Question 1
Paper 4 short-answer style1 markName the process by which water enters a root hair cell from the soil. (1 mark)
Model answer
Osmosis.
Why this scores
One mark for 'osmosis'.
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Detailed notes on Transport in Plants for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Plants drink through their roots. Water enters by osmosis through ROOT HAIR cells, crosses the cortex, and reaches the xylem.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
A root hair is a cell extension that pushes between soil particles. Massive surface area, thin wall, lots of mitochondria.
Where they are. Just behind the GROWING TIP of every root branch. Older roots lose their root hairs and grow new ones.
Adaptations:
| Feature | Why it helps |
|---|---|
| Long, finger-like extension (the "hair") | Pushes between soil particles → massive contact area with soil water. |
| THIN cell wall | Water and ions pass through easily. |
| Thin cell membrane | Short diffusion path. |
| Many MITOCHONDRIA | Provide ATP for active transport of mineral ions. |
| Large vacuole | Helps maintain water potential gradient (concentrated cell sap). |
| No chloroplasts | Underground — no light. |
Surface area math.
Worked qualitative. Why does a transplanted seedling sometimes wilt for a few days?
Cambridge tip. Always say "long thin extension to GIVE A LARGE SURFACE AREA". Both the feature and the consequence are needed for full marks.
Soil water is dilute; cell sap is concentrated. Water flows IN.
The setup.
The result. Water moves DOWN the water potential gradient — from soil into the cell — by OSMOSIS through the partially permeable cell membrane.
Then what? Water moves cell-to-cell across the root cortex, each cell having slightly lower water potential than the one before. The endodermis funnels water into the xylem.
At the xylem. The xylem has very low water potential (because water is constantly being drawn up to the leaves) → water rushes in.
Worked qualitative. Why does fertiliser kill a plant if you use too much?
Cambridge tip. Always describe BOTH ends of the gradient (soil + cell). "Water moves by osmosis" alone earns 0; "soil has higher water potential than the cell, so water moves in by osmosis" earns full marks.
Soil → root hair → cortex → endodermis → xylem. All passive (osmosis).
Stage 1 — soil to root hair cell. Osmosis across the cell membrane.
Stage 2 — through the cortex. Cell-to-cell osmosis. The cortex cells are arranged so each has slightly lower water potential than the previous, creating a continuous gradient.
Stage 3 — endodermis. A specialised layer of cells with a waterproof "Casparian strip" forces water through the cell cytoplasm (not just between cells). This lets the plant control what enters the xylem.
Stage 4 — into the xylem. Water enters the hollow xylem vessels. From here it's pulled UP by transpiration (see Transpiration topic).
Why an endodermis? It acts like a filter — minerals can be selectively pumped IN; root pathogens kept OUT.
Worked qualitative. Why is the root system usually larger (more spread out) than the shoots?
Cambridge tip. Memorise the four stages in order. "Soil to xylem" alone is incomplete.
Verbatim phrases and definitions Cambridge mark schemes credit.
Water uptake appears Paper 4 (5-7 marks). Common formats: label a root cross-section, explain root hair adaptations, describe osmosis. Examiner reports flag students confusing osmosis (water) with active transport (ions).
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (8.1); 0610/42 Oct/Nov 2024 — Q5 (water uptake); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on water uptake, written exactly the way a tutor would explain them at the board.
Question
Describe how a root hair cell is adapted for the absorption of water and mineral ions.
Step-by-step solution
Step 1
Long, thin extension of the cell — gives a large SURFACE AREA in contact with the soil water.
Step 2
Thin cell wall and membrane — short diffusion distance for water to enter.
Step 3
Many mitochondria — release energy for the active transport of mineral ions.
Step 4
Cell sap of low water potential — encourages water to enter by osmosis.
Answer
Long thin extension (large surface area), thin wall (short distance), many mitochondria (energy for active transport), low water potential (draws water in).
Examiner tip
State 'large surface area' AND 'thin wall' explicitly. 'Adapted to absorb' alone earns 0.
Question
Explain how water moves from the soil into a root hair cell.
Step-by-step solution
Step 1
The root hair cell contains dissolved solutes, so its water potential is LOWER than the dilute soil water.
Step 2
The soil water has a HIGHER water potential.
Step 3
Water moves down the water potential gradient by OSMOSIS, through the partially permeable cell membrane, into the cell.
Answer
Soil water has a higher water potential than the cell, so water moves in by osmosis through the partially permeable membrane.
Question
Trace the path of a water molecule from the soil to the xylem of the root.
Step-by-step solution
Step 1
Soil water enters the root hair cell by osmosis through the cell membrane.
Step 2
Water then moves through the cortex cells, passing from cell to cell by osmosis down the water potential gradient.
Step 3
It reaches the centre of the root and enters the xylem vessels.
Step 4
From the xylem the water travels up through the stem to the leaves.
Answer
Soil → root hair cell → cortex (cell to cell by osmosis) → xylem → up to the leaves.
Question
Explain why water and mineral ions are taken up into a root hair cell by different processes.
Step-by-step solution
Step 1
Water moves in by OSMOSIS, a passive process, because the soil water has a higher water potential than the cell.
Step 2
Mineral ions are usually at a LOWER concentration in the soil than inside the cell.
Step 3
So the ions must move AGAINST their concentration gradient, which osmosis/diffusion cannot do.
Step 4
They are taken in by ACTIVE TRANSPORT, using energy from respiration and protein carriers — which is why root hair cells have many mitochondria.
Answer
Water enters by osmosis (passive, down the water potential gradient). Mineral ions enter by active transport (against their gradient, using energy from respiration).
Examiner tip
Different molecules, different mechanisms: water = osmosis; ions = active transport (usually).
