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

These notes condense Topic B7, Practical skills, in the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for practical activity groups (PAGs) 1 to 4 and the working scientifically practical skills 2a to 2d. No Higher Tier only statements appear here. Fuller explanations are in the study guide. Then try the practice questions, the OCR GCSE Biology hub, the printable checklist and the free 10-minute diagnostics.

The four groups at a glance

PAG	Core idea	Typical practical
1 Microscopy	Observe, draw, measure, calculate magnification	Stained onion epidermis or cheek cells
2 Testing for biological molecules	Safe heating; qualitative reagents; continuous sampling	Food tests; amylase and starch
3 Sampling techniques	Distribution and abundance; biotic and abiotic factors	Quadrats along a transect
4 Rates of enzyme-controlled reactions	Rate from gas, water uptake or colour change	Catalase and hydrogen peroxide

All four also need accurate measurement of the common quantities (length, area, mass, time, temperature, volumes and pH).

PAG 1: key facts

Item	Revise
Microscope parts	Eyepiece, objective lenses, stage, lamp, focus wheels
Slide	Specimen on a drop of water or stain, cover slip lowered slowly
Stains	Make colourless structures visible; iodine for starch and plant tissue, methylene blue for cheek cells
Starting power	Shortest objective first, large then small focus wheel

Formulae:

Formula	Notes
magnification = image size ÷ actual size	Same units on both
image size = actual size × magnification	Rearranged
actual size = image size ÷ magnification	Rearranged
total magnification = eyepiece × objective	e.g. 10 × 40 = ×400
1 mm = 1000 µm	Multiply mm by 1000 for µm

Method in steps for a magnification question:

1. Write the formula.
2. Convert the image size and the actual size to the same unit.
3. Substitute and divide.
4. Give the unit (or a plain number with a multiplication sign for magnification).

Worked reminder: image 24 mm, magnification ×300, actual size = 24 ÷ 300 = 0.08 mm = 80 µm. A 12 mm drawing of a 40 µm cell: 12 000 ÷ 40 = ×300.

Drawing rules: sharp pencil; single clear lines; no shading; correct proportions; ruled labels ending on the structure, never intersecting; title; magnification or scale bar.

Measuring: ruler for length; squared paper for area; balance for mass; stopwatch for time; thermometer or probe for temperature; read a meniscus at eye level; gas syringe or inverted measuring cylinder for gas; indicator, paper or probe for pH.

PAG 2: key facts

Safe heating: yellow safety flame when lighting and idle, blue flame to heat; water bath or electric heater for gentle heating; eye protection; tubes in a holder, pointing away; hair tied back.

Reagent	Tests for	Heat?	Negative	Positive
Iodine solution	Starch	No	Orange-brown	Blue-black
Benedict’s	Reducing sugar	Yes, hot water bath	Blue	Green, yellow, orange, brick-red
Biuret	Protein	No	Blue	Purple (lilac)
Ethanol, then water	Lipid	No	Clear	Cloudy white emulsion

• These tests are qualitative. Benedict’s colour gives only a rough guide to amount.
• Continuous sampling: take samples from one mixture at set intervals and test each, for example drops of iodine on a spotting tile. The end point is the first sample that no longer turns blue-black.
• Rate from an end point = 1 ÷ time. A time of 50 s gives 1 ÷ 50 = 0.02 s⁻¹.

PAG 3: key facts

Term or method	Revise
Distribution	Where organisms are found
Abundance	How many are present
Random sampling	Random coordinates for quadrats avoid bias
Transect	A line or belt across a gradient, quadrats at regular intervals
Quadrat	Square frame; count individuals or estimate percentage cover
Pooter, net, key	Collect small insects; sample flying or pond animals; identify organisms
Capture-recapture	Mark, release, resample
Scaling up	Mean per quadrat ÷ quadrat area × total area

Equations:

• Capture-recapture population = (first sample × second sample) ÷ marked animals recaptured.
• Estimate for an area = mean per m² × area in m².

Worked reminder: mean 2.4 per 0.25 m² quadrat is 2.4 ÷ 0.25 = 9.6 per m². Over 50 m² that is 9.6 × 50 = 480. Capture-recapture: 24 marked, second sample 30, of which 6 marked: (24 × 30) ÷ 6 = 120.

