Water UptakeCambridge IGCSE Biology: Revision notes

Section 1

Root Hair Cells and Water Uptake

Water uptake begins at root hair cells, specialised cells on the surface of roots.

Each root hair cell has a long, thin extension that greatly increases its surface area
A large surface area allows more water (and mineral ions) to be absorbed from the soil
Root hair cells are the entry point for water into the plant

Key termsroot hair cell

Exam tip

If asked why root hairs are effective at water uptake, the answer is always about their large surface area.

Section 2

The Pathway of Water Through the Plant

Once absorbed, water follows a set pathway through the plant:

Root hair cells absorb water from the soil
Water crosses the root cortex cells
Water enters the xylem
Water moves upwards through the stem to the mesophyll cells of the leaf

Section 3

Investigating Water Movement

The pathway of water through a plant can be investigated using a coloured stain.

A cut plant stem is placed in water containing a stain (e.g. a dye)
Over time, the dye is drawn up through the xylem
Cutting the stem and leaf at intervals reveals which structures the dye has reached, showing the route water takes through the root, stem and leaf

Example

A stained xylem shows up as coloured strands in a cross-section of stem, confirming that the xylem (not the phloem) carries water upwards.

Section 4

How Does Water Move Up the Stem? The Transpiration Pull

Water moves upwards through the xylem against gravity because of the transpiration pull.

As water evaporates from leaf cells and diffuses out through the stomata (transpiration), it creates a 'pull' on the water column beneath it
Water molecules are held together by forces of attraction between them (cohesion)
This cohesion means that as water is lost at the top, a continuous column of water is drawn up through the xylem from the roots

Key termstranspiration pull

Think of it like this

Think of the water column in the xylem like a continuous chain - pulling one end (evaporation at the leaf) drags the whole chain (the water column) upward.

Must Know

Root hair cells have a large surface area, increasing uptake of water and mineral ions
Water pathway: root hair cells to root cortex cells to xylem to mesophyll cells
A stain can be used to trace the pathway of water through a plant
Water moves up the xylem due to the transpiration pull, caused by water loss at the leaves and cohesion between water molecules

That's the notes covered.

Carry on to the next subtopic.