Cells pay for almost everything with one molecule: ATP, adenosine triphosphate. It is a close cousin of the nucleotides in RNA: the base adenine, the sugar ribose, and a tail of three phosphate groups.
Key idea
ATP releases energy when its last phosphate is removed, leaving ADP (adenosine diphosphate) and a free phosphate. Cells couple that release to work, and they recharge ADP back into ATP with energy from food.
$$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$$ Under standard laboratory conditions this releases about 7.3 kcal (30.5 kJ) per mole of ATP; inside a living cell, where ATP is kept far more plentiful than ADP, it is larger, roughly 50–60 kJ per mole.
Why it works
Why does removing a phosphate release energy? Not because one bond "snaps" and lets energy out: breaking any bond takes energy. The three phosphate groups are all negatively charged and crowded together, so ATP is strained and unstable. ADP and the free phosphate are far more stable, and the new bonds they form with water release more energy than it took to break the old one. The net result is energy released. Cells use it by moving the phosphate onto another molecule, such as a pump protein or a reactant, which couples an energy-releasing reaction to one that needs energy.
- Mechanical work: muscle proteins pulling.
- Transport work: the sodium-potassium pump of Channels, Carriers and Pumps.
- Chemical work: building polymers by dehydration synthesis.
Worked example: Energy from a pile of ATP
Under standard conditions, how much energy is released by hydrolyzing 4 moles of ATP? (7.3 kcal per mole)
- $4 \times 7.3 = 29.2$ kcal.
Answer: $29.2$ kcal
Worked example: Paying for a reaction
A reaction needs 20 kcal/mol to go forward. At 7.3 kcal per ATP (per mole), what is the smallest whole number of ATP the cell must spend per reaction?
- Two ATP give $2 \times 7.3 = 14.6$ kcal: not enough.
- Three ATP give $3 \times 7.3 = 21.9$ kcal: enough.
- So the cell must spend 3 ATP.
Answer: $3$ ATP
ATP is a currency, not a savings account. A cell holds only a small supply and recycles it constantly, turning ADP back into ATP as fast as it is used. Long-term energy is stored in fats and in starch or glycogen.
Common mistake
Picturing energy stored in a single "high-energy bond" that explodes when broken. Breaking any bond takes energy in. ATP hydrolysis releases energy overall because the products (ADP and phosphate, surrounded by water) are much more stable than ATP. Check your wording: "ATP hydrolysis releases energy," not "breaking the bond releases its energy."