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

This study guide follows the OCR GCSE (9-1) Gateway Science Biology A specification (J247), Version 4.0 (August 2026), for first assessment in 2018. It covers topic B1.3, Respiration, outcomes B1.3a to B1.3f. The specification marks no B1.3 statement in bold, so none is labelled Higher Tier only. Revise it with the revision notes and test yourself on the practice questions. Course links: OCR GCSE Biology hub, printable checklist and the free diagnostics. For the wider topic, see Cell level systems.

What this unit covers

Outcome	You must be able to
B1.3a	Describe cellular respiration as a universal chemical process, continuously occurring, that supplies ATP in all living cells
B1.3b	Describe cellular respiration as an exothermic reaction
B1.3c	Compare aerobic and anaerobic respiration: in plants/fungi and animals, the different conditions, substrates, products and relative yields of ATP
B1.3d	Explain the importance of sugars in the synthesis and breakdown of carbohydrates (use the terms monomer and polymer)
B1.3e	Explain the importance of amino acids in the synthesis and breakdown of proteins (monomer and polymer)
B1.3f	Explain the importance of fatty acids and glycerol in the synthesis and breakdown of lipids

The specification notes a common misconception: ventilation is not respiration. It also warns against confusing “breakup” with “breakdown”. Breakdown means a large molecule split into smaller ones by a chemical reaction.

B1.3a Respiration supplies ATP in every living cell

Respiration is the chemical process in cells that transfers energy from organic molecules (fuels) such as glucose. It is not breathing. Breathing (ventilation) is the movement of air into and out of the lungs. Respiration happens inside cells.

Key points:

• It is universal: plant, animal, fungal and bacterial cells all do it.
• It is continuous: it happens day and night, at rest and during exercise. A plant cell respires in the dark and in the light.
• The energy is transferred to ATP (adenosine triphosphate), a small molecule that carries energy to wherever the cell needs it. ATP is made from ADP and phosphate, and the energy is released when ATP is broken back down to ADP and phosphate.
• The cell uses the ATP for other chemical reactions: building large molecules, contracting muscle, active transport, cell division and sending nerve impulses.

Respiration is controlled by enzymes, so its rate changes with temperature and pH in the same way as other enzyme-controlled reactions.

B1.3b Respiration is exothermic

An exothermic reaction transfers energy to the surroundings. Respiration is exothermic: part of the energy from the fuel is captured in ATP and the rest is released as heat. This is why you feel warm after exercise, why compost heaps and piles of damp grain get hot, and why mammals and birds can keep their bodies warmer than their surroundings.

Worked example. A student puts 20 g of germinating peas in one vacuum flask and 20 g of boiled, cooled peas in another. Both start at 24.0 °C. After 24 hours the live peas read 28.5 °C and the boiled peas read 24.2 °C.

1. Temperature rise of the live peas = 28.5 − 24.0 = 4.5 °C.
2. Rise in the control = 24.2 − 24.0 = 0.2 °C.
3. Rise due to respiration = 4.5 − 0.2 = 4.3 °C.

The boiled peas are the control because boiling denatured their enzymes, so they cannot respire. The result supports the claim that respiration transfers heat to the surroundings.

B1.3c Aerobic and anaerobic respiration compared

Aerobic respiration needs oxygen. Glucose is fully broken down:

glucose + oxygen → carbon dioxide + water (C6H12O6 + 6O2 → 6CO2 + 6H2O)

Anaerobic respiration happens without oxygen. Glucose is only partly broken down, so much of the chemical energy stays in the products and far less ATP is made.

	Aerobic	Anaerobic in animals	Anaerobic in plants and fungi (yeast)
Oxygen needed	Yes	No	No
Substrate	Glucose	Glucose	Glucose
Products	Water and carbon dioxide	Lactic acid only	Carbon dioxide and ethanol
Relative ATP yield	Large (about 30 per glucose)	Small (2 per glucose)	Small (2 per glucose)
Where in the cell	Mostly mitochondria	Cytoplasm	Cytoplasm

Anaerobic respiration in animals: glucose → lactic acid. In plants and fungi: glucose → ethanol + carbon dioxide. The second is called fermentation when yeast does it. It is used in brewing and baking. Plant roots in waterlogged soil also respire anaerobically because the soil contains little oxygen.

The ATP figures vary slightly between sources. In the exam the point is “much more” against “much less”. The worked example below shows what the difference means in practice.

Worked example (yield). A muscle cell needs 600 ATP. How much glucose does it use under each type?

• Aerobic, 30 ATP per glucose: 600 ÷ 30 = 20 glucose molecules.
• Anaerobic, 2 ATP per glucose: 600 ÷ 2 = 300 glucose molecules.
• 300 ÷ 20 = 15, so anaerobic respiration uses 15 times as much glucose for the same ATP.

