The big idea: Cells cannot use the energy locked in food directly. Instead they transfer it to a small carrier molecule called ATP (adenosine triphosphate), and every energy-needing process in the cell is paid for in ATP.
That is why ATP is called the cell's energy currency — like cash that any process can 'spend'.
ATP carries energy in the bond to its third phosphate group. When the cell needs energy, this bond is broken, turning ATP into ADP + a phosphate (Pi) and releasing the energy.
Respiration then re-adds the phosphate, recharging ADP back into ATP — a continuous ATP–ADP cycle.
The ATP–ADP cycle: breaking the third phosphate bond turns ATP into ADP + Pi and releases energy for cell work; respiration re-adds the phosphate to recharge ADP back to ATP.
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- ATP (adenosine triphosphate)
- The cell's energy-carrying molecule. It has three phosphate groups; the bond to the third one stores readily usable energy.
- ADP (adenosine diphosphate)
- The 'discharged' form of ATP, with only two phosphate groups. It is formed when ATP releases its energy.
- Pi (inorganic phosphate)
- The single phosphate group released when ATP is broken down to ADP.
- Energy currency
- A description of ATP: it is the universal, spendable form of energy that powers every energy-requiring process in the cell.
- ATP–ADP cycle
- The continuous interconversion in which ATP is broken to ADP + Pi (releasing energy) and rebuilt from ADP + Pi by respiration (storing energy).
Where the energy sits: The energy is not stored in ATP forever — it is stored in the bond to the third phosphate.
Break that bond → energy is released (ATP → ADP + Pi).
Remake it (using energy from respiration) → energy is stored again (ADP + Pi → ATP).
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ATP works because it can be broken and rebuilt easily, again and again.
Splitting off the third phosphate releases a small, ready-to-use amount of energy — and respiration puts the phosphate straight back, so the same molecule is reused rather than thrown away.
Releasing energy — ATP → ADP + Pi: When a process needs energy, the third phosphate bond is broken:
ATP → ADP + Pi, and energy is released.
This released energy powers cell work such as active transport, muscle contraction and building macromolecules. Only one phosphate is removed, so only a small, usable amount of energy is freed at a time — little is wasted.
Storing energy again — ADP + Pi → ATP: The reverse step recharges the currency: ADP + Pi → ATP.
This needs an input of energy, which comes from cell respiration (releasing energy from glucose and other carbon compounds).
Because ATP is constantly recharged, a cell only holds a tiny amount at any moment — each ATP molecule is recycled thousands of times a day.
| Feature | ATP | ADP |
|---|---|---|
| Full name | adenosine triphosphate | adenosine diphosphate |
| Number of phosphate groups | three | two |
| Energy state | the 'charged' form — energy stored in the bond to the third phosphate | the 'discharged' form — that energy has been released |
| Made from the other by | respiration adding a phosphate to ADP | breaking the third phosphate off ATP |
Why ATP is so well suited to the job: ATP is a near-perfect energy currency because:
it releases a small amount of energy in a single step, so the cell gets just enough for one task; it is soluble, so it moves freely through the cytoplasm to wherever energy is needed; and it is quickly recharged, so the same molecules are reused endlessly.
These are exactly the features an exam will ask you to explain when a question links ATP to a specific process such as nerve-impulse transmission.
| Feature of ATP | What it means | Why it suits powering cell processes |
|---|---|---|
| Releases a small amount of energy | Splitting off one phosphate gives a small, manageable burst | The cell gets just enough energy for one task, with little wasted as heat |
| Releases energy in a single step | One bond is broken, releasing energy instantly | Energy is available immediately, exactly when and where it is needed |
| Soluble in water | ATP dissolves and moves freely in the cytoplasm | It can be delivered to wherever in the cell the energy is required |
| Quickly recharged | Respiration rapidly rebuilds ATP from ADP + Pi | The same molecules are reused over and over — a renewable currency, not used up |
Breaking ATP (ATP → ADP + Pi)
- Breaks the third phosphate bond
- Releases a small, usable amount of energy
- Powers cell work (active transport, muscle contraction, synthesis)
- Produces ADP + Pi
Recharging ATP (ADP + Pi → ATP)
- Re-adds a phosphate to ADP
- Stores energy in the new bond
- Powered by cell respiration
- Produces ATP again, ready to reuse
A memory hook: Triphosphate = Three phosphates = the charged battery. Take one phosphate off and you get Diphosphate (Discharged).
Break to use, respire to recharge.
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How this is tested: A common Paper 1B task gives 3 marks to explain the features of ATP that make it suited to powering a specific process (such as nerve-impulse transmission) — score with separate features: releases a small/usable amount of energy, releases it quickly in one step, is soluble/moves freely, and is rapidly recharged.
A 2-mark State question (Paper 2) can ask you to name cellular processes powered by converting ATP into ADP — give two from active transport, muscle contraction, synthesis of macromolecules or nerve-impulse transmission.
ATP also turns up in active-transport data questions: spot the energy-using process and link it back to the ATP–ADP cycle.
IB-style question — explain ATP's features for nerve cells
A neuron uses a large amount of ATP to keep transmitting impulses. Explain three features of ATP that make it well suited to supplying this energy. [3]
How to score all three marks
- Small, usable release. Breaking ATP to ADP + Pi releases a small amount of energy in one step, so the neuron gets just enough energy for each task without wasting it as heat.
- Fast and on demand. The energy is released quickly, exactly when the neuron needs it, so impulses can fire rapidly one after another.
- Soluble and reusable. ATP is soluble and moves freely through the cytoplasm to where it is needed, and it is rapidly recharged by respiration (ADP + Pi → ATP), so the same molecules are reused continuously. (Award 1 mark for each distinct, correctly explained feature, up to 3.)
Final answer
ATP releases a small, usable amount of energy in a single step; it releases that energy quickly and on demand; and it is soluble (moving freely to where it is needed) and rapidly recharged by respiration, so it is reused over and over.
Why this scores full marks: Each point is a separate feature of ATP — small/single-step release, fast release, soluble/reusable — and each is linked to the job of powering the neuron.
An 'explain [3]' needs three distinct features, each explained, not the same idea rephrased three times.
| Feature of ATP | What it means | Why it suits powering cell processes |
|---|---|---|
| Releases a small amount of energy | Splitting off one phosphate gives a small, manageable burst | The cell gets just enough energy for one task, with little wasted as heat |
| Releases energy in a single step | One bond is broken, releasing energy instantly | Energy is available immediately, exactly when and where it is needed |
| Soluble in water | ATP dissolves and moves freely in the cytoplasm | It can be delivered to wherever in the cell the energy is required |
| Quickly recharged | Respiration rapidly rebuilds ATP from ADP + Pi | The same molecules are reused over and over — a renewable currency, not used up |