All subjects · Biology · WBI13 Practical Skills in Biology I

Skills applied: CP8 fibre strength, CP9 antimicrobial plants

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

CP8 and CP9 bring safety, standardising natural material and calculations such as force, gradient and the area of an inhibition zone.

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CP8: tensile strength of plant fibres

Method recap (see Core practical 8, Unit 2): clamp one end of a fibre (for example from celery, nettle or flax) and hang masses on the other end, adding small masses one at a time until it breaks. Tensile strength is the force (or mass) needed to break the fibre, often per unit cross-sectional area.

Force = mass (kg) x 9.81 N kg⁻¹.
Standardise fibre length and how it is extracted; add masses gently and in small steps (large steps make results imprecise; the January 2025 report noted masses far too large).
Safety: wear eye protection (a snapping fibre can whip), keep feet clear of falling masses (a tray of sand below).
Explanation: fibres with more cellulose microfibrils and lignified sclerenchyma walls are stronger.

CP9: antimicrobial properties of plants

Method recap (see Core practical 9, Unit 2): extract plant material (garlic, mint, basil) by crushing in ethanol, soak sterile paper discs, let the ethanol evaporate, place discs on agar spread with bacteria, incubate, measure the zone of inhibition.

Area of a circular zone = πr² (r = diameter ÷ 2). Area is better than diameter if zones are not perfect circles; measure two diameters at right angles and take a mean.
Control: disc with ethanol only (evaporated).
Aseptic technique throughout (flame loop, open lid minimally, sterile forceps).
Same volume of extract per disc, same bacteria culture density, same incubation time and temperature.

Exam favourites

Calculate and compare zone areas.
Explain why the ethanol must evaporate before discs go on the plate.
Explain why different microbes need different incubation conditions (fungi grow more slowly; enzymes have different optimum temperatures).
Explain why one plant species and one fibre is not enough to generalise.

Worked example

A zone of inhibition has a diameter of 14 mm. Calculate its area.

r = 14 ÷ 2 = 7 mm
Area = π x 7² = 153.9

Answer: 154 mm² (3 s.f.: 154)

Key idea: CP8: add small masses until the fibre breaks; force = mass x 9.81; standardise length and extraction; eye protection and a sand tray. CP9: discs in ethanol extract, ethanol evaporated, ethanol-only control, zone area = πr², aseptic technique.

Check yourself

A zone of inhibition is 18 mm in diameter. Calculate its area in mm² to 3 significant figures.
Show answer
254 mm². r = 9 mm; π x 81 = 254 mm².
Two zones have diameters of 10 mm and 20 mm. How many times larger is the area of the larger zone?
Show answer
4. Area ∝ r², and the ratio of radii equals the ratio of diameters: (20 / 10)² = 4.
A fibre broke when 0.85 kg was hanging from it. Calculate the force in N, using g = 9.81 N kg⁻¹, to 3 significant figures.
Show answer
8.34 N. 0.85 x 9.81 = 8.34 N.
Why is the ethanol allowed to evaporate from the discs before they go on the agar?
- Ethanol itself kills bacteria, which would hide the effect of the plant extract
- Wet discs do not stick
- Ethanol feeds the bacteria
- It makes the disc heavier
Show answer
Ethanol itself kills bacteria, which would hide the effect of the plant extract. The control checks any remaining solvent effect too.
Why should small masses be added one at a time in CP8?
- So the fibre stretches less
- So the clamp holds better
- So the breaking mass is found precisely
- To make the fibre stronger
Show answer
So the breaking mass is found precisely. Large steps make the breaking mass imprecise.
Which safety precaution suits CP8 with its reason?
- Wear eye protection because a breaking fibre can whip back
- Wear gloves because cellulose is toxic
- Work in a fume cupboard because fibres give off gas
- Use a Bunsen to sterilise the fibres
Show answer
Wear eye protection because a breaking fibre can whip back. Snapping fibres and falling masses are the main hazards.
Why might fungal plates be incubated longer than bacterial plates?
- Fungi need no nutrients
- Fungi die at room temperature
- Fungi grow more slowly
- Fungi are not affected by plant extracts
Show answer
Fungi grow more slowly. The June 2024 mark scheme credited longer incubation for slower fungal growth.

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