OCR GCSE Biology A: Respiration and the synthesis and breakdown of biological molecules (J247) – Revision Notes

Condensed notes for OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), first assessment 2018, topic B1.3 Respiration (outcomes B1.3a to B1.3f). No B1.3 statement is bold in the specification, so nothing here is Higher Tier only. For full explanations use the study guide, then try the practice questions. Also useful: the OCR GCSE Biology hub, the printable checklist, the free diagnostics and the Cell level systems notes.

Key definitions

Term	Meaning
Cellular respiration	Chemical process in all living cells that transfers energy from fuels such as glucose to make ATP
ATP	Small molecule that carries energy to where the cell needs it; made from ADP and phosphate
Exothermic	Transfers energy to the surroundings (heat released)
Aerobic	Needs oxygen
Anaerobic	Without oxygen
Fermentation	Anaerobic respiration by yeast: ethanol and carbon dioxide
Monomer	A small unit that repeats to make a polymer
Polymer	A long chain of many monomers
Condensation	Joining two molecules with loss of a water molecule
Hydrolysis	Splitting a bond by adding water (what digestive enzymes do)

B1.3a and B1.3b Respiration in every cell

• Universal: all living cells, all the time (plants too, in the dark and the light).
• Not the same as breathing or ventilation.
• Supplies ATP, which powers building molecules, muscle contraction, active transport, cell division and nerve impulses.
• Exothermic: some energy is captured in ATP, the rest is released as heat.
• Enzyme-controlled: rate rises with temperature up to the optimum, then falls as enzymes denature.

B1.3c Compare aerobic and anaerobic

	Aerobic	Anaerobic (animals)	Anaerobic (plants, fungi)
Oxygen	Needed	Not needed	Not needed
Substrate	Glucose	Glucose	Glucose
Products	CO2 and water	Lactic acid only	Ethanol and CO2
ATP per glucose	About 30 (large)	2 (small)	2 (small)
Why the difference	Complete breakdown	Incomplete: energy left in lactic acid	Incomplete: energy left in ethanol

Word equations:

• glucose + oxygen → carbon dioxide + water
• glucose → lactic acid
• glucose → ethanol + carbon dioxide

Symbol equation for aerobic respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O.

Method in steps: investigating yeast respiration

1. Mix yeast into glucose solution in a flask. For anaerobic conditions, use boiled, cooled glucose topped with a layer of oil.
2. Connect a gas syringe, or a delivery tube to an inverted measuring cylinder.
3. Change one independent variable (for example temperature) and keep the others constant: identical yeast mass, glucose volume and glucose strength.
4. Measure the volume of gas in a fixed time, then calculate rate = volume ÷ time.
5. Repeat and calculate a mean. Spot anomalies.

CO2 tests: limewater turns cloudy; hydrogen-carbonate indicator changes from red-orange to yellow.

Monomers and polymers

Large molecule	Monomers	Joined by	Broken down by	Products
Carbohydrate (starch, glycogen, cellulose)	Sugars (glucose)	Condensation	Carbohydrase	Sugars
Protein	Amino acids	Condensation (peptide bond)	Protease	Amino acids
Lipid (triglyceride)	1 glycerol + 3 fatty acids	Condensation	Lipase	Glycerol + fatty acids

Note: lipids are not true polymers, because they are not chains of one repeating unit.

B1.3d Sugars and carbohydrates

• Glucose is the main respiratory substrate.
• Linked into starch (plant store), glycogen (animal store) and cellulose (plant cell walls).
• Breakdown releases glucose for respiration.
• Test: Benedict’s (blue to brick red on heating, reducing sugar); iodine (blue-black, starch).

B1.3e Amino acids and proteins

• Order of amino acids set by DNA decides how the chain folds and what the protein does.
• Used to build enzymes, antibodies, haemoglobin, collagen and more.
• Test: biuret (blue to purple).

B1.3f Fatty acids, glycerol and lipids

• Triglyceride: glycerol + 3 fatty acids, 3 water molecules released.
• Energy store, insulation, phospholipids in membranes.
• Test: emulsion test (ethanol, then water: cloudy white).

Method in steps: food tests (qualitative)

1. Starch: add a few drops of iodine solution to the sample. Orange-brown to blue-black means starch.
2. Reducing sugar: warm the sample with Benedict’s solution in a hot water bath. Blue stays blue with no sugar; green, yellow, orange then brick red show increasing amounts.
3. Protein: add biuret reagent. Blue to purple (lilac) means protein.
4. Lipid: shake the sample with ethanol, then pour the liquid into water. A cloudy white emulsion means lipid.

These tests are qualitative: they tell you whether a molecule is present, not how much. Write a conclusion that names the molecule and the colour you saw.

