AP BIOLOGY

Detailed explanation-1: -The tricarboxylic acid (TCA) cycle, also known as the Krebs or citric acid cycle, is the main source of energy for cells and an important part of aerobic respiration.

Detailed explanation-2: -Thus, if oxygen is not present, the respiratory cycle cannot function, which shuts down the Krebs cycle. For this reason, the Krebs cycle is considered an aerobic pathway for energy production.

Detailed explanation-3: -The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism.

Detailed explanation-4: -TCA cycle occurs in mitochondria during aerobic respiration to release energy, water, and carbon dioxide through the oxidation process. So, anaerobic respiration does not use the TCA cycle.

Detailed explanation-5: -Under anaerobic conditions, especially during fermentative growth, during which NADH cannot be reoxidized by the respiratory chain, Krebs cycle activity is reduced to the minimum level needed to supply biosynthetic pathways and is transformed from a cyclic pathway to two oppositely oriented half-cycles.