Topic 8.2 · Core
Water Uptake in Plants
Water doesn’t travel from soil to leaf by one vague process called ‘absorption’ — Cambridge wants a specific, ordered route through named cells and tissues, starting at the root hair and ending at the mesophyll. Learn the route in that order and the rest of this page’s reasoning falls into place around it.
Root hair cells
Root hair cells sit near the surface of a young root, and you should be able to recognise one in a diagram by its long, hair-like extension projecting out from the root surface into the soil. They absorb water and mineral ions, and the extension is the reason why: it gives the cell a large surface area, which means more of the cell’s membrane is in contact with the water and dissolved ions surrounding soil particles. A larger surface area in contact with the soil solution means more uptake — that structure–function link, not simply “the root hair is long,” is what an examiner is actually checking for.
Water and mineral ions don’t necessarily enter by the same mechanism. Water typically enters root hair cells by osmosis, following its own concentration gradient without any input of energy. Mineral ions are often a different story: the concentration of a given ion in the soil water can already be lower than its concentration inside the root hair cell, and diffusion only ever moves a substance from high concentration to low. Once that’s the situation, the plant needs active transport — using energy released by respiration — to keep bringing ions in against their own concentration gradient. Don’t assume every mineral ion simply diffuses in; where uptake works against the gradient, active transport is the required mechanism, not diffusion.
This chapter doesn’t re-teach osmosis or active transport in full — that mechanism sits with cell membranes more generally — but you are expected to connect it here: root hair cells are where water and mineral ion uptake actually begins.
The pathway of water through the plant
Once water is inside the root, it follows a fixed, ordered route to the leaf, and Cambridge wants that route stated in this exact order:
root hair cells → root cortex cells → xylem → mesophyll cells
- Water is absorbed from the soil by root hair cells.
- It passes across the root through the cortex cells.
- It enters the xylem.
- Xylem carries the water upward through the root and stem and into the leaf veins.
- Water reaches the mesophyll cells inside the leaf.
Two things are worth being precise about here. First, the named route runs through specific cells and tissues — root hair cells, cortex cells, xylem, mesophyll cells — rather than the looser “root, then stem, then leaf” version some students default to. Second, phloem plays no part in this pathway at all: this is exclusively a xylem story, and bringing phloem into an answer about water movement is a sign the two transport systems have been mixed together.
Investigating the pathway using a stain
The pathway of water through the above-ground parts of a plant can be made visible using a coloured stain, because the dye travels along with the water rather than staying behind. A typical method places the freshly cut base of a leafy shoot, or something like a celery stalk, into water containing a suitable stain, and leaves it long enough for the coloured water to be drawn upward. Transverse sections are then cut through the stem, and whichever tissue has taken up the colour is identified.
The reasoning runs in one direction: observing the stain in a particular tissue lets you identify that tissue as xylem, which is then evidence that water moved through the xylem to reach that point. The colour should appear specifically in the xylem, not scattered across the whole cross-section, and that specificity is the whole point of the investigation — it demonstrates that xylem, not phloem or any other tissue, is the transport pathway for water. It’s also worth being clear that this particular investigation demonstrates the pathway through the above-ground parts of the plant; it doesn’t by itself stand in for the complete root hair to mesophyll route described above.
This is a directly examinable practical skill on Cambridge’s Paper 6, so expect to interpret an unfamiliar stained specimen or section, distinguish what you actually observed from the conclusion you drew from it, and identify the transport tissue from the result rather than from memory alone.
Cross-links
This page assumes you already know what xylem is and where to find it in a section — see xylem and phloem if you need that first. What happens to the water once it reaches the mesophyll cells at the end of this pathway is covered on the transpiration page.