What is ATP? How cells store and spend energy

ATP, short for adenosine triphosphate, is the molecule every living cell uses to carry energy from where it is released to where it is spent. Breaking down food releases energy in steps; cells catch part of it by building ATP, then split ATP to power muscle, pump ions and build proteins. It is spent within seconds and remade constantly, so it works less like a savings account and more like cash.

In short

• ATP is a carrier, not a store: cells keep only a little at a time and recycle each molecule about a thousand times a day.
• Splitting off ATP's last phosphate group releases about 30 kJ per mole in the lab and nearer 50–57 kJ per mole inside a cell.
• Respiration turns one glucose molecule into roughly 30 ATP, almost all of it made in the mitochondria.

What is ATP made of?

ATP has three parts: adenine, a base also found in DNA; ribose, a five-carbon sugar; and a chain of three phosphate groups. Adenine plus ribose is adenosine, which is why the molecule is named adenosine triphosphate. Lose one phosphate and it becomes ADP, adenosine diphosphate; lose two and it is AMP, the monophosphate.

The phosphate groups each carry negative charge, and packing three of them in a row is chemically strained. When water breaks off the last one, ATP + H₂O → ADP + Pᵢ (Pᵢ is a free phosphate), the products are more stable than what went in, and the difference is released as usable energy. OpenStax's biology textbook gives the standard figure as −7.3 kcal/mol (−30.5 kJ/mol), and about −14 kcal/mol (−57 kJ/mol) under the conditions inside a living cell, where ATP is kept far more concentrated than ADP. The minus sign means energy is given out.

How does ATP power the work a cell does?

A cell rarely just "burns" ATP for heat. Instead, it couples the split to a job that would not happen on its own. An enzyme moves ATP's end phosphate onto another molecule, and that phosphorylated molecule changes shape or becomes reactive enough for the next step. The sodium-potassium pump in a nerve cell's membrane works this way: one ATP phosphorylates the pump, the pump flips, and three sodium ions are pushed out while two potassium ions come in, against their gradients.

Every known organism, from bacteria to people, spends ATP this way, which is one reason using energy through chemistry is on the list of traits that define life. The same trick runs muscle contraction (myosin heads pull on actin once per ATP), copying DNA, building proteins at the ribosome and moving cargo along the cell's internal tracks. Cells also use ATP as a signal: it is turned into cyclic AMP inside cells to relay hormone messages, and released between cells as a messenger. Caffeine works by blocking receptors for adenosine, the molecule left when all three phosphates are gone, as the note on how caffeine works explains.

How do cells make ATP from glucose?

Cells make ATP by metabolism, the network of reactions that break food down. For a sugar molecule in a human cell, the route is cellular respiration, in four stages. The first happens in the cytosol, the fluid of the cell; the rest happen in the mitochondria.

Stage	Where	ATP made directly	Electron carriers loaded
Glycolysis: glucose split into two pyruvate	Cytosol	2 (net)	2 NADH
Pyruvate oxidation: pyruvate to acetyl-CoA, CO₂ released	Mitochondrial matrix	0	2 NADH
Citric acid cycle: acetyl-CoA taken apart to CO₂	Mitochondrial matrix	2	6 NADH, 2 FADH₂
Electron transport and ATP synthase	Inner mitochondrial membrane	about 26–28	Carriers emptied; O₂ becomes water

The first three stages make only 4 ATP directly. Most of the energy leaves them riding on NADH and FADH₂, molecules that carry high-energy electrons. In the last stage, those electrons pass down a chain of proteins to oxygen, and the chain uses the energy to pump protons across the inner membrane. The protons flow back through ATP synthase, an enzyme that spins like a turbine as they pass and builds ATP from ADP and phosphate. Paul Boyer and John Walker shared half the 1997 Nobel Prize in Chemistry for working out that mechanism.

A worked count: why textbooks say 30, 32 or 38

The total depends on how many ATP each carrier is worth, and that is measured, not fixed.

