Part 1: Structure of ATP

ATP stands for adenosine triphosphate. It is a nucleotide derivative made from ribose, adenine, and three phosphate groups. Adenine joined to ribose is called adenosine, and the three phosphate groups are attached to this adenosine.

Because ATP has the same basic features as a nucleotide, it is described as a nucleotide derivative. The phosphate groups are especially important because ATP transfers energy when its terminal phosphate group is removed.

Part 2: Hydrolysis of ATP

When energy is needed, ATP is hydrolysed to ADP and inorganic phosphate (Pi). Water is used in the reaction, and the enzyme ATP hydrolase catalyses it.

ATP + H2O -> ADP + Pi + energy

Hydrolysis of ATP is a single reaction, so energy can be released quickly where it is needed. Each ATP molecule releases a small, manageable amount of energy, which is why ATP is useful as an immediate energy source rather than a long-term store.

Part 3: Coupling and phosphorylation

The energy released by ATP hydrolysis can be coupled to reactions inside cells that require energy. In a coupled reaction, energy from ATP is transferred directly to an energy-requiring process so that the process can proceed.

The inorganic phosphate released from ATP can also be used to phosphorylate other molecules. Phosphorylation changes a molecule's chemical state and often makes it more reactive, so it can take part in the next step of a metabolic pathway more easily.

ATP is therefore useful to cells in two linked ways: it transfers energy and it supplies phosphate groups that can make other compounds easier to react.

Part 4: Resynthesis of ATP

ATP is not stored in large amounts, so cells must resynthesise it continuously. ATP is formed again when ADP combines with inorganic phosphate in a condensation reaction. This reaction is catalysed by ATP synthase.

ADP + Pi -> ATP + H2O

ATP synthase resynthesises ATP during photosynthesis and during respiration. This means cells can keep recycling ATP between its hydrolysed form and its resynthesised form as energy is transferred around the cell.

A simple way to remember the cycle is this: hydrolysis of ATP releases energy for cell processes, and resynthesis of ATP stores energy again in a form the cell can use later.