Edexcel International GCSE Biology 4BI1: Respiration – Revision Notes

These notes condense sub-topic 2(f), Respiration (points 2.34–2.39), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3, for the November and June series examined under Issue 3. The course is untiered and none of these points is a bold B statement, so all of them can appear on Paper 1 and Paper 2. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions. Mitochondria and other organelles are in the cell structure revision notes. The rest of the course is on the Edexcel IGCSE Biology hub, the printable checklist lists every point, and the free diagnostics show where to focus.

2.34 Respiration produces ATP

• Respiration: chemical reactions in living cells that break down glucose to release energy, which is used to make ATP.
• Happens in every living cell, in plants, animals and microorganisms, day and night.
• Controlled by enzymes, so affected by temperature and pH.
• Respiration ≠ breathing. Breathing moves air; respiration is a chemical process in cells.
• Energy is released, never “made”. Some is lost as heat.

2.35 ATP provides energy for cells

• ATP = adenosine triphosphate.
• Glucose → (respiration) → energy stored in ATP → used by the cell.
• Uses of energy from ATP:
  • muscle contraction
  • active transport
  • building large molecules (e.g. proteins from amino acids)
  • cell division and growth
  • nerve impulses
  • keeping body temperature constant in mammals and birds (heat from respiration).
• High-energy cells (muscle, sperm) have many mitochondria.

2.37–2.38 The three equations

Type	Word equation	Symbol equation needed?
Aerobic (all organisms)	glucose + oxygen → carbon dioxide + water	Yes
Anaerobic in animals	glucose → lactic acid	No
Anaerobic in plants (and yeast)	glucose → ethanol + carbon dioxide	No

Aerobic symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Method in steps: checking it is balanced

1. Count C, H and O on the left (include the 6 O inside glucose).
2. Count C, H and O on the right (multiply by the big number in front).
3. C: 6 = 6   H: 12 = 12   O: 18 = 18

2.36 Aerobic vs anaerobic

	Aerobic	Anaerobic
Oxygen	Needed	Not needed
Glucose broken down	Completely	Partly
Energy / ATP per glucose	Much more	Much less
Products	CO₂ + water	Lactic acid (animals); ethanol + CO₂ (plants, yeast)
Site	Mitochondria	Cytoplasm

Must-know distinctions

• Lactic acid vs ethanol: animals make lactic acid only; plants and yeast make ethanol and carbon dioxide.
• Less energy vs no energy: anaerobic releases less energy, not none. Energy is left in lactic acid or ethanol.
• Respiration vs photosynthesis: the aerobic equation looks like photosynthesis reversed, but respiration happens in all cells all the time, while photosynthesis needs light and chloroplasts.
• When anaerobic respiration happens: human muscle during vigorous exercise when oxygen cannot be supplied fast enough; plant roots in waterlogged soil; yeast in a sealed container.

Worked reminder: a 3-mark “describe the differences” answer

Each point names both types and uses a linking word such as “but” or “whereas”:

1. Aerobic respiration needs oxygen, but anaerobic respiration does not.
2. Aerobic respiration releases much more energy from each glucose molecule than anaerobic respiration.
3. Aerobic respiration produces carbon dioxide and water, whereas anaerobic respiration in animals produces only lactic acid.

A list such as “uses oxygen, makes water, happens in mitochondria” describes aerobic respiration only and does not compare.

Why anaerobic respiration releases less energy

• Glucose is only partly broken down.
• Lactic acid and ethanol still contain a lot of chemical energy.
• Carbon dioxide and water, the products of aerobic respiration, contain very little.
• So less energy is transferred to ATP from each glucose molecule.

Links to other topics

• Yeast making carbon dioxide for bread and ethanol for brewing is applied in Topic 5; see the use of biological resources revision notes.
• How respiration and photosynthesis together set the net gas exchange of a leaf by day and night is point 2.44B in sub-topic 2(g), Paper 2 only.
• Root hair cells need ATP from respiration for active transport of mineral ions, which links to 2.15.

2.39 Practical: carbon dioxide and heat from respiring organisms

Carbon dioxide

Indicator	Starts	More CO₂	Less CO₂
Hydrogen-carbonate	Red / orange	Yellow	Purple
Limewater	Clear	Milky / cloudy	–

Method in steps

1. Same volume of indicator in each tube.
2. Organisms on a gauze platform above the indicator.
3. Control: boiled (dead) seeds or glass beads of the same volume.
4. Seal with bungs; same temperature; dark for seeds.
5. Record colour (from ... to ...) or time to change colour.

