Krebs cycleCitric acid cycle · TCA cycle

The Krebs cycle, also called the citric acid cycle or TCA cycle, is the hub of aerobic metabolism. It sits in the mitochondrial matrix and takes in acetyl-CoA, the two carbon fragment that carbohydrate, fat and, when needed, protein are all broken down into.

Each turn of the cycle strips carbon off as carbon dioxide, which is why you breathe it out, and loads electrons onto carrier molecules. Those carriers then feed the electron transport chain, where oxidative phosphorylation converts their cargo into the bulk of your ATP.

It matters conceptually because it is the point where the different fuels become interchangeable. Whatever you ate, once it reaches acetyl-CoA the machinery downstream is the same, which is the biochemical reason the body can run on a wide range of diets.

It is a cycle rather than a line because the molecule it starts with is regenerated at the end, ready to accept the next fragment. The intermediates in between do other jobs too, feeding into the synthesis of amino acids and other molecules, so the cycle is a distribution point as well as an energy pathway.

Its capacity adapts to training. Endurance work increases both mitochondrial number and the concentration of the enzymes running this cycle, so a trained muscle processes more fuel aerobically at any given intensity.

It is also, unfortunately, the biochemistry most often invoked to sell supplements. Ingredients that appear as intermediates in the cycle are marketed on that appearance, as though adding a molecule the body already makes would speed the whole thing up. The rate limiting factor is the machinery, not the parts list.

In practice, this is background rather than actionable, and the useful version is short: the cycle is the aerobic engine, and endurance training is what enlarges it.