OCR GCSE Biology A: Practical skills I: microscopy, food tests, sampling and enzyme practicals (PAG 1-4) (J247)

This guide covers the first four practical activity groups (PAGs) in Topic B7, Practical skills, of the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026): microscopy, testing for biological molecules, sampling techniques and rates of enzyme-controlled reactions. It also covers the working scientifically practical skills 2a to 2d. None of these statements carries the Higher Tier only marking. OCR states that 15% of the written-paper questions test practical skills.

Related pages: the OCR GCSE Biology hub, the printable checklist, the revision notes for this unit, the practice questions and the free 10-minute diagnostics.

What this unit covers

Group	What the specification expects	Linked content
PAG 1 Microscopy	Microscopes and magnification to observe specimens and make labelled scientific drawings; accurate measurements, including pH	B1.1a
PAG 2 Testing for biological molecules	Safe heating with a Bunsen burner or water bath; qualitative reagents for molecules and processes, including continuous sampling	B1.3d to B1.3f
PAG 3 Sampling techniques	Field sampling of distribution and abundance, with biotic and abiotic factors; the same measurement skills	B4.1f, B6.1a
PAG 4 Rates of enzyme-controlled reactions	Heating devices; observing and measuring change; rates from gas produced, water taken up and indicator colour change	B1.2f, B1.2g
Working scientifically 2a to 2d	Carry out experiments, make and record observations, present observations, communicate the rationale and conclusions	all of the above

Every learner must be offered at least eight practical activities across the eight groups, and you must be able to describe, explain and evaluate each method on paper.

PAG 1: Microscopy

The light microscope

Know the lenses, stage, lamp, slides, cover slips and stains. Colourless specimens are stained so that structures show up. Iodine solution stains starch and cell contents in plant tissue; methylene blue is a common stain for cheek cells.

Method for a temporary slide, for example onion epidermis:

1. Place a drop of water or stain on a clean slide.
2. Peel a thin layer of epidermis with forceps and lay it flat on the drop.
3. Touch one edge of a cover slip to the liquid and lower it slowly, supported by a needle, so air does not get trapped.
4. Begin on the shortest objective lens. Bring the image into view with the large focus wheel, then sharpen it with the small one.
5. Switch to a longer objective only once the specimen is centred and sharp.

Magnification

magnification = image size ÷ actual size, or image size = actual size × magnification

Total magnification = eyepiece magnification × objective magnification. Use one unit throughout (1 mm = 1000 µm).

Worked example 1. A student uses a ×10 eyepiece and a ×40 objective, and measures a cheek cell in a photograph as 30 mm long.

• Total magnification = 10 × 40 = ×400.
• Actual size = 30 ÷ 400 = 0.075 mm.
• 0.075 mm × 1000 = 75 µm.

Worked example 2. A drawing of an onion cell has a scale bar of 15 mm labelled 100 µm. The cell in the drawing is 33 mm long.

• Convert the bar: 15 mm = 15 000 µm.
• Magnification = 15 000 ÷ 100 = ×150.
• Actual length = 33 ÷ 150 = 0.22 mm = 220 µm.

Labelled scientific drawings

• Use a sharp pencil, one clear continuous line, and no shading or colouring.
• Draw what you see, in proportion, and large.
• Label using ruled lines that end on the structure and never intersect.
• Give a title, and record the magnification or a scale bar.

Measuring length, area, mass, time, temperature, volume and pH

Quantity	Apparatus	Good practice
Length	Ruler, or a stage micrometer under the microscope	Read at eye level from the same edge
Area	Squared paper or a grid; or length × width for regular shapes	Count part-squares consistently
Mass	Balance	Zero with the container on
Time	Stopwatch	Use the same end point each time
Temperature	Thermometer or temperature probe	Stir first; keep the bulb off the glass
Volume of liquid	Measuring cylinder, pipette or syringe	Read the bottom of the meniscus at eye level
Volume of gas	Gas syringe, or a measuring cylinder inverted in a trough	Check for leaks
pH	Indicator solution or paper, or a pH meter or probe	Rinse the probe between solutions

PAG 2: Testing for biological molecules

Safe heating

Light a Bunsen burner on the safety flame (yellow), open the air hole for the blue flame to heat, and close it again when not in use. Heat a boiling tube in a water bath (a beaker of hot water on a heatproof mat, or an electric water bath) rather than in the flame, so heating is gentle and even. Wear eye protection, tie back long hair, use a holder and point tubes away from everyone.

Qualitative tests

A qualitative test tells you whether a molecule is present, not how much.

Test	Detects	Method	Positive result
Iodine solution	Starch	Add a few drops to the sample	Orange-brown to blue-black
Benedict’s reagent	Reducing sugars, such as glucose	Add reagent, heat in a hot water bath for a few minutes	Blue to green, yellow, orange or brick-red
Biuret reagent	Protein	Add the reagent to the sample and mix	Blue to purple (lilac)
Ethanol emulsion	Lipids	Mix the sample with ethanol, then add to water	Cloudy white emulsion

A negative Benedict’s result stays blue. The colour moves towards brick-red as the amount of reducing sugar increases, so comparing colours gives a rough, semi-quantitative idea of concentration.

Worked example 3. A food extract gives these results: iodine stays orange-brown, Benedict’s turns brick-red, biuret stays blue, ethanol test stays clear. Conclusion: it contains a reducing sugar and no detectable starch, protein or lipid.

Continuous sampling

In continuous sampling you take small samples from one reaction mixture at regular intervals and test each one. For starch digestion by amylase, put drops of iodine in the wells of a spotting tile. Every 30 seconds, move a drop of the reaction mixture into a new well. The mixture is blue-black while starch remains. The time at which the drop first stays orange-brown is the end point.

