Aerobic cellular respiration

Aerobic cellular respiration is a process that oxidizes the major products of glycolysis to produce ATP. These major products are pyruvate, which is oxidized via the citric acid cycle (also known as the Tricarboxylic Acid (TCA) Cycle or Krebs Cycle), and NADH, which is oxidized via the electron transport chain. This chemical reaction is called Oxidative Phosphorylation. Aerobic respiration takes place in the mitochondria of the cell when oxygen is present. By contrast, anaerobic cellular respiration takes place when oxygen is limiting, and occurs independently of mitochondria.

The citric acid cycle:
During the citric acid cycle, also known as the Tricarboxylic acid (TCA) cycle or Krebs cycle,  pyruvate molecules produced by glycolysis are actively transported from the cytosol, where they are produced, across the inner mitochondrial membrane, and into the matrix. In the matrix, pyruvate is oxidized and combined with coenzyme A to produce CO2, 1 GTP, 3 NADH and 1 FADH2. The GTP is converted to ATP, and the NADH and FADH2 serve as a source of electrons for the electron transport chain.

The electron transport chain:
The redox energy (in the form of electrons) from NADH and FADH2 (byproducts of the Citric Acid Cycle) is transferred to oxygen (O2) in several steps, via protein complexes in the mitochondrial inner membrane (NADH dehydrogenase, cytochrome c reductase and cytochrome c oxidase). The incremental release of energy is used to pump protons (H+) into the mitochondrial intermembrane space, creating an electrochemical proton gradient with the potential to do work. This potential is realized by ATP synthase (ATP-ase), an intermembrane spanning enzyme, which, by allowing these protons to flow passively down the gradient, back into the matrix, generates ATP from adenosine diphosphate (ADP) and inorganic phosphate. This chemical process is called oxidative phosphorylation.

Oxidative phosphorylation:
ADP3- + H+ + Pi ↔ ATP4- + H2O