Aerobic respiration includes those portions of the respiratory metabolic pathway that are oxygen dependent. This includes the transition reactions, the Krebs cycle, and the electron transport system. All of these occur within the mitochondria [in eukaryotic cells. In prokaryotes, the enzymes and carrier molecules involved in the aerobic pathways are embedded in the plasma membrane, and the reactions occur in the cytoplasm at the surface of the membrane.
Pyruvate is one of the substances that is allowed to cross the membrane of the mitochondria into the organelle. As pyruvate enters the transition reactions, it is oxidized and decarboxylated, becoming a 2-carbon acetate molecule. The acetate is then joined to the carrier molecule acetyl coA, forming acetyl coA, which is delivered to the Krebs cycle. In the course of the transition reactions, one carbon dioxide is produced from each pyruvate molecule, and one molecule of NAD+ is reduced to NADH.
To begin the Krebs cycle, the acetate group is delivered to a molecule of oxaloacetate [a 4-carbon compound found in the matrix of the mitochondrion]. The 2-carbon acetate group joins to the 4-carbon oxaloacetate to form the 6-carbon tricarboxylic compound citrate. [The Krebs cycle is also known as the tricarboxylic acid cycle or the citric acid cycle, named for this first step of the reaction.] The Coenzyme A molecule is released to be recycled, picking up the next acetyl unit that passes through the transition reactions.
In a two step process, citrate if first dehydrated, then rehydrated, to produce isocitrate, an isomer of citrate.The isomerization prepares the molecule for an oxidative decarboxylation in the next step of the cycle.
In the fourth step, a carboxyl group is removed, and two hydrogens are removed, creating a 5-carbon molecule of [[alpha]] ketoglutarate. The hydrogens [one proton and 2 electrons] are passed on to NAD+ [the second proton remains free].
In another complex reaction, a second oxidative decarboxylation occurs. Coenzyme A temporarily binds to the resulting 4-carbon compound, producing a molecule of succinyl coA. NAD+ again accepts the electrons and one of the protons removed from the [[alpha]]-ketoglutarate.
The coenzyme A is removed, and enough energy is released to allow a substrate level phosphorylation. The phosphate group is bound briefly to the succinyl group, then is transferred to a molecule of GDP [guanosine diphosphate] to produce GTP. GTP transfers the phosphate group to ADP, producing a molecule of ATP. The 4-carbon succinate continues in the Krebs cycle, while the coenzyme A and the GDP are recycled.
Succinate is oxidized, passing the 2 removed hydrogens to the carrier molecule FAD. The remaining 4-carbon compound is fumarate.
Water is added to the fumarate to produce malate, in preparation for the last step of the cycle.
In the final step, a last oxidation occurs, removing 2 hydrogens, which are passed on to NAD+, and re-creating oxaloacetate. The cycle has been completed, and is ready to begin again.
In a single cycle of the Krebs cycle, 3 molecules of NAD+ have been reduced, and 1 molecule of FAD was reduced. One ATP molecule was produced through substrate level phosphorylation during the Krebs cycle. Two carbon dioxide molecules were released. Another NAD+ was reduced during the transition reactions, and one carbon dioxide was released during this reaction. All of the carbon-carbon and carbon-hydrogen bonds of the original pyruvate molecule have been broken.
If two pyruvates [the products of a single glucose molecule from the glycolyitic pathway] pass through these aerobic pathways, the total recovery will be doubled: 8 NADH, 2 FADH2, and 2 ATP will be recovered.
If the two ATP from the glycolytic pathway are added, this makes a total of 4 ATP recovered at the end of the Krebs cycle. 4 X 7.3 = 29.2 kcal of energy recovered from the original glucose molecule. Although this is better than the recovery of glycolysis alone, it is still inefficient. [about a 4 % efficiency rating.]
Aerobic respiration, however, is not yet over. The carrier molecules FADH2 and NADH hold electrons that remain at a high energy level, and can release that energy through continued oxidation. In the final steps of aerobic respiration, these electrons are passed through a series of oxidation-reduction reactions in the electron transport system.