Water uptake in roots
Water uptake in roots happens mainly by osmosis, as root hair cells with a large surface area draw in water from the soil down a water potential gradient, while mineral ions are absorbed separately by active transport.
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Where it happens
Water uptake happens at the root hair cells found near the tips of young roots, in the region just behind the root cap.
Inputs and outputs
- Input: soil water, a dilute solution with a high water potential held in the spaces between soil particles.
- Input: dissolved mineral ions in the soil water, such as nitrate, phosphate, potassium and magnesium ions.
- Input: ATP from respiration in the root hair cells, used for the active transport of mineral ions.
- Output: water delivered to the root xylem, ready to be pulled upward by the transpiration stream.
- Output: mineral ions loaded into the xylem sap and carried to the leaves for making proteins, chlorophyll and other compounds.
The steps
- Root hair cells have long, thin extensions (root hairs) that greatly increase the surface area in contact with soil water.
- Soil water is a dilute solution and so has a higher water potential than the cell sap inside the root hair cell.
- Water moves into the root hair cell by osmosis, down the water potential gradient, through the partially permeable cell membrane.
- Water then passes from cell to cell across the root cortex, each cell having a slightly lower water potential than the one before, until it reaches the xylem.
- Mineral ions such as nitrate and magnesium ions are absorbed into root hair cells separately, by active transport against their concentration gradient, using energy from respiration.
- At the endodermis, a waterproof band called the Casparian strip blocks the route between the cells, so water and ions must pass through the cytoplasm of the endodermal cells before entering the xylem; this lets the root control what reaches the xylem.
- Once inside the xylem, the water joins the continuous column that is drawn upward by transpiration pull, and the lowered water potential in the root xylem keeps drawing fresh water in from the cortex.
Why it matters and how it is controlled
The large surface area, thin wall and large vacuole of root hair cells make water uptake efficient. Because mineral ions in the soil are often more dilute than inside the root, the plant relies on active transport, not osmosis, to obtain enough of them, which is why root hair cells contain abundant mitochondria.
The rate of uptake is controlled from above rather than by the root itself. When transpiration is rapid, water is removed from the root xylem quickly, the water potential gradient across the cortex steepens, and uptake speeds up.
When the stomata close at night, uptake slows. This coupling means that a plant losing water faster than its roots can supply it will wilt.
Soil conditions also set limits. Waterlogged soil has little oxygen, so root cells cannot respire aerobically to produce the ATP needed for active transport of mineral ions, and root hairs may die.
Soil with a high salt concentration can have a lower water potential than the root hair cell sap, so water leaves the root by osmosis and the plant wilts even though the soil is wet. This is why over-fertilising a plant damages it.
Mineral ion uptake is selective. The carrier proteins in the root hair cell membrane recognise particular ions, so the plant takes up nitrate and magnesium in the proportions it needs rather than whatever is most abundant in the soil.
How it is examined
You may be asked to explain how root hair cells are adapted for water uptake, to explain why water moves in by osmosis while mineral ions need active transport, or to explain why root hair cells have abundant mitochondria.
Structured questions often show a diagram of a root hair cell in soil and ask you to draw an arrow for the direction of water movement, then explain it using water potential. Marks are awarded for stating that soil water has a higher water potential than the cell sap and that water moves by osmosis across a partially permeable membrane.
A second style presents data on ion uptake when roots are supplied with oxygen or a respiratory inhibitor, and asks you to explain why uptake falls without oxygen. The link between respiration, ATP and active transport must be stated clearly.
Common misconceptions
Worked exam-style question
Question. Two identical seedlings were grown in solutions containing nitrate ions. Solution A was bubbled with air; solution B was bubbled with nitrogen gas so that it contained no oxygen.
After 24 hours the nitrate concentration inside the roots of seedling A was five times higher than in the solution, while in seedling B it was about the same as the solution. (a) Name the process by which seedling A absorbed nitrate ions.
(b) Explain the difference between the two seedlings. (c) State how root hair cells are adapted for this process.
(d) Explain why water uptake by both seedlings was less affected than nitrate uptake.
Model answer. (a) Active transport. (b) Seedling A had oxygen, so its root cells carried out aerobic respiration and produced ATP.
The ATP powered carrier proteins that moved nitrate ions against the concentration gradient, so ions accumulated inside the root. Seedling B had no oxygen, so little ATP was produced; nitrate could only enter by diffusion until the concentrations were equal.
(c) Root hair cells have abundant mitochondria to supply ATP and a large surface area with carrier proteins in the membrane. (d) Water enters by osmosis, which is passive and needs no ATP, so it continues as long as a water potential gradient exists between the solution and the cell sap.
Source:SRC-DSKP-EN
Frequently asked questions
Why do mineral ions need active transport but water does not?
How are root hair cells adapted for absorbing water?
What happens to water uptake if a plant is over-fertilised?
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