Co-ordinated Sciences Biology B8: Transport in Plants

Topic 8 of Cambridge IGCSE Co-ordinated Sciences 0654 and 0973 separates two transport systems. Xylem transports water and mineral ions and supports the plant. Phloem translocates sucrose and amino acids between sources and sinks. Official sections 8.1 to 8.4 connect tissue positions to water uptake, transpiration, environmental effects, wilting and changing source-sink relationships.

Xylem and phloem ownership

Xylem transports water and dissolved mineral ions from roots through stems to leaves and other tissues. It also provides mechanical support. Phloem transports sucrose and amino acids between plant regions.

In a non-woody dicotyledonous root, xylem is central and often forms a cross or star shape, with phloem between its arms. In a stem, vascular bundles are arranged around the outside of the central region; xylem lies toward the inside of each bundle and phloem toward the outside. In a leaf vascular bundle, xylem lies closer to the upper surface and phloem closer to the lower surface.

Use tissue relationships when a section is rotated. "Top" and "bottom" are not reliable labels in an unfamiliar root or stem image.

The 0654 outcome requires tissue functions and positions, not detailed xylem-vessel adaptations. Keep structure answers at the root, stem and leaf tissue-position level unless a question supplies additional information.

Water uptake and the pathway to leaves

Root hair cells are specialised epidermal cells with long extensions. Their large surface area increases contact with water around soil particles and increases uptake of water and mineral ions.

Water enters root hair cells by osmosis down a water-potential gradient through a partially permeable cell membrane. Mineral-ion uptake may involve active transport when ions move against a concentration gradient. Those mechanisms are established in Topic 3; Topic 8 focuses on the route through the plant.

The required water pathway is:

1. root hair cells
2. root cortex cells
3. xylem
4. mesophyll cells

Water crosses the root cortex and enters xylem. It travels upward in the xylem of root and stem, enters the leaf vascular bundles and reaches mesophyll cells.

A suitable stain can make the pathway through above-ground parts visible. Coloured regions in stem and leaf sections indicate where the water-carried stain travelled. This supports identification of xylem, but the stain itself does not prove every molecular part of the transpiration mechanism. Preparation, sectioning and controls belong in the practical hub.