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4.2.7Topic 4 · Biodiversity and Natural Resources

Practical- Determining the Tensile Strength of Plant Fibres — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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4.2.7 Practical- Determining the Tensile Strength of Plant Fibres — Edexcel A Level Biology A (SNAB) 9BI0 notes: Plan and carry out an investigation into the tensile strength of plant fibres, controlling variables and analysing results.Tensile strength is the maximum pulling forc…

  • Independent variable
  • Control variables
  • breaking stress
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Section 1 of 8Principle

Tensile strength is the maximum pulling force a fibre can bear before it breaks. In school laboratories it is measured by hanging increasing masses from a fibre until it snaps; the weight at breaking point is a measure of strength. Fibres from different plants, or from plants grown under different conditions (for example with differing mineral supply), are compared.

Fibres can be obtained by stripping strands from the stems of plants such as flax, or by using commercial strands such as sisal, cotton, hemp or jute. Fibres are mostly sclerenchyma, so strength depends on cellulose content, lignin and fibre length and diameter.

Section 2 of 8Method

1
Cut equal lengths of each fibre, for example 20 cm, and measure the diameter with a micrometer or use a fixed number of fibres per strand.
2
Clamp the top of the fibre firmly to a retort stand or boss so it cannot slip. Fix a hook or container to the lower end.
3
Add masses (for example 100 g slotted masses) one at a time, waiting a few seconds between additions, until the fibre breaks. Record the total mass supported just before the break; alternatively slowly add sand or water to a bucket and measure the final mass.
4
Convert the mass (kg) to weight in newtons using force = mass x 9.8 N kg-1.
5
Repeat several times for each type of fibre and calculate a mean.

Safetyplace a cushion or tray under the masses and wear safety glasses, as masses can fall and fibres can snap back.

Section 3 of 8Variables and reliability

Independent variable
Independent variable type of fibre (or growth conditions).
Dependent variable
Dependent variable breaking mass or force.
Control variables
Control variables fibre length, thickness, number of strands, humidity, how it is attached and how the load is applied (the rate at which mass is added).

Weak points or knots affect results and the method of attachment can cause fibres to break at the clamp, so these should be avoided.

Repeating gives a mean and a range; with sufficient repeats a statistical test such as a t-test can be used to see whether differences between two fibre types are significant.
Breaking stress
Because fibres vary in diameter, it is often fairer to compare breaking stress: force divided by cross-sectional area.

Section 4 of 8Interpreting results

Higher breaking force indicates greater tensile strength.

Interpreting results
For example if hemp fibres support a mean of 38 N and cotton 12 N, hemp has about three times the tensile strength under these conditions.
Explanations link to structure: greater lignin and cellulose content, longer fibre cells, orientation of microfibrils along the fibre axis and more extensive hydrogen bonding between cellulose chains all increase strength.
Plants grown with a deficiency of some minerals, for example calcium, may produce fibres with lower strength.

Linkthe results support decisions about using plant fibres as renewable materials (4.2.5 and 4.2.10).

Section 5 of 8Evaluation and improvements

When evaluating this practical consider sources of error and uncertainty.

1
The resolution of the masses used (for instance 100 g steps) limits precision: a fibre might break part-way between 700 g and 800 g, so the true value is uncertain by up to 100 g.
2
Using smaller masses near the expected breaking point improves resolution.
3
Natural fibres are variable, so using more replicates gives a more reliable mean, and outliers should be examined, not just thrown away.
4
If the fibre breaks near the clamp this suggests damage by gripping, and the result should be repeated.

Environmental conditions matter as wellfibres absorb water, which can change their strength, so they should be conditioned at the same humidity or soaked for the same time before testing.

Evaluation and improvements
Different plant varieties and growing conditions also change fibre properties, so if you are comparing the effect of a mineral deficiency, the plants should be the same variety grown in the same conditions apart from the nutrient.
Valid conclusions are limited to the sample tested: concluding that all hemp is stronger than all cotton would be an over-generalisation from a small sample.
The same method could be adapted to test the effect of soaking in different pH solutions or the effect of treating fibres with enzymes.

Section 6 of 8Exam-style questions

  1. State three variables that should be controlled when comparing the tensile strength of two fibres. [3]3 marks

  2. A fibre broke when a mass of 3.2 kg was supported. Calculate the force in newtons (g = 9.8 N kg−1). If its cross-sectional area was 0.8 mm2, calculate the breaking stress in N mm−2. [3]3 marks

  3. Suggest why results from the fibres might vary considerably between repeats. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Tensile strengthGreatest pulling force per unit area a material can bear before breaking.
Breaking forceForce at which a fibre breaks.
StressForce divided by cross-sectional area.
Independent variableThe variable changed by the investigator.
Dependent variableThe variable measured as the outcome.
Control variableA variable kept constant to make the test fair.
ReliabilityHow consistent results are on repeating the investigation.
Cellulose microfibrilStrong structural unit of cell walls and fibres.

Section 8 of 8Exam technique

1
Recording the mass that broke the fibre instead of the last mass it supported, or being inconsistent.
2
Forgetting to convert mass to force in newtons.
3
Comparing fibres of different thickness without allowing for cross-sectional area.
4
Using only one fibre per type and drawing firm conclusions.
5
Failing to control the rate of loading.

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