Respiration

All living cells release the energy in substrate molecules using aerobic or anaerobic respiration. The respiratory process is a sequence of interconnected enzyme controlled steps called a metabolic pathway. Other pathways include photosynthesis and the synthesis of steroid hormones such as oestrogen from cholesterol. During glycolysis, the link reaction and the Krebs cycle, some of the steps include oxidation by dehydrogenase enzymes. This oxidation involves the transfer of hydrogen ions and electrons from the substrate and passing them to a coenzyme which becomes reduced. For example, in the cytoplasm, when triose phosphate molecules are oxidised to pyruvate as part of glycolysis, the coenzyme NAD is reduced forming reduced NAD. The coenzyme forms part of the active site of the dehydrogenase enzyme allowing it to function as a catalyst and