citric acid (Krebs) cycle
/ SIT-rik AS-id; KREBZ SY-kuhl /
Imagine a recycling loop where a worker grabs a raw scrap, runs it around a circular conveyor that strips off every reusable part, drops the worker back at the start, and is ready to grab the next scrap. The citric acid cycle is exactly this kind of loop, and the "scraps" it strips down are the two-carbon fuel pieces handed over from the previous stage.
Inside the mitochondrion, each acetyl-CoA (the two-carbon fuel) is joined to a four-carbon molecule to make the six-carbon citric acid that gives the cycle its name. Through a series of enzyme steps, the cycle then carefully dismantles it: it releases two molecules of carbon dioxide, harvests electrons onto carriers to make NADH and FADH2, and makes a small amount of ATP directly. By the end it has regenerated the four-carbon starter, ready to accept the next acetyl-CoA. Because each glucose supplies two acetyl-CoA, the cycle turns twice per glucose.
The cycle's real product is not ATP but loaded electron carriers: it is where the last electrons are pulled out of the original glucose. Those NADH and FADH2 then feed the electron transport chain, which makes most of the ATP. It is also a metabolic crossroads, where fats and amino acids enter or leave, so it is far more than a sugar pathway. It is named for Hans Krebs, who worked out its looping logic in 1937.
For every acetyl-CoA fed in, the cycle spins once and spits out 2 CO2, 3 NADH, 1 FADH2, and 1 ATP — the carriers being the truly valuable haul.
The Krebs cycle mainly harvests electron carriers (NADH, FADH2), not ATP directly.
The cycle makes little ATP itself; its true value is loading NADH and FADH2 for the electron transport chain. It also runs only when oxygen lets that chain keep going.