Water, Lipids, Proteins and Ions (2.1.2): OCR A Level Biology Exam Questions by Topic

OCRFoundations in BiologyExam Questions
Written by Tom Whitburn

Biological molecules is the longest spec point in Module 2, so it has two pages. My Carbohydrates page covers sugars, starch, glycogen, cellulose, the Benedict's and iodine tests and colorimetry. This page covers the rest of spec point 2.1.2: how hydrogen bonding gives water its properties, triglycerides and phospholipids, amino acids and peptide bonds, the four levels of protein structure, globular and fibrous proteins, the key inorganic ions, the biuret test and chromatography.

The examiners' reports show where the marks go. Students call the carboxyl group a hydroxyl group, forget that hydrolysing a triglyceride breaks three ester bonds, and say that hydrogen bonds explain the melting points of fats when the answer is kinked chains. They describe a membrane when the question asks them to explain how phospholipids form one, and they draw a general amino acid next to a figure instead of completing the R group they were given.

Work through the twenty questions below in order. They start with recall, move on to water, lipids and proteins in new contexts, and finish with practical questions on the biuret test, ions and chromatography. If you'd like to go through questions like these with me each week, my OCR A masterclasses run on Wednesdays at 5.15pm for Year 12 and on Tuesdays at 6.30pm or Thursdays at 5.15pm for Year 13.

What you need from GCSE

Each question on this page assumes you're secure on these GCSE ideas. If any of them feels shaky, look it up before you start.

  • Proteins are made of amino acids, and lipids are made of fatty acids and glycerol.
  • Digestive enzymes break large molecules into small ones: proteases make amino acids and lipases make fatty acids and glycerol.
  • A protein's shape decides its function, and high temperature or extreme pH can denature it.
  • The biuret test turns purple with protein, and the emulsion (ethanol) test goes cloudy white with lipid.
  • Paper chromatography separates a mixture, and Rf = distance moved by the substance ÷ distance moved by the solvent.

Key terms

These words come up in the questions and mark schemes. Examiners expect you to use them exactly.

  • Hydrogen bond: a weak attraction between the slightly positive hydrogen of one polar water molecule and the slightly negative oxygen of another.
  • Cohesion and adhesion: water molecules sticking to each other (cohesion) and to other surfaces such as xylem walls (adhesion).
  • Triglyceride: one glycerol joined to three fatty acids by three ester bonds, made by three condensation reactions.
  • Saturated and unsaturated: a saturated fatty acid has no C=C double bonds; an unsaturated one has at least one, which puts a kink in the chain (polyunsaturated means more than one).
  • Phospholipid: glycerol joined to two fatty acids and a phosphate group: a hydrophilic head and two hydrophobic tails, which arrange themselves into a bilayer in water.
  • Amino acid: a central carbon carrying an amine group (NH₂), a carboxyl group (COOH), a hydrogen atom and a variable R group.
  • Peptide bond: the bond between two amino acids, formed by condensation and broken by hydrolysis.
  • Levels of protein structure: primary (sequence of amino acids), secondary (α-helix and β-pleated sheet, held by hydrogen bonds), tertiary (3D folding, held by hydrogen, ionic and disulfide bonds and hydrophobic interactions), quaternary (more than one polypeptide).
  • Conjugated protein: a protein with a non-protein prosthetic group, such as the haem groups in haemoglobin.
  • Rf value: distance moved by the solute ÷ distance moved by the solvent front, measured from the origin.

