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This lesson covers the mechanisms of water uptake by roots and mineral ion transport as required by the Edexcel A-Level Biology specification (9BI0). You need to understand how water moves from the soil into the root xylem, the three pathways water takes across the root cortex, the role of the Casparian strip, and how mineral ions are absorbed.
To understand water uptake, you need to know the structure of a typical dicotyledonous root in cross-section:
| Layer (from outside to inside) | Description |
|---|---|
| Root hair cells (part of the epidermis) | Elongated epidermal cells with hair-like extensions that project into the soil; greatly increase the surface area for water and mineral ion absorption |
| Cortex | Several layers of parenchyma cells with thin cellulose cell walls and many intercellular air spaces |
| Endodermis | A single layer of cells surrounding the vascular tissue; contains the Casparian strip (a band of suberin, a waterproof waxy substance, in the cell walls) |
| Pericycle | A layer of cells inside the endodermis; can give rise to lateral roots |
| Xylem | Star-shaped (in cross-section) in the centre of the root; transports water and mineral ions upward |
| Phloem | Located between the arms of the xylem star; transports sucrose and amino acids |
Root hair cells are highly adapted for absorption:
| Adaptation | Function |
|---|---|
| Long, thin extension (hair) | Massively increases the surface area in contact with soil water |
| Thin cell wall | Short diffusion pathway |
| Many mitochondria | Provide ATP for active transport of mineral ions |
| Large vacuole with a dilute solution | Maintains a low (negative) water potential to draw water in by osmosis |
| No cuticle | Allows free movement of water across the cell surface |
Key Definition: Water potential (ψ) — a measure of the tendency of water to move from one place to another. Pure water has a water potential of zero (ψ = 0). The addition of solutes lowers the water potential (makes it more negative). Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential.
Water enters the root hair cell by osmosis — the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Once water has entered the root hair cell, it must travel across the cortex to reach the xylem. There are three pathways:
| Pathway | Route | Membranes crossed | Speed |
|---|---|---|---|
| Apoplast | Cell walls and intercellular spaces | None | Fastest |
| Symplast | Cytoplasm and plasmodesmata | Cell surface membrane (once) | Moderate |
| Vacuolar | Cytoplasm, plasmodesmata, and vacuoles | Cell surface membrane and tonoplast | Slowest |
Exam Tip: Make sure you can draw a labelled diagram showing all three pathways across a root cross-section. Examiners often ask you to describe the difference between the apoplast and symplast pathways and to explain where and why the apoplast pathway is blocked.
The apoplast pathway is the fastest route for water across the cortex, but if water could pass freely through the cell walls all the way to the xylem, there would be no way to control which substances enter the xylem. The plant needs a checkpoint.
The Casparian strip is a band of waterproof, waxy substance called suberin deposited in the cell walls of the endodermal cells (the innermost layer of the cortex, surrounding the vascular tissue).
| Function | Explanation |
|---|---|
| Selective control | Endodermal cells can actively pump mineral ions into the xylem and exclude harmful substances |
| Maintains the water potential gradient | Active transport of ions into the xylem lowers the water potential in the xylem, pulling water in by osmosis |
| Prevents backflow | Suberin is waterproof, so water in the xylem cannot leak back out through the apoplast |
Key Definition: Casparian strip — a band of suberin in the cell walls of endodermal cells that blocks the apoplast pathway, forcing water and solutes to pass through the living cytoplasm of the endodermis and allowing the plant to control which substances enter the xylem.
Once water has passed through the endodermis (via the symplast), it enters the xylem by osmosis.