Worked example (investigating yeast). A student measures carbon dioxide from yeast in glucose solution at four temperatures. The gas collects in a syringe for 10 minutes. Volumes: 15 °C, 6 cm³; 25 °C, 21 cm³; 35 °C, 48 cm³; 55 °C, 3 cm³.

1. Rate = volume ÷ time: 0.6, 2.1, 4.8 and 0.3 cm³ per minute.
2. Percentage increase in rate from 25 °C to 35 °C = (4.8 − 2.1) ÷ 2.1 × 100 = 128.6%, so about 129%.
3. At 55 °C the rate collapses because the enzymes are denatured: their active sites change shape and substrate no longer fits.

To make the yeast respire anaerobically, boil and cool the glucose solution (to drive off dissolved oxygen) and float a layer of oil on top. Carbon dioxide can be detected with limewater (turns cloudy) or hydrogen-carbonate indicator (red-orange to yellow). Immobilising yeast in alginate beads makes the yeast easy to remove and re-use.

B1.3d Sugars: carbohydrates

Large biological molecules are polymers: long chains made of many repeating smaller units called monomers. Glucose is a sugar and the monomer of the carbohydrates starch, glycogen and cellulose.

• Synthesis: glucose molecules join in a condensation reaction that releases a water molecule at each join. Plants make starch (an energy store) and cellulose (cell walls). Animals make glycogen (a store in liver and muscle).
• Breakdown: carbohydrase enzymes such as amylase break the bonds, adding water, to release sugars. Glucose then goes into respiration, or into synthesis of other molecules.

Sugars matter because they are the main respiratory substrate, and because they are the building blocks that cells can link into storage and structural carbohydrates.

Worked example. Two glucose (relative formula mass 180) join to make maltose (342).

• Mass of two glucose = 2 × 180 = 360.
• Mass lost as water = 360 − 342 = 18, which is one water molecule.
• A starch chain of 500 glucose units has 499 joins, so 499 water molecules are released on synthesis.

Test: Benedict’s solution is heated with a reducing sugar (blue to green, yellow, orange or brick red). Iodine solution turns blue-black with starch.

B1.3e Amino acids: proteins

Amino acids are the monomers of proteins. About twenty different amino acids occur in proteins. Cells join them in a sequence set by DNA, forming a chain called a polypeptide, which folds into a protein.

• Synthesis: amino acids join in condensation reactions, one water lost per peptide bond.
• Breakdown: protease enzymes add water to split the peptide bonds and release amino acids.
• Importance: the amino acids from digested protein are absorbed and used to build the cell’s own proteins, including enzymes, antibodies, haemoglobin and structural proteins such as collagen. The order of amino acids decides how the protein folds and so what it does.

Worked example. Glycine (75) and alanine (89) join to make a dipeptide.

• 75 + 89 = 164.
• Dipeptide mass = 164 − 18 = 146.
• A polypeptide of 120 amino acids has 119 peptide bonds and 119 water molecules released.

Test: biuret reagent turns from blue to purple (lilac) if protein is present.

B1.3f Fatty acids and glycerol: lipids

Lipids (fats and oils) are not true polymers because they are not long chains of one repeating unit. They are built from two kinds of smaller molecule: glycerol and fatty acids.

• Synthesis: one glycerol joins to three fatty acids, losing three water molecules, to make a triglyceride. Phospholipids, used in cell membranes, are built from glycerol and fatty acids too.
• Breakdown: lipase enzymes add water to split a lipid into glycerol and three fatty acids.
• Importance: lipids store energy compactly, insulate, and form membranes. The glycerol and fatty acids from breakdown can be respired or used to synthesise the body’s own lipids.

Worked example. Glycerol (92) joins three palmitic acid molecules (256 each).

• Mass of the four reactants = 92 + 3 × 256 = 860.
• Mass lost = 3 × 18 = 54.
• Mass of the triglyceride (tripalmitin) = 860 − 54 = 806.

Test: the emulsion test. Shake the sample with ethanol, then pour into water. A cloudy white layer shows lipid.

Common errors

• Writing that respiration is “breathing in oxygen”. Breathing supplies oxygen; respiration happens in cells.
• Saying plants respire only at night. They respire all the time and photosynthesise only in light.
• Giving lactic acid for yeast, or ethanol for muscle. Check the organism first.
• Saying anaerobic respiration “makes no energy”. It makes a small yield.
• Writing “respiration produces energy”. Energy is transferred, not made.
• Describing lipids as made from amino acids, or proteins from glycerol.
• Saying breakdown “breaks up” molecules without naming the enzyme type and the product.

Using this guide

Read each outcome, cover the page and explain it aloud, then check the practice questions. A weak area is a cue to try the diagnostics or to book a free trial class.

Official syllabus

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