1.2.4—ATP in cells

ATP is the immediate energy-transfer molecule used by cells. When ATP is hydrolysed to ADP and inorganic phosphate (Pi), energy is released and can be coupled to an energy-requiring process.

Respiration continually supplies energy to rebuild ATP from ADP + Pi. ATP then carries that immediately usable energy to cellular machinery; hydrolysis releases it at the site of work, and the ADP and Pi can be recycled into ATP. This links energy release in respiration to work elsewhere in the cell without treating glucose as the direct fuel for each process.

ATP from respiration is used for energy-requiring processes such as:

• building larger molecules in anabolic reactions;
• moving substances across membranes by active transport or within the cell;
• muscle contraction and nerve impulse transmission in animals.

Glucose is broken down in respiration to help make ATP, but ATP—not glucose—is the immediate energy source used directly by cellular processes. ATP is a short-term transfer molecule, not the cell’s main long-term energy store.