Overview
What you’ll learn
- Write both bioenergetics equations and label each as endothermic or exothermic.
- Identify the limiting factor on any photosynthesis graph section.
- Describe the pondweed practical and its controls, and use the inverse-square law.
- Name five fates of glucose and when a plant uses each.
- Compare aerobic and anaerobic respiration in muscles, yeast and roots.
- Link exercise responses to oxygen delivery, and define metabolism with examples.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 7.1Describe photosynthesis as an endothermic reaction in which carbon dioxide and water react, using light energy absorbed by chlorophyll, to produce glucose and oxygen, and state the word and balanced symbol equations.
- 7.2Explain the effect of light intensity, carbon dioxide concentration, temperature and (for higher tier) chlorophyll availability on the rate of photosynthesis, and interpret graphs to identify the limiting factor.
- 7.3Carry out the required practical: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed, and explain the inverse-square relationship between distance and light intensity.
- 7.4Describe the uses of glucose produced by photosynthesis: respiration, conversion into insoluble starch for storage, into cellulose for cell walls, into amino acids (with nitrate ions) for protein, and into lipids for seeds.
- 7.5Describe respiration as an exothermic reaction that transfers energy from glucose, and state the aerobic equation; explain the anaerobic pathway in muscles (lactic acid, oxygen debt) and in yeast and plant root cells (ethanol and carbon dioxide, or carbon dioxide and water in roots).
- 7.6Explain how the body responds during exercise — increased heart rate, breathing rate and breath volume — to supply muscles with more oxygenated blood, and describe metabolism as the sum of all reactions in a cell or body, giving examples.
- 7.7Calculate rates from practical data, use the inverse-square law, and interpret response data from exercise experiments.
Key points
- 1Photosynthesis: carbon dioxide + water → glucose + oxygen, powered by light — endothermic (energy taken in), catalysed inside chloroplasts.
- 2Limiting factors: light intensity, carbon dioxide concentration, temperature (enzymes denature when too hot); the flat part of a rate graph reveals which factor is holding the rate down.
- 3Inverse-square law: doubling the lamp distance quarters the light intensity (I ∝ 1/d²).
- 4Glucose fates: respiration for energy · starch for storage · cellulose for walls · amino acids + nitrate ions → proteins · lipids in seeds.
- 5Respiration is exothermic: glucose + oxygen → carbon dioxide + water, transferring energy for every cell job.
- 6Anaerobic in muscles: glucose → lactic acid (+ little energy); the oxygen needed afterwards to break the lactic acid down is the oxygen debt.
- 7Anaerobic in yeast and plant root cells: glucose → ethanol + carbon dioxide (fermentation); in roots the products are ethanol and carbon dioxide, or carbon dioxide and water.
- 8Exercise raises heart rate, breathing rate and breath volume, flooding muscles with oxygenated blood; metabolism is the sum of all the reactions in a cell or body.
Notes · 1 of 6
The photosynthesis reaction
Energy banks glucose — with carbon dioxide and water as raw materials.
Photosynthesis happens inside chloroplasts, where the green pigment chlorophyll absorbs light energy. That energy lets carbon dioxide from the air join water from the soil, building glucose and releasing oxygen as a by-product. Because light energy is taken in, photosynthesis is an endothermic reaction — the mirror image of respiration, which releases energy.
| Reaction | Equation | Energy |
|---|---|---|
| Photosynthesis | carbon dioxide + water → glucose + oxygen (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) | Endothermic — light energy absorbed |
| Aerobic respiration | glucose + oxygen → carbon dioxide + water (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) | Exothermic — energy transferred to cells |
Plants and animals run the two reactions in opposite directions, which is why forests and oceans keep the atmosphere stocked with oxygen. Every gram of glucose a leaf makes is banked sunlight.
Notes · 2 of 6
Limiting factors and rate graphs
Whichever factor runs short sets the speed — learn to read the graph.
Three factors can limit photosynthesis. Light intensity: more light, more energy for the reaction. Carbon dioxide concentration: usually the scarcest raw material in fresh air (about 0.04%). Temperature: enzymes speed up as it warms — until denaturation above roughly 40–45 °C, when the rate collapses. A chlorophyll shortage (from disease or magnesium-poor soil) can also limit the Higher-tier answer.
| Graph shape | Limiting factor | Why |
|---|---|---|
| Steep rise (rate climbing) | The factor on the x-axis | Adding more of it speeds the reaction |
| Flat plateau | Something else — carbon dioxide or temperature | Light is no longer in short supply |
| Peak then fall (against temperature) | Temperature past the optimum | Photosynthesis enzymes denature |
Worked example — naming the limiting factor
A greenhouse trial at dawn on a cold March morning: light is dim, carbon dioxide is at normal air level, and the leaf temperature is 8 °C. Which factor limits the rate? Temperature — enzyme reactions crawl when cold, so even extra lamps would barely help until the greenhouse warms. Name the factor AND say why: “enzyme activity is low at 8 °C, so temperature limits the rate”.
