which, in the case of these biochemical reactions, becomes the final electron receptor in order to make CO2 and water. Basically, the reaction involved in aerobic respiration is this: C6H12O6 + 6 O2 goes to 6 CO2 + 6 H2O + heat (this reaction is spontaneous and does not innately require energy). There are also two important molecules made in these biochemical reactions: NADH and FADH2, which are used for the electron transport chain, which will be discussed. Flavin adenine dinucleotide, or FADH2, is a cofactor made during the Krebs cycle and contains hydrogen atoms, that give off electrons in the electron transport chain. Nicotinamide adenine dinucleotide, or NADH, is a related compound used in the electron transport chain as well. These are energy molecules like ATP, but are used in different reactions than ATP. So, how much ATP is made in aerobic respiration after a molecule of glucose is used up? We’ll break it down soon but, if the system worked perfectly, 38 molecules of ATP are made from a molecule of glucose (2 from glycolysis, 2 from the Krebs cycle, and 34 from the electron transport chain) but because these reactions rely on membranes and because membranes are inherently leaky, the reactions are inefficient so that only about 28 to 30 molecules are actually created in these reactions. This means that aerobic metabolism is up to 15 times more efficient than anaerobic metabolism. On the other hand, some anaerobic organisms, like those that use methane (methanogens), will use other organic molecules as their final electron receptor, yielding more ATP molecules than can be gotten from the typical anaerobic reactions that are seen in other animal organisms.
GLYCOLYSIS This is the part of the ATP-producing process that takes place in the cytoplasm of the cell. It takes glucose (a single molecule per reaction) and turns it into 2 pyruvate molecules. This reaction uses up two ATP molecules but yields 4 ATP molecules so that the net gain is 2 ATP molecules. Figure 20 shows what the reactions in glycolysis look like: 86
