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

These notes condense sections 1.2.1 and 1.2.2 of OCR A Level Biology A (H420, specification Version 4.1, April 2026, first assessment in 2025). For the full explanations, use the study guide, then test yourself with the practice questions. Related pages: practical skills revision notes, the OCR A Level Biology hub, the printable checklist and the free diagnostics.

The Practical Endorsement in brief

• A minimum of 12 assessed practical activities, covering PAG1 to PAG12.
• Teacher assessment against the Common Practical Assessment Criteria; reported separately from the A level grade.
• Five competencies: follows written procedures; applies investigative approaches when using instruments and equipment; uses equipment and materials safely; makes and records observations; researches, references and reports.
• You keep your own record and show independent competence, even when you work in a group.

1.2.1 Skills at a glance

Ref	Skill	Evidence
(a)	Investigative approaches, problem solving	pilot trial, controls, adjusted method
(b)	Safe use of equipment	hazard, risk, control measure
(c)	Follow written instructions	method read first, steps in order
(d), (e)	Observations, measurements, records	dated table, units in headings
(f)	Present data scientifically	labelled axes, sensible scales
(g)	Software to process data, research, report	spreadsheet or graph software
(h), (i)	Research and cite sources	reference list, in-text citations
(j)	Wide range of instruments	the 1.2.2 techniques

1.2.2 Techniques: the key recall

Ref	Technique	Must remember
(a)	Mass, time, volume, temperature, length, pH	choose by resolution; zero first; calibrate pH meter
(b)	Colorimeter	blank, filter, calibration curve
(b)	Potometer	airtight, bubble, volume = πr² × distance
(c)	Serial dilutions	same factor each step
(d)	Light microscope	low power first, calibrate graticule
(e)	Drawings	pencil, no shading, labels not crossing
(f)	Qualitative reagents	Benedict’s, iodine, biuret, emulsion
(g)	Paper or thin layer chromatography, electrophoresis	Rf; charge and size
(h)	Organisms	consent, welfare, return
(i)	Aseptic technique	flame, minimal lid opening, upside down
(j)	Dissection	sharp, away from body, tile
(k)	Fieldwork	random or systematic
(l)	ICT	logger, modelling, spreadsheet

Formulas

Quantity	Formula
Magnification	image size / actual size
Volume in capillary	πr² × length
Rf	spot distance from baseline / solvent front distance from baseline
Percentage change	(final − initial) / initial × 100
Area of a circular zone	πr²
Stock needed	(C₂ × V₂) / C₁
Population by recapture	(M × n) / m
Rate from a logger	change in value / time

Rearrangements matter: actual size = image size / magnification. An image of 45 mm at ×1500 is 45 / 1500 = 0.030 mm = 30 µm.

Reagent results

Test	Positive result
Benedict’s (reducing sugar, hot water bath)	blue to green, yellow, orange, brick red
Hydrolyse with acid, neutralise, then Benedict’s	positive means non-reducing sugar present
Iodine in potassium iodide (starch)	blue-black
Biuret (protein)	purple
Ethanol then water (lipid)	white emulsion

Methods in steps

Calibrating a graticule

1. Place the stage micrometer on the microscope and focus using one objective.
2. Line up the two scales at one end.
3. Count graticule divisions that match a known stage length.
4. Divide: length per division = known length / divisions.
5. Repeat for each objective, because the value changes.

Reminder. 22 divisions at 2.5 µm each is 55 µm. Drawn at 66 mm (66 / 0.055), that is ×1200.

Using a serial dilution for counting bacteria

1. Transfer 1 cm³ into 9 cm³ of sterile diluent at each step.
2. Spread a measured volume of a chosen dilution on agar.
3. Incubate, then count only plates with a manageable number of colonies.
4. Divide by the volume spread, then divide by the dilution.

Reminder. 52 colonies from 0.1 cm³ of a 10⁻³ dilution: 52 / 0.1 = 520 per cm³, / 10⁻³ = 5.2 × 10⁵ per cm³.

Paper chromatography

1. Pencil baseline; small concentrated spots.
2. Solvent below the baseline; lid on.
3. Remove before the front reaches the top; mark the front.
4. Dry, apply the locating agent if needed.
5. Measure to the spot centre; calculate Rf. A spot at 4.2 cm with the front at 7.0 cm has Rf 0.60.

Aseptic inoculation

1. Disinfect bench; wash hands.
2. Flame loop; cool it in air.
3. Open lids just enough, near a flame.
4. Seal with two pieces of tape; incubate upside down.
5. Dispose of cultures by sterilising.