What these questions cover (OCR 2.1.2, water, lipids, proteins and ions)

The questions test these learning outcomes from the OCR A Level Biology A specification (H420):

  • (a) how hydrogen bonding occurs between water molecules (hydrogen bonding, and water as solvent, transport medium, coolant and habitat)
  • (c) the chemical elements that make up biological molecules
  • (h) the structure of a triglyceride and a phospholipid as examples of macromolecules (including saturated and unsaturated fatty acids)
  • (i) the synthesis and breakdown of triglycerides by the formation and breakage of ester bonds between fatty acids and glycerol
  • (j) how the properties of triglyceride, phospholipid and cholesterol molecules relate to their functions in living organisms
  • (k) the general structure of an amino acid
  • (l) the synthesis and breakdown of dipeptides and polypeptides, by the formation and breakage of peptide bonds
  • (m) the levels of protein structure
  • (n) the structure and function of globular proteins including a conjugated protein (haemoglobin as a conjugated protein, a named enzyme and insulin)
  • (o) the properties and functions of fibrous proteins (collagen, keratin and elastin)
  • (p) the key inorganic ions that are involved in biological processes (recognise the symbols of the key ions)
  • (q) how to carry out and interpret the results of the following chemical tests (the biuret test is here; Benedict's and iodine tests are on the Carbohydrates page)
  • (s)(i) the principles and uses of paper and thin layer chromatography to separate biological molecules / compounds (including calculating Rf values)
  • (s)(ii) practical investigations to analyse biological solutions using paper or thin layer chromatography

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.2 minutes per mark: the 20 questions are worth 41 marks, so about 49 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

Recall

Question 1

AS Paper 2 · June 2024 · Q6(a)(i) · 2 marks · Easy

Name the two groups in an amino acid apart from the R group.

Question: the general structure of an amino acid has one R group and two other groups; name the two other groups. 2 marks.
Show the mark scheme Mark scheme: amine (amino allowed) and carboxyl (carboxylic acid allowed).

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From the examiners' report: Most scored both marks. The group at the end is the whole COOH, so call it carboxyl, not hydroxyl.

Question 2

AS Paper 2 · June 2024 · Q6(a)(iii) · 1 mark · Easy

Name the type of reaction that breaks a peptide bond.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question showing a dipeptide diagram and asking for the type of reaction involved in breaking the peptide bond. 1 mark.
Show the mark scheme Mark scheme: hydrolysis.

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From the examiners' report: Most got this. Bonds are broken by hydrolysis and made by condensation, so don't swap them.

Question 3

A level Paper 2 · June 2018 · Q22(a) · 1 mark · Easy

State one structural difference between a hydrocarbon and a saturated fatty acid.

Diagram of three hydrocarbons, W, X and Y, from crude oil; W is a straight chain of ten carbons with hydrogens. State one structural difference between W and a saturated fatty acid. 1 mark.
Show the mark scheme Mark scheme: a saturated fatty acid has a carboxyl group (COOH), or OH, or oxygen atoms. Hydroxide ignored.

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From the examiners' report: Fewer than half scored this easy mark. A fatty acid has a carboxyl group at one end; it has no double bonds if saturated, and no ester bond or glycerol on its own.

Question 4

AS Paper 2 · June 2018 · Q4(a)(i) · 2 marks · Medium

Match ions to their roles in organisms.

Table of roles (production of nitrate ions by bacteria: NH₄⁺, given; loading of phloem; DNA structure; cofactor for amylase) with a list of ions NH₄⁺, Cl⁻, H⁺, OH⁻, PO₄³⁻ and Ca²⁺. 2 marks.
Show the mark scheme Mark scheme: loading of phloem = H⁺; DNA structure = PO₄³⁻; cofactor for amylase = Cl⁻. All three for 2 marks, one or two for 1. Symbols must be fully correct.

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From the examiners' report: Most got one or two. The least known was the chloride ion as the cofactor for amylase, and the symbols must be written exactly, charges included.

Applying what you know

Question 5

AS Paper 1 · June 2022 · Q25(a)(i) · 3 marks · Medium

Draw two water molecules and label the bond between them.

Question: water molecules are transported in a sunflower stem and are polar, so they attract each other. Draw two water molecules and label the bond between the two molecules. 3 marks.
Show the mark scheme Mark scheme: water molecules correctly drawn; a dashed line between the H of one molecule and the O of the next; labelled hydrogen bond.

