What is ATP?
ATP stands for adenosine triphosphate. It is the molecule cells use to pass energy from food (or light) to the jobs that need it. It is often called the energy currency of the cell: like money, it is earned in one place and spent in another.
ATP is a nucleotide (a close relative of the building blocks of RNA). It has three parts:
- adenine: a nitrogen-containing base
- ribose: a five-carbon (pentose) sugar
- three phosphate groups in a chain
Adenine + ribose together are called adenosine. With two phosphates the molecule is ADP (adenosine diphosphate); with one it is AMP.
Hydrolysis of ATP releases energy
The enzyme ATP hydrolase breaks the bond to the last phosphate using a molecule of water:
ATP + H₂O → ADP + Pi (Pi = inorganic phosphate)
This is a hydrolysis reaction ("splitting with water"). It releases about 30.5 kJ per mole of ATP. Note: energy is not "stored in the bond" like a spring. The products (ADP + Pi) are more stable than ATP, so the reaction as a whole gives out energy.
The released Pi can be added to another molecule (phosphorylation). This makes that molecule more reactive, for example glucose at the start of respiration, or a carrier protein that changes shape.
Synthesis of ATP: ATP synthase
ATP is remade from ADP and Pi by the enzyme ATP synthase. This is a condensation reaction because water is released:
ADP + Pi → ATP + H₂O
Energy is needed. It comes from:
- respiration in mitochondria and the cytoplasm (oxidative and substrate-level phosphorylation)
- photosynthesis in chloroplasts (photophosphorylation)
ATP synthase sits in membranes. A flow of hydrogen ions through it spins part of the enzyme like a tiny turbine, and each turn helps make ATP.
Why ATP and not glucose?
- Small packets: one hydrolysis releases a small, right-sized amount of energy, so little is wasted as heat.
- One step: energy is released in a single reaction, so it is fast. Glucose needs many steps.
- Quickly remade: ADP + Pi can be turned back into ATP again and again.
- Soluble and mobile: it moves easily around the cell.
- Phosphorylates: it can pass Pi to other molecules to activate them.
But ATP is not a good long-term store: its phosphate bonds are unstable, so cells keep only a tiny amount and remake it constantly. Long-term stores are fats and carbohydrates such as glycogen and starch.
What cells use ATP for
- Metabolic processes: building large molecules (proteins, DNA, starch).
- Movement: muscle contraction, cilia and flagella.
- Active transport: carrier proteins pump ions against a gradient, e.g. the sodium–potassium pump in nerve cells.
- Secretion: forming vesicles to release substances.
- Activating molecules: adding phosphate so a molecule reacts more easily.
Heat from these reactions also helps birds and mammals keep warm.
Energy yield of metabolism
How much ATP does one glucose molecule give?
| Stage | Where | Net ATP (approx.) |
|---|---|---|
| Glycolysis | cytoplasm | 2 |
| Link reaction + Krebs cycle | mitochondrial matrix | 2 |
| Oxidative phosphorylation | inner mitochondrial membrane | about 26–28 |
| Total, aerobic | about 30–32 |
Older books say 36–38; today's estimate is lower because some energy is lost moving molecules across membranes.
In anaerobic respiration (no oxygen) only glycolysis gives ATP: 2 ATP per glucose, plus lactate (muscles) or ethanol and CO₂ (yeast). So aerobic respiration yields about 15 times more ATP. Fats give even more ATP per gram than carbohydrates because they contain more hydrogen.
Try it: count the cost
In the 3D, press Split 10 times. Each split is one "job". Predict: how many times must ATP synthase spin (Join) to be ready again? Check by pressing Join.
At home: do 30 quick squats. The burning feeling near the end comes when muscles cannot get oxygen fast enough and start making ATP anaerobically, producing lactate.
Key formulas and definitions
- ATP + H₂O → ADP + Pi + energy (≈ 30.5 kJ/mol), enzyme: ATP hydrolase
- ADP + Pi + energy → ATP + H₂O, enzyme: ATP synthase
- ATP = adenine + ribose + 3 phosphates
- Aerobic: ≈ 30–32 ATP per glucose; anaerobic: 2 ATP per glucose
Worked examples
1. Name the three parts of an ATP molecule and the bond-breaking enzyme.
Adenine (base), ribose (pentose sugar) and three phosphate groups. ATP hydrolase splits off the last phosphate.
2. Why is ATP formation called condensation and its breakdown hydrolysis?
Joining ADP and Pi releases a water molecule (condensation). Splitting ATP uses a water molecule (hydrolysis).
3. A muscle cell hydrolyses 2 × 10⁻³ mol of ATP. How much energy is released? (30.5 kJ/mol)
Energy = 2 × 10⁻³ × 30.5 = 0.061 kJ = 61 J.
4. Yeast respires 10 glucose molecules anaerobically. A muscle respires 10 glucose aerobically (take 30 ATP each). Compare the ATP made.
Yeast: 10 × 2 = 20 ATP. Muscle: 10 × 30 = 300 ATP. Aerobic gives 15 times more.
5. Complete oxidation of glucose releases 2870 kJ/mol. If 32 ATP are made and each holds 30.5 kJ/mol, what is the efficiency?
Energy in ATP = 32 × 30.5 = 976 kJ. Efficiency = 976 / 2870 × 100 ≈ 34%. The rest is released as heat.
Common mistakes
- Saying ATP is a long-term energy store. It is unstable and used within seconds; fats and glycogen are the stores.
- Saying "the bond stores lots of energy". Energy is released because ADP + Pi are more stable than ATP + water; breaking any bond takes energy.
- Mixing up the enzymes: ATP hydrolase breaks ATP, ATP synthase makes it.
- Calling the sugar deoxyribose. ATP contains ribose, like RNA.