What Is ATP and How Is It Used in Cells?

ATP (adenosine triphosphate) is a molecule that transfers energy between reactions in cells. It is called an energy currency because cells can make it in energy-releasing processes and use it to drive processes that need energy.

How ATP works

ATP consists of adenine, ribose and three phosphate groups. When ATP reacts with water, ATP hydrolase catalyses its hydrolysis to ADP (adenosine diphosphate) and inorganic phosphate, often written as ${\mathrm{P}}_{\mathrm{i}}$:

$$\mathrm{ATP}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{ADP}+{\mathrm{P}}_{\mathrm{i}}$$

This reaction releases energy that can be coupled to an energy-requiring process. The released phosphate can also be added to another molecule. This is called phosphorylation, and it can make the molecule more reactive.

ATP is well suited to this role because it releases energy in small, manageable amounts and can be regenerated rapidly. It therefore couples energy-releasing reactions to energy-requiring reactions.

A simple example is the phosphorylation of a carrier protein during active transport. The phosphate transferred from ATP changes the protein's shape, allowing it to move a substance across the membrane. The phosphate is then released, so the protein returns to its original shape.

For example, cells use ATP in active transport. ATP hydrolysis supplies energy for a carrier protein to change shape and move a substance across a membrane against its concentration gradient. ATP is also used in processes such as muscle contraction and the synthesis of large molecules.

How cells replace ATP

ATP is continually resynthesised from ADP and inorganic phosphate in a condensation reaction catalysed by ATP synthase. Energy from respiration or photosynthesis drives this process, so ATP can be used again to transfer energy where the cell needs it.

Exam mistake to avoid: do not say that breaking a phosphate bond by itself releases energy. Breaking a bond requires energy; ATP hydrolysis releases energy overall because of the complete reaction. Also, active transport means movement against a concentration gradient, not simply any movement that uses ATP.