Worked example 4. The end point is reached at 80 s. Rate = 1 ÷ time = 1 ÷ 80 = 0.0125 per second (s⁻¹).

PAG 3: Sampling techniques

You must be able to plan a field study of distribution (where organisms are) and abundance (how many), and work out numbers in a given area. The methods listed are random sampling, transects, capture-recapture, quadrats, pooters, nets, keys and scaling up.

Random quadrats

A quadrat is a square frame; the one used here is 0.5 m by 0.5 m, so its area is 0.25 m². To avoid bias, lay two tape measures along adjacent edges of the area and take each quadrat position from random numbers. Count the organisms in each quadrat; use percentage cover for plants that are hard to count. A larger sample gives a more representative mean.

Worked example 5. A meadow of 450 m² is sampled with nine 0.25 m² quadrats. Buttercup counts: 6, 2, 9, 4, 0, 7, 3, 5, 8.

• Total = 44, so mean = 44 ÷ 9 = 4.9 per quadrat (2 significant figures).
• Per m² = 4.89 ÷ 0.25 = 19.6 (using the unrounded mean).
• Scaling up: (44 ÷ 9 ÷ 0.25) × 450 = 8800 buttercups, an estimate.

Transects

A transect is a line or belt across a gradient, such as from a path into woodland. Place quadrats at regular intervals along it and record organisms and abiotic readings at each point. It shows distribution when conditions change gradually.

Capture-recapture for animals

Catch a sample, mark each animal harmlessly, release them, then catch a second sample.

population = (first sample × second sample) ÷ marked animals found in the second sample

Worked example 6. 40 beetles are marked. A second sample has 35 beetles, of which 7 are marked. Population = (40 × 35) ÷ 7 = 200. The estimate assumes marks do not wear off or harm survival, marked animals mix back in, and the population is closed.

Pooters collect small insects, nets sample flying or pond animals, and a key identifies them.

Biotic and abiotic factors

Measure abiotic factors where you sample: a light meter for light intensity, a thermometer for temperature, a moisture meter for soil moisture and a pH probe for soil pH. Biotic factors include predators and food. Finding that an organism and a factor change together shows a correlation, not proof of cause.

PAG 4: Rates of enzyme-controlled reactions

rate = amount of product formed (or substrate used) ÷ time, or rate = 1 ÷ time taken for a fixed change

Three ways to follow a rate:

1. Gas production. Catalase breaks down hydrogen peroxide to water and oxygen. Collect the oxygen in a gas syringe or an upturned measuring cylinder over water and record the volume at fixed times.
2. Water uptake. A potometer measures how fast a cut shoot takes up water; the bubble in a capillary tube moves as water is lost.
3. Colour change of an indicator. Amylase and starch with iodine fade from blue-black to orange-brown; an indicator that changes colour as pH falls can follow a lipase reaction.

Control the other variables: water bath for temperature, buffers for pH, equal volumes and concentrations of enzyme and substrate. Repeat each condition and take a mean, leaving out anomalies.

Worked example 7. Oxygen collected from catalase and hydrogen peroxide:

Time (s)	0	30	60	90	120	150	180
Volume (cm³)	0	12	22	30	36	39	40

• Rate from 0 to 30 s = 12 ÷ 30 = 0.40 cm³/s.
• Rate from 90 to 120 s = (36 − 30) ÷ 30 = 0.20 cm³/s.
• Mean rate over 180 s = 40 ÷ 180 = 0.22 cm³/s (2 significant figures).
• The rate falls because substrate is used up, so fewer collisions with active sites. The curve levels off at 40 cm³ when the substrate has gone.

Worked example 8. Repeat times to clear: 62 s, 58 s, 85 s, 60 s. The 85 s is anomalous, so mean = (62 + 58 + 60) ÷ 3 = 60 s.

Worked example 9. In a potometer, a bubble moves 42 mm in 6 minutes in a capillary of radius 0.5 mm. Volume = π r² × length = π × 0.5² × 42 = 33 mm³. Rate = 33 ÷ 6 = 5.5 mm³ per minute.

Working scientifically skills 2a to 2d

• 2a Carry out experiments with correct handling of apparatus, accurate measurements, safety and written instructions.
• 2b Make and record observations and measurements, keeping appropriate records.
• 2c Present observations in descriptive, tabular, diagrammatic or graphical form: independent variable in the first column and on the x-axis, units in headings, even scales, labelled axes, points marked clearly.
• 2d Communicate the rationale, methods, findings and reasoned conclusions in words, diagrams, graphs, numbers and symbols.

OCR also expects scientific diagrams of apparatus: ruled lines, labelled parts and a clear set-up for any procedure.

Accuracy means close to the true value. Precision means repeats close together. Repeatable results come back when you rerun the work yourself; reproducible results come back when someone else, or another method, is used.

Common errors

• Dividing a drawing size in mm by a magnification and calling the answer µm. Convert units.
• Reading Benedict’s result before heating.
• Using biuret reagent to test for sugar; it detects protein.
• Placing quadrats where the organisms look thickest.
• Describing a correlation as proof that a factor caused a distribution.
• Forgetting to scale up a mean per quadrat to the whole area.
• Averaging an anomalous result into the mean.

For the content behind these practicals see the cell level systems guide, the microscopy guide, the enzyme action guide and the field investigations guide.

Official syllabus

OCR, GCSE (9-1) Gateway Science Suite Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018: Topic B7 Practical skills (PAG 1 to PAG 4) and working scientifically skills 2a to 2d, with the linked content statements in B1, B4 and B6.