All subjects · Biology · WBI13 Practical Skills in Biology I

The Unit 3 paper: what is assessed and how

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Unit 3 is a written exam about practical work. There is no lab test: you answer questions about experiments you should already have done, especially core practicals 1 to 9.

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The paper

WBI13/01, 1 hour 20 minutes, 50 marks, all questions compulsory, calculator allowed.
Worth 20% of the International AS and 10% of the full International A level.
At least 5 marks test maths at Level 2 or above (for example percentages, gradients, standard deviation, significant figures).
Questions are set in the context of core practicals 1 to 9 (Units 1 and 2), the recommended additional practicals in those topics (heart dissection, tissue water potential, biuret protein estimation, pollen tube growth, plant mineral deficiencies) and unfamiliar experiments built from the same skills.

CP	Practical	Lesson that teaches the method
1	Semi-quantitative reducing sugar and starch tests	Core practical 1 (Unit 1)
2	Vitamin C content with DCPIP	Core practical 2 (Unit 1)
3	Membrane permeability (beetroot)	Core practical 3 (Unit 1)
4	Factors affecting initial rate of enzyme reactions	Core practical 4 (Unit 1)
5	Light microscope and graticule	Core practical 5 (Unit 2)
6	Root tip squash and mitotic index	Core practical 6 (Unit 2)
7	Plant tissues under the microscope	Core practical 7 (Unit 2)
8	Tensile strength of plant fibres	Core practical 8 (Unit 2)
9	Antimicrobial properties of plants	Core practical 9 (Unit 2)

The skills tested

The specification groups the skills into three areas:

Planning: apparatus, independent, dependent and controlled variables, how to measure and control them, repeats, health and safety, how the data will be used, sources of uncertainty and systematic error, and the implications of the biology.
Implementation and measurements: number and range of readings, significant figures, checking an inconsistent reading, improving the experiment.
Processing results: calculations to the right significant figures, graphs with a sensible scale, correct units, relationships and gradients, modifications to reduce error, and uncertainties discussed qualitatively and quantitatively.

Command words and exam habits

Examiners repeatedly say marks are lost by not doing what the command word asks:

Explain needs a reason (a precaution plus why it is needed, not just the precaution).
Compare and contrast needs similarities and differences, written as matched pairs.
Evaluate means judge the method or data: say what limits the conclusion and why (no repeats, no control, no standard deviations).
Calculate to the precision asked: if a question says 2 significant figures, 9.06 loses the mark; 9.1 gains it.
Write one answer per line where lines are numbered: only the first answer on a line is marked.

Generic words such as accurate, reliable or valid score nothing unless tied to the experiment.

Worked example

A question asks you to explain one safety precaution when growing non-pathogenic soil bacteria on agar plates. A student writes: 'Wear gloves.' How many marks would this score out of 2, and how should it be improved?

Explain needs the precaution and the reason.
The answer only states a precaution.
Improve: 'Tape the lid with two strips, not all the way round, so microbes cannot escape but anaerobic pathogens are not encouraged to grow.'

Answer: Usually 0 or 1 out of 2: a precaution with no reason does not explain. Always add 'so that...' to link the precaution to the hazard.

Key idea: 50 marks, 80 minutes, all on CP1 to CP9, the recommended additional practicals and new contexts. Planning, measuring and processing skills; follow the command word exactly and tie every point to the experiment.

Check yourself

How many marks is the WBI13 paper worth?
Show answer
50. WBI13/01 is a 50-mark written paper lasting 1 hour 20 minutes.
How long is the WBI13 paper, in minutes?
Show answer
80 minutes. 1 hour 20 minutes = 80 minutes.
Which core practicals form the main contexts for Unit 3?
- Core practicals 10 to 18, from Units 4 and 5
- Only core practicals 5 to 7
- Core practicals 1 to 9, from Units 1 and 2
- Any experiment except the core practicals
Show answer
Core practicals 1 to 9, from Units 1 and 2. Unit 3 is the AS practical paper, so it draws on the AS core practicals 1 to 9.
A question says 'Compare and contrast the structure of a fish heart and a mammal heart.' What must the answer include?
- Only the differences
- At least one similarity and at least one difference
- A description of one heart only
- A labelled drawing only
Show answer
At least one similarity and at least one difference. Compare and contrast means similarities and differences; the June 2024 and January 2025 reports both note candidates who gave only one kind.
What does the command word 'evaluate' require when applied to a set of results?
- Repeating the results in words
- Drawing a graph of the results
- Stating the independent variable
- Judging the strength of the evidence, giving limitations and why they matter
Show answer
Judging the strength of the evidence, giving limitations and why they matter. Evaluate is a judgement: for example, no control and no standard deviations mean significance cannot be judged.
A question asks for an answer to 2 significant figures. The calculator shows 9.0625. Which answer gains the mark?
- 9.06
- 9.1
- 9.0625
- 9
Show answer
9.1. 2 s.f. means 9.1; the June 2024 report says many lost the mark by writing 9.06.
Why do answers such as 'to make it more accurate' usually score nothing?
- Accuracy is never important in biology
- Examiners only accept the word 'precise'
- They are too long
- They are generic and not linked to the specific experiment
Show answer
They are generic and not linked to the specific experiment. Examiners want the specific reason, for example 'equal sized pieces give equal surface area for the enzyme'.

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Last updated 8 November 2026. Spotted a mistake? Tell us.