the ATP/ADP cycle
/ A-T-P / A-D-P SY-kuhl /
Cash is useful because you can earn it from many jobs and spend it on almost anything. A cell needs the same convenience for energy: a single, standard form it can earn from breaking down food and spend on almost any task. That common energy currency is a molecule called ATP, and the cell recharges and spends it over and over in the ATP/ADP cycle.
ATP stands for adenosine triphosphate — adenosine carrying three phosphate groups in a row. The last two phosphates are joined by bonds that, when broken with water, release a useful burst of free energy. Spending ATP means snapping off the end phosphate to give ADP (adenosine diphosphate) plus a free phosphate, releasing energy for work. Recharging means the cell uses energy from food to stick that phosphate back on, turning ADP into ATP again. This spend-and-recharge loop runs millions of times per second, which is why a cell holds only a small amount of ATP at any moment but cycles a body weight's worth of it per day.
The cycle is the heart of cellular energy handling. It does not store energy long-term — that is what fats and carbohydrates are for — but it shuttles energy from where it is released to where it is needed, on a moment-to-moment basis. Calling the bonds "high-energy" is common but a bit misleading: the energy comes from the whole reaction with water, not from a magic spring inside one bond.
When a muscle contracts, it splits ATP to ADP for the energy to pull; moments later, respiration adds the phosphate back, recharging ADP into ATP for the next pull.
ATP is spent into ADP, then recharged back to ATP — a rechargeable battery, not a fuel tank.
ATP is the cell's spending money, not its savings. It stores very little energy overall; long-term energy is kept in fats and carbohydrates, and ATP is regenerated continuously from them.