ATP (adenosine triphosphate) is the immediate source of energy for reactions in cells. It is a phosphorylated nucleotide made of three parts: the nitrogenous base adenine, the pentose sugar ribose and a chain of three phosphate groups.
ATP + H2O → ADP + Pi The reverse reaction, a condensation, is catalysed by ATP synthase during respiration and photosynthesis. Adding a phosphate group to a molecule is called phosphorylation.ATP is a good energy currency because:
• hydrolysis is a single step, so energy is released quickly;
• it releases energy in small, usable amounts, so little is wasted as heat;
• it is small and soluble, so it moves easily around the cell;
• the released phosphate can be transferred to other molecules, making them more reactive (for example, glucose at the start of glycolysis).ATP cannot be stored in large amounts, so cells make it continuously from ADP and Pi. A resting human turns over roughly their own body mass of ATP each day.
• hydrolysis is a single step, so energy is released quickly;
• it releases energy in small, usable amounts, so little is wasted as heat;
• it is small and soluble, so it moves easily around the cell;
• the released phosphate can be transferred to other molecules, making them more reactive (for example, glucose at the start of glycolysis).ATP cannot be stored in large amounts, so cells make it continuously from ADP and Pi. A resting human turns over roughly their own body mass of ATP each day.