AP BIOLOGY

Detailed explanation-1: -In addition, the respiratory cycle converts NADH and FADH2 into reactants that the Krebs cycle requires to function. 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-2: -The Krebs cycle, by itself, does not require the presence of oxygen; this element is necessary for the last stage of aerobic cellular respiration, i.e., oxidative phosphorylation. Organic molecules endowed with energy (carbohydrates, lipids, proteins) are split in previous reactions.

Detailed explanation-3: -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-4: -Glycolysis can take place without oxygen in a process called fermentation. The other three stages of cellular respiration-pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation-require oxygen in order to occur.

Detailed explanation-5: -Glycolysis, which is the first step in all types of cellular respiration is anaerobic and does not require oxygen. If oxygen is present, the pathway will continue on to the Krebs cycle and oxidative phosphorylation. However, if oxygen is not present, some organisms can undergo fermentation to continually produce ATP.