OCR A Level Biology A: Proteins, inorganic ions and biochemical tests (H420) – Revision Notes

These notes condense OCR A Level Biology A (H420, Version 4.1, April 2026, first assessment 2025), module 2.1.2 Biological molecules, statements (k) to (s). For full explanations go to the study guide, then test yourself with the practice questions.

Also useful: the OCR A Level Biology hub, the printable checklist, the Foundations in Biology revision notes and the free 10-minute diagnostics.

Amino acids and peptide bonds: 2.1.2(k), (l)

• Elements in proteins: C, H, O, N and usually S.
• General structure: central carbon bonded to NH₂ (amine), COOH (carboxyl), H and a variable R group.
• About twenty amino acids occur in proteins; only the R group differs.
• Condensation: COOH + NH₂ gives a peptide bond (C–N) plus H₂O.
• Hydrolysis: adds H₂O and breaks the peptide bond.
• n amino acids in one chain gives n − 1 peptide bonds and n − 1 water molecules released.

Quick reminder. A polypeptide of 85 amino acids: 84 peptide bonds, 84 water molecules. Two chains of 85 joined only by disulfide bonds: 168 peptide bonds.

Levels of structure: 2.1.2(m)

Level	Description	Bonding
Primary	Sequence of amino acids	Peptide bonds
Secondary	α-helix or β-pleated sheet	Hydrogen bonds along the backbone
Tertiary	3D shape of one polypeptide	Hydrogen, ionic and disulfide bonds; hydrophobic interactions
Quaternary	Several polypeptides, with any prosthetic group	The same bonds and interactions between chains

• Hydrogen bonds: weak, many, broken by heat.
• Ionic bonds: between oppositely charged R groups, affected by pH.
• Disulfide bonds: covalent, between two cysteine residues.
• Hydrophobic interactions: non-polar R groups in the centre. Hydrophilic R groups sit on the outside.

Globular and fibrous proteins: 2.1.2(n), (o)

Feature	Globular	Fibrous
Shape	Compact, rounded	Long strands
Solubility	Usually soluble	Insoluble
Main role	Metabolic: transport, catalysis, hormones	Structural
Examples	Haemoglobin, amylase, insulin	Collagen, keratin, elastin

• Haemoglobin: conjugated protein, four polypeptides plus four haem prosthetic groups, each with Fe²⁺ that binds oxygen.
• Enzyme (amylase): the active site is complementary to the substrate.
• Insulin: two polypeptides joined by disulfide bonds; soluble hormone that binds receptors.
• Collagen: high tensile strength (tendons, bone, artery walls).
• Keratin: tough and sulfur-rich (hair, nails, feathers).
• Elastin: stretches and recoils (artery walls, lungs, skin).

Inorganic ions: 2.1.2(p)

Ion	One role to quote
Ca²⁺	Muscle contraction and bone strength
Na⁺	Nerve impulses; co-transport of glucose
K⁺	Nerve impulse recovery; stomatal opening
H⁺	pH; proton gradients for ATP synthesis
NH₄⁺	Nitrogen cycle, oxidised by bacteria to nitrate
NO₃⁻	Plants use it to make amino acids and nucleotides
HCO₃⁻	Carbon dioxide transport and buffering of blood
Cl⁻	Charge balance when HCO₃⁻ leaves red blood cells
PO₄³⁻	ATP, nucleotides, phospholipids
OH⁻	Raises pH

Reagent tests: 2.1.2(q)

Test	Method	Positive	Negative
Biuret	NaOH, then dilute copper(II) sulfate	Purple	Blue
Benedict’s	Reagent, hot water bath	Green, yellow, orange, brick-red	Blue
Iodine	Iodine solution	Blue-black	Yellow-brown
Emulsion	Ethanol, shake, add to water	Cloudy white	Clear

Non-reducing sugar method, in steps:

1. Run Benedict’s on a first portion: stays blue.
2. Boil a second portion with dilute HCl.
3. Cool and neutralise with sodium hydrogencarbonate until fizzing stops.
4. Add Benedict’s and heat: a colour change means non-reducing sugar.

• Biuret detects peptide bonds, not free amino acids.
• Only Benedict’s and the acid hydrolysis step need heat.
• Use a negative control (water) to compare colours.

Colorimetry: 2.1.2(r)

1. Pick a filter complementary to the solution colour.
2. Zero with a blank.
3. Make standards by serial dilution.
4. Plot a calibration curve of absorbance against concentration.
5. Read the unknown from the curve, and multiply by any dilution factor.

• Dilution volume: stock volume = (target concentration ÷ stock concentration) × final volume.
• Benedict’s filtrate: more reducing sugar gives less blue, so lower absorbance.
• Suitable check: repeat readings and calculate a mean.

