ATP and ADP at Higher Level: The same statement as SL. At HL the energy to make ATP is traced in detail: glycolysis, the Krebs cycle and chemiosmosis at ATP synthase.
Practise this as you read
- State which conversion releases energy.
- Outline how ATP is regenerated.
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ATP works because it can be broken and rebuilt easily, again and again.
Break, release, rebuild: Splitting off the third phosphate releases a small, ready-to-use amount of energy. 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 is removed by hydrolysis:
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.
Making ATP 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 — removing its third phosphate releases energy | 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 of three features.
The three features
- 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.
- It is quickly recharged, so the same molecules are reused endlessly.
What the exam asks: 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 phosphate is hydrolysed off, releasing energy at once | 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)
- Hydrolyses off the third phosphate
- 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
- Needs an input of energy
- Powered by cell respiration
- Produces ATP again, ready to reuse
An easy way to remember: 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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The big idea: Hydrolysis of ATP to ADP and phosphate releases energy. Making ATP from ADP and phosphate needs energy, which comes from respiration.
The points to remember
- ATP + water → ADP + phosphate: energy is released (hydrolysis).
- ADP + phosphate → ATP + water: energy is needed (from respiration).
- The energy released by one ATP is enough for many tasks in the cell.
- ATP and ADP are converted back and forth constantly, so ATP is recycled, not used up.
Remember it as: Hydrolysis out, respiration in.
Real example: ATP synthase. In mitochondria, the enzyme ATP synthase joins ADP and phosphate into ATP. A busy human cell can make millions of ATP molecules every second this way, using energy from respiration.
Trap: Making ATP uses energy; it does not release it. Energy is released only when ATP is hydrolysed.
How it is asked: Expect the direction of the energy change: which conversion releases energy and which needs it.
IB-style questionOutline[3 marks]
Outline the energy changes when ATP is converted to ADP and back again.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.