Key Idea: Every living cell needs a constant supply of energy — for active transport, building molecules, movement and just staying organised. Cells can't use the energy in food directly. Instead they carry out cell respiration: glucose is broken down to release its energy, and that energy is captured in ATP — the cell's spendable energy currency. This whole topic is one chain: glucose → respiration releases energy → ATP → cell work. It is a regular on Paper 1A/1B (quick MCQs and data questions on the ATP–ADP cycle, aerobic vs anaerobic, and respirometers) and on Paper 2 (outline/explain how cells make and use ATP).
⚡ ATP — the cell's energy currency
ATP (adenosine triphosphate) carries energy in the bond to its third phosphate group. When the cell needs energy, this bond is broken: ATP → ADP + Pi, releasing a small, usable amount of energy. Respiration then re-adds the phosphate, recharging ADP + Pi → ATP — the continuous ATP–ADP cycle.
The ATP–ADP cycle: breaking the third phosphate bond gives ADP + Pi and releases energy for cell work; respiration re-adds the phosphate to recharge ADP back to ATP.
🔒 Interactive diagram
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| Step of the cycle | Direction | Energy | Powered by / used for |
|---|---|---|---|
| ATP broken down | ATP → ADP + Pi | released | powers cell work (e.g. active transport) |
| ATP rebuilt | ADP + Pi → ATP | stored | powered by cell respiration |
The energy is stored in the bond to the third phosphate. Break it → energy released (ATP → ADP + Pi). Remake it → energy stored again (ADP + Pi → ATP). ATP is recycled, not used up — recharged thousands of times a day.
ATP is a near-perfect currency because it releases a small amount of energy in a single step (just enough for one task), it is soluble (moves freely to where energy is needed), and it is quickly recharged and reused. Processes it powers: active transport, muscle contraction, synthesis of macromolecules, nerve-impulse transmission.
🔋 Why cells respire & how ATP is generated
ATP isn't made from scratch each time — cells recycle between ATP and ADP. Using ATP turns it into ADP; respiration then recharges it. The energy for that recharge comes from respiring glucose.
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.
Key Idea: glucose → (respiration releases energy in controlled steps) → ADP + phosphate → ATP Glucose is broken down in small, controlled steps (not one wasteful burst); the energy released is used to bond a phosphate onto ADP, regenerating ATP. Memory hook: glucose = money in the bank, ATP = cash in your pocket; respiration is the cash machine that keeps your ATP topped up.
The fuel that is respired is the respiratory substrate — usually glucose. In a yeast experiment, the sugar (sucrose) supplied is the respiratory substrate the yeast respires; it does not supply oxygen or absorb CO₂.
🌬️ Aerobic vs anaerobic respiration
Cells can release energy from glucose in two ways, and which one they use depends mainly on whether oxygen is available. The cause of every difference traces back to that one thing: with oxygen the cell finishes the job; without it, it stops early.
Glucose branches two ways: with oxygen → carbon dioxide + water + a lot of ATP (mitochondria); without oxygen → lactate (animals) or ethanol + carbon dioxide (yeast), with only a little ATP (cytoplasm).
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| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen needed? | Yes — uses oxygen | No — oxygen is absent |
| Where in the cell | Mitochondria | Cytoplasm only |
| How fully glucose is broken down | Fully broken down | Only partly broken down |
| ATP yield per glucose | A lot of ATP | Only a little ATP |
| Products in animals / human muscle | Carbon dioxide + water | Lactate |
| Products in yeast (and plants) | Carbon dioxide + water | Ethanol + carbon dioxide |
Aerobic respiration gives the same products in everything: carbon dioxide + water. Anaerobic products differ: in animals / human muscle → lactate (made in the cytoplasm); in yeast (and plant cells) → ethanol + carbon dioxide — the basis of bread, brewing and bioethanol fuel.
'An-' means 'without' → anaerobic = without oxygen. Oxygen = more ATP (full breakdown). For the products: Animals make lactate; yeast makes ethanol (think brewing).
🧪 Measuring respiration rate
Respiration rate is simply how fast an organism respires — usually the oxygen used (or carbon dioxide produced) per minute. We can't watch a cell respire, so we measure it indirectly by tracking something that changes.
| What you measure | What happens during respiration | How you detect it |
|---|---|---|
| Oxygen used up | Aerobic respiration consumes oxygen | Gas volume falls → coloured liquid in a respirometer moves towards the organism |
| Carbon dioxide produced | Respiration releases CO₂ | An indicator changes colour, or you measure the gas given off |
| Temperature rise | Respiration releases some energy as heat | A thermometer/data-logger in an insulated flask shows it climbing |
| Mass lost | Carbon leaves as CO₂ gas over time | Weigh the organisms (e.g. seeds) before and after — dry mass falls |
Key Idea: In a respirometer, germinating seeds respire and use up oxygen, so the coloured liquid moves towards them. Potassium hydroxide (KOH) is added to absorb the carbon dioxide, so the only gas change left is the oxygen used — a clean measurement. A tube of dead (boiled) seeds is the control: it should show no movement, proving the movement in the live tube is caused by respiration, not by temperature or pressure.
An inhibitor (e.g. cyanide) slows respiration → less oxygen used → liquid moves less far in the same time. 'Suggest another measurement' → CO₂ produced, temperature rise, or mass lost (never 'oxygen' again).
✍️ Worked examples
IB-style question — outline how cells generate ATP
Outline how a cell generates ATP. [3]
Model answer:
The cell carries out respiration, breaking down glucose (the respiratory substrate) to release the energy stored in it.
This released energy is used to bond a phosphate group onto ADP.
Adding the phosphate to ADP regenerates ATP, ready for the cell to use. (Mark 1: respiration releases energy from glucose. Mark 2: energy adds a phosphate to ADP. Mark 3: this regenerates ATP.)
