21. which step in cellular respiration occurs outside of the inner membrane space of the cell mitochondria? a. krebs cycle c. transition reaction b. citric acid cycle d. phosphorylation reaction

c. Transition Reaction

Explanation:

During the transition reaction, Acetyl-CoA is formed and connects the first stage of glycolysis with the Krebs Cycle (Citric Acid Cycle). In the presence of oxygen, pyruvate enters the mitochondria and is oxidized to form a compound of 2 carbon, acetate, with energy and CO2 release. During this process, the acetate binds to a coenzyme(coenzyme A (CoA)) - forming the acetyl-coenzyme A.

The 3 steps:

1. pyruvate is oxidized and forms acetate with liberation of CO2;

2. the energy released in the oxidation of pyruvate is stored in the reduction reaction of NAD+ to NADH + H+

3. The acetate molecule combines with coenzyme A to form acetyl-coenzyme A.