AQA GCSE Biology 8461: Bioenergetics – Revision Notes

These are condensed recall notes for section 4.4 Bioenergetics (4.4.1.1 to 4.4.2.3) of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. The topic is assessed on Paper 1, set at Foundation and Higher Tier. Points the specification marks (HT only) are labelled Higher tier only.

For full explanations and worked examples, use the Bioenergetics study guide. Then test yourself with the Bioenergetics practice questions. The course hub is AQA GCSE Biology, and the printable checklist lists every point.

The equations

Process	Word equation	Energy change
Photosynthesis	carbon dioxide + water → glucose + oxygen (light needed)	Endothermic – energy from the environment to the chloroplasts by light
Aerobic respiration	glucose + oxygen → carbon dioxide + water	Exothermic
Anaerobic respiration in muscles	glucose → lactic acid	Exothermic, much less energy
Anaerobic respiration in plants and yeast	glucose → ethanol + carbon dioxide	Exothermic, much less energy

Formulae to recognise: CO₂, H₂O, O₂, C₆H₁₂O₆. Anaerobic respiration in yeast is called fermentation (bread and alcoholic drinks).

4.4.1.2 Rate of photosynthesis

Four factors: temperature, light intensity, carbon dioxide concentration, amount of chlorophyll.

Factor	Why it affects rate
Light intensity	Light transfers the energy needed
CO₂ concentration	CO₂ is a reactant
Temperature	Reactions are enzyme-controlled; too hot and enzymes denature
Chlorophyll	Absorbs the light; less chlorophyll, less light absorbed

Limiting factor: the one in shortest supply, which holds the rate back. On a graph, the factor on the x-axis is limiting where the line rises. Where the line is flat, another factor is limiting.

Interacting factors (Higher tier only): any one factor may be limiting. On graphs with two or three lines, if the lines separate at a point, the factor that differs between them is limiting there. If a line is flat, the x-axis factor is not limiting at that point.

Rate calculations

rate = amount of product (or bubbles) ÷ time

Example: 45 bubbles in 5 minutes → 45 ÷ 5 = 9 bubbles per minute.

Inverse square law (Higher tier only)

light intensity ∝ 1 / distance²

Change in distance	Change in light intensity
× 2	× 1/4
× 3	× 1/9
÷ 2	× 4

Relative light intensity is often written as 1/d². At 20 cm, 1/d² = 1/400 = 0.0025.

Greenhouse economics (Higher tier only)

Adding heat, light or CO₂ is worth it only if the value of the extra yield is greater than the cost. Adding a factor that is not limiting raises cost, not yield.

Required practical 6 – method in steps

1. Pondweed in a boiling tube of water; lamp at a measured distance.
2. Heat shield (beaker of water) between lamp and tube; thermometer to keep temperature constant.
3. Let the weed adjust at each new distance before counting.
4. Count bubbles or collect oxygen in a fixed time.
5. Take multiple readings at each light intensity; calculate a mean.
6. Plot light intensity (or distance) against rate, choosing a sensible scale.

Control variables: temperature, CO₂ concentration, same piece of pondweed, time for each reading.

4.4.1.3 Uses of glucose

• Respiration
• Insoluble starch for storage
• Fat or oil for storage
• Cellulose to strengthen the cell wall
• Amino acids for protein synthesis – also needs nitrate ions from the soil

4.4.2.1 Aerobic vs anaerobic – must-know comparison

	Aerobic	Anaerobic (muscles)	Anaerobic (plants, yeast)
Oxygen	Needed	Not needed	Not needed
Products	CO₂ + water	Lactic acid	Ethanol + CO₂
Glucose oxidation	Complete	Incomplete	Incomplete
Energy transferred	Much more	Much less	Much less

Organisms need energy for: building larger molecules, movement, keeping warm.

Respiration is exothermic and happens continuously in all living cells – plants respire day and night.

4.4.2.2 Response to exercise – method in steps

1. Muscles need more energy.
2. Heart rate, breathing rate and breath volume increase.
3. More oxygenated blood reaches the muscles.
4. If oxygen supply is still too low, muscles respire anaerobically.
5. Incomplete oxidation of glucose → lactic acid builds up → oxygen debt.
6. Long vigorous activity → muscles fatigued, stop contracting efficiently.

Higher tier only:

• Blood carries lactic acid to the liver, where it is converted back into glucose.
• Oxygen debt = the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.

4.4.2.3 Metabolism

Metabolism: the sum of all the reactions in a cell or the body. Respiration supplies the energy for these enzyme-controlled reactions.