Notes · 3 of 6
The pondweed required practical
Bubbles as a speedometer — method, controls and inverse-square maths.
- 11. Set the shoot
Place fresh pondweed under water in a beaker; cut the stem cleanly so bubbles escape freely.
- 22. Add carbon dioxide
Dissolve sodium hydrogencarbonate in the water to keep the carbon dioxide concentration constant.
- 33. Light it
Place a lamp at a measured distance (e.g. 10 cm). Leave the shoot a few minutes to adjust before counting.
- 44. Count bubbles
Count bubbles per minute (or collect gas in a syringe for a volume rate). Repeat three times and average.
- 55. Move the lamp
Repeat at 15, 20 and 30 cm, changing only the distance; shield the setup from room light.
| Element | Detail |
|---|---|
| Independent variable | Distance of the lamp (hence light intensity) |
| Dependent variable | Bubbles per minute (or gas volume per minute) |
| Controls | Temperature (heat shield), carbon dioxide level (hydrogencarbonate), the same shoot, acclimatisation time |
| Improvements | Gas syringe instead of bubbles; LED lamp (less heat); darkened room |
| Main error source | The lamp warming the water, speeding enzymes rather than light alone |
Worked example — inverse-square law
Light intensity falls with the square of distance: I ∝ 1/d². A lamp gives 40 arbitrary units at 10 cm; move it to 20 cm — double the distance — and the intensity becomes 40 ÷ 2² = 10 units. At 30 cm: 40 ÷ 3² ≈ 4.4 units. Inverse-square predictions are favourite Higher-tier marks: double distance, quarter intensity.
Notes · 4 of 6
Five fates of glucose
Fuel, store, build, strengthen, or pack into seeds.
| Fate | Form | Purpose |
|---|---|---|
| Respiration | Glucose itself | Transfers energy for every cell job |
| Storage | Insoluble starch (in leaves, roots, tubers) | Energy bank that does not dissolve away or attract pests |
| Cell walls | Cellulose | Strong structural fibres |
| Proteins | Amino acids (glucose + nitrate ions) | Growth and enzymes |
| Seeds | Lipids (oils) | Energy-dense packing for the next generation |
The storage trick matters: starch is insoluble, so it stays put in the cell instead of drawing water osmotically or leaking through membranes — exactly why plants bank energy as starch rather than free glucose.
Notes · 5 of 6
Respiration, exercise and oxygen debt
The sprint, the pant and the payback.
Respiration is exothermic — it transfers energy from glucose to every process in the body. During aerobic exercise, muscles demand far more energy, so the heart beats faster and the lungs breathe faster and deeper, raising breath volume to deliver more oxygenated blood. When the oxygen supply cannot keep up, muscles switch to anaerobic respiration: glucose → lactic acid, releasing much less energy. Lactic acid builds, causing fatigue and ache; afterwards, deep panting repays the oxygen debt — the oxygen needed to break the lactic acid down in the liver.
| Where | Equation | Note |
|---|---|---|
| Human muscles | glucose → lactic acid | Little energy; causes fatigue and oxygen debt |
| Yeast (fermentation) | glucose → ethanol + carbon dioxide | Basis of brewing and bread-making |
| Plant root cells in waterlogged soil | glucose → ethanol + carbon dioxide (or carbon dioxide + water) | Oxygen-poor conditions force anaerobic chemistry |
Worked example — heart rate response data
A runner’s heart rate is 64 bpm at rest, rising to 158 bpm after 3 minutes of hard exercise, then returning to 70 bpm ten minutes later. The rise is 158 − 64 = 94 bpm (about 2.5× the resting rate), supplying respiring muscles with extra oxygen and glucose; the slow fall shows the body still repaying the oxygen debt as it clears the lactic acid. Interpret the whole curve — rise, plateau, fall — never just one number.
Notes · 6 of 6
Metabolism
The sum of every reaction in you.
Metabolism is the sum of all the reactions happening in a cell or in the body. Examples worth quoting: converting glucose to starch or cellulose in plants; building amino acids into proteins; the aerobic and anaerobic respiration reactions themselves; breaking down excess protein into urea in the liver. Energy transferred by respiration drives all of this chemistry — metabolism is where bioenergetics becomes a whole organism.