Potometer set-up

1. Cut the shoot under water at an angle.
2. Assemble under water; check for leaks.
3. Dry the leaves; introduce a bubble.
4. Allow to equilibrate; record start position.
5. Reset the bubble with the reservoir between runs.

Reminder. Radius 0.35 mm, bubble moves 30 mm in 4.0 min: volume = π × 0.35² × 30 = 11.5 mm³; rate = 2.9 mm³ per minute.

Colorimetry and ICT

• Zero with a blank of solvent. Choose a filter the sample absorbs strongly.
• Make standards by serial dilution, plot absorbance (y) against concentration (x), and read unknowns from the line. Do not extrapolate beyond the standards.
• A data logger gives frequent, continuous readings; set the sampling interval and the run time first. Check the sensor is calibrated.
• A computer model is only as good as its assumptions; say what it simplifies.
• Spreadsheet output still needs checking for sensible values and correct units.

Organisms, dissection and drawings

• Human volunteers. Informed consent, the right to stop, no strenuous activity where a health condition makes it unsafe, results kept anonymous.
• Animals. Handle briefly and gently, keep in suitable conditions, return where found, and wash hands.
• Dissection. Animal or plant organ: cut onto a tile and away from your body; identify, then measure; dispose of tissue and disinfect.
• Drawings. Title, magnification or scale bar, continuous lines, proportions as seen, only what is visible.
• Magnification from a scale bar: bar length drawn / length it represents. A 20 mm bar for 10 µm is 20 / 0.010 = ×2000.

Research and referencing

• Use more than one source and say why each is trustworthy: author, date, evidence, independent agreement.
• Cite in the text and list full details at the end: author, year, title, publisher or website and the date accessed.
• Do not copy text; put it in your own words or use quotation marks and a citation.

Sampling

Method	Use	Weakness
Random quadrats	unbiased abundance or cover in an area	misses patterns across a gradient
Line or belt transect	change along an environmental gradient	fewer independent samples
Capture–mark–recapture	mobile animals	needs assumptions

Mark–recapture assumptions: population closed, marks last and cause no harm, marked animals mix fully. With M = 30, n = 25 and m = 5, N = 30 × 25 / 5 = 150.

Must-know distinctions

• Label and annotation. A label names a structure; an annotation describes or explains it.
• Plan and detail drawing. A plan shows tissue regions; a detail drawing shows cells.
• Magnification and resolution. Magnification is size of image over actual size; resolution is how close two points can be and still look like two.
• Qualitative and quantitative. Colour judged by eye versus a colorimeter reading.
• Random and systematic sampling. Random removes bias; systematic follows a gradient.
• Potometer and transpiration. The potometer measures uptake; most of it is lost by transpiration.
• Ethics and safety. Safety protects the worker; ethics protects the organism and volunteers.

Quick self-test

1. Name the control measure for a scalding hazard from a hot water bath.
2. An antibiotic zone has a radius of 7 mm. What is its area (nearest mm²)?
3. A stock is 0.50 mol dm⁻³. How much stock is needed for 25 cm³ of 0.10 mol dm⁻³?
4. Why must a graticule be recalibrated for each objective?
5. Give the percentage change from a pulse of 72 to 90 beats per minute.
6. Which reagent turns blue-black with starch?
7. Why are chromatography baselines drawn in pencil?
8. Why does DNA move towards the positive electrode?
9. Why are plates incubated upside down?
10. Logger temperature goes from 20 to 35 °C in 10 minutes. State the mean rate.

Answers

1. Place the bath away from the bench edge on a tray, and let it cool before moving it.
2. Radius 7 mm, so π × 7² = 154 mm².
3. 0.10 × 25 / 0.50 = 5.0 cm³ of stock plus 20 cm³ of water.
4. The magnification changes the apparent size of one division.
5. (90 − 72) / 72 × 100 = +25 %.
6. Iodine in potassium iodide solution.
7. Pencil does not dissolve in the solvent, whereas ink would run.
8. Its phosphate groups are negatively charged.
9. Condensation collects in the lid, not on the agar surface.
10. 15 / 10 = 1.5 °C per minute.

Where marks are usually lost

• Writing a risk as a hazard (“hot water”) with no control measure.
• Forgetting to convert micrometres to millimetres before using the magnification formula.
• Shading or sketchy lines in drawings, and label lines that cross.
• Using the word “reliable” without saying what was repeated.
• Quoting Rf with units, or measuring to the spot edge.
• Saying “no colour change” proves a sugar is absent without an acid hydrolysis step.
• Describing aseptic technique as “keep it clean” instead of naming actions.
• Calling any random sample “systematic”, or the reverse.
• No stated reason when suggesting an improvement, such as a data logger.
• Leaving out the unit in calculated answers.

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 notes.