The water potential in the xylem is kept very low (very negative) by two mechanisms:
Plants require a range of mineral ions for healthy growth:
| Mineral ion | Use in the plant |
|---|---|
| Nitrate (NO₃⁻) | Needed to synthesise amino acids and proteins, nucleic acids, and chlorophyll |
| Magnesium (Mg²⁺) | Component of the chlorophyll molecule; essential for photosynthesis |
| Potassium (K⁺) | Involved in stomatal opening and closing; enzyme activation |
| Phosphate (PO₄³⁻) | Component of ATP, DNA, RNA, and phospholipids |
| Calcium (Ca²⁺) | Component of the middle lamella (calcium pectate); involved in cell wall structure and signalling |
| Iron (Fe²⁺/Fe³⁺) | Required for electron carriers in respiration and photosynthesis; component of some enzymes |
Mineral ions are typically present in the soil at very low concentrations — often lower than inside the root cells. This means they cannot be absorbed by diffusion alone. Instead, most mineral ions are taken up by active transport.
| Evidence | Explanation |
|---|---|
| Mineral ions accumulate against their concentration gradient | Root cells often contain ion concentrations 10–100 times higher than the soil solution — this cannot be achieved by diffusion alone |
| Metabolic inhibitors reduce uptake | Treating roots with cyanide (which blocks respiration) or removing oxygen (anaerobic conditions) greatly reduces mineral ion uptake, confirming that ATP is required |
| Uptake rate increases with temperature — up to a point | Higher temperature increases enzyme and carrier protein activity; this is consistent with an active, enzyme-dependent process |
| Selective uptake | Plants absorb some ions preferentially over others, even when they are present at similar concentrations in the soil — this requires specific carrier proteins |
Exam Tip: When explaining mineral ion uptake, always state that it occurs by active transport because ions are taken up against their concentration gradient. Link this to the need for ATP (from respiration), the role of carrier proteins, and the evidence from metabolic inhibitor experiments.
Many plants form a mutualistic association with mycorrhizal fungi. The fungal hyphae extend from the root surface into the soil, greatly increasing the effective surface area for water and mineral ion absorption (especially phosphate, which is often present at very low concentrations in the soil).
| Benefit to the plant | Benefit to the fungus |
|---|---|
| Increased absorption of water and mineral ions (especially phosphate) | Receives sugars (organic carbon) produced by the plant through photosynthesis |
This is a mutualistic (mutually beneficial) relationship.
The following terms are important for understanding water movement:
| Symbol | Term | Description |
|---|---|---|
| ψ | Water potential | Overall tendency of water to move; measured in kPa |
| ψs | Solute potential | Contribution of dissolved solutes; always negative or zero |
| ψp | Pressure potential | Contribution of physical pressure (turgor); usually positive in plant cells |
Water potential equation:
ψ = ψs + ψp
Water always moves from a region of higher (less negative) ψ to a region of lower (more negative) ψ.
Exam Tip: In calculations, remember that water potential values for biological solutions are usually negative. The less negative the value, the higher the water potential. Water moves from less negative to more negative.
A common practical involves placing pieces of plant tissue (e.g. potato cylinders) in solutions of different sucrose concentrations and measuring changes in mass or length.
| Feature | Detail |
|---|---|
| Root hair cells | Long extensions; large SA; many mitochondria for active transport |
| Apoplast pathway | Through cell walls; fast; blocked by Casparian strip at endodermis |
| Symplast pathway | Through cytoplasm and plasmodesmata; crosses membranes |
| Casparian strip | Suberin band in endodermal cell walls; forces symplast route; selective barrier |
| Mineral ion uptake | Active transport (against concentration gradient); requires ATP and carrier proteins |
| Key minerals | Nitrate (proteins), Mg²⁺ (chlorophyll), K⁺ (stomata), PO₄³⁻ (ATP, DNA) |
| Mycorrhizae | Fungal hyphae increase absorption of water and phosphate |
| Water potential | ψ = ψs + ψp; water moves from high ψ to low ψ |
Water uptake and mineral ion transport complete the picture of plant transport systems — together with xylem, phloem, and transpiration, they form a coherent topic that is frequently examined in both short-answer and extended-response questions.