Revise
Key terms
The words examiners expect you to use precisely.
- Endothermic reaction
- A reaction that takes in energy from its surroundings — photosynthesis is the classic.
- Limiting factor
- The factor in shortest supply that holds back the rate of photosynthesis.
- Inverse-square law
- Light intensity falls with the square of distance from the source (I ∝ 1/d²).
- Aerobic respiration
- Glucose broken down with oxygen, releasing plenty of energy as carbon dioxide and water form.
- Anaerobic respiration
- Glucose broken down without oxygen, giving lactic acid in muscles or ethanol and carbon dioxide in yeast and roots.
- Oxygen debt
- The extra oxygen needed after exercise to break down the lactic acid built up anaerobically.
Show 1 more key termShow fewer key terms
- Metabolism
- The sum of all reactions in a cell or the body.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Calling photosynthesis exothermic
It stores energy rather than releasing it.
Instead: Photosynthesis is endothermic — light energy is taken in.
Mistake: Writing the photosynthesis products the wrong way round
Oxygen is released; glucose is made.
Instead: Carbon dioxide + water → glucose + oxygen.
Show 10 more mistakesShow fewer mistakes
Mistake: Reading a flat graph as “the light is limiting”
A plateau means adding that factor changes nothing.
Instead: On the flat section something ELSE is limiting — carbon dioxide or temperature.
Mistake: Ignoring temperature in rate questions
Enzymes run the dark stage of photosynthesis.
Instead: Too hot denatures the enzymes and the rate collapses.
Mistake: Treating the lamp as the only light source
Room light adds an uncontrolled background.
Instead: Work in a darkened room or shield the apparatus so only the lamp’s light counts.
Mistake: Forgetting the heat shield in the pondweed practical
The lamp warms the water and speeds enzymes, faking a light effect.
Instead: A heat shield (or LED lamp) keeps temperature constant.
Mistake: Saying glucose is stored as soluble glucose granules
Soluble sugar would wreck the cell’s water balance.
Instead: Plants store glucose as insoluble starch.
Mistake: Omitting nitrate ions when naming amino-acid production
Glucose alone lacks nitrogen.
Instead: Glucose + nitrate ions → amino acids → proteins.
Mistake: Writing anaerobic respiration in muscles as ethanol production
Muscles make lactic acid; yeast makes ethanol.
Instead: Muscles: glucose → lactic acid. Yeast: glucose → ethanol + carbon dioxide.
Mistake: Saying the oxygen debt is paid during the sprint
The debt builds during the sprint.
Instead: The debt is repaid afterwards by deep panting that supplies oxygen to break down lactic acid.
Mistake: Saying breathing rate rises “for more glucose”
Lungs deliver oxygen, not food.
Instead: Breathing rate and breath volume rise to deliver more oxygen and remove carbon dioxide.
Mistake: Defining metabolism as digestion
Metabolism covers every reaction, not just the gut.
Instead: Metabolism is the sum of ALL reactions in a cell or body — respiration, protein building, urea production and more.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
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Explain the photosynthesis reaction
The raw materials of photosynthesis are
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 7 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. Bioenergetics: photosynthesis and its limiting factors, the pondweed required practical and inverse-square law, uses of glucose, aerobic and anaerobic respiration, exercise responses and metabolism.
Mark scheme
PROChapter 7 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How does this chapter fit the real exams?
Biology Paper 1
1 h 15 · 70 marks (Trilogy)
Cell biology, organisation, infection and response, bioenergetics (chapters 1–7)
Biology Paper 2
1 h 15 · 70 marks (Trilogy)
Homeostasis, inheritance variation and evolution, ecology (chapters 8–10)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
How is Combined Science bioenergetics different from triple Biology?
The core is identical — equations, limiting factors, the pondweed practical and inverse-square law, respiration and metabolism. Triple papers simply ask harder application questions on the same content.
Which maths skills appear in this chapter?
Inverse-square predictions (double distance → quarter intensity), means from repeated trials, percentage change in rates, and reading limiting-factor graphs.
What is the fastest way to find the limiting factor?
Look at the graph: where the line climbs steeply, the x-axis factor limits; where it flattens, something else (carbon dioxide or temperature) limits.
Why do I ache after sprinting?
Hard-working muscles outpace the oxygen supply and respire anaerobically, building lactic acid. The ache fades as deep breathing repays the oxygen debt and clears it.
Does respiration only happen at night in plants?
No — respiration runs 24/7 in every cell. At night photosynthesis stops, so the plant becomes a net oxygen absorber and carbon dioxide releaser.
Up next
Chapter 8: Homeostasis and response