Glossary
ATP
ATP is the molecule cells use to carry energy — breaking off its third phosphate releases the energy a reaction needs.
ATP, adenosine triphosphate, is the molecule a cell uses to carry energy from where it is released to where it is spent. It is a nucleotide with three phosphates attached, and the last two of those bonds hold a large amount of usable energy. Take off the third phosphate and the molecule becomes ADP, releasing energy that drives some other reaction, and the cell rebuilds ATP from ADP later. That cycle is constant: a cell does not store ATP in any quantity but makes and spends it continuously, which is why a cell deprived of oxygen fails within minutes. ATP is spent on everything that does not happen by itself — pumping ions across membranes, building proteins, contracting a muscle, moving a vesicle to the membrane. A question that asks for the cell's energy currency, or the molecule produced by glycolysis and the Krebs cycle, is asking for this.
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.
- 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.
- sodium-potassium pump — The sodium-potassium pump is a membrane protein that spends ATP to push three sodium ions out and pull two potassium ions in.
- exocytosis — Exocytosis is a vesicle fusing with the cell membrane and releasing its contents outside the cell — how a cell secretes what it makes.
- nucleotide — A nucleotide is the building block of DNA and RNA: a phosphate, a five-carbon sugar, and one of the nitrogenous bases.
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)