Glossary
Krebs cycle
The Krebs cycle is the second stage of cellular respiration, in the mitochondrial matrix, where acetyl-CoA is broken down and most of the electron carriers are loaded.
The Krebs cycle — also the citric acid cycle, or TCA cycle — is the second stage of cellular respiration, and it runs inside the mitochondrion. Pyruvate from glycolysis is converted to acetyl-CoA, which enters the cycle; the two carbons it brings are stripped off as carbon dioxide, and what remains is rearranged and released to start the cycle again. The yield per glucose is small in ATP — two molecules — and large in electron carriers: NADH and FADH2, loaded with the energy those electrons carry. The big ATP payoff comes after the cycle, when those carriers feed the electron transport chain and the energy is finally converted to ATP. The cycle needs oxygen only indirectly, as the acceptor at the end of that chain; that is why a cell without oxygen cannot keep the cycle running, even though no oxygen is used in the cycle itself. The name to remember is the product: acetyl-CoA in, carbon dioxide and carriers out.
Taught in
Related terms
- glycolysis — Glycolysis is the first stage of cellular respiration: in the cytoplasm, one glucose is split into two pyruvate, with a small net gain of ATP.
- ATP — ATP is the molecule cells use to carry energy — breaking off its third phosphate releases the energy a reaction needs.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)