Hydrolysis and Synthesis of ATP — AQA A Level Biology 7402 Revision Notes & Topical Questions

ATP is hydrolysed in a reaction catalysed by the enzyme ATP hydrolase (ATPase):

ATP + H₂O → ADP + P_i + energy

Water is added across the terminal phosphoanhydride bond. This releases about 30.6 kJ mol^-1. The reaction is reversible, so ADP and P_i can be recombined to form ATP, using energy from respiration or photosynthesis. The energy released is used in endergonic processes, and the inorganic phosphate may be passed to other molecules.

ATP is synthesised from ADP and P_i in a condensation reaction catalysed by ATP synthase. Energy must be supplied. There are three main routes:

Because ATP is made and broken down so rapidly, the ATP-ADP cycle never stops and a cell has only a small pool of ATP at any moment.

Many reactions in cells need energy (they are endergonic) while hydrolysis of ATP releases energy (it is exergonic). The two are linked or coupled. In a coupled reaction, the phosphate released from ATP is transferred to another reactant (a phosphorylated intermediate), which raises its energy level and makes it more reactive. Examples include:

A resting adult uses and remakes roughly 40 kg of ATP per day, which shows how fast the cycle turns over.

The organelles that make ATP (mitochondria, chloroplasts) are covered in section 2. Respiration in section 5.2 builds on this lesson, and ATP is also needed for protein synthesis, DNA replication and active transport across membranes. Remember that ATP hydrolysis is catalysed by an enzyme, so it follows the rules of enzyme-controlled reactions (temperature, pH).

If a question gives a graph showing ATP concentration in a muscle during exercise, remember that the concentration of ATP stays almost constant even when demand rises sharply, because synthesis keeps pace with hydrolysis.

In the muscle, creatine phosphate provides a rapid store of phosphate groups that can regenerate ATP from ADP for the first seconds of intense activity, before respiration speeds up.

Always state clearly whether you are describing the breakdown or the synthesis: hydrolysis uses water and releases energy, while synthesis is a condensation reaction that needs an energy input.

As a summary, remember the cycle

• respiration and photosynthesis supply energy to join ADP and P_i into ATP;
  • the ATP moves to where it is needed;
  • hydrolysis there releases energy and phosphate for work;
  • and the products return to be recycled.
• No ATP is ever lost from the system except by the organism's death.