Aerobic Respiration

Aerobic Respiration: The Overview

  • Aerobic respiration is a metabolic process in which glucose is transformed into ATP in the presence of oxygen throughout four major stages: Glycolysis, Link Reaction, Krebs Cycle, and the Electron Transport Chain.


  • Glycolysis occurs in the cytoplasm and involves the breakdown of glucose (a six-carbon compound) into two molecules of pyruvate (a three-carbon compound).
  • This process has a net gain of two ATP molecules and produces two molecules of NADH.
  • The Link Reaction happens in the mitochondrial matrix once pyruvate enters the mitochondrion.
  • Each pyruvate molecule is transformed into a molecule of Acetyl CoA, producing one molecule of CO2 and one molecule of NADH.

Krebs Cycle

  • The Krebs Cycle is the third stage taking place within the mitochondrial matrix.
  • Acetyl CoA enters the Krebs Cycle and goes through a series of redox reactions that generate two molecules of CO2, one ATP, three NADH, and one FADH2.

Electron Transport Chain (ETC)

  • The Electron Transport Chain is the final stage of aerobic respiration, taking place across the inner mitochondrial membrane.
  • In the ETC, electrons carried by NADH and FADH2 are transferred via several protein complexes, generating a proton (H+) gradient across the membrane.
  • Energy from the electrons is used to synthesise ATP during a process known as oxidative phosphorylation.
  • In total, the ETC is able to produce about 34 molecules of ATP. However, this number can vary depending on whether conditions are optimal.
  • Oxygen serves as the final electron acceptor in the ETC, combining with electrons and H+ ions to form water.

Efficiency of Aerobic Respiration

  • Aerobic respiration is a highly efficient process, able to generate up to 38 ATP molecules from a single glucose molecule under optimum conditions.
  • However, actual ATP yield in cells is often slightly lower than the maximum theoretical yield due to losses during the transport of ATP and NADH, as well as the differing efficiency of glycolysis in different cells.
  • Despite these constraints, aerobic respiration is still vastly more efficient than anaerobic respiration which only yields 2 ATP molecules per glucose molecule.

Role of Oxygen in Aerobic Respiration

  • Oxygen plays a critical role in aerobic respiration as it’s the final electron acceptor in the ETC.
  • Without oxygen to accept the electrons, the ETC comes to a halt, and no ATP can be generated through oxidative phosphorylation.
  • This results in cells having to resort to less efficient methods for producing ATP, such as glycolysis followed by fermentative processes.