Question
Describe how you could show that a leafy shoot takes up water, and how the rate could be compared in still and moving air.
Step-by-step solution
Step 1
Place a leafy shoot in a measuring cylinder of water and add a thin layer of oil on top to stop evaporation from the water surface.
Step 2
Record the water level (or the mass of the whole apparatus) at the start and again after a set time — a fall shows water has been taken up.
Step 3
To compare conditions, set up two identical shoots and place a fan near one (moving air) and leave the other in still air, keeping everything else the same.
Step 4
The shoot in moving air takes up water faster, because faster water loss from the leaves (transpiration) pulls up more water — showing uptake is linked to water loss.
Answer
Measure the fall in water level (or mass) of a shoot in water with oil on top. Comparing still vs moving air shows faster uptake in moving air, because more water is lost and so more is pulled up.
Examiner tip
Oil prevents evaporation from the water surface so the measured loss is due to the plant. A potometer measures uptake more precisely.
Question
Explain why a plant placed in very salty soil cannot take up water and may wilt, even though the soil is wet.
Step-by-step solution
Step 1
Salty soil water contains a high concentration of dissolved salt, so it has a low water potential.
Step 2
If the soil water potential is lower than that inside the root cells, water will not move into the root by osmosis.
Step 3
In fact water may move out of the root cells into the soil by osmosis, so the plant loses water.
Step 4
The cells lose turgor and become flaccid, so the plant wilts despite the soil being wet.
Answer
Salty soil has a low water potential, so water does not move into the roots by osmosis (and may even leave). The cells lose turgor and the plant wilts even though the soil is wet.
Examiner tip
The key is comparing water potentials: osmosis depends on the gradient, not just on whether water is present.
High-scoring sample answers for water uptake on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the process by which water enters a root hair cell from the soil. (1 mark)
Model answer
Osmosis.
Why this scores
One mark for 'osmosis'.
State two ways in which a root hair cell is adapted for absorbing water. (2 marks)
Model answer
It has a long, thin extension giving a large surface area in contact with the soil water, and a thin cell wall giving a short distance for water to enter.
Why this scores
Any two of: large surface area (long extension); thin wall; low water potential of cell sap. Each linked to water uptake.
Explain how water moves from the soil into a root hair cell. (3 marks)
Model answer
The soil water is dilute and has a higher water potential than the cell contents, which contain dissolved solutes and so have a lower water potential. Water therefore moves down the water potential gradient by osmosis, through the partially permeable cell membrane, into the root hair cell.
Why this scores
Three marks: soil higher water potential / cell lower; osmosis down the gradient; through a partially permeable membrane.
Describe the pathway taken by water from the soil to the xylem of a root. (4 marks)
Model answer
Water from the soil enters a root hair cell by osmosis. It then passes through the cells of the cortex, moving from cell to cell by osmosis down a water potential gradient. The water reaches the centre of the root and enters the xylem vessels, which carry it up through the stem to the leaves.
Why this scores
Four marks: soil → root hair cell; across the cortex; by osmosis (cell to cell); into the xylem.
Explain how a root takes up water and mineral ions, and why the two are absorbed by different processes. (5 marks)
Model answer
Water is taken up by osmosis: the soil water has a higher water potential than the root cells, so water moves down the gradient into the root through partially permeable membranes — a passive process needing no energy. Mineral ions, however, are usually at a lower concentration in the soil than inside the root cell, so they must move against their concentration gradient. Diffusion cannot do this, so the ions are taken in by active transport, using energy from respiration and protein carriers in the membrane. This is why the two are absorbed by different processes — water by passive osmosis, ions by energy-requiring active transport.
Why this scores
Five marks: water by osmosis; down the water potential gradient/passive; ions lower in soil than cell; active transport against the gradient; uses energy from respiration.
Explain why a plant growing in very dry or very salty soil cannot take up enough water and may wilt. (6 marks)
Model answer
Water enters the roots by osmosis, moving from the soil into the root cells only when the soil water has a higher water potential than the cells. In very dry soil there is little water available, so the plant cannot take up enough to replace what it loses by transpiration. In very salty soil the soil water contains a high concentration of dissolved salts, giving it a low water potential — possibly lower than that of the root cells. In that case water will not move into the roots, and may even move out of the root cells into the soil by osmosis. Because the cells then take in too little water (or lose water), they lose turgor and become flaccid, so the plant can no longer support its leaves and stems and it wilts, even though the salty soil is wet.
Why this scores
Up to 6 marks: water enters by osmosis; depends on soil having higher water potential; dry soil = too little water; salty soil = low water potential; water does not enter / may leave; cells lose turgor / become flaccid → wilting.
Definitions to memorise and the exact keywords mark schemes credit for water uptake answers — sharpened from recent examiner reports for the 2026 0610 sitting.
Specialised cell with a long thin extension that increases surface area for water and mineral uptake.
Movement of water from the soil (higher water potential) through root hair cells towards the xylem (lower water potential), down the gradient.
Layer of cells between the root hair cells and the xylem. Water passes through by osmosis from cell to cell.
Hollow, dead, lignified vessels that carry water up from the roots through the stem to the leaves.
The traps other students keep falling into on water uptake questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Mineral ions enter by active transport.
How to avoid it
Water = OSMOSIS (passive, no energy). Ions = active transport (usually). Different molecules, different mechanisms.
Why it happens
The name 'water uptake' biases students.
How to avoid it
Root hairs absorb water (osmosis) AND mineral ions (active transport). Both happen at the same surface.
Why it happens
Higher water potential sounds like 'more pull'.
How to avoid it
Water moves DOWN the water potential gradient (high → low). Soil water (high) → root cell (lower).
The things students keep getting wrong in this sub-topic, answered.