Respiration breaks glucose down to release energy; that energy adds a phosphate to ADP, regenerating ATP.
IB-style question — explain ATP's features
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]
Model answer:
Small, usable release. Breaking ATP to ADP + Pi releases a small amount of energy in a single step, so the neuron gets just enough for each task with little wasted as heat.
Fast and on demand. The energy is released quickly, exactly when needed, so impulses can fire rapidly one after another.
Soluble and reusable. ATP is soluble (moves freely through the cytoplasm to where it is needed) and is rapidly recharged by respiration, so the same molecules are reused. (Award 1 mark per distinct, explained feature, max 3.)
ATP releases a small, usable amount of energy in one step; releases it quickly on demand; and is soluble and rapidly recharged, so it is reused over and over.
IB-style question — respiration during a sprint
During a hard sprint, the oxygen supply to a person's leg muscles cannot keep up with demand. State the type of respiration the muscle cells switch to, the product they form, and where in the cell it is made. [3]
Model answer:
The muscle cells switch to anaerobic respiration (because oxygen is in short supply).
They produce lactate.
This happens in the cytoplasm of the muscle cells. (Mark 1: anaerobic respiration. Mark 2: lactate. Mark 3: cytoplasm.)
Anaerobic respiration; the product is lactate; made in the cytoplasm of the muscle cells.
IB-style question — interpret a respirometer
In a respirometer, germinating seeds with potassium hydroxide cause a drop of coloured liquid to move towards them, while an identical tube of dead seeds shows no movement. Explain what these results demonstrate. [3]
Model answer:
The living seeds are using up oxygen as they respire, lowering the gas volume, so the coloured liquid is drawn towards them.
The potassium hydroxide absorbs the carbon dioxide released, so the only gas change left is the oxygen being used — that is what the liquid's movement measures.
The dead seeds do not respire, so the liquid does not move; this shows the movement in the live tube is caused by respiration, not by temperature or pressure. (Mark 1: oxygen used / liquid drawn in. Mark 2: KOH absorbs CO₂. Mark 3: control shows movement is due to respiration.)
The seeds use up oxygen so the liquid moves towards them; KOH absorbs the CO₂ so only the oxygen change is measured; the unmoving dead-seed control proves the movement is caused by respiration.
✅ Quick self-check
Tap each card to check yourself.
Why is ATP called the cell's 'energy currency'? It is the universal, spendable form of energy — every energy-requiring process in the cell is paid for by breaking ATP down to ADP + Pi.
Which conversion releases energy, and which stores it? ATP → ADP + Pi RELEASES energy for cell work; ADP + Pi → ATP STORES energy (supplied by respiration). Getting the direction right is the most common exam slip.
How is ATP regenerated from glucose? Respiration breaks glucose down in controlled steps to release energy; that energy bonds a phosphate onto ADP, remaking ATP.
What single feature distinguishes aerobic from anaerobic respiration? Whether oxygen is used. Aerobic uses oxygen (mitochondria, full breakdown, a lot of ATP); anaerobic uses none (cytoplasm, partial breakdown, a little ATP).
What are the anaerobic products in animals vs yeast? Animals (and human muscle) → lactate. Yeast (and plant cells) → ethanol + carbon dioxide (the basis of bioethanol, brewing and bread).
In a respirometer, why does the coloured liquid move, and why add KOH? The seeds use up oxygen so the gas volume falls and the liquid is drawn towards them. KOH absorbs the CO₂ so the only gas change measured is the oxygen used.
One picture for the whole topic: oxygen present → full breakdown → much more ATP; oxygen absent → partial breakdown → much less ATP, plus lactate (animals) or ethanol + CO₂ (yeast).
🔒 Interactive diagram
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The whole topic on one card
- ATP = the cell's energy currency; energy sits in the bond to the third phosphate
- ATP → ADP + Pi releases energy for work; ADP + Pi → ATP stores it (the ATP–ADP cycle)
- ATP suits the job: small release, single step, soluble, quickly recharged
- Respiration releases energy from glucose in controlled steps to regenerate ATP
- Glucose = the respiratory substrate (fuel); ATP = the usable energy
- Aerobic = with oxygen → mitochondria → full breakdown → a lot of ATP → CO₂ + water
- Anaerobic = no oxygen → cytoplasm → partial breakdown → a little ATP
- Anaerobic products: lactate (animals/human muscle); ethanol + CO₂ (yeast)
- Respiration rate = oxygen used (or CO₂ made) per minute; measure with a respirometer
- Respirometer: seeds use O₂ → liquid moves; KOH absorbs CO₂; dead-seed control proves it's respiration
Exam Tips
- Get the ATP direction right: breaking ATP RELEASES energy; recharging it (powered by respiration) STORES energy.
- For 'explain ATP's features [3]', give THREE separate features each linked to the job — small/single-step, fast/on-demand, soluble, quickly recharged.
- For 'outline how cells generate ATP', give the full chain: respiration releases energy from glucose → adds a phosphate to ADP → regenerates ATP. 'Respiration makes ATP' alone is one mark.
- Anaerobic in humans makes LACTATE (cytoplasm); ethanol + CO₂ is the YEAST product — don't swap them.
- Aerobic = mitochondria; anaerobic = cytoplasm. Don't reverse the locations or the ATP yields.
- On a 'compare/distinguish' question, contrast each point explicitly (aerobic does X whereas anaerobic does Y).
- In a respirometer, the moving liquid shows OXYGEN being used (KOH has removed the CO₂); always credit the dead-organism control.
- An inhibitor (e.g. cyanide) SLOWS respiration → less oxygen used → liquid moves less far. For 'another measurement', pick CO₂ produced, temperature rise or mass lost.