Many steps in respiration are oxidation reactions in which hydrogen atoms are removed from a substrate. These reactions are catalysed by dehydrogenase enzymes. The hydrogen is not simply released; it is picked up by a hydrogen acceptor, a coenzyme that is reduced as the substrate is oxidised.Remember: oxidation is the loss of hydrogen (or electrons, or the gain of oxygen); reduction is the gain of hydrogen (or electrons).The two hydrogen acceptors in respiration are:
• NAD (nicotinamide adenine dinucleotide), used in glycolysis, the link reaction and the Krebs cycle. When it accepts hydrogen it becomes reduced NAD.
• FAD (flavin adenine dinucleotide), used at one step of the Krebs cycle. It becomes reduced FAD.Both are coenzymes, so they are not used up. Reduced NAD and reduced FAD carry the hydrogen to the electron transport chain on the inner mitochondrial membrane. There they give up the hydrogen, which splits into protons and electrons, and the coenzymes return to their oxidised form, ready to accept more hydrogen.
The final hydrogen acceptor in aerobic respiration is oxygen, which combines with the electrons and protons to form water. If there is no oxygen, reduced NAD cannot be reoxidised this way, so the cell must use another acceptor (pyruvate in animals, ethanal in yeast) to regenerate NAD.In photosynthesis a similar coenzyme, NADP, is the hydrogen acceptor.
• NAD (nicotinamide adenine dinucleotide), used in glycolysis, the link reaction and the Krebs cycle. When it accepts hydrogen it becomes reduced NAD.
• FAD (flavin adenine dinucleotide), used at one step of the Krebs cycle. It becomes reduced FAD.Both are coenzymes, so they are not used up. Reduced NAD and reduced FAD carry the hydrogen to the electron transport chain on the inner mitochondrial membrane. There they give up the hydrogen, which splits into protons and electrons, and the coenzymes return to their oxidised form, ready to accept more hydrogen.
The final hydrogen acceptor in aerobic respiration is oxygen, which combines with the electrons and protons to form water. If there is no oxygen, reduced NAD cannot be reoxidised this way, so the cell must use another acceptor (pyruvate in animals, ethanal in yeast) to regenerate NAD.In photosynthesis a similar coenzyme, NADP, is the hydrogen acceptor.