2.6Structures and Functions in Living Organisms
Respiration — Edexcel Modular - OL Revision Notes & Topical Questions
15 min read 8 parts Lesson 6 of 12 2 quizzes
Lesson summary
Respiration Edexcel O Level Modular Biology notes: understand how the process of respiration produces ATP in living organismsRespiration happens in all living cells, all the time.Plants respire as well as photosynthesise.Energy from respiration is used for movement, growth and ke…
- Word equation (aerobic)
- Balanced symbol equation (aerobic)
- Anaerobic in animals
- Anaerobic in plants and yeast
- Heat practical
- Write the balanced symbol equation for aerobic respiration
Section 1 of 8Process & sequence
- Glucose
- + Oxygen
- → Carbon dioxide
- + Water
- + Energy
1
Food is digested into glucose.
2
Blood carries glucose and oxygen to cells.
3
Mitochondria respire → energy released.
4
Carbon dioxide travels back to the lungs.
5
It is breathed out.
1
During vigorous exercise the muscles have a high energy demand.
2
Oxygen delivery to the muscle fibres is insufficient for aerobic respiration.
3
Glucose is therefore broken down anaerobically, giving a small amount of ATP.
4
Lactic acid builds up in the muscle.
5
The pH falls and becomes acidic, so enzymes can no longer work at their optimum.
6
The lactic acid is broken down later, using oxygen, to carbon dioxide and water.
Aerobic respiration in the bodywrite these steps in this order; each step is a marking point.
1
Food is digested to glucose
2
Blood carries glucose to the cells
3
Oxygen is breathed in and carried to the cells
4
Glucose and oxygen enter the mitochondria
5
Energy is released as ATP
6
Carbon dioxide diffuses into the blood
7
Carbon dioxide is breathed out
Anaerobic respiration during vigorous exerciseorder these.
1
Muscles need more energy
2
Oxygen supply is insufficient
3
Glucose is broken down anaerobically
4
Lactic acid forms and the pH falls
5
Enzymes denature in the acidic conditions
6
Lactic acid is later broken down using oxygen
Section 2 of 8Compare
| Feature | Breathing | Respiration | Where | Involves | Product |
|---|---|---|---|---|---|
| Type of process | Physical (movement) | Chemical reaction | — | — | — |
| Location | Lungs / chest | Every living cell | Mitochondria | — | — |
| Purpose | Move air in and out | Release energy | — | — | Energy |
| Gases | Moves O₂ in, CO₂ out | Uses O₂, makes CO₂ | — | Oxygen + glucose | CO₂ + water |
| Organisms | Animals with lungs | All living things | Plants too | — | — |
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen | Used | Not used |
| Breakdown of glucose | Complete | Incomplete |
| Products | Carbon dioxide + water | Lactic acid (animals) or ethanol + carbon dioxide (plants, yeast) |
| Energy released per glucose | A lot | A little |
| Where it happens | Mitochondria | Muscle cells (animals); cytoplasm of yeast/plants |
Section 3 of 8Worked examples
1Write the balanced symbol equation for aerobic respiration.
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Check the atoms balance: 6 carbon on each side, 12 hydrogen, and 6 + 12 = 18 oxygen.
2Cloudy
Tube A contains germinating seeds and tube B boiled seeds, both with limewater. Predict and explain.
- A turns cloudy because the living seeds respire and release carbon dioxide.
- B shows no change because boiling killed the seeds, so no respiration takes place.
- B is the control.
3Reduce heat loss
A student respiring-seed flask warmed by 5.5 °C over three days while the dead-seed flask did not warm. Why use vacuum flasks and why sterilise the seeds?
- Vacuum flasks reduce heat loss, so the temperature rise from respiration can be detected.
- Seeds are sterilised with weak bleach so that microorganisms on the surface do not respire and add heat that is not from the seeds.
4Explain why a runner's muscles fatigue near the end of a race.
