<Dd> PEP + CO → oxaloacetate </Dd> <P> PEP carboxylase has a lower Km for HCO − 3--and, hence, higher affinity--than RuBisCO . Furthermore, O is a very poor substrate for this enzyme . Thus, at relatively low concentrations of CO, most CO will be fixed by this pathway . </P> <P> The product is usually converted to malate, a simple organic compound, which is transported to the bundle - sheath cells surrounding a nearby vein . Here, it is decarboxylated to produce CO and pyruvate . The CO now enters the Calvin cycle and the pyruvate is transported back to the mesophyll cell . </P> <P> Since every CO molecule has to be fixed twice, first by 4 - carbon organic acid and second by RuBisCO, the C pathway uses more energy than the C pathway . The C pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C pathway requires 30 molecules of ATP . This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss . </P>

Where does calvin cycle occur in c4 plants