PAG 6 Chromatography and Electrophoresis: OCR A Level Biology Exam Questions by Topic

OCRPAGPractical SkillsExam Questions
Written by Tom Whitburn

OCR A PAG 6 · 18 questions · mark schemes and examiners’ tips included

PAG 6 covers two ways of separating biological molecules. In chromatography, a solvent carries substances such as amino acids, sugars or photosynthetic pigments through paper or a thin layer of silica, and you identify them from their Rf values. In gel electrophoresis, an electric current moves charged DNA fragments or proteins through agarose gel, and the fragments separate by size.

Most of the marks here are for practical details: keeping the origin line above the solvent, letting a spot dry before adding more, measuring to the middle of a spot, and giving an Rf value to a sensible number of figures. For electrophoresis, the examiners want you to explain each step, such as why you add a buffer or a dye, not just name it.

Work through the eighteen questions below in order. They start with running a chromatogram, then Rf calculations and interpreting chromatograms, and finish with electrophoresis and DNA profiling. 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 these questions cover (OCR PAG 6: chromatography or electrophoresis)

PAG 6 is one of the twelve practical activity groups in the OCR A Level Biology A specification (H420). The questions test these parts of the specification:

  • 1.2.2(g) separation of biological compounds using thin layer/paper chromatography or electrophoresis
  • 2.1.2(s)(i) the principles and uses of paper and thin layer chromatography to separate biological molecules / compounds (to include calculating Rf values)
  • 2.1.2(s)(ii) practical investigations to analyse biological solutions using paper or thin layer chromatography
  • 5.2.1(c)(ii) practical investigations using thin layer chromatography (TLC) to separate photosynthetic pigments
  • 6.1.3(c) the principles of DNA profiling and its uses (including how electrophoresis is used in a DNA profile)
  • 6.1.3(e) the principles and uses of electrophoresis for separating nucleic acid fragments or proteins

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.2 minutes per mark: the 18 questions are worth 40 marks, so about 48 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.

Running a chromatogram

Question 1

A level Paper 3 · June 2023 · Q2(a)(i) · 1 mark · Easy

Suggest one way to see colourless vitamins on a TLC plate.

Question 2(a)(i): thin layer chromatography was used to separate a mixture of vitamins from a supplement tablet; most vitamins are colourless; suggest one method for visualising them.
Show the mark scheme Mark scheme: a stain or dye (any named stain), or exposure to UV light.

Tap an image to open it full size.

From the examiners' report: Most students got this. Mistakes were 'a solvent', or attaching a chemical or a tagged antibody to the vitamins before running the TLC, which would change their Rf values.

Question 2

A level Paper 3 · June 2017 · Q1(b)(i) · 1 mark · Medium

State a material that can be used as the stationary phase in TLC.

Part (b)(i): thin layer chromatography can also separate photosynthetic pigments; state a material that can be used as the stationary phase.
Show the mark scheme Mark scheme: silica gel; aluminium oxide, cellulose or silicon dioxide are also accepted, but not paper, silicon or aluminate.

Tap an image to open it full size.

From the examiners' report: Only a small number of students could name one. Many wrongly said paper, which is used in paper chromatography, not TLC, or named a solvent.

Question 3

AS Paper 2 · June 2025 · Q1(c) · 2 marks · Medium

Outline the role of the solvent in paper chromatography.

Part (c) asking for an outline of the role of the solvent in paper chromatography of carbohydrates.
Show the mark scheme Mark scheme: the solvent is the mobile phase that dissolves the carbohydrates and carries them through the stationary phase; they travel different distances depending on their solubility in it.

Tap an image to open it full size.

From the examiners' report: Many students knew the solvent is the mobile phase and dissolves the solute. Weaker answers talked vaguely about separating colours or dyes, or said the solvent 'pushes' or 'drags' the solute along.

Question 4

AS Paper 2 · June 2018 · Q1(c)(i) · 4 marks · Hard

Describe four ways to refine a TLC method for amino acids, with a reason for each.