Method in steps for a quadrat estimate:

1. Choose positions with random coordinates, not by eye.
2. Count (or estimate percentage cover) in every quadrat.
3. Find the mean per quadrat.
4. Divide by the quadrat area to get a number per m².
5. Multiply by the total area and say the result is an estimate.

A bigger sample, more quadrats and repeating the transect all make the estimate more representative.

Capture-recapture assumptions: marks stay and do no harm, marked animals mix with the rest, and no significant births, deaths or migration.

Abiotic factors: light intensity, temperature, moisture level and soil pH, each measured at the sampling point. Biotic factors: predators, food, competition. A matching pattern shows correlation only.

PAG 4: key facts

Method	What you measure	Example
Gas production	Volume of gas per unit time	Oxygen from catalase
Water uptake	Distance a bubble moves per unit time	Potometer
Colour change	Time to reach an end point	Iodine clearing with amylase

• rate = change ÷ time; rate = 1 ÷ time to end point.
• Volume of water in a capillary = π r² × length of bubble movement.
• Control: temperature (water bath), pH (buffer), enzyme and substrate volume and concentration. Repeat and take a mean without anomalies. Method in steps for a rate from a table of results:

1. Pick the time interval asked for.
2. Subtract the earlier volume from the later volume.
3. Divide by the time interval.
4. Give the unit, such as cm³/s.

• Initial rate is the steepest part of a volume-time graph. The curve flattens when substrate is used up.
• Too high a temperature or the wrong pH changes the shape of the active site; the enzyme is denatured.

Worked reminder: 9 cm³ in the first 30 s is 9 ÷ 30 = 0.30 cm³/s. A bubble that moves 25 mm in 5 minutes in a capillary of radius 0.4 mm: volume = π × 0.4² × 25 = 12.6 mm³, so rate = 2.5 mm³ per minute.

Working scientifically skills 2a to 2d

Skill	What you do
2a	Carry out experiments, handling apparatus correctly and safely
2b	Make and record observations and measurements, keeping records
2c	Present observations in words, tables, diagrams or graphs
2d	Communicate the reason for the investigation, the method, the findings and the conclusion

Graph checklist: independent variable on the x-axis; labelled axes with units; even scales that use over half the grid; points plotted exactly; line of best fit through the trend, not joined dot to dot unless the data call for it.

Must-know distinctions

• Distribution is where; abundance is how many.
• Qualitative is present or absent; quantitative is how much.
• Accuracy is closeness to the true value; precision is closeness of repeats.
• Repeatability is the same person again; reproducibility is a different person or method.
• Correlation is a link in the data; cause needs more evidence.

Quick self-test

1. A cell measures 18 mm in a photograph taken at ×600. What is its actual length in µm?
2. State the positive result for the biuret test.
3. Why is Benedict’s reagent heated in a water bath rather than directly in a flame?
4. A mean of 3.5 organisms per 0.25 m² quadrat is found. How many per m²?
5. 36 beetles are marked, a second sample is 45, and 9 are marked. Estimate the population.
6. Name one assumption of capture-recapture.
7. Starch is broken down at the end point of 125 s. Calculate 1 ÷ time.
8. Three repeats give 18, 20 and 19 s. What is the mean?
9. Name two ways of measuring the rate of an enzyme reaction.
10. What does an ethanol emulsion test detect and what is the result?

Answers

1. 18 ÷ 600 = 0.03 mm = 30 µm.
2. Blue to purple (lilac).
3. A water bath heats gently and evenly, which lowers the risk of the liquid spitting out.
4. 3.5 ÷ 0.25 = 14 per m².
5. (36 × 45) ÷ 9 = 180.
6. Marks do not harm the animals or rub off; marked animals mix back; there is no migration.
7. 1 ÷ 125 = 0.008 s⁻¹.
8. (18 + 20 + 19) ÷ 3 = 19 s.
9. Any two: gas volume per time, water uptake, time for a colour change.
10. Lipid; a cloudy white emulsion.

Where marks are usually lost

• Not converting mm to µm before calculating, or giving the wrong unit.
• Writing “it turns red” when the answer is “blue to brick-red after heating”.
• Stating “protein turns purple” without naming the reagent.
• Describing sampling as “pick some quadrats” rather than random coordinates.
• Forgetting that a mean per quadrat has to be scaled to the whole area.
• Plotting a line through every point when a trend line is needed.
• Calling a correlation a proof.
• Not saying which variables were controlled and how.
• Leaving out units in the heading of a table column.

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.