Respiration and ATP

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.34 — How respiration produces ATP in living organisms
• 2.35 — ATP provides energy for cells
• 2.37 — Word and balanced symbol equation for aerobic respiration

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Glucose is a store of chemical energy, and a cell cannot use it. That sounds like a contradiction and it is the key to the whole topic. Energy in glucose is like money in a house: real, valuable, and useless at the moment you want to buy a coffee. What a cell needs is small amounts of energy, available instantly, anywhere in the cell, at the moment a job has to be done. Respiration is the process that breaks the large store into that usable form, and the usable form is a molecule called ATP.

What respiration is actually doing

1. Glucose is broken down in a controlled series of reactions, and energy is released as its bonds are rearranged.
2. That energy is not simply let go as heat. It is used immediately to build ATP molecules.
3. ATP is then carried to wherever it is needed in the cell.
4. When a cell needs energy for a job, an ATP molecule is broken down, and the energy released drives that job.
5. So respiration does not 'make energy' or 'give a cell energy' directly. It transfers energy from a store the cell cannot use into a form it can.

Once you see ATP as the usable currency, the list of what it pays for stops being a list to memorise. Anything a cell does that requires energy is paid for in ATP: muscle contraction, active transport across membranes, building large molecules like proteins from amino acids, cell division, and in mammals and birds the deliberate release of heat to keep body temperature constant. If a process in this course needs energy, ATP is where that energy comes from.

Aerobic respiration is the version that uses oxygen, and it is the one that releases by far the most energy. Read the equation as the reverse of photosynthesis and something clicks: photosynthesis builds glucose and oxygen out of carbon dioxide and water using light, and aerobic respiration takes glucose and oxygen and returns carbon dioxide and water, releasing the energy that was stored. The same atoms, the same molecules, run in the opposite direction.

The two mistakes that are worth killing now

1. RESPIRATION IS NOT BREATHING. Breathing is a physical movement of air in and out of the lungs. Respiration is a chemical reaction happening inside every cell — in every plant, in every bacterium, and in your own cells at this moment. Plants respire; they do not breathe.
2. RESPIRATION DOES NOT CREATE ENERGY. Energy cannot be created. It is transferred, from the chemical store in glucose to the chemical store in ATP, and eventually to heat and to work done.
3. Both errors are so common that examiners write mark schemes specifically to catch them, and both are avoided by the same habit: say what is transferred, and where from and to.

One last thing, because it explains an observation you will meet in the practical next. Not all of the energy released ends up in ATP — a substantial part is transferred to the surroundings as heat. In a mammal that is genuinely useful, since it is what keeps body temperature above the surroundings. But it is true of every respiring organism, including germinating seeds in a vacuum flask, which is why a flask of living seeds warms up while a flask of dead ones does not.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Respiration

The chemical process, taking place in all living cells, in which glucose is broken down and the energy released is used to produce ATP. It is not the same as breathing.

Learn this definition · ATP

The molecule that acts as the immediate energy supply in cells. Energy released by respiration is used to make it, and it is broken down to release that energy wherever a cell needs it.

glucose + oxygen → carbon dioxide + water

Conditions: aerobic respiration; energy is released and used to produce ATP. Learn this word equation exactly.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Conditions: the balanced symbol equation for aerobic respiration. Check the balance: 6 C, 12 H and 18 O on each side.

What cells spend ATP on — the five uses Edexcel expects

• Muscle contraction, and so all movement
• Active transport of substances against a concentration gradient
• Building large molecules from small ones — proteins from amino acids, starch from glucose
• Cell division for growth and repair
• In mammals and birds, releasing heat to maintain a constant body temperature

Respiration against breathing — stated in every mark scheme for a reason

A. RESPIRATION is a chemical reaction inside every living cell that transfers energy from glucose to ATP. Plants, bacteria and animals all respire, all the time.

B. BREATHING (ventilation) is the physical movement of air into and out of the lungs. Only some animals do it, and it merely supplies the oxygen respiration uses.

Learn this definition · Where aerobic respiration happens

Aerobic respiration takes place in the MITOCHONDRIA of the cell. A cell with an unusually large ATP bill — one doing a great deal of active transport, or dividing rapidly — contains unusually large numbers of them, so the count of mitochondria is evidence of how much ATP that cell has to produce.

Model answer [3 marks]

Explain why a cell needs to make ATP rather than using the energy in glucose directly. [3]

Glucose is a large store of energy that cannot be used directly by the processes in a cell. Respiration breaks glucose down and uses the energy released to make ATP. ATP releases a small, usable amount of energy immediately, wherever in the cell it is needed, so it acts as the cell's available energy supply.

Model answer [3 marks]

A student says plants do not respire because they photosynthesise instead. Explain why this is wrong. [3]

Plants are made of living cells, and every living cell must respire to produce ATP. Photosynthesis makes glucose but does not supply usable energy to the cell directly; the glucose still has to be respired. Plants therefore respire continuously, day and night, as well as photosynthesising when there is light.

Mark-losing trap. Respiration is not breathing. Breathing moves air; respiration is a chemical reaction in every cell, including in plants.

Mark-losing trap. Energy is never made or created. Say it is transferred or released, from glucose to ATP — mark schemes are written to catch 'produces energy'.

Mark-losing trap. Learn the symbol equation with all its numbers: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. An unbalanced version scores nothing.

Mark-losing trap. Respiration happens in all living cells all of the time, not only during exercise and not only in animals.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Complete the word equation for aerobic respiration: glucose + ______ → carbon dioxide + water
2. Grade 7 · Identify [1 mark] — Which statement about respiration is correct?
3. Grade 8 · Explain [4 marks] — Explain why a cell breaks glucose down to make ATP, rather than using the energy in glucose directly. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Explain [4 marks] — Cells lining the small intestine and cells in the tip of a plant root both contain unusually large numbers of mitochondria. Select every statement that belongs in a full-mark explanation of why.
5. 9+ · Analyse [5 marks] — A sealed flask of germinating seeds is left overnight in the dark. The temperature inside rises by several degrees, and the concentration of carbon dioxide increases. A second identical flask of seeds that have been boiled and cooled shows neither change. A student concludes that the seeds are photosynthesising. Select every statement that belongs in a full-mark response.

The people behind this science

Two ways into the same idea — the one who thought breathing was for cooling, and the one who proved that a living body burns. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Aristotle — the one who thought breathing was for cooling

Aristotle taught that the heart was the seat of the body's innate heat and that breathing existed to cool it, like bellows working in reverse. It is wrong, but it is a serious attempt to explain a real observation — respiring bodies genuinely are warm — and arguing with it is the sharpest way to separate breathing from respiration, which is the distinction this lesson turns on.

• “Why did you conclude that the purpose of breathing was to cool the body?”
• “What did you think was making a living body warm in the first place?”
• “If plants do not breathe, did you think they were alive in the same way?”
• “What would have persuaded you that air is consumed rather than just moved?”

Antoine Lavoisier — the one who proved that a living body burns

With Laplace, Lavoisier sealed a guinea pig inside a jacket of ice and weighed the water its warmth melted, while measuring the oxygen it consumed and the fixed air it gave out. The proportions matched a burning candle exactly. That experiment is where this lesson's equation comes from, and it is the moment respiration stopped being a mystery of life and became chemistry.

• “How did melting ice let you measure the heat coming from a living animal?”
• “What made you certain that breathing and burning are the same process?”
• “Why must a living body consume the same part of the air a flame does?”
• “What did you think happened to the oxygen once the animal had taken it in?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Antoine Lavoisier in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Absorption and the Energy in Food
• Next lesson: Anaerobic Respiration
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Incandio Science