7.2.17 · Transport in Plants
Water and Mineral Ion Transport in Plants — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 8 parts Lesson 8 of 15 Unit quiz
Lesson summary
7.2.1 Water and Mineral Ion Transport in Plants — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how water and mineral ions enter a root and move across the cortex to the xylem by the apoplast, symplast and vacuolar pathways.
- Root hairs
- Effect on water potential
- Shared features
- Casparian strip
- Forced entry to the symplast
- Selective uptake
Part 1 of 6Uptake at the root
1Root hairs
- Root hairs extensions of epidermal cells, thousands of which give a very large surface area, with thin walls and membranes giving a short diffusion path.
2Water
- Water it enters by osmosis because the soil solution is dilute and has a higher water potential than the root hair cytoplasm.
3Mineral ions
- Mineral ions ions such as nitrate, phosphate and magnesium are usually at a lower concentration in the soil,
- so they are taken up by active transport through carrier proteins using ATP from respiration.
- Some ions move in by facilitated diffusion when the gradient allows.
4Effect on water potential
- Effect on water potential uptake of minerals lowers the water potential of the root hair cell, which drives water uptake by osmosis.
Part 2 of 6Pathways across the cortex
- Gradient
- water potential falls progressively from the root hair, through the cortex, to the xylem.
| Pathway | Route | Membranes crossed | Features |
|---|---|---|---|
| Apoplast | Cell walls and intercellular spaces | None until the endodermis | Fast, carries most water; cellulose walls are porous and water is pulled along by cohesion and tension |
| Symplast | Cytoplasm, via plasmodesmata between cells | One, on entry | Slower; water moves by osmosis into cells and by diffusion through plasmodesmata |
| Vacuolar | Vacuole to vacuole, through cytoplasm | Several, including the tonoplast | Slowest, with most resistance |
- Shared features
- all three are driven by the water potential gradient and take water towards the xylem.
Part 3 of 6The endodermis and Casparian strip
1Casparian strip
- Casparian strip a waterproof band of suberin in the walls of endodermal cells, the innermost cortex layer, which blocks the apoplast pathway.
2Forced entry to the symplast
- Forced entry to the symplast water and dissolved ions must pass through the cell surface membrane into the cytoplasm of endodermal cells.
3Selective uptake
- Selective uptake membrane proteins control which ions enter the stele, so harmful substances can be excluded.
4Root pressure
- Root pressure ions are actively pumped into the xylem, which lowers its water potential, so water follows by osmosis and creates a push.
- Root pressure is small and cannot account for water reaching the top of tall trees, but it contributes at night and in small plants.
Part 4 of 6