This material sits in Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport) at the root pole, completing the trio with xylem (lesson 8) and phloem (lesson 9). Candidates must (i) describe root-hair cell structure as a single epidermal-cell extension increasing SA without increasing cell number, with thin walls, no cuticle, many mitochondria and a low (negative) vacuolar Psi, (ii) explain water uptake as osmosis down a Psi gradient from dilute soil to more negative cell sap, (iii) contrast the three cortical pathways — apoplast (cell walls + intercellular spaces; no membranes; fastest), symplast (cytoplasm via plasmodesmata; one membrane), vacuolar (cytoplasm + vacuoles via tonoplast; slowest), (iv) explain the role of the Casparian strip — a band of suberin in endodermal cell walls that blocks the apoplast and forces water and solutes into the symplast where membrane transporters control entry to the xylem, and (v) explain mineral ion uptake as active transport against a concentration gradient, powered by H+-ATPase proton-pumping plus secondary co-transport, with evidence from concentration ratios, metabolic inhibitors (cyanide, anoxia) and selective uptake. Synoptic with lesson 8 (root water uptake feeds the transpiration stream), lesson 9 (H+/sucrose symporter shares the secondary-active-transport architecture of root H+/anion symporters), Topic 1 lesson 1 (water polarity drives osmosis; inorganic ions), Topic 5 (Mg2+ in chlorophyll; nitrate / ammonium for amino-acid, nucleotide and chlorophyll synthesis; phosphate for ATP, DNA, RNA, phospholipids), and Topic 5 ecology (mycorrhizal symbiosis extends effective root SA by orders of magnitude). Refer to the official Pearson Edexcel 9BI0 specification for exact wording.
Question (8 marks):
A student measures the uptake of nitrate ions (NO3−) by barley roots. The soil solution contains 0.1 mmol dm−3 NO3−; the root tissue accumulates NO3− to 12 mmol dm−3. The student then divides the roots into three batches: (i) a control in aerated solution at 20 °C, (ii) roots bubbled with N2 to remove O2, (iii) roots treated with 1 mmol dm−3 KCN (cyanide) at 20 °C. After 1 hour, NO3− uptake rates are: control 100% (relative); anoxic 8%; cyanide-treated 6%.
(a) Explain why nitrate uptake by the root cannot occur by simple diffusion alone. (2)
(b) Use the data to explain the mechanism by which nitrate is taken up. (4)
(c) Suggest one reason why anoxia and cyanide give similar (but not identical) reductions in uptake. (2)
Solution with mark scheme:
(a) M1 (AO1) — Diffusion is net movement down a concentration gradient.
A1 (AO2) — Root tissue NO3− (12 mmol dm−3) is 120-fold higher than soil NO3− (0.1 mmol dm−3); uptake is therefore against the gradient, which simple diffusion cannot achieve.
(b) M1 (AO1) — Mineral ion uptake at the root is by active transport, which requires ATP and specific carrier (transporter) proteins.
M1 (AO1) — The mechanism in plants: an H+-ATPase in the root-cell plasma membrane hydrolyses ATP to pump H+ out of the cell, establishing an electrochemical proton gradient across the membrane.
M1 (AO2) — H+ then re-enters via an H+/NO3− symporter, dragging NO3− in against its gradient — secondary active transport.
A1 (AO3) — Data support active transport: removing O2 (no aerobic respiration → no ATP) and adding cyanide (blocks cytochrome oxidase → no ATP) both collapse uptake to ~6–8%, consistent with ATP-dependence.
(c) M1 (AO3) — Both treatments cut off ATP supply (anoxia stops aerobic respiration; cyanide blocks the electron-transport chain at complex IV); both should therefore reduce uptake to a similar degree.
A1 (AO3) — They are not identical because anoxic cells can still produce a small ATP yield via anaerobic respiration (lactate / ethanol fermentation, ~2 ATP per glucose); cyanide-poisoned cells in O2 may also have slightly different secondary effects on membrane integrity. The residual 6–8% may also reflect a small passive (channel-mediated) flux down the electrochemical gradient.
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