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4.2.5Topic 4 · Biodiversity and Natural Resources

Plant Fibres — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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4.2.5 Plant Fibres — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the structure and properties of sclerenchyma and xylem fibres and explain how they are used by humans.Plant fibres are long, narrow cells with thick walls that give strength to stems, leaves and bark.

  • sclerenchyma fibres
  • xylem tissue
  • tensile strength
  • flexibility
  • durability
  • Exam-style question 1

Section 1 of 8What are plant fibres?

Plant fibres are long, narrow cells with thick walls that give strength to stems, leaves and bark. Humans have used them for thousands of years for textiles, rope, paper and building materials. There are two main fibre-forming tissues in plants: sclerenchyma fibres and xylem tissue.

Section 2 of 8Sclerenchyma fibres

Sclerenchyma fibres are long cells, tapered at both ends, with very thick secondary walls containing cellulose and lignin.

Sclerenchyma fibres
They die when mature, leaving a hollow lumen, and are fixed in place by overlapping tapering ends, so a stem behaves like a rope of many strands.
They are located in a ring around the vascular tissue, in the phloem region and in the cortex of many plants,
  • giving the stem its strength and flexibility.

Examples used commercially include fibres from flax (linen), hemp and jute. Because the cellulose microfibrils are oriented almost parallel to the fibre axis, the fibres have a high tensile strength; they are also fairly flexible so they can be spun and woven.

Section 3 of 8Xylem fibres and vessels

Xylem contains fibres with thick lignified walls, along with vessels.

Xylem fibres and vessels
Their main role is to support the plant against gravity and wind.
Lignin makes the wall rigid and resistant to compression, so wood from trees, which is mostly xylem, is used for construction and fuel.
Compared with sclerenchyma fibres in phloem, xylem fibres are stiffer and stronger under compression, but less suitable for spinning into thread.
FeatureSclerenchyma fibresXylem tissue
Main wall componentsCellulose with some ligninCellulose with much lignin
Main functionTensile support and flexibilityWater transport and rigid support
Human useTextiles, rope, paper, compositesTimber, paper pulp, fuel

Section 4 of 8Properties and uses

The usefulness of a fibre depends on tensile strength (force needed to break it), flexibility, durability and whether it can be grown cheaply and renewably.

Properties and uses
Cotton fibres,
  • from seed hairs, are almost pure cellulose and are soft;
    • flax and hemp give stronger fibres used in rope, canvas and fabrics;
    • sisal is used for twine.
Newer uses include fibres blended into composite materials for cars and insulation, replacing synthetic materials.
These plant fibres are biodegradable and require less energy to produce than synthetic fibres from oil.

Fibre quality is affected by growth conditions, the plant species, and how fibres are separated from the stem. Retting uses microorganisms or moisture to rot away softer tissue and release the fibres.

Link

  • the practical for tensile strength (4.2.7) tests these differences experimentally;
    • the sustainability topic (4.2.10) considers fibre crops versus synthetic material.

Section 5 of 8Evaluating fibres and linking to other topics

When asked to evaluate a plant fibre you should consider both its biological structure and its economic and environmental profile.

Evaluating fibres and linking to other topics
Flax and hemp fibres are long, strong and relatively cheap, and the crops grow quickly with little pesticide, which improves sustainability.
Cotton produces a soft fibre, but it is thirsty: large volumes of water for irrigation can cause problems for local supplies.
Synthetic fibres such as nylon or polyester are strong and cheap but come from oil, are not biodegradable and release microplastics when washed.

Fibres also form a link between cellular structure and the tissues learned about in stems.

1
The strength of a sclerenchyma fibre comes from the three-dimensional arrangement of cellulose microfibrils, themselves made of chains linked by hydrogen bonds.
2
Strength can be enhanced by lignin, but lignin makes fibres stiffer and less able to be spun.
3
Data questions may give breaking forces for different fibres and ask you to compare them; remember to quote figures with units, calculate the difference or ratio,
  • and link it to structure, for instance the proportion of cellulose or the length of the fibres.
4
Fibres can also be tested for quality by measuring how far they stretch before breaking, though this is not required at this level.

Remember that fibre strength is also influenced by moisture and processing.

Evaluating fibres and linking to other topics
Linen from flax is stronger when wet, whereas some other fibres weaken.
Fibres can be combined with resins in composite materials, in which the fibre carries tension and the resin spreads the load, an idea similar to the way lignin and cellulose cooperate in wood.
Recognising such parallels between natural structures and engineered materials is a useful way to show higher-level understanding.

Section 6 of 8Exam-style questions

  1. Explain why sclerenchyma fibres are able to provide support in plant stems. [3]3 marks

  2. Suggest why plant fibres might be preferred to synthetic fibres for some products. [2]2 marks

  3. Describe two differences between sclerenchyma fibres and xylem vessels. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Sclerenchyma fibreLong dead cell with thick lignified wall giving support to plants.
LigninRigid waterproof polymer in xylem and sclerenchyma walls.
Tensile strengthMaximum pulling force a material can withstand before breaking.
LumenHollow central space of a dead fibre or vessel.
RettingProcess using microbes or moisture to free fibres from stem tissue.
FlaxPlant whose fibres are made into linen.
HempPlant grown for strong stem fibres.
BiodegradableAble to be broken down by decomposers.

Section 8 of 8Exam technique

1
Believing all plant fibres are livingmature sclerenchyma and xylem fibres are dead.
2
Stating that fibres carry waterthis is the job of vessels.
3
Confusing tensile strength (resisting pulling) with compressive strength.
4
Assuming lignin is present in cottoncotton is almost pure cellulose.
5
Describing retting as chemical extraction when it is rotting of other tissues by microorganisms.

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