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From the examiners' report: Most drew this well. Each water molecule is one oxygen and two hydrogens, and the hydrogen bond goes from H on one molecule to O on the other, never H to H.

Question 6

AS Paper 1 · June 2022 · Q25(a)(ii) · 3 marks · Hard

Explain how the properties of water relate to its transport role in a stem.

Question: explain how the properties of water are related to the transport role of water in a stem. 3 marks.
Show the mark scheme Mark scheme: cohesion holds water molecules together; so a chain of water molecules is pulled up the xylem; adhesion lets water molecules stick to the xylem walls; capillary action; water is a solvent for ions in the xylem or sucrose in the phloem.

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From the examiners' report: Students knew cohesion and adhesion but used them loosely. Say water molecules stick to each other and to the xylem walls, and link each property to transport, not to heat capacity.

Question 7

AS Paper 1 · June 2018 · Q22(c)(i) · 3 marks · Medium

Name three functions of triglycerides other than making phospholipids.

Question: living organisms use triglycerides to produce phospholipids; name three other functions of triglycerides. 3 marks.
Show the mark scheme Mark scheme: respiratory substrate; energy storage; thermal insulation; electrical insulation; buoyancy; physical protection, e.g. around the kidneys. "Energy for respiration" not allowed.

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From the examiners' report: Most scored one or two. One-word answers rarely scored: say thermal or electrical insulation, and energy storage rather than just energy.

Question 8

AS Paper 1 · June 2018 · Q22(c)(ii) · 2 marks · Medium

Describe and explain the pattern in the melting points of three methyl esters.

Table of three methyl esters from sunflower oil: methyl stearate C₁₉H₃₈O₂ melting at 39.1 °C, methyl oleate C₁₉H₃₆O₂ at −19.9 °C and methyl linoleate C₁₉H₃₄O₂ at −35.0 °C. Describe and explain the pattern. 2 marks.
Show the mark scheme Mark scheme: fewer hydrogens, more double bonds or less saturated gives a lower melting point; more kinked chains; so the molecules pack together less uniformly.

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From the examiners' report: Spot the trend first: fewer hydrogens, lower melting point. Then explain it with double bonds that kink the chains so they pack loosely. Hydrogen bonds have nothing to do with it.

Question 9

AS Paper 2 · June 2022 · Q3(b)(i) · 1 mark · Medium

Describe how a polyunsaturated triglyceride differs from a saturated one.

Structural formula (Fig. 3.2) of a saturated triglyceride with three fatty acid chains; describe how the structure of a polyunsaturated triglyceride would differ. 1 mark.
Show the mark scheme Mark scheme: more than one C=C double bond, or more than one kink, or fewer H atoms.

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From the examiners' report: Most knew saturated versus unsaturated, but polyunsaturated means more than one C=C double bond. One double bond or one kink didn't score.

Question 10

A level Paper 2 · June 2019 · Q21(c) · 2 marks · Medium

Use triglycerides to explain what hydrolysis means.

Question: triglycerides can be broken down by hydrolysis; using their structure as an example, explain what is meant by hydrolysis. 2 marks.
Show the mark scheme Mark scheme: uses water; to break three ester bonds; lysis means splitting, so the fatty acids are split from glycerol. Points from an annotated diagram credited.

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From the examiners' report: Most got the water mark, but few said three ester bonds are broken. Name the bond correctly (not glycosidic or phosphodiester) and label any diagram you draw.

Question 11

A level Paper 2 · November 2021 · Q18(a) · 3 marks · Medium

Explain how the structure of phospholipids lets them form plasma membranes.

Question: cells are surrounded by a plasma membrane containing phospholipids; explain how the structure of phospholipid molecules allows the formation of plasma membranes. 3 marks.
Show the mark scheme Mark scheme: the phosphate head is hydrophilic; the two fatty acid tails are hydrophobic; heads face the water and tails face each other, away from water, so a bilayer forms.

