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

This guide is written for OCR A Level Biology A (H420), using the specification Version 4.1 (April 2026), for first assessment in 2025. It covers section 1.2, the skills and techniques behind the Practical Endorsement: 1.2.1(a) to (j) and 1.2.2(a) to (l). Pair it with the revision notes and the practice questions. Course links: OCR A Level Biology hub, printable checklist and the free diagnostics.

How the Practical Endorsement works

The specification asks for a minimum of 12 assessed practical activities covering PAG1 to PAG12. Your teacher assesses you against the Common Practical Assessment Criteria, and the result is reported separately from the A level grade. You keep your own record and show independent competence even in group work.

Spec reference	What you must be able to do	PAG
1.2.1(a) to (c)	Apply investigative approaches, use equipment safely, follow instructions	all
1.2.1(d) to (g)	Record, present and process data	all
1.2.1(h) to (j)	Research, cite sources, use a range of instruments	12 and all
1.2.2(a), (c)	Quantitative measurements, glassware, serial dilutions	4, 5, 7, 8, 9
1.2.2(b)	Colorimeter or potometer	5
1.2.2(d), (e)	Light microscope, graticule, drawings	1, 2, 3
1.2.2(f), (g)	Qualitative reagents, chromatography or electrophoresis	9, 6
1.2.2(h)	Safe, ethical use of organisms	11
1.2.2(i), (j), (k), (l)	Aseptic technique, dissection, sampling, ICT	7, 2, 3, 4, 8, 10

The practical skills guide covers variables, error and uncertainty; this one covers the hands-on methods.

1.2.1 Practical skills

(a) Investigative approaches and problem solving. Plan with a pilot trial, then fix the range, intervals, controls and repeats. If a method fails, adjust it and record what you changed and why.

(b) Safe use of equipment, with hazards identified. A risk assessment gives three separate items: hazard, resulting risk, and control measure. Example: hot water bath (hazard), scald if knocked (risk), use a tray away from the edge (control).

(c) Read the whole method first and collect all apparatus.

(d) and (e) Observations, measurements and records. Record in the lab at the time, in a table with the units in the column headings. Give raw readings to the instrument’s resolution: 2.40 g, not 2.4 g, on a balance reading to 0.01 g. Record colours and textures as seen, not as expected.

(f) and (g) Presenting and processing data. Software can produce graphs, means and test results; check the output makes sense.

(h) and (i) Research and referencing. Use websites, textbooks and printed sources. Judge each source: author, date, evidence cited, and whether another source agrees. Cite what you use in the text and list it at the end with author, year, title, and publisher or website with the date you accessed it. The specification points to the OCR Practical Skills Handbook for citation methods.

(j) The techniques below make up the range of instruments.

1.2.2(a) Quantitative measurements

Mass, time, volume, temperature, length and pH must all be measured. A smaller scale division gives a smaller percentage uncertainty. Zero the balance with the container on. Human reaction time is much larger than a stopwatch’s display resolution. Calibrate a pH meter with buffer solutions first.

1.2.2(b) Colorimeter and potometer

Colorimeter. It measures how much light of one colour passes through a solution, shown as absorbance or percentage transmission. Choose the filter colour the solution absorbs most, zero the meter with a solvent-only blank, wipe the cuvette, remove bubbles, then read.

Worked example: calibration curve. Protein standards of 0, 0.5, 1.0, 1.5 and 2.0 g per 100 cm³ gave absorbances of 0.00, 0.11, 0.23, 0.34 and 0.46. A sample gave 0.28.

gradient = 0.46 / 2.0 = 0.23 per g per 100 cm³
concentration = 0.28 / 0.23 = 1.2 g per 100 cm³ (2 s.f.)

Never read beyond the standards.

Potometer. It measures water uptake by a cut shoot, a good indicator of transpiration but not the same thing. Set up: cut the shoot at an angle under water, join all parts under water, keep joints airtight, dry the leaves, introduce an air bubble and let it settle before timing. Volume taken up = πr² × distance moved by the bubble.

Worked example. Capillary radius 0.50 mm; the bubble moves 42 mm in 5.0 minutes.

volume = π × 0.50² × 42 = 33 mm³
rate = 33 / 5.0 = 6.6 mm³ per minute

Two readings on a ruler with 1 mm divisions give ±1 mm in 42 mm, about 2.4 %. Divided by a leaf area of 48 cm², the rate is 0.14 mm³ per cm² per minute.

1.2.2(c) Glassware and serial dilutions

A serial dilution makes a range of concentrations from one stock with the same factor at each step. For a doubling series from 1.0 mol dm⁻³, mix 5 cm³ of the previous solution with 5 cm³ of water each time: 0.50, 0.25, 0.125. For a tenfold series use 1 cm³ plus 9 cm³ of diluent.

To make a volume V₂ of concentration C₂ from stock C₁: volume of stock = (C₂ × V₂) / C₁. For 50 cm³ of 0.20 from 1.00: stock = 0.20 × 50 / 1.00 = 10 cm³, plus 40 cm³ of water.

Worked example: counts. A culture is diluted to 10⁻⁴. A 0.1 cm³ sample is spread on agar and gives 36 colonies.

colonies per cm³ in the diluted sample = 36 / 0.1 = 360
original culture = 360 / 10⁻⁴ = 3.6 × 10⁶ per cm³

1.2.2(d) The light microscope and graticule

Start on low power with coarse then fine focus, then move to high power using fine focus only. Use a thin section and lower the cover slip at an angle to avoid bubbles. Total magnification = eyepiece × objective.

An eyepiece graticule has arbitrary divisions. Calibrate it for each objective against a stage micrometer, then convert.