Fig. 1.4: a TLC plate standing in a beaker with the origin line 1 cm up and solvent about 0.9 cm deep, spots including amino acid X run towards the solvent front; the students held the plate in the middle with their hands.
Show the mark scheme Mark scheme: any four changes with reasons, such as raising the origin above the solvent so spots don't dissolve into it, spacing spots further apart so they don't merge, holding the edges or wearing gloves to avoid contamination, covering the beaker to stop evaporation, and using ninhydrin to see the amino acids.

Tap an image to open it full size.

From the examiners' report: Most students made two sensible suggestions, but very few made four. Every point needs a reason as well as a change, so write each one as 'do this, because of that'.

Calculating Rf values

Question 5

AS Paper 2 · June 2018 · Q1(c)(ii) · 2 marks · Easy

Calculate the Rf value of amino acid X and use it to identify X.

Table 1 of Rf values: alanine 0.31, cysteine 0.40, glutamine 0.13 and phenylalanine 0.59; identify amino acid X in Fig. 1.4 by calculating its Rf value and showing working.
Show the mark scheme Mark scheme: an Rf value between 0.10 and 0.15 with the working shown (for example 0.65 ÷ 4.95 = 0.13), and glutamine.

Tap an image to open it full size.

From the examiners' report: Most students chose glutamine, but not all showed an Rf calculated from the figure to back it up. Some measured so carelessly that their value fell outside the generous range allowed.

Question 6

A level Paper 1 · June 2025 · Q16(b)(i) · 3 marks · Easy

Measure a TLC plate and calculate the Rf of the most soluble pigment, to 2 significant figures.

A TLC plate of chloroplast pigments drawn sideways, with the origin line and spot X at the left, four separated pigment spots, and the solvent front line at the right; answer lines for distance travelled in mm and the Rf.
Show the mark scheme Mark scheme: the most soluble pigment travelled 91 mm (87 to 95 allowed); Rf = 91 ÷ 109 = 0.83, with a table of allowed measurement combinations.

Tap an image to open it full size.

From the examiners' report: Most students picked the pigment nearest the solvent front. Measure to the middle of the spot, and to the solvent front line rather than the end of the plate. Many measured in cm even though the answer line said mm.

Question 7

AS Paper 1 · June 2023 · Q23(b)(i) · 2 marks · Medium

Calculate the Rf value of valine from a TLC chromatogram.

A TLC chromatogram with the origin near the bottom, a dashed solvent front near the top, and spots labelled, from the top, leucine, valine, serine and aspartic acid.
Show the mark scheme Mark scheme: 3.6 ÷ 6 = 0.6 (0.58 to 0.62 allowed); more than two decimal places limits it to 1 mark.

Tap an image to open it full size.

From the examiners' report: This was very well answered. A few students lost a mark by not rounding to a sensible number of decimal places, and some worked out the Rf for leucine instead of valine.

Question 8

A level Paper 3 · June 2023 · Q2(a)(ii) · 2 marks · Hard

Calculate the Rf value of vitamin B2 from a chromatogram.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Fig. 2.1: a chromatogram with spots for vitamins B6, B1, B2 and C between the origin and the dashed solvent front.
Show the mark scheme Mark scheme: 2.4 ÷ 8 = 0.3 (0.29 to 0.31 allowed); more than 2 significant figures limits it to 1 mark.

Tap an image to open it full size.

From the examiners' report: Most students got both marks. Those who scored nothing usually divided the wrong way round (8 ÷ 2.4) or measured the wrong vitamin. Some lost a mark by giving more than 2 significant figures.

Interpreting and improving chromatograms

Question 9

AS Paper 1 · June 2023 · Q23(b)(ii) · 1 mark · Medium

Suggest what the chromatogram shows about the R groups of serine and leucine.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Part (b)(ii): serine and leucine have different R groups; with reference to the chromatogram, suggest what can be concluded about the chemical properties of their R groups.
Show the mark scheme Mark scheme: the R groups have different polarity, charges or functional groups (serine more polar); answers about size or solubility are ignored.

Tap an image to open it full size.

From the examiners' report: Only a few students scored. Many just described differences in size or solubility. TLC of amino acids uses an organic solvent, so how far each one travels reflects the polarity of its R group.

