AP BIOLOGY

Detailed explanation-1: -The primary fuel for cellular respiration is glucose. Fats can be broken down to make ATP. Proteins and nucleic acids can also be used to make ATP, but they are usually used for building important cell parts. In glycolysis, enzymes break down one six-carbon molecule of glucose into two three-carbon pyruvate molecules.

Detailed explanation-2: -Proteins are used as an energy source only if protein intake is high, or if glucose and fat sources are depleted, in which case amino acids from protein breakdown are converted into molecules that can enter the TCA Cycle.

Detailed explanation-3: -Amino acids, lipids, and other carbohydrates can be converted to various intermediates of glycolysis and the citric acid cycle, allowing them to slip into the cellular respiration pathway through a multitude of side doors.

Detailed explanation-4: -The most common respiratory substrate is glucose, which has a 6-carbon compound. The substrate is metabolized through glycolysis, the TCA cycle, electron transport chain, and oxidative phosphorylation.