Final answer:
Fats are the greatest source of ATP when oxidized because they are rich in hydrocarbons and yield a high number of ATP molecules upon metabolism, more so than sucrose, starch, or glycogen.
Step-by-step explanation:
The question asks which substrate, when oxidized, provides the greatest source of ATP. Among the options given—sucrose, starch, fats, and glycogen—the fats oxidation yields the most ATP. Fats, being rich in hydrocarbons, provide a dense source of chemical energy that, when metabolized through processes like beta-oxidation and the citric acid cycle, yield a high number of ATP molecules.
Each of the other substrates mentioned also contributes to ATP production. Glycogen and starch, as polymers of glucose, can be broken down into glucose and then utilized in glycolysis and the Krebs cycle to yield ATP. Sucrose, consisting of glucose and fructose, also goes through metabolic pathways that produce ATP upon catabolism. However, compared to fats, they all provide fewer ATP molecules per unit mass due to their structure and how they are metabolized within cellular respiration.