The big idea: Every living cell needs a constant supply of energy to stay alive and do its work.
Cells cannot use the energy locked inside food (like glucose) directly. Instead they carry out cell respiration: glucose is broken down in controlled steps to release its energy a little at a time.
That released energy is used to make ATP — the molecule the cell actually 'spends' to power everything it does.
- Cell respiration
- The controlled breakdown of glucose (and other carbon compounds) inside a cell to release energy and regenerate ATP.
- ATP (adenosine triphosphate)
- The molecule cells use as their immediate, usable energy supply — sometimes called the cell's 'energy currency'.
- ADP (adenosine diphosphate)
- ATP with one phosphate removed. Adding a phosphate back to ADP (using energy from respiration) regenerates ATP.
- Respiratory substrate
- The fuel molecule that is respired to release energy — most often glucose.
So why bother breaking glucose down step by step instead of all at once? Releasing all the energy in one burst would be wasteful and could damage the cell.
By releasing it in small, controlled steps, the cell can capture a useful share of the energy in ATP, ready to use exactly when and where it is needed.
What cells spend energy on: Cells need energy for active transport (pumping ions), building large molecules (proteins, DNA), movement (muscle contraction, beating cilia) and keeping the cell organised.
All of these are paid for by ATP — and ATP is supplied by respiration.
| Energy-using process | What the cell is doing | Why it needs ATP |
|---|---|---|
| Active transport | Pumping ions across a membrane against their gradient | Moving substances 'uphill' costs energy |
| Synthesis of macromolecules | Building proteins, DNA, polysaccharides from subunits | Joining monomers by condensation needs energy |
| Movement | Muscle contraction, beating cilia/flagella, cytoplasmic streaming | Mechanical work needs an energy supply |
| Maintaining the cell | Keeping concentrations, repairing structures, generating heat | Staying alive and organised is not free |
ATP is not made from scratch each time. The cell keeps recycling between ATP and ADP, over and over.
When a cell uses ATP, the molecule loses its third phosphate and becomes ADP. Respiration then recharges it: energy released from glucose is used to bond that phosphate back on, turning ADP back into ATP.
The ATP–ADP cycle: Using energy: ATP → ADP + phosphate (energy released to do work)
Regenerating ATP: ADP + phosphate → ATP (energy from respiration stored in the bond)
The energy needed to drive the second step — turning ADP back into ATP — comes from respiring glucose.
| Stage of the cycle | What happens to the phosphate | Energy |
|---|---|---|
| ADP → ATP (recharging) | A third phosphate is bonded onto ADP | Energy from respiration is stored in the new bond |
| ATP → ADP (using) | The third phosphate is removed | Energy is released to power cell work |
| Where the energy comes from | Respiration of glucose regenerates the ATP | The cycle runs continuously, all day |
Where the energy comes from: Glucose is an energy-rich molecule: a lot of chemical energy is stored in its bonds.
During respiration the cell breaks those bonds in stages. The energy released at each stage is used to add a phosphate to ADP, regenerating ATP.
So the path is always: glucose → (respiration releases energy) → ADP + phosphate → ATP.
Glucose
- An energy-rich carbon compound (a 'fuel')
- Energy is stored in its chemical bonds
- Released slowly, in controlled steps
- Cannot be used by the cell directly
ATP
- The cell's immediate, usable energy supply
- Energy stored in the bond to its third phosphate
- Released instantly when that phosphate is removed
- Used directly to power cell processes
A memory hook: Think of glucose as money in the bank and ATP as cash in your pocket.
Respiration is the cash machine: it draws on the glucose 'savings' to keep your ATP 'cash' topped up, so you can spend it the moment you need it.
Feeling unprepared for exams?
Get a clear study plan, practice with real questions, and know exactly where you stand before exam day. No more guessing.
Get Exam Ready Free7-day free trial • No card required
How this is tested: A classic Paper 2 short question (3 marks, Outline) asks you to outline how cells generate ATP — give the chain: respiration releases energy from glucose, and that energy is used to add a phosphate to ADP, regenerating ATP.
A bigger Paper 2 question (Explain) can ask why cells need energy and how respiration releases it — here you score by listing several energy-using processes AND describing the controlled breakdown of glucose.
On a Paper 1B data question you might be given a yeast experiment and asked the function of the sugar (sucrose/glucose) provided — it is the respiratory substrate the yeast respires to release energy.
IB-style question — outline how cells generate ATP
Outline how a cell generates ATP. [3]
How to score all three marks
- Start with the fuel. The cell carries out respiration, breaking down glucose (the respiratory substrate) to release the energy stored in it.
- Link energy to ADP. This released energy is used to bond a phosphate group onto ADP.
- Name the product. Adding the phosphate to ADP regenerates ATP, which the cell can then use. (Mark 1: respiration releases energy from glucose. Mark 2: energy adds a phosphate to ADP. Mark 3: this regenerates ATP.)
Final answer
Respiration breaks down glucose to release energy; that energy is used to add a phosphate to ADP, regenerating ATP.
✓ Why this scores full marks: It gives the full chain of cause and effect — glucose broken down, energy released, phosphate added to ADP, ATP regenerated.
An answer that just says 'respiration makes ATP' is only one of the three marks; the examiner wants the mechanism: energy → ADP + phosphate → ATP.
| Stage of the cycle | What happens to the phosphate | Energy |
|---|---|---|
| ADP → ATP (recharging) | A third phosphate is bonded onto ADP | Energy from respiration is stored in the new bond |
| ATP → ADP (using) | The third phosphate is removed | Energy is released to power cell work |
| Where the energy comes from | Respiration of glucose regenerates the ATP | The cycle runs continuously, all day |