- Oxygen cannot be delivered fast enough, so glucose is broken down anaerobically. Lactic acid accumulates, the pH falls, and enzymes stop working at their optimum, so the muscle contracts less effectively.
Section 4 of 8Keep going
Respiration Edexcel O Level Modular Biology: what to do next
These Respiration Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Respiration Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Respiration Edexcel O Level Modular Biology point you were unsure about before moving on.
Respiration Edexcel O Level Modular Biology: revision checklist
1
Revise the main Respiration Edexcel O Level Modular Biology definitions until you can write them from memory.
2
Practise Respiration Edexcel O Level Modular Biology exam questions under timed conditions.
3
Link each Respiration Edexcel O Level Modular Biology idea to a diagram or a worked example.
4
List the common Respiration Edexcel O Level Modular Biology mistakes and how to avoid them.
5
Explain one Respiration Edexcel O Level Modular Biology idea out loud as if teaching a friend.
6
Summarise Respiration Edexcel O Level Modular Biology in five key points on a revision card.
7
Check which command words appear most often in Respiration Edexcel O Level Modular Biology questions.
2
Previous lesson: 2.5 Nutrition — Notes
3
Next lesson: 2.7 Gas exchange — Notes
4
Course: Edexcel Modular - OL — all units
5
Official syllabus: Pearson Edexcel International GCSEs
Section 5 of 8Key terms
Section 6 of 8Syllabus checklist
1
understand how the process of respiration produces ATP in living organisms
2
know that ATP provides energy for cells
3
describe the differences between aerobic and anaerobic respiration
4
know the word equation and the balanced chemical symbol equation for aerobic respiration in living organisms
5
know the word equation for anaerobic respiration in plants and in animals
6
practical: investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms (g) Gas exchange Learners should
- Flowering plants 2.40B understand the role of diffusion in gas exchange 2.41B understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis 2.42B understand how the structure of the leaf is adapted for gas exchange 2.43B describe the role of stomata in gas exchange 2.44B understand how respiration continues during the day and night
- but that the net exchange of carbon dioxide and oxygen depends on the intensity of light 2.45B practical: investigate the effect of light on net gas exchange from a leaf
- using hydrogen- carbonate indicator Humans
7
describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes
8
understand the role of the intercostal muscles and the diaphragm in ventilation
9
explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries
10
understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease
11
practical: investigate breathing in humans, including the release of carbon dioxide and the effect of exercise
Section 7 of 8Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Respiration · Aerobic respiration · Glucose · Mitochondria · Energy · Reactant · Product · Word equation · Blood · Anaerobic respiration
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Keywords the mark scheme looks for in this lesson
- Respiration — ATP — Aerobic — Anaerobic — Fermentation — Mitochondria — Glucose — Lactic acid — Ethanol — Limewater — CORMS
- Never write "energy is made". Respiration transfers or releases energy as ATP.
Equations
- Learn both forms
- the word equation and the balanced symbol equation, with the 6 in front of O₂ and CO₂.
- Writing only the reactants scores nothing.
- State which product belongs to which pathway: lactic acid is animals;
- ethanol + carbon dioxide is plants and yeast.
Investigation questions
Investigation questions use CORMS
- Change
- what differs between the tubes or flasks.
- (the contents: germinating seeds, boiled seeds, glass beads)
- what differs between the tubes or flasks.
- Organism
- the same type and number of seeds in every tube.
- Repeat
- repeat to obtain reliable results.
- Measure
- the same measurement in each: how long the limewater takes to turn cloudy, or the temperature, measured after the same set time.
- Same
- the controlled variables: number of seeds, temperature, volume of indicator, size of flask.
Comparison questions
- Compare one feature per line (oxygen, completeness of breakdown, products, ATP yield, site).
- Do not describe aerobic respiration in full and then anaerobic in full — the marks are awarded for the pairs.
Marked practiceQuizzes for this lesson
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