Plant transport, support, minerals and sustainable materials

What you’ll learn

• Compare xylem vessels, phloem and sclerenchyma fibres.
• Explain the importance of water, nitrate, calcium and magnesium to plants.
• Interpret fibre-strength evidence and evaluate plant-based materials.

Understand the idea

Xylem vessels are formed from dead cells whose contents and end walls have largely disappeared, creating a water-conducting pathway. Lignified secondary walls strengthen the vessels and resist collapse; pits permit lateral movement. Xylem transports water and mineral ions and also supports the plant. Phloem contains living sieve-tube elements linked through sieve plates, with companion cells supplying metabolic support. It translocates organic solutes between sources and sinks, which can be in different directions in different sieve tubes.

Sclerenchyma fibres have thick, often lignified walls and are usually dead at maturity; their long, narrow form gives mechanical support. In a typical young dicot stem, vascular bundles form a ring, with xylem towards the centre and phloem towards the outside; sclerenchyma may reinforce the outer part. Roots and leaves have different arrangements, so identify a tissue using structure and context. Core practical 7 develops tissue drawings, while core practical 8 compares fibre tensile strength, controlling length, diameter and preparation. A force comparison alone can be misleading if fibres differ in cross-sectional area.

Water supplies a solvent, transport medium, photosynthetic reactant and turgor. Nitrate provides nitrogen for amino acids and nucleic acids; magnesium is part of chlorophyll; calcium contributes to wall and membrane functions, including pectate in the middle lamella. Deficiency symptoms require controlled comparison because several causes can look similar. Plant fibres and starch-based materials can replace some petroleum-based products, but sustainability depends on land use, water, fertiliser, processing energy, durability and disposal. Renewable does not automatically mean impact-free or biodegradable under every condition.

Key formulae

$$\text{tensile stress}=\frac{\text{force}}{\text{cross-sectional area}}$$

Worked example

Fibre A breaks at 12 N and has cross-sectional area 0.20 mm². Fibre B breaks at 18 N and has area 0.40 mm². Compare their breaking stress in N per mm².

1. For A, stress is 12/0.20 = 60 N per mm².
2. For B, stress is 18/0.40 = 45 N per mm².
3. B withstands a larger total force, but A withstands a larger force per unit area.

Answer: A has the greater breaking stress: 60 compared with 45 N per mm².