Method in steps: showing that respiration is exothermic

1. Put germinating seeds in one vacuum flask and boiled, cooled seeds in another. Rinse both in disinfectant so that microbes do not respire and affect the result.
2. Put a thermometer through a cotton wool plug in each flask.
3. Record both temperatures at the start, then at regular intervals for a day or two.
4. The living seeds warm up more. The boiled seeds are the control.

Data and graph reminders

• A rate-against-temperature graph rises, peaks at the optimum, then drops steeply as enzymes denature. Describe the shape first, then explain it.
• When respiration data show a rise with exercise, link the result to ATP demand: more muscle contraction needs more ATP.
• If the question gives lactic acid levels that rise sharply at high speed, say that oxygen supply cannot meet demand, so more anaerobic respiration occurs.
• Percentage change always divides by the original value.
• Compare numbers: “the rate at 35 °C is 8 times the rate at 15 °C” earns more than “it is faster”.

Explain-answer templates

• Why do muscles use anaerobic respiration in a sprint? Oxygen is delivered too slowly, so glucose is broken down without oxygen. It is quick, but gives only 2 ATP per glucose and makes lactic acid.
• Why does yeast make bread rise? Yeast respires, releases carbon dioxide, and the gas bubbles expand the dough.
• Why is starch broken down before cells can use it? Starch is a large polymer. Carbohydrase splits it into small sugars that can be absorbed and respired.
• Why must amino acids be absorbed after protein digestion? Cells build their own proteins from them; the order in the chain decides the protein made.

Small worked reminders

• Water released = number of joins. A chain of n monomers has n − 1 joins. A starch of 500 glucose has 499 joins.
• Mass of product = sum of reactant masses − 18 for each water lost. Glucose 180 + glucose 180 − 18 = maltose 342.
• Rate: 36 cm³ in 12 minutes is 36 ÷ 12 = 3.0 cm³ per minute.
• Percentage change = (new − old) ÷ old × 100. From 2.5 to 4.0: 1.5 ÷ 2.5 × 100 = 60%.
• Corrected temperature rise = rise with living tissue − rise in dead control.

Respiration in plants and fungi

• Plants respire in every living cell, including roots, which need oxygen from air spaces in the soil.
• Photosynthesis stores energy in glucose; respiration releases it. Both happen in plants, but photosynthesis needs light.
• In waterlogged soil, root cells may switch to anaerobic respiration and make ethanol and carbon dioxide.
• Yeast is a fungus. It can respire aerobically when oxygen is present and by fermentation when it is not.
• Brewing uses the ethanol. Baking uses the carbon dioxide.

Must-know distinctions

• Respiration (in cells, makes ATP) against breathing (moving air).
• Lactic acid (animals) against ethanol + CO2 (plants and fungi).
• Aerobic (30 ATP) against anaerobic (2 ATP): the same fuel gives much less ATP when the breakdown is incomplete.
• Breakdown (a chemical reaction that splits a molecule into smaller ones) against breakup (a loose word that does not say what happened).
• Monomer (one unit) against polymer (chain).

Quick self-test

1. Name the process that supplies ATP in all living cells.
2. Is respiration exothermic or endothermic?
3. Give the word equation for anaerobic respiration in yeast.
4. State the products of anaerobic respiration in human muscle.
5. Name the monomer of a protein.
6. How many water molecules are released when glycerol joins three fatty acids?
7. A starch chain has 300 glucose units. How many water molecules are released making it?
8. A cell needs 240 ATP. How many glucose molecules does it use anaerobically (2 ATP each)?
9. Yeast makes 45 cm³ of gas in 15 minutes. Give the rate.
10. Which enzyme type breaks down lipids, and what does it produce?
11. What colour change shows protein with biuret reagent?
12. A dipeptide forms from amino acids of mass 75 and 117. What is its mass?

Answers

1. Cellular respiration.
2. Exothermic.
3. Glucose → ethanol + carbon dioxide.
4. Lactic acid only (no carbon dioxide).
5. Amino acid.
6. Three.
8. 240 ÷ 2 = 120.
9. 45 ÷ 15 = 3.0 cm³ per minute.
10. Lipase, producing glycerol and fatty acids.
11. Blue to purple (lilac).
12. 75 + 117 − 18 = 174.

Where marks are usually lost

• Writing “respiration is breathing” or “plants only respire at night”.
• Mixing up the products for muscle and for yeast.
• Writing “makes energy” instead of “transfers energy” or “makes ATP”.
• Saying anaerobic respiration releases no ATP, rather than less ATP.
• Forgetting to say the yeast investigation needs a fixed time and equal masses.
• Using n joins for n monomers.
• Naming lipids as a polymer of fatty acids only; glycerol is also needed.
• Leaving out units, or giving rates without “per minute”.

Official syllabus

OCR, GCSE (9-1) Gateway Science Suite Biology A (J247), Version 4.0 (August 2026), first assessment 2018, topic B1.3 Respiration.