8.2 Water uptake
- Syllabus
- 0610–2026–2027
- Topic
- 8.2
- Level
- —
Learning objectives
8.2.1Root hair cells and state their• Identify in diagrams and images root hair cells and state their functions8.2.2Large surface area of root hairs• State that the large surface area of root hairs increases the uptake of water and mineral ions8.2.3Pathway taken by water through• Outline the pathway taken by water through the root, stem and leaf as: root hair cells, root cortex cells, xylem, mesophyll cells8.2.4Investigate, using a suitable stain• Investigate, using a suitable stain, the pathway of water through the above-ground parts of a plant
Identify root hair cells and their functions
A root hair cell is an epidermal cell near the tip of a young root. Its long, thin projection extends between soil particles.
| Substance taken up | Root hair cell function |
|---|---|
| water | absorbs water from the soil |
| mineral ions | absorbs dissolved mineral ions from the soil |
In a diagram, look for one cell on the outside of a root with a single hair-like extension. The projection is part of the cell, not a separate root.
Root hairs absorb water and mineral ions; they do not absorb glucose from soil and they are not the branching whole roots visible to the naked eye.
Explain why root hairs have a large surface area
Each root hair is a long extension of a root epidermal cell. Many root hairs together create a very large contact area with the thin films of water around soil particles.
many long root hairs → larger surface area in contact with soil → more membrane available for uptake at the same time → increased uptake of water and mineral ions
A root system with many root hairs can explore more soil-water contact than a smooth root surface of the same size, increasing the rate and total opportunity for uptake.
Large surface area increases uptake; it does not mean each root hair contains a larger volume of cell sap.
Trace water from soil through root, stem and leaf
The required pathway is: soil water → root hair cells → root cortex cells → xylem → mesophyll cells in the leaf.
| Stage | Where the water is |
|---|---|
| 1 | enters root hair cells at the root surface |
| 2 | crosses root cortex cells toward the centre |
| 3 | enters xylem and travels through root and stem to the leaf |
| 4 | leaves xylem and enters mesophyll cells |
The sequence connects uptake at the root surface to delivery inside the leaf. Xylem is the long-distance transport stage between the root cortex and leaf mesophyll.
Do not place xylem before the root cortex: water must cross from root hair cells through cortex cells before entering the central xylem.
Investigate the pathway of water using a stain
A coloured, water-soluble stain can reveal which above-ground plant tissues carry water.
| Step | Action and purpose |
|---|---|
| 1 | place the cut base of a leafy shoot or celery stalk in coloured water |
| 2 | leave it long enough for the colour to move up the shoot |
| 3 | cut thin transverse sections of the stem and examine leaf veins |
| 4 | record the positions that are stained and compare them with vascular-bundle structure |
The stain appears in the xylem of the stem and leaf veins. This shows that water moves through the above-ground parts of the plant in xylem.
The stain marks the water-conducting xylem, not every tissue in the section. Use a freshly cut surface and make thin sections carefully so the stained positions can be identified.