Aerobic Respiration: An Overview

When oxygen is available, a respiring cell abandons the energetically wasteful route of fermentation and instead sends the pyruvic acid produced by glycolysis into the mitochondria, where it undergoes complete, or aerobic, oxidation all the way to carbon dioxide and water. Because this complete breakdown releases very much more of the chemical energy originally stored in the glucose molecule than fermentation ever can, aerobic respiration is, cell for cell, an overwhelmingly more efficient way of generating usable energy in the form of ATP, and it is the pathway relied upon by the great majority of plant, animal, fungal and microbial cells whenever oxygen is present.

Aerobic respiration can be thought of as unfolding in three connected stages, each taking place within, or at, the mitochondrion -- the organelle popularly described as the cell's powerhouse precisely because of the central role it plays in this process. The first stage is the oxidative decarboxylation of pyruvic acid, sometimes called the link reaction because it connects glycolysis (which occurs in the cytoplasm) to the rest of aerobic respiration (which occurs inside the mitochondrion). Pyruvic acid produced by glycolysis is actively transported across the mitochondrial membranes into the mitochondrial matrix, where a large multi-enzyme assembly called the pyruvate dehydrogenase complex catalyses its oxidative decarboxylation: one carbon atom is removed as carbon dioxide, the remaining two-carbon fragment is oxidised (reducing one NAD+ to NADH), and the resulting two-carbon acetyl group is attached to coenzyme A to form acetyl coenzyme A (acetyl CoA). Because two molecules of pyruvic acid are produced from each original glucose molecule, this link reaction runs twice per glucose, yielding two molecules of acetyl CoA, two molecules of carbon dioxide, and two molecules of NADH.

The acetyl CoA produced by the link reaction then enters the second stage, the tricarboxylic acid (TCA) cycle -- described in detail in the next section -- which takes place in the mitochondrial matrix and completes the oxidation of the two-carbon acetyl group all the way to carbon dioxide, in the process generating a further, much larger, supply of the reduced coenzymes NADH and FADH2. …