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From the examiners' report: The command word is explain. Don't just describe the membrane; say which way the heads and tails face relative to the water inside and outside the cell.

Question 12

AS Paper 1 · June 2022 · Q21(a) · 3 marks · Hard

Complete the structure of cysteine from its R group.

Question showing the amino acid sequence of the two chains of human insulin (Fig. 21.1) and the R group of cysteine (CH₂SH, Fig. 21.2); complete Fig. 21.2 to show the whole amino acid. 3 marks.
Show the mark scheme Mark scheme: R group attached to a central carbon; an amine group on one side of the central carbon; a carboxyl group on the other.

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From the examiners' report: Many drew a general amino acid to the side and scored nothing. Complete the figure you are given: add a central carbon to the R group, then the amine and carboxyl groups.

Question 13

AS Paper 1 · June 2024 · Q24(a)(i) · 3 marks · Medium

Match statements about haemoglobin to levels of protein structure.

Question: haemoglobin is a conjugated protein with quaternary structure. Table of four statements (disulfide bonds in an α-globin chain; two α and two β chains; each chain folded into a spherical shape; 147 amino acids in a specific sequence) to match to a level of structure. 3 marks.
Show the mark scheme Mark scheme: tertiary; quaternary; tertiary; primary. Four correct for 3 marks, three for 2, one or two for 1.

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From the examiners' report: Write a level of structure in each box; you can use a level more than once, and you don't have to use them all. Disulfide bonds and folding are tertiary.

Question 14

A level Paper 2 · June 2022 · Q16(a)(iii) · 1 mark · Medium

State a feature of a haemoglobin model that shows it is globular.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Model of haemoglobin (Fig. 16.1) with one part labelled X; state one feature visible in the figure that shows haemoglobin is a globular protein. 1 mark.
Show the mark scheme Mark scheme: spherical (round, ball-shaped) shape; no fibres; contains prosthetic groups. "Coiled" and "compact" ignored.

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From the examiners' report: Over half scored, usually by describing the round shape. "Coiled" doesn't count, because fibrous proteins are coiled too.

Question 15

AS Paper 2 · June 2023 · Q5(b)(ii) · 2 marks · Medium

State the properties of collagen that suit it to ligaments.

Question: collagen is a protein found in ligaments, which attach bone to bone and stabilise joints; state the properties of collagen that make it suitable. 2 marks.
Show the mark scheme Mark scheme: any two of fibrous; insoluble; high tensile strength; resistant to stretching.

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From the examiners' report: Say what kind of strength: high tensile strength, so it resists stretching. Collagen isn't elastic; that's elastin.

Question 16

A level Paper 2 · June 2022 · Q21(c)(iii) · 2 marks · Medium

Explain why a fungus grown in a fermenter is supplied with ammonia.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question about a fermenter growing a fungus; the air inlet supplies oxygen for respiration and ammonia. Suggest and explain why the fungus is provided with ammonia. 2 marks.
Show the mark scheme Mark scheme: a source of nitrogen (amine groups); to make amino acids, polypeptides or proteins (nucleic acids allowed).

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From the examiners' report: Only those who saw ammonia as a nitrogen source scored, but they usually got both marks. Link the nitrogen to making amino acids and proteins, not to the nitrogen cycle or respiration.

Practical and maths

Question 17

AS Paper 2 · June 2023 · Q3(a)(i) · 1 mark · Medium

Correct a student's test for protein.

Question: a student proposes testing an unknown liquid for protein by adding an equal volume of sodium hydroxide solution, mixing and leaving for 5 minutes. State the change needed to detect protein. 1 mark.
Show the mark scheme Mark scheme: add biuret reagent (or copper sulfate solution) instead of just sodium hydroxide.

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From the examiners' report: Well answered, but spell biuret correctly so it can't be mistaken for burette, and don't use Benedict's reagent.

