Cells whose function requires the expenditure of large amounts of energy usually contain more mitochondria than other cells (e.g., intestinal epithelial cells need energy for absorption of nutrients by active transport). The mitochondria may be packed into the part of the cell where the energy is required, e.g. tails of sperm cells. Red blood cells lack mitochondria and derive energy from glycolysis only.

Note that acetyl coenzyme A is an intermediate in the formation of fatty acids, while the GP is an intermediate in the formation of glycerol. So, glycolysis and the Krebs cycle can, and do, provide intermediates for lipogenesis (formation of lipids. Alternatively, fats can be broken down to yield GP and acetyl CoA to be fed into glycolysis and the Krebs cycle.

Similarly, as acetyl coenzyme A is an intermediate in the formation of amino acids, proteins can be broken down to amino acids which can be converted to acetyl co A and used to produce energy.

Because 2 molecules of acetyl CoA are produced from each glucose molecule, two turns of the Krebs cycle are required for each molecule of glucose. After 2 turns of the cycle, the original glucose has lost all of its carbon. The Krebs cycle produces 4CO₂, 6NADH/H⁺, 2FADH₂ and 2ATP from each molecule of glucose. Most of the energy of the glucose is still in the high-energy reduced coenzymes (NADH/H⁺ and FADH₂). The energy in these molecules will be used to synthesize ATP through the electron transport system.

The ETS is a series of acceptor molecules (carriers) embedded in the inner membrane of the mitochondrion or the plasma membrane of aerobic prokaryotes. Each carrier is reduced as it accepts hydrogen atoms from a previous carrier. The carrier is then oxidised as it passes on these hydrogens to the next carrier in the series. In this way the carriers are alternately reduced and oxidised as the high-energy hydrogen atoms are shuttled from one to the next.

Hydrogen atoms entering the chain have a relatively high energy content, but along the way the hydrogen atoms lose energy as each carrier is at a lower energy level than the previous one. Some of this energy that is ‘lost’ is used to make ATP from ADP and Pi. We say that the transfer of hydrogen atoms is coupled to the synthesis of ATP.

For each pair of H-atoms transferred from NADH/H⁺, 3 molecules of ATP are made, while each pair from FADH₂ leads to the production of 2 molecules of ATP. The final acceptor of hydrogen atoms is oxygen, so that a pair of hydrogen atoms (actually 2 protons and 2 electrons) combine with an oxygen atom to produce a molecule of water.

This final transfer is catalysed by cytochrome oxidase. In this step, hydrogen atoms are transferred from cytochrome to oxygen to form water. Cyanide (and some other poisons) can prevent this reaction from occurring. This stops the Krebs cycle, leading to reduced energy production in important muscles like those responsible for breathing and the heartbeat and, soon, death.

The net yield of ATP from 1 glucose molecule, when oxygen is present is:

The chemiosmotic model was proposed by Peter Mitchell in 1961 to explain the coupling of hydrogen and electron transport to ATP synthesis.