Core Practical 8 – From Topic 4 (Biodiversity and Natural Resources)
Aim: To determine the tensile strength of plant fibres.
Independent Variable: The type of fibre used (from different plants)
Dependent Variable: The amount of mass that can be added before the fibre snaps
- Length of fibre – each fibre should be roughly the same length for a fair comparison
- Size of each individual mass – a set of the same weights can be used
- Stems of stinging nettles or celery
- Paper towels
- 2 clamp stands
- A set of the same type of weights (that can be added in increments)
- White tile
- Sharp knife
- The plant material should be left to soak in a bucket of water for about a week in order for the fibres to be easily extracted (retting).
- Once the fibres have been removed, connect them between 2 clamp stands and gradually add mass in the middle until the fibre snaps. Note the mass required to snap the fibre.
- Try this again but with individual fibres from different plants and different ways of combining fibres (e.g. twists). You can also compare the tensile strength of the stem to the individual fibres.
Results: You should observe that different species of plants have different tensile strengths of their fibres. This is related to the plants ecological niche and how the plant has adapted to it. You may also observe that more fibres bundled together results in greater strength, requiring a greater mass to snap.
Conclusion: The strength of the fibres is thanks to the chemical nature and structure of the cells that make up the fibre. Cellulose is a key component of cell walls and has cross-linking thanks to strong, horizontal glycosidic bonds between glucose molecules and vertical hydrogen bonds between neighbouring chains (forming microfibrils). A mesh of microfibrils is then glued together with pectin and hemicellulose which allows for greater strength and flexibility.
Lignin is a chemical found in cell walls as well which gives cells support and waterproof capabilities. Middle lamella join adjacent cell walls together with calcium pectate – adding to the strength of the plant fibres. The fibres may also contain sclerenchyma fibres – these form long tubes for strength and support, featuring lignified walls as well. All of these features give the plant fibres great tensile strength.
- Variation within fibres (random error) – use a large sample of fibres with repeats for reliability. Also, try to pick plants of roughly the same age and in the same location for validity.