Question 18

AS Paper 2 · June 2023 · Q3(a)(ii) · 2 marks · Hard

Give the symbol of the anion in sodium hydroxide and one biological use.

Table to complete: state the chemical symbol of the anion in sodium hydroxide, and one use of this anion in a biological process. 2 marks.
Show the mark scheme Mark scheme: OH⁻; one use: regulating pH of body fluids, neutralising stomach acid, catalysing reactions or determining protein shape.

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From the examiners' report: Few named an anion. Write the symbol OH⁻, then a role linked to pH; it isn't the same as hydrolysis, which uses water.

Question 19

A level Paper 3 · June 2017 · Q1(a)(ii) · 1 mark · Medium

Conclude what a chromatogram shows about leucine and amino acid X.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Thin layer chromatogram (Fig. 1.2) of amino acids with spots for leucine, X and Y between the origin and solvent front, where leucine and X are close together. What can you conclude about the chemical properties of leucine and X? 1 mark.
Show the mark scheme Mark scheme: similar solubility in the solvent or interaction with the stationary phase; or similar R groups.

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From the examiners' report: "Similar chemical properties" or "similar Rf values" didn't score. Say they have similar solubility in the solvent, or similar R groups.

Question 20

A level Paper 3 · June 2017 · Q1(a)(iii) · 3 marks · Hard

Plot where amino acid Z would be, from its Rf value.

Question: amino acid Z has an Rf value 0.20 lower than amino acid Y; place a dot on the chromatogram (Fig. 1.2) to show how far Z moved, showing working. 3 marks.
Show the mark scheme Mark scheme: Rf of Y = 2.5 ÷ 5 = 0.5; Rf of Z = 0.5 − 0.2 = 0.3; distance = 0.3 × 5 = 1.5 cm, so a dot 1.4–1.6 cm from the origin.

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From the examiners' report: Measure Y and the solvent front carefully, subtract 0.2 from Y's Rf, then multiply by the solvent front distance to get the position. Don't just multiply 0.2 by 5.

Where students lose marks (from OCR examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • Call the COOH end carboxyl: Some students named the carboxyl group a hydroxyl group because of the OH in it.

    ✔ Gains credit: "amine; carboxyl" (carboxylic acid allowed).

  • Water molecules stick, to each other and to xylem: Answers mentioned cohesion and adhesion loosely, without saying water molecules, or wandered into heat capacity.

    ✔ Gains credit: "cohesion, holds water molecules together … adhesion allows water molecules to stick to xylem vessel walls".

  • One triglyceride, three ester bonds: Most students said water was used but very few said that three ester bonds are broken; some called them glycosidic.

    ✔ Gains credit: "uses water (to) break 3 ester bonds … fatty acids are split from glycerol".

  • Polyunsaturated means more than one C=C: Many students described a single double bond or a single kink.

    ✔ Gains credit: "more than one, kink / bend" or "fewer H atoms".

  • Kinked chains, not hydrogen bonds, lower the melting point: Many thought more hydrogen atoms meant more hydrogen bonds between the molecules.

    ✔ Gains credit: "more double bonds … more kinked chain … molecules less uniformly packed together".

  • Say thermal or electrical insulation: Single-word answers such as "insulation" or "energy" seldom scored.

    ✔ Gains credit: "energy storage; thermal insulation; electrical insulation; buoyancy".

  • Explain which way the heads face: Many described membrane structure instead of explaining why phospholipids form a bilayer.

    ✔ Gains credit: "phosphate head is hydrophilic … tails are hydrophobic … heads orientate towards water".

  • Build the amino acid onto the given R group: Many drew a general amino acid beside Fig. 21.2 instead of completing the cysteine side chain shown.

    ✔ Gains credit: "R group attached to central carbon; amine group … carboxyl group on central carbon".

Download the questions

Download the PDF pack (20 questions, mark schemes at the end)

Keep practising

Questions, mark schemes and examiners' report extracts © OCR.

Tom Whitburn
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