Question 10

A level Paper 1 · November 2020 · Q17(a)(iii) · 2 marks · Medium

Identify the spot for the least polar pigment when the solvent is non-polar, with a reason.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Part (a)(iii): the solvent used was non-polar; identify the spot corresponding to the least polar pigment and give a reason.
Show the mark scheme Mark scheme: spot 5, because it is the most soluble in the mobile phase (or least attracted to the stationary phase).

Tap an image to open it full size.

From the examiners' report: Use the mark scheme wording: the least polar pigment is the most soluble in the non-polar solvent, so it travels furthest, to the spot nearest the solvent front.

Question 11

A level Paper 1 · June 2025 · Q16(b)(ii) · 2 marks · Medium

Describe how to make faint pigment spots more intense using the same extract.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Part (b)(ii): the spots were very faint; describe how the student could increase their intensity using the same chloroplast extract.
Show the mark scheme Mark scheme: let the spot at the origin dry, then add more extract to the same spot; or evaporate some solvent from the extract.

Tap an image to open it full size.

From the examiners' report: Answers often didn't say where the extra extract goes. Simply adding a bigger volume spreads the spot out, so let it dry between additions. A stronger solvent or a stain doesn't make these spots more intense.

Question 12

A level Paper 1 · June 2025 · Q16(b)(iv) · 1 mark · Medium

Suggest why the pigments couldn't be identified from data-book Rf values.

Part (b)(iv): the student could not identify any of the pigments using Rf values from a data book; suggest an explanation.
Show the mark scheme Mark scheme: the data book used a different solvent, TLC plate or temperature, or the extract came from a different plant.

Tap an image to open it full size.

From the examiners' report: This part sorted students well. Stronger answers said that Rf values depend on the conditions, such as a different solvent, temperature or plant species.

Question 13

A level Paper 1 · June 2025 · Q16(b)(v) · 2 marks · Medium

Describe how TLC could identify the pigments without calculating Rf values.

Part (b)(v) asking how the student could use TLC to identify the pigments in the mixture without calculating Rf values.
Show the mark scheme Mark scheme: run known pigments on the same plate with the same solvent, then compare their positions, or compare colours with known pigments or a colour chart.

Tap an image to open it full size.

From the examiners' report: Many students said they would compare the colours but didn't say what with. Run known pigments alongside on the same plate and compare positions or colours.

Electrophoresis and DNA profiling

Question 14

A level Paper 3 · June 2017 · Q1(c) · 3 marks · Medium

Describe three differences between TLC and the electrophoresis used to sequence DNA.

Part (c): when sequencing DNA, fragments are separated by electrophoresis; describe three differences between thin layer chromatography and this form of electrophoresis.
Show the mark scheme Mark scheme: TLC separates by solubility or interaction with the stationary phase, electrophoresis by size; electrophoresis only separates charged molecules; it uses electricity and a buffer; TLC uses dyes and electrophoresis uses fluorescent or radioactive tags; sequencing is automated.

Tap an image to open it full size.

From the examiners' report: Most students knew this well, especially that electrophoresis uses an electric current and separates DNA by size or length, while TLC separates by solubility. Tags to see the fragments and the buffer were also credited.

Question 15

A level Paper 2 · November 2021 · Q16(b)(i) · 4 marks · Medium

Outline the steps that follow DNA extraction in producing a DNA profile.

Part (b)(i): DNA profiling uses variation in DNA; DNA has been extracted from a tissue sample; outline the subsequent steps in producing a DNA profile.
Show the mark scheme Mark scheme: amplify with PCR, cut with restriction enzymes, separate by electrophoresis, transfer to a membrane (Southern blot), add a radioactive or fluorescent probe, and view with X-rays or UV light.

Tap an image to open it full size.

From the examiners' report: Many students wrote a long description when the command word was 'outline'. Name the probe or tag: 'use radioactivity' on its own isn't enough.

Question 16

A level Paper 3 · June 2018 · Q4(c)(v) · 2 marks · Medium

Describe and explain two changes to a student's electrophoresis procedure.

Part (c)(v): a student plans to load DNA into wells at the cathode, pass a voltage through agarose gel for one minute, then place the gel in purified water to see the bands; describe two changes and explain how they would improve electrophoresis.
Show the mark scheme Mark scheme: use a buffer solution, which carries the current; Southern blotting onto a membrane; add probes or dyes to see the bands; or run it for longer so the fragments separate.

Tap an image to open it full size.

From the examiners' report: Many students named a buffer, a longer run or a stain but didn't explain it. A longer run lets the fragments separate, not just move further, and the buffer lets the current flow. Many also mixed up the anode and the cathode.

Question 17

A level Paper 3 · June 2025 · Q4(d)(i) · 4 marks · Medium

State two other solutions added to an electrophoresis box and explain the purpose of each.

Diagram of an electrophoresis box: agarose gel with a row of wells next to the negative electrode, three of them loaded with DNA samples, and the positive electrode at the other end.
Show the mark scheme Mark scheme: buffer, to maintain the pH or carry the current; a reference DNA ladder, to compare fragment sizes; a dye or fluorescent marker, to see the bands. An explanation only scores with its matching solution.

Tap an image to open it full size.

From the examiners' report: Few students got all four marks. A buffer or a dye were the most common correct answers; fewer remembered a DNA ladder of known fragment sizes to compare the samples with.

Question 18

A level Paper 3 · June 2025 · Q4(d)(ii) · 2 marks · Medium

Use a gel of HBB gene fragments to identify homozygous and heterozygous genotypes.

Electrophoresis gel with five lanes below the loading wells: lanes 2 and 5 have one larger band, lanes 1 and 4 have two smaller bands, and lane 3 has all three bands.
Show the mark scheme Mark scheme: homozygous sickle cell 2 and 5; homozygous normal 1 and 4; heterozygous 3.

Tap an image to open it full size.

From the examiners' report: Students did very well on this despite the unfamiliar context. The sickle cell allele gives one larger fragment and the normal allele two smaller ones, so a heterozygote shows all three bands.

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

Each point below is followed by wording that gained marks.

  • Silica, not paper: Asked for the stationary phase in thin layer chromatography, many students said paper or named a solvent.

    ✔ Gains credit: "Silica (gel)"; aluminium oxide, cellulose or silicon dioxide are also accepted.

  • The solvent dissolves and carries: Vague answers about 'separating colours', or the solvent 'pushing' or 'dragging' the solute, didn't explain its role.

    ✔ Gains credit: "(Solvent) dissolves the carbohydrates / acts as mobile phase; carries (dissolved) carbohydrates through stationary phase".

  • Every change needs a reason: Most students suggested changes to a chromatography method without saying why each one would improve the results.

    ✔ Gains credit: "Put pencil line / origin higher (than the solvent) to stop spots dissolving into / mixing with the solvent".

  • Measure to the middle of the spot: Students measured to the end of the plate instead of the solvent front, and gave distances in cm when the answer line asked for mm.

    ✔ Gains credit: "Distance travelled by most soluble = 91 (mm); Rf = 91 ÷ 109 = 0.83".

  • Spot over solvent, to 2 s.f.: Some students divided the wrong way round, and answers with more than 2 significant figures lost a mark.

    ✔ Gains credit: "2.4/8 = 0.3" (0.29 to 0.31 allowed).

  • Dry between spots: To make faint spots stronger, students suggested a larger volume, a stronger solvent or a stain, none of which was credited.

    ✔ Gains credit: "Allow (existing) spot to dry; (once dry) add another spot / more extract to the origin".

  • Polarity, not size: To explain why two amino acids travelled different distances, many described differences in size or solubility of the R groups.

    ✔ Gains credit: "Different polarity / charges / functional groups" (serine more polar).

  • Say why the buffer is there: Students named a buffer, a longer run or a stain for electrophoresis but didn't explain it, and many said the buffer just keeps the pH constant.

    ✔ Gains credit: "Use buffer (solution) AND solution carries charge / current (to separate fragments)"; "dye / stain to see bands / DNA fragments".

Download the questions

Download the PDF pack (18 questions, mark schemes at the end)

Keep practising

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

Tom Whitburn
Previous

PAG 7 Microbiological Techniques: OCR A Level Biology Exam Questions by Topic
Next

PAG 5 Colorimeter and Potometer: OCR A Level Biology Exam Questions by Topic