Building up or breaking down	From	To
Carbohydrates	Glucose (sugars)	Starch, glycogen, cellulose
Lipids	1 glycerol + 3 fatty acids	Lipid molecule
Proteins	Glucose + nitrate ions → amino acids	Proteins
Excess protein	Protein	Urea, for excretion
Respiration	Glucose	Energy transferred

Small worked reminders

Rate from a gas syringe. Pondweed gives 7.5 cm³ of oxygen in 5 minutes.

rate = 7.5 ÷ 5 = 1.5 cm³ per minute

Inverse square (Higher tier only). A meter reads 900 units at 10 cm. At 30 cm:

distance × 3  ->  intensity × 1/9
900 ÷ 9 = 100 units

Percentage increase. Heart rate rises from 70 to 147 beats per minute.

(147 − 70) ÷ 70 × 100 = 110%

Greenhouse decision (Higher tier only). Extra CO₂ costs £50 a week and adds crop worth £35 a week. Gain = 35 − 50 = −£15, so it is not cost effective. CO₂ was probably not the limiting factor.

Reading graphs of rate

• Label axes with quantity and unit; the independent variable goes on the x-axis.
• Choose a scale that uses at least half the grid, and draw a smooth curve or line of best fit.
• “Describe” means give the trend with data: “rate rises from 5 to 20 units, then levels off at 20”.
• “Explain” means give the reason: “light is limiting while the line rises; after that another factor limits”.
• Translate between a table and a graph in both directions, reading values carefully from the grid.

Must-know distinctions

• Endothermic vs exothermic: photosynthesis takes in energy; respiration releases it.
• Lactic acid vs ethanol: lactic acid in animal muscles; ethanol and CO₂ in plants and yeast.
• Starch vs glycogen: plants store starch; glycogen is made in animals.
• Limiting vs not limiting: only raising the limiting factor raises the rate.
• Breathing vs respiration: breathing moves air; respiration is a chemical reaction in cells.

Quick self-test

1. Give the word equation for photosynthesis.
2. Why is photosynthesis described as endothermic?
3. Name the four factors that affect the rate of photosynthesis.
4. Pondweed gives off 45 bubbles in 5 minutes. Calculate the rate.
5. (Higher tier only) A lamp is moved from 30 cm to 15 cm. By what factor does light intensity change?
6. (Higher tier only) A lamp is moved from 10 cm to 30 cm. What fraction of the original light intensity reaches the plant?
7. Give three uses of glucose in a plant.
8. Why does anaerobic respiration transfer less energy than aerobic respiration?
9. Yeast releases 24 cm³ of carbon dioxide in 8 minutes. Calculate the rate.
10. Why do muscles become fatigued during long vigorous exercise?
11. (Higher tier only) What happens to lactic acid after exercise?
12. Which molecules combine to make a lipid?

Answers

1. carbon dioxide + water → glucose + oxygen (in light).
2. Energy is transferred from the environment to the chloroplasts by light.
3. Temperature, light intensity, carbon dioxide concentration, amount of chlorophyll.
4. 45 ÷ 5 = 9 bubbles per minute.
5. Distance halves, so intensity × 2² = × 4.
6. Distance × 3, so intensity × 1/3² = 1/9.
7. Any three: respiration; starch for storage; fat or oil for storage; cellulose for cell walls; amino acids for proteins.
8. The oxidation of glucose is incomplete.
9. 24 ÷ 8 = 3 cm³ per minute.
10. Lactic acid builds up from anaerobic respiration, so muscles stop contracting efficiently.
11. Blood carries it to the liver, where it is converted back into glucose.
12. One glycerol molecule and three fatty acid molecules.

Where marks are usually lost

• Writing “light” as a reactant in the photosynthesis equation. It goes above the arrow as the energy source.
• Calling photosynthesis exothermic, or respiration endothermic.
• Adding carbon dioxide to the anaerobic equation for muscles.
• Saying anaerobic respiration releases “no energy” rather than “much less”.
• Stating that plants respire only at night.
• Naming a limiting factor from a graph without referring to where the line is flat or rising.
• (Higher tier only) Halving the light intensity when the distance doubles, instead of quartering it.
• Forgetting the unit on a rate, or dividing time by amount.
• Saying the oxygen debt is “oxygen owed to the muscles” without linking it to removing lactic acid (Higher tier only).
• Confusing glycogen (animal store) with glucagon, a hormone from a different topic.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.4 Bioenergetics. Check your recall with the free 10-minute diagnostics.