Every drop of water a plant uses enters through its roots. Root hair cells reach out into the soil and give the root a huge surface for absorbing water and mineral ions. From there the water crosses the root, rises in the xylem and reaches the cells of the leaf. A coloured stain lets you watch the route for yourself.
Root hair cells absorb water (by osmosis) and mineral ions from the soil. Their long, thin extensions give a large surface area, which increases the uptake of water and mineral ions. Water then follows the pathway root hair cells → root cortex cells → xylem → mesophyll cells. Standing a plant in a coloured dye shows that water travels up the above-ground parts in the xylem.
Fig. 1: the pathway of water from the soil, through a root hair cell and the root cortex, up the xylem to the mesophyll cells of a leaf
Section 01
Root hairs and the pathway of water.
Each root hair is a long, thin outgrowth of a single cell in the outer layer of the root. In the sim, follow the water: watch the readout that says where the water is now, then untick Show root hairs and compare the uptake.
Functions of root hair cells: absorb water from the soil (by osmosis) and absorb mineral ions such as nitrate and magnesium (by active transport). They also help anchor the root.
Large surface area: thousands of root hairs greatly increase the surface area of the root in contact with soil water, so water and mineral ions are taken up faster.
The pathway: soil water → root hair cells → root cortex cells → xylem (in the centre of the root) → up the stem in the xylem → mesophyll cells of the leaf.
Transplanting: pulling up a seedling tears off many root hairs, so it takes up less water and may wilt. Moving it with a ball of soil protects the root hairs.
Examiner trap
A root hair is part of one cell, not a separate cell and not a whole root. And the hairs increase the surface area; they do not make the water move faster through the plant.
Section 02
Tracing water with a stain.
To find out which tissue carries water up a plant, stand a cut celery stalk (or a white flower) in water coloured with a dye such as food colouring or eosin. Leave it for a few hours, then cut thin slices across the stalk. Use the time slider, then Cut a slice.
Time in the dye
What you see
at the start
stalk and leaves are their normal colour
after an hour or two
separate coloured strands rising near the outside of the stalk
after a few hours
the dye reaches the leaves and shows up in the veins
slice across the stalk
coloured dots in a ring near the outside; the rest of the tissue is unstained
Conclusion: each coloured dot is the xylem of one vascular bundle. Water (with the dye) travels only in the xylem.
Method points: cut the stalk end just before putting it in the dye; keep the plant in the light (and warm) so it transpires and draws the dye up faster; use a sharp blade and thin slices; view with a hand lens or microscope.
Examiner trap
The dye is carried up with the water, so it shows the xylem, not the phloem. Say the dye "moves with the water in the xylem", not that the dye is "absorbed by the leaves".
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the water uptake sim, what happens to the uptake readout when you untick Show root hairs?
Quick check+10 XP
As the sim follows the water inwards, in which order does the readout name the places?
Quick check+10 XP
In the stain sim, where does the red dye appear in the slice across the celery stalk?
Quick check+10 XP
After several hours in the dye, where does the stain show up in the celery leaves?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which is the correct pathway of water through a plant?
Exam style+20 XP
Why do root hair cells increase the rate of water uptake by a root?
Exam style+20 XP
A white carnation stands in blue ink for a day and its petals turn blue. Which tissue carried the ink?
Exam style+20 XP
A seedling wilts after transplanting even though the soil is wet. The most likely reason is:
Skill unlocked
Water uptake, mastered.
This skill is now recorded on your profile. Keep the chain going.