Chromatography: 2.1.2(s)

• Stationary phase: paper (cellulose) or a thin layer of silica on a plate. Mobile phase: solvent.
• Pencil origin line; spots above the solvent level; mark the solvent front immediately.
• Locating agent for colourless spots (ninhydrin gives purple with amino acids) or UV light.
• Proteins are hydrolysed to amino acids first.
• Rf = distance moved by solute ÷ distance moved by solvent. Range 0 to 1, no units.
• Identify by matching Rf, or by spots travelling level with standards on the same plate.
• Distance moved by a spot = Rf × distance moved by solvent.

Worked reminders

Peptide count. Insulin has chains of 21 and 30 amino acids: (21 − 1) + (30 − 1) = 49 peptide bonds, so 49 water molecules were released making it.

Calibration reading. Standards: 0.4 g per 100 cm³ gives absorbance 0.61 and 0.6 g per 100 cm³ gives 0.47. A sample reads 0.54, halfway between, so its concentration is 0.50 g per 100 cm³. If it was diluted 1 in 10 first, the original is 5.0 g per 100 cm³.

Serial dilution. To make 10 cm³ of a 0.6 standard from a 1.0 stock: (0.6 ÷ 1.0) × 10 = 6 cm³ stock plus 4 cm³ water.

Rf rearranged. Spots at 2.4 and 6.3 cm with the solvent front at 9.0 cm give Rf 0.27 and 0.70. For another run with a front at 11.0 cm, a spot with Rf 0.27 sits 0.27 × 11.0 = 3.0 cm from the origin.

Writing good answers

• Explain how structure gives function. Link the feature to the job in one sentence: “Haemoglobin is soluble because hydrophilic R groups are on the outside, so it can be carried inside red blood cells.”
• Name the bond or interaction. Saying “bonds hold it together” gains little. Say which R groups interact and which bond forms.
• Describe tests in order. Reagent, quantity, heat or no heat, positive colour and negative colour.
• Show the structure of a calculation. Write the equation, substitute, then give the answer with the right unit (none for Rf).
• Use data. In a table or graph question, quote values and say what they show about concentration or identity.

Protein structure at a glance

Bond or interaction	Strength	Affected by
Peptide	Strong, covalent	Hydrolysis by protease
Disulfide	Strong, covalent	Reducing agents
Ionic	Moderate	pH change
Hydrogen	Weak	Heat and pH
Hydrophobic	Depends on environment	Change of solvent

Must-know distinctions

• Peptide bond (between amino acids) versus disulfide bond (between cysteine R groups).
• Primary structure (sequence) versus tertiary (3D fold of one chain) versus quaternary (more than one chain).
• Conjugated protein (has a prosthetic group) versus simple protein.
• Globular (soluble, metabolic) versus fibrous (insoluble, structural).
• Qualitative test (colour) versus quantitative method (colorimeter reading).

Quick self-test

1. What four groups are attached to the central carbon of an amino acid?
2. How many peptide bonds are there in a polypeptide of 120 amino acids?
3. Name the bond that links two cysteine R groups.
4. Which type of protein structure involves hydrogen bonds between the backbone C=O and N–H groups?
5. Why is haemoglobin called a conjugated protein?
6. State one property and one location of elastin.
7. Give the symbol for the hydrogencarbonate ion and one role.
8. What colour does a positive biuret test give, and what does it detect?
9. A sample stays blue with Benedict’s, but gives orange after boiling with acid and neutralising. What does it contain?
10. A pigment travelled 3.6 cm while the solvent reached 8.0 cm. Calculate its Rf.
11. Why is the origin line drawn in pencil?
12. A solution of 4.0 g per 100 cm³ must be diluted to 1.0 g per 100 cm³ in 20 cm³. What volumes of stock and water?

Answers

1. Amine group, carboxyl group, hydrogen atom and R group.
2. 119 (n − 1).
3. A disulfide bond.
4. Secondary structure.
5. It is a globular protein with a non-protein prosthetic group (haem).
6. It stretches and recoils; found in artery walls, lungs or skin.
7. HCO₃⁻; carries carbon dioxide in blood or buffers pH.
8. Purple or lilac; peptide bonds, so proteins.
9. A non-reducing sugar, such as sucrose.
10. 3.6 ÷ 8.0 = 0.45.
11. Pencil does not dissolve in the solvent or run.
12. (1.0 ÷ 4.0) × 20 = 5 cm³ stock and 15 cm³ water.

Where marks are usually lost

• Writing “protein” for a single amino acid, or the reverse.
• Saying the biuret test turns blue-black or that the iodine test turns purple.
• Describing hydrolysis of sucrose without stating the acid, the neutralisation and the second Benedict’s test.
• Giving Rf with units, or a value greater than 1.
• Stating that hydrogen bonds hold the primary structure.
• Naming “an ion” without its role, or confusing NH₄⁺ with NO₃⁻.
• Omitting the blank when describing colorimeter use.
• Calling collagen soluble, or describing a globular protein as long and thin.
• Forgetting to multiply by the dilution factor.

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025, published by OCR. Module 2.1.2 Biological molecules, statements (k) to (s).