ATP, the cell's energy carrier

Section 3 of 4

The energy from glucose has to reach the muscles that move you, the pumps in the walls of your cells and the enzymes that build new molecules. One small molecule carries it to all of them: ATP.

ATP is short for adenosine triphosphate. It starts with adenosine, the messenger that builds up in your brain and makes you sleepy: adenine, one of the bases in DNA, joined to a ribose sugar ring. In place of the H on the ribose's -⁠CH₂OH group, ATP carries a chain of three phosphate groups. Each one is a phosphorus atom, P, with four oxygen atoms round it, and neighbouring phosphates share an oxygen atom between them.

ATP, adenosine triphosphate A skeletal formula in two shaded parts. On the right, shaded blue, adenosine: adenine, a six-membered ring and a five-membered ring sharing one edge, with four nitrogen atoms in the rings and an -NH2 group at the top, joined below to a ribose ring of four carbon atoms and one oxygen atom, with two -OH groups. On the left, shaded amber, three phosphate groups in a row, joined to the ribose ring through a CH2 group and an oxygen atom. Each phosphate group is a phosphorus atom, P, with a double-bonded oxygen above it and an -OH group below, and neighbouring phosphates share an oxygen atom. The phosphate at the left end carries one more -OH group. NNNNNH₂OOHOHOPOHOOPOHOOPOHOHO three phosphate groups adenosine
ATP, C₁₀H₁₆N₅O₁₃P₃: adenosine (blue) carrying a chain of three phosphate groups (amber). Inside a cell, the -⁠OH groups on the phosphates have lost their H⁺, so the chain carries four negative charges.

Spending a phosphate

When a cell needs energy, an enzyme uses a water molecule to break the end phosphate off. ATP becomes ADP, adenosine diphosphate, with two phosphates, and a free phosphate is left over. The reaction gives out energy. As in any reaction that does, the bonds in the products are stronger overall. The negative charges crowded along the phosphate chain also push each other apart, and they spread out once the end phosphate is free.

Muscles use that energy to pull, pumps use it to move ions in and out of cells, and enzymes use it to join small molecules into proteins and DNA. When cells work hard, some of their ATP loses all three phosphates, and the adenosine left behind builds up.

ATP gives out energy as it loses a phosphate, and is made again At the top, ATP: a blue block labelled adenosine joined to a chain of three amber circles labelled P, the phosphate groups. At the bottom, ADP plus phosphate: the same blue block with a chain of two P circles, and a third P circle on its own. An amber arrow on the left points down from ATP to ADP plus phosphate, labelled gives out energy. An arrow on the right points back up from ADP plus phosphate to ATP, labelled takes in energy. adenosine P P P ATP adenosine P P + P ADP + phosphate gives out energy takes in energy
ATP and ADP go round and round. Water taking the end phosphate off ATP gives out energy for the cell to use. Energy from glucose and fat joins ADP and phosphate back into ATP.

Making ATP again

Joining ADP and phosphate back into ATP takes in energy, and that energy comes from breaking down glucose and fat. The joining gives off a water molecule, a condensation like the one that links glucose units into starch. A cell makes about 30 ATP from each glucose molecule, most of them in its mitochondria. About a third to a half of the glucose's energy ends up in ATP, and the rest comes out as heat, which keeps your body at about 37 °C.

Your body holds only a small amount of ATP at any moment, and it makes and uses about its own weight of ATP every day.

Quick check

What happens to ATP when a cell uses its energy?