Summary of respiration to see how much ATP is made from each glucose molecule. ATP is made in two different ways:
- Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate level phosphorylation (since ADP is being phosphorylated to form ATP).
- Most of the ATP molecules are made by the ATP synthase enzyme in the respiratory chain. Since this requires oxygen it is called oxidative phosphorylation. Scientists don’t yet know exactly how many protons are pumped in the respiratory chain, but the current estimates are: 10 protons pumped by NADH; 6 by FADH; and 4 protons needed by ATP synthase to make one ATP molecule. This means that each NADH can make 2.5 ATPs (10/4) and each FADH can make 1.5 ATPs (6/4).
Two ATP molecules are used at the start of glycolysis to phosphorylate the glucose, and these must be subtracted from the total.
The table below is an “ATP account” for aerobic respiration, and shows that 32 molecules of ATP are made for each molecule of glucose used in aerobic respiration. This is the maximum possible yield; often less ATP is made, depending on the circumstances. Anaerobic respiration only produces the 2 molecules of ATP from the first two rows.
Other substances can also be used to make ATP. Glycogen of course is the main source of glucose in humans.
Triglycerides are broken down to fatty acids and glycerol, both of which enter the Krebs Cycle. A typical triglyceride molecule might make 50 acetyl CoA molecules, yielding 500 ATP molecules. Fats are thus a very good energy store, yielding 2.5 times as much ATP per g dry mass as carbohydrates. Proteins are not normally used to make ATP, but in starvation they can be broken down and used in respiration.
They are first broken down to amino acids, which are converted into pyruvate and Krebs Cycle metabolites and then used to make ATP.