Heat

Method in steps

1. Soak peas so they germinate; boil a second batch to kill them (control).
2. Rinse both in disinfectant -- kills microbes that would respire and give heat.
3. Put each batch in a vacuum flask -- insulation keeps heat in.
4. Cotton wool plug -- lets oxygen in, keeps most heat in.
5. Read the thermometer daily for several days.

Result: germinating peas warm up; boiled peas stay at room temperature.

Worked reminder (rate of gas release): seeds release 2.4 cm³ of carbon dioxide in 8 minutes.

Rate = 2.4 ÷ 8 = 0.3 cm³ per minute

Worked reminder (rate of temperature rise):

Rate = (final temperature − starting temperature) ÷ time

Worked reminder (explaining a result with a control): a flask of germinating beans rises from 19.0 °C to 23.0 °C, while a flask of boiled beans stays at 19.0 °C.

• The germinating beans are alive and respire, releasing energy as heat.
• The vacuum flask stops this heat escaping, so the temperature rises by 4.0 °C.
• The boiled beans are dead, so they cannot respire and release no heat.
• The boiled beans show the rise is caused by respiration of living cells, not by the surroundings.

Command words for this practical

Command word	What earns the marks
Describe (a method)	Numbered, practical steps, with named equipment and quantities
Explain (a result)	What happened and why, linked to respiration
Suggest (a reason)	A sensible reason applied to the set-up in the question
Calculate	Working shown, final answer with a unit

Variables to name

• Independent: living vs dead seeds (or temperature, or seed type).
• Dependent: temperature rise, or time for indicator to change colour.
• Control: mass and type of seeds, volume of indicator, starting temperature, flask size.
• Repeat and take a mean for reliability.

Quick self-test

1. What does ATP stand for?
2. Name the organelle where most aerobic respiration takes place.
3. Give the word equation for aerobic respiration.
4. Give the balanced symbol equation for aerobic respiration.
5. Give the word equation for anaerobic respiration in human muscle.
6. Give the word equation for anaerobic respiration in plant root cells.
7. State two ways anaerobic respiration differs from aerobic respiration.
8. State three uses of the energy from ATP in cells.
9. Hydrogen-carbonate indicator over respiring woodlice changes colour. From what colour, to what colour?
10. Why are seeds rinsed in disinfectant before the vacuum flask experiment?
11. Germinating seeds rise by 9.0 °C over 6 days. Calculate the mean rate of temperature rise.
12. The rate of carbon dioxide release from some seeds rises from 0.25 to 0.40 cm³ per minute when they are warmed. Calculate the percentage increase.

Answers

1. Adenosine triphosphate.
2. Mitochondrion (mitochondria).
3. glucose + oxygen → carbon dioxide + water.
4. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
5. glucose → lactic acid.
6. glucose → ethanol + carbon dioxide.
7. Any two: anaerobic needs no oxygen; releases less energy (less ATP) per glucose; glucose only partly broken down; different products (lactic acid, or ethanol and CO₂, instead of CO₂ and water); happens in the cytoplasm, not mainly in mitochondria.
8. Any three: muscle contraction; active transport; building proteins (large molecules); cell division/growth; nerve impulses; keeping body temperature constant.
9. From red (orange) to yellow.
10. To kill microorganisms on the seed surface, which would respire and release heat, so any temperature rise is due to the seeds alone.
11. 9.0 ÷ 6 = 1.5 °C per day.
12. (0.40 − 0.25) ÷ 0.25 × 100 = 60%.

Where marks are usually lost

• “Respiration produces energy” instead of “releases energy” or “produces ATP”.
• Putting carbon dioxide in the animal anaerobic equation, or leaving it out of the plant/yeast one.
• Writing “oxygen” as O, or “water” as H₂O₂, in the symbol equation.
• Stating that anaerobic respiration releases no energy.
• Confusing respiration with breathing or with gas exchange.
• Describing a colour change with no starting colour.
• Not saying the boiled seeds are dead and so cannot respire (the reason they are a control).
• Missing the reason for disinfectant (microbes respire too) or for the vacuum flask (insulation).
• A rate with no unit, such as “1.5” instead of “1.5 °C per day”.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Section 2, Structure and functions in living organisms, sub-topic (f) Respiration, points 2.34–2.39.