Aerobic Respiration

Aerobic respiration is the complete oxidation of organic food in the presence of oxygen, and it is the pathway that releases the bulk of the energy stored in respiratory substrates. In eukaryotic plant cells it takes place inside the mitochondria. The pyruvic acid formed by glycolysis in the cytoplasm is transported into the matrix of the mitochondrion, where it is completely broken down to carbon dioxide and water, with a large amount of energy captured as ATP.

For aerobic respiration to proceed, the cell needs to accomplish two things:

• The complete oxidation of pyruvate, step by step, so that the carbon atoms are released as CO2 and the hydrogen atoms (with their electrons) are handed over to carrier molecules such as NAD+ and FAD.
• The passing on of these electrons through a series of carriers to molecular oxygen, coupling that electron flow to the synthesis of ATP.

The first of these — the systematic oxidation of pyruvate — is carried out by the reactions of the citric acid cycle (also called the Krebs cycle or the TCA cycle). The second — electron transfer to oxygen with ATP synthesis — is the work of the electron transport system (ETS) and oxidative phosphorylation. These two are dealt with in the sections that follow.

Before the citric acid cycle can begin, pyruvate must first be prepared. In a crucial link reaction inside the matrix, pyruvic acid undergoes oxidative decarboxylation, catalysed by the enzyme complex pyruvate dehydrogenase and requiring coenzymes such as coenzyme A and NAD+:

Pyruvic acid + CoA + NAD⁺ → (Pyruvate dehydrogenase; Mg²⁺) Acetyl CoA + CO₂ + NADH + H⁺

Each three-carbon pyruvate molecule loses one carbon as CO2 and the remaining two-carbon fragment is joined to coenzyme A to form acetyl CoA. This step also reduces one molecule of NAD+ to NADH for every pyruvate. Since glycolysis yields two molecules of pyruvate per glucose, two molecules of acetyl CoA (and two of CO2 and two of NADH) are produced. The acetyl CoA then enters the citric acid cycle, which is the true beginning of the aerobic breakdown of the respiratory substrate. …