pyruvate into another group of cells called mesophyll cells. Now the pyruvate is converted to phosphoenolpyruvate or PEP by a special enzyme called pyruvate phosphate dikinase. This conversion is energy-hungry, however, and to power it the cell must burn up ATP. Each molecule of pyruvate converted to PEP uses up the equivalent of two ATP by converting one ATP to AMP. PEP is a relatively high-energy molecule, so by building it the plant paves the way for the ensuing reaction.
  
  

PEP Carboxylase

• Next the mesophyll cell couples PEP to bicarbonate through a reaction catalyzed by PEP carboxylase. This is a key step, because bicarbonate forms from dissolved CO2, so the plant is incorporating the equivalent of a molecule of CO2 into an organic compound. The result of this reaction is oxaloacetate, another important metabolic intermediate in its own right. The mesophyll cell now expends some reducing power in the form of a molecule of NADPH to turn oxaloacetate into malate -- a reaction catalyzed by malate dehydrogenase.
  
  

Full Cycle

• This sequence of reactions causes an accumulation of malate in the mesophyll cell, so the malate diffuses down its concentration gradient and back into the bundle-sheath cell via channels called plasmodesmata. Here another enzyme called malic enzyme oxidizes the malate, reducing a molecule of NADPH and converting the malate back into pyruvate while casting a molecule of CO2 into solution. The increasing concentration of pyruvate in the bundle-sheath cell will cause the pyruvate to diffuse back into the mesophyll cell. This cyclic process acts a little like a conveyor belt for CO2, increasing CO2 concentrations in the bundle-sheath cell in a way that benefits the plant during photosynthesis.