1. Carriers from one glucose: 2 + 2 + 6 = 10 NADH and 2 FADH₂.
2. Modern measurements put one NADH at about 2.5 ATP and one FADH₂ at about 1.5. So 10 × 2.5 = 25, plus 2 × 1.5 = 3, gives 28.
3. Add the 4 ATP made directly: 28 + 4 = 32.
4. The 2 NADH from glycolysis are made outside the mitochondria and have to be passed in. Cells that use the glycerol phosphate shuttle hand those electrons on at the FADH₂ level, so each is worth only about 1.5, which takes 1 ATP off each: 32 − 2 = 30.
5. Older textbooks counted 3 per NADH and 2 per FADH₂: 30 + 4 + 4 = 38, the figure many people learned.

The honest answer is "about 30", because membranes leak protons and moving pyruvate and ADP into the mitochondria costs energy too. Cellular respiration is usually cited at around 29 to 30 ATP per glucose in practice. Even at that rate, the cell captures about a third of the energy in glucose, counted at standard conditions (more under the conditions inside a cell); the rest leaves as heat, which is part of why the body stays warm.

How much ATP does a body use?

An adult has only a small amount of ATP at any moment, yet recycles around 50 kg of it a day, roughly their own body weight. A rough check with real numbers shows why that is plausible.

• ATP weighs about 507 grams per mole, so 50 kg is about 50,000 ÷ 507 ≈ 99 moles.
• At about 50 kJ released per mole inside cells, 99 moles deliver around 5,000 kJ, or about 1,200 kcal.
• An adult eating 2,000 kcal a day spends a large share of it this way, with much of the rest lost as heat while the ATP is being made.

Each ATP molecule is therefore built and split again over and over within a single day. When demand jumps, in a sprint, a muscle cell's ATP would run out in seconds. It is topped up first from creatine phosphate, a quick reserve, then by glycolysis, and then by respiration as breathing catches up. The long-term stores are glycogen and fat, not ATP.

How does ATP connect respiration and photosynthesis?

Plants make ATP too, in two ways. In the light reactions of photosynthesis, the energy of sunlight drives a proton pump and an ATP synthase very like the one in mitochondria; that ATP is spent at once building sugar. Then plant cells, like animal cells, run respiration on that sugar to make ATP for everything else.

	Photosynthesis	Cellular respiration
Inputs	CO₂, water, light	Glucose, oxygen
Outputs	Glucose, oxygen	CO₂, water, ATP
Where	Chloroplasts	Cytosol and mitochondria
Role of ATP	Made from light, spent to build sugar	Made from sugar, spent on the cell's work

The two run as near mirror images. That is why the energy in almost every meal can be traced back through ATP to sunlight, and, further back, to the rules for energy that say it is moved and transformed, never made from nothing.

What do people get wrong about ATP?

The most common mistake is to say energy is "stored in the phosphate bond" and released when it breaks. Breaking any chemical bond takes energy; it never gives it out. The energy comes from the whole reaction: ATP and water on one side, ADP and phosphate on the other, and the new bonds formed with water on the product side are more stable. The phrase "high-energy bond" is shorthand for that, not a description of one bond.

The second mistake is to treat ATP as a fuel tank. It is closer to a rechargeable battery that is drained and recharged so fast that the supply never builds up. A cell low on ATP is not short of stored energy; it is short of the machinery or oxygen to turn that energy over quickly enough.

Check yourself

• Without looking back, explain why ATP is described as a carrier of energy rather than a store.
• Where do most of the 30 or so ATP from one glucose molecule come from, and why do counts differ?
• What is wrong with saying energy is released when ATP's phosphate bond breaks?

Sources

• 6.4 ATP: Adenosine Triphosphate — OpenStax, Biology 2e, 2018
• 7.4 Oxidative Phosphorylation — OpenStax, Biology 2e, 2018
• Adenosine triphosphate — Wikipedia
• Cellular respiration — Wikipedia
• The Nobel Prize in Chemistry 1997 — Nobel Prize Outreach