Worked example. At ×10, 100 µm on the stage micrometer spans 20 graticule divisions: 1 division = 5.0 µm. At ×40, 100 µm spans 80 divisions: 1 division = 1.25 µm. A cell is 14 divisions long at ×40, so its length is 14 × 1.25 = 17.5 µm.

1.2.2(e) Scientific drawings

Use a sharp pencil, continuous lines and no shading. Draw large, in proportion, with a title and magnification or scale bar. Label lines are straight, touch the structure and do not cross. Annotations add function or description, such as “thick wall, gives support”.

A low-power plan shows tissue regions, never single cells; a high-power drawing shows a few cells in detail.

Worked example. The 17.5 µm cell is drawn 52.5 mm long. Convert units first: 17.5 µm = 0.0175 mm. Magnification = 52.5 / 0.0175 = ×3000.

1.2.2(f) Qualitative reagents for biological molecules

Molecule	Reagent and method	Positive result
Reducing sugar	Benedict’s, heat in a hot water bath	blue to green, yellow, orange or brick red
Non-reducing sugar	Boil with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then Benedict’s	colour change as above
Starch	iodine in potassium iodide solution	yellow-brown to blue-black
Protein	sodium hydroxide then dilute copper(II) sulfate (biuret)	blue to purple
Lipid	ethanol, shake, pour into water	cloudy white emulsion

Worked example. Sample A is Benedict’s negative, but gives brick red after acid hydrolysis and neutralising, so it contains a non-reducing sugar. Colour is only a rough guide to amount.

1.2.2(g) Chromatography and electrophoresis

Chromatography (paper or thin layer). Draw the baseline in pencil, spot on it, stand the paper in solvent below the baseline, cover the tank, and mark the solvent front before it reaches the top. Colourless compounds need a locating agent, such as ninhydrin for amino acids (fume cupboard).

Rf = spot distance from baseline / solvent front distance from baseline

Worked example. Front at 9.5 cm; spots at 2.85, 5.70 and 7.60 cm give Rf of 0.30, 0.60 and 0.80. Compare with known standards on the same sheet, because Rf depends on the solvent.

Electrophoresis. Samples go into wells in a gel in buffer and a current is applied. Molecules separate by charge and size. DNA is negatively charged (phosphate groups), so it travels toward the positive electrode, and small fragments pass through the gel faster, ending further along. Compare unknown bands with a ladder of known sizes. Wear gloves for stains.

1.2.2(h) Safe and ethical use of organisms

For plant or animal responses (PAG11) and physiological functions:

• Human volunteers: informed consent, right to stop, no strenuous exercise with a relevant health condition, anonymous data.
• Animals: invertebrates where possible, minimum handling, suitable conditions, returned to where found, hands washed afterwards.
• Plants: take only what you need and avoid damaging the habitat.
• Justify every procedure and keep harm to a minimum.

Worked example. Mean resting pulse 68 beats per minute; after exercise 112.

percentage change = (112 − 68) / 68 × 100 = +64.7 %

1.2.2(i) Aseptic technique

The aim is to keep contaminants out and your culture in. Disinfect the bench, flame bottle necks, heat the loop until red hot and cool it in air, and open lids only as far as needed. Seal an agar plate with two short pieces of tape, not all round, incubate it upside down at about 25 °C in schools (not body temperature) and never open it afterwards. In broth, growth can be judged by turbidity in a colorimeter.

Worked example. Antibiotic discs on a lawn: A gives a clear zone radius 9 mm, B gives 6 mm. Area = πr²: A = 254 mm², B = 113 mm². A’s zone is 2.25 times the area. A disc with no antibiotic is the control.

1.2.2(j) Dissection

Use a sharp scalpel on a tile, cut away from your body and wear gloves and eye protection. Plant organs such as a stem or root are cut into thin sections. Identify structures before cutting, dispose of tissue as instructed and disinfect surfaces.

Worked example. In a mammalian heart the left ventricle wall is 12.5 mm and the right is 4.0 mm. Ratio = 3.1 : 1. The left side pumps blood around the body at higher pressure, so a thicker wall fits its function. A ruler gives ±1 mm on 4.0 mm (25 %), so the ratio is approximate.

1.2.2(k) Fieldwork sampling

• Random sampling removes bias: generate coordinates with random numbers and place a quadrat at each.
• Systematic sampling suits a gradient: a belt or line transect with quadrats at regular intervals along it.

Record abiotic factors such as light at each point, and take enough samples for a stable mean.

Worked example. Ten 0.25 m² quadrats gave counts 6, 3, 0, 4, 7, 2, 5, 1, 4 and 8. Mean = 4.0 per quadrat = 16 per m². A field of 600 m² holds about 9600 plants.

For mobile animals, capture–mark–recapture: N = (M × n) / m, where M is the number first marked, n the second catch and m the marked animals recaptured. With 45, 38 and 9, N = 190. It assumes no births, deaths or migration, and harmless marks.

1.2.2(l) ICT

A data logger with a sensor (temperature, light, pH, oxygen) takes frequent readings without timing error from you. Set the sampling interval and duration first. Computer modelling shows molecular structure or predicts outcomes, but only as well as its assumptions.

Worked example. A logger records oxygen from 2.1 to 5.7 mg dm⁻³ over 6 minutes. Mean rate = (5.7 − 2.1) / 6 = 0.60 mg dm⁻³ min⁻¹.

Common errors

• Giving Rf units (it is a ratio), or measuring to the spot edge instead of its centre.
• Quoting magnification without converting micrometres to millimetres.
• Writing “wear goggles” as a full risk assessment.

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. Section 1.2.1 and 1.2.2 are the source for this guide.