OCR A Level Biology A: Practical Endorsement skills, apparatus and techniques (H420) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sections 1.2.1 and 1.2.2 of OCR A Level Biology A (H420), specification Version 4.1 (April 2026), for first assessment in 2025. Read the study guide first, or use the revision notes. Course links: OCR A Level Biology hub, printable checklist and free diagnostics.

Questions

1. A student heats tubes of sugar solution with Benedict’s solution in a water bath at 80 °C. Complete a risk assessment line by stating one hazard, the risk it creates and a control measure. [3]

2. A colorimeter was used with protein standards of 0, 20, 40, 60 and 80 g dm⁻³. The absorbances were 0.00, 0.18, 0.36, 0.54 and 0.72.

(a) A sample gave an absorbance of 0.45. Calculate its concentration. [2] (b) State why the meter is zeroed with a blank before the readings. [1] (c) A second sample reads 0.95. Explain what should be done. [2]

3. A student has a 100 g dm⁻³ stock and makes a doubling dilution series.

(a) State the volumes of previous solution and water used to make 10 cm³ at each step. [2] (b) Give the concentration of the fourth dilution made. [1] (c) State one action that makes the series accurate. [1]

4. A bacterial culture is diluted to 10⁻⁵. A sample of 0.2 cm³ is spread on agar and 40 colonies grow.

(a) Calculate the number of bacteria per cm³ in the original culture. [3] (b) Suggest why this may differ from the true number of cells. [1]

5. A potometer capillary has a radius of 0.40 mm. A bubble moves 36 mm in 3.0 minutes.

(a) Calculate the volume of water taken up. [2] (b) Calculate the rate in mm³ per minute. [1] (c) The shoot has 25 cm² of leaf. Calculate the rate per cm². [1] (d) State two actions in setting up that make the results valid. [2]

6. At ×40, 0.10 mm on a stage micrometer spans 40 eyepiece graticule divisions.

(a) Calculate the length of one division in µm. [1] (b) A cell is 18 divisions wide. Calculate its width in µm. [1] (c) A drawing of the cell is 90 mm wide. Calculate its magnification. [2] (d) Explain why the graticule needs recalibrating for another objective. [1]

7. Two unknown samples gave these results.

Test	P	Q
Benedict’s	stays blue	orange
Benedict’s after boiling with acid and neutralising	brick red	brick red
Iodine in potassium iodide	yellow-brown	blue-black
Biuret	blue	blue

(a) State what each sample contains. [4] (b) Explain why the acid-treated sample is neutralised before Benedict’s. [1] (c) Suggest how the Benedict’s test could give more than a yes or no result. [1]

8. A chromatogram has a solvent front at 7.2 cm. The mixture gives spots at 3.6 cm and 5.4 cm. Known amino acids J, K and L have Rf values of 0.50, 0.62 and 0.75.

(a) Calculate the Rf values. [2] (b) State which amino acids are in the mixture. [1] (c) Explain why the baseline is drawn in pencil. [1] (d) Explain why ninhydrin is used. [1]

9. Two antibiotic discs gave clear zones of radius 11 mm and 7 mm on a bacterial lawn.

(a) Calculate the area of each zone. [2] (b) State why a disc without antibiotic is included. [1] (c) Describe two aseptic steps when spreading the bacteria. [2]

10. Eight 1 m² quadrats, placed randomly, gave counts of 12, 9, 15, 7, 11, 10, 14 and 6 plants. The area is 450 m².

(a) Calculate the mean number per quadrat. [1] (b) Estimate the population. [2] (c) Describe how to place the quadrats at random. [2] (d) Explain when a belt transect would be better. [2] (e) In a beetle study, 28 were marked, a second catch of 24 had 6 marked. Estimate the population. [2]

11. An oxygen sensor on a data logger shows a rise from 4.5 to 9.0 mg dm⁻³ over 15 minutes.

(a) Calculate the mean rate. [1] (b) Give two advantages of a data logger over a student with a stopwatch. [2] (c) Explain a limitation of using a computer model of a molecule. [1]

12. A class measures pulse before and after exercise. The mean was 76 beats per minute before and 120 after.

(a) Describe three measures that make the use of volunteers ethical. [3] (b) Calculate the percentage change. [2] (c) State what to include when citing a website used for background. [2] (d) State one hazard of the exercise and a control. [1]

Answers

1. Hazard: hot water or steam [1]. Risk: scalds or burns if spilt or knocked [1]. Control: stand the bath on a tray away from the edge and use tongs or a holder [1]. [3] Examiner insight: Answers gain credit when hazard, risk and control are separate statements; “be careful” is too vague.

2. (a) Gradient = 0.72 / 80 = 0.009 per g dm⁻³ [1]. Concentration = 0.45 / 0.009 = 50 g dm⁻³ [1]. (b) To remove the absorbance of the solvent and cuvette so readings are from the sample only [1]. (c) The reading is above the highest standard [1]. Dilute the sample with a known factor, re-read, then multiply back [1]. Examiner insight: Show how the value was read from the line; extrapolating beyond the standards is usually treated as unreliable.

