Adenosine triphosphate (ATP) is a nucleotide used by cells to transfer energy between reactions. It is often called the cell's energy currency because cells continually make it, break it down, and use the released free energy to drive work.
ATP is found in all known living cells. It is not a long-term energy store like fat or glycogen. It is a short-term carrier that links energy-releasing reactions to energy-requiring reactions.
Structure
ATP is made from:
- Adenine, a nitrogen-containing base.
- Ribose, a five-carbon sugar.
- Three phosphate groups.
The related molecules ADP and AMP have two and one phosphate groups respectively.
ATP Hydrolysis
Cells commonly use ATP by hydrolysing it to ADP and inorganic phosphate:
ATP + H2O -> ADP + Pi + free energy
The useful energy does not come from simply "breaking a bond" in isolation. It comes from the overall reaction, including formation of more stable products and changes in water, phosphate, and protein interactions. This distinction matters because simple textbook wording can make ATP sound like a tiny battery with energy stored only in one bond.
Cellular Work
ATP supports many types of cellular work, including:
- Muscle contraction and relaxation.
- Active transport across membranes.
- Protein phosphorylation in signalling pathways.
- Biosynthesis of macromolecules.
- Vesicle transport and other motor-protein activity.
- Maintenance of ion gradients in nerve and muscle cells.
Kinases use ATP to transfer phosphate groups to proteins or other molecules. ATPases use ATP hydrolysis to drive mechanical or transport processes.
Production
Cells regenerate ATP from ADP and phosphate. Main routes include glycolysis, substrate-level phosphorylation, oxidative phosphorylation in mitochondria, and photosynthetic phosphorylation in plants and some microbes.
In aerobic eukaryotic cells, mitochondria make much of the ATP by using energy from food molecules and oxygen to maintain a proton gradient that drives ATP synthase.
Signalling Roles
ATP also has signalling roles. Inside cells it is used by enzymes such as kinases. Outside cells, ATP can act through purinergic receptors and contribute to communication between cells, including in nerves, immune responses, and tissue injury.
See Also
References
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