Cellular Respiration (advanced)

Aerobic respiration involves four key metabolic events: glycolysis, the linked reaction, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (including the electron transport chain and chemiosmosis). The overall process converts one glucose molecule and six oxygen molecules into 38 ATP, 6 carbon dioxide, and 6 water molecules.

Glycolysis occurs in the cytosol, with or without oxygen, and is a 10-step enzymatic process. It has a preparatory phase (steps 1-5) where 2 ATP are consumed, and a payoff phase (steps 6-10) where 4 ATP and 2 NADH are created. The net output is 2 ATP, 2 NADH, and 2 pyruvate molecules. ATP is used immediately, while NADH and pyruvate move to the mitochondria.

The linked reaction takes place in the fluid matrix inside the inner mitochondrial membrane. Pyruvate molecules react with coenzyme A. During this process, NAD is reduced to NADH, and pyruvate is decarboxylated, releasing carbon dioxide and forming acetyl CoA. NADH goes to the final steps of aerobic respiration, carbon dioxide diffuses out, and acetyl CoA moves to the next step. In total, two pyruvate molecules produce two acetyl CoA, two carbon dioxide, and two NADH.

Also known as the citric acid cycle or tricarboxylic acid cycle, this enzymatic pathway occurs in the mitochondrial matrix. It's a cyclic process where each turn involves two decarboxylations and four oxidations. Acetyl CoA reacts with oxaloacetic acid to form citric acid. Throughout the cycle, NAD is reduced to NADH, FAD is reduced to FADH2, carbon dioxide is released, and ATP is phosphorylated. Since two acetyl CoA molecules are produced from one glucose, the cycle occurs twice, generating NADH and FADH2 for the final phase.

This multi-step process occurs between the mitochondrial matrix and the intermembrane space, across the inner membrane called the cristae. Embedded in the cristae are four carrier protein complexes forming the electron transport chain. NADH and FADH2, produced in earlier steps, supply electrons and hydrogen ions. As electrons move through the chain, energy is released, pumping protons into the intermembrane space. Oxygen acts as the final electron acceptor, reacting with hydrogen ions to form water. The high concentration of protons in the intermembrane space drives them through ATP synthase, which phosphorylates ADP to make up to 34 ATP molecules per glucose.

From one glucose molecule: 2 ATP are made in glycolysis, 2 ATP in the Krebs cycle, and up to 34 ATP via ATP synthase in oxidative phosphorylation. Oxygen is used at the end of the electron transport chain to form water, while carbon dioxide is produced during the linked reaction and the Krebs cycle.