3. (a) 5 cm³ previous solution [1] plus 5 cm³ water [1]. (b) 100 → 50 → 25 → 12.5 → 6.25 g dm⁻³ [1]. (c) Mix each tube thoroughly before transferring, using a clean pipette or measuring syringe [1]. Examiner insight: Give both volumes; stating only “dilute it” does not show the dilution factor.

4. (a) 40 / 0.2 = 200 per cm³ of dilution [1]. Divide by 10⁻⁵ [1]. 2.0 × 10⁷ per cm³ [1]. (b) A colony may come from a clump of cells, and only living cells form colonies [1]. Examiner insight: Show the dilution step and the division by the volume spread so that each stage of the method can be seen.

5. (a) Volume = π × 0.40² × 36 [1] = 18 mm³ [1]. (b) 18.1 / 3.0 = 6.0 mm³ min⁻¹ [1]. (c) 6.0 / 25 = 0.24 mm³ min⁻¹ cm⁻² [1]. (d) Any two: cut the shoot under water at an angle, or make airtight joints assembled under water [1]; dry the leaves, or let the bubble settle before timing [1]. Examiner insight: Keep the unrounded volume in the next step; units such as mm³ per minute should appear on each answer.

6. (a) 100 µm / 40 = 2.5 µm [1]. (b) 18 × 2.5 = 45 µm [1]. (c) 45 µm = 0.045 mm [1]. 90 / 0.045 = ×2000 [1]. (d) Changing the objective changes the apparent size of each division [1]. Examiner insight: Convert units before dividing; mixing mm and µm is a common reason for answers out by a factor of 1000.

7. (a) P: non-reducing sugar present [1]; no reducing sugar, starch or protein [1]. Q: reducing sugar present [1]; starch present and no protein [1]. (b) Benedict’s requires alkaline conditions, which the acid would prevent [1]. (c) Compare colour with standards, or filter the precipitate and measure with a colorimeter [1]. Examiner insight: Link each result to its conclusion; listing colours without naming the molecule does not answer “state what it contains”.

8. (a) 3.6 / 7.2 = 0.50 [1] and 5.4 / 7.2 = 0.75 [1]. (b) J and L [1]. (c) Pencil is insoluble in the solvent, so it does not run [1]. (d) Amino acids are colourless; ninhydrin reacts to make them visible [1]. Examiner insight: Rf has no unit because it is a ratio; compare with standards run on the same sheet.

9. (a) π × 11² = 380 mm² [1]; π × 7² = 154 mm² [1]. (b) To show that the zone is caused by the antibiotic and not the disc or the medium [1]. (c) Any two: flame the loop or spreader and let it cool [1]; open the lid only slightly, or flame the bottle neck [1]. Examiner insight: Name what is done, such as “flame the loop”; “keep everything sterile” repeats the question.

10. (a) 84 / 8 = 10.5 [1]. (b) 10.5 × 450 [1] = 4725 (about 4700) [1]. (c) Use a grid with random number tables or a generator for coordinates [1]; place a quadrat at each pair [1]. (d) When conditions change gradually, such as with distance from a shore [1]; quadrats at set intervals show the trend [1]. (e) N = 28 × 24 / 6 [1] = 112 [1]. Examiner insight: Say how randomness is achieved; “throw the quadrat” is not usually given credit as random.

11. (a) (9.0 − 4.5) / 15 = 0.30 mg dm⁻³ min⁻¹ [1]. (b) Frequent readings [1]; no human timing error or continuous record [1]. (c) The model simplifies, so it only reflects its assumptions [1]. Examiner insight: An advantage gains credit when it is compared with the manual method, not just named.

12. (a) Informed consent [1]; right to stop [1]; excluding anyone for whom exercise is unsafe, with anonymous data [1]. (b) (120 − 76) / 76 × 100 [1] = +57.9 % [1]. (c) Author, year, page title, website name and date accessed [1] [1]. (d) Hazard: tripping or overexertion; control: clear space and stop if unwell [1]. Examiner insight: A percentage change needs the sign or the word increase, and divides by the starting value.

Where marks are usually lost

• Stating only a hazard in a risk assessment.
• Not converting micrometres to millimetres in magnification.
• Giving Rf with units, or measuring to the spot edge.
• Omitting the dilution factor from plate counts.
• Using vague words such as “careful”, “clean” or “fair” in method answers.
• Extrapolating a calibration line.
• Describing random sampling without saying how coordinates are chosen.
• Dividing a percentage change by the final value.

Next steps

• Revision notes
• Study guide
• OCR A Level Biology hub and checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420), specification Version 4.1 (April 2026), for first assessment in 2025, published by OCR. Sections 1.2.1 and 1.2.2 are the source for these questions.