ATP

Cells need energy for reactions that would not occur without an energy input. Adenosine triphosphate (ATP) acts as an energy carrier: its breakdown releases energy that can be coupled to these energy-requiring reactions.

ATP is a nucleotide derivative made of adenine, ribose and a chain of three phosphate groups. Adenine is attached to ribose, which is attached to the phosphate chain.

ATP hydrolysis

In hydrolysis, water is used to split a molecule. The enzyme ATP hydrolase catalyses the hydrolysis of ATP, removing the phosphate at the end of its chain. This produces adenosine diphosphate (ADP), which has two phosphate groups, and inorganic phosphate, written as $\mathrm{P_i}$ .

\mathrm{ATP} + \ce{H2O} \xrightarrow{\text{ATP hydrolase}} \mathrm{ADP} + \mathrm{P_i}

This reaction releases energy. By coupling ATP hydrolysis to an energy-requiring reaction, the cell can use that energy to drive the reaction.

The released inorganic phosphate can also be transferred to another compound. This transfer is called phosphorylation and often makes the receiving compound more reactive.

ATP resynthesis

ATP can be made again from ADP and inorganic phosphate. This is a condensation reaction: the phosphate joins ADP and water is formed. The reaction requires an input of energy.

\mathrm{ADP} + \mathrm{P_i} \xrightarrow{\text{ATP synthase}} \mathrm{ATP} + \ce{H2O}

The enzyme ATP synthase catalyses ATP resynthesis during respiration or photosynthesis. ATP can then be hydrolysed again, allowing energy to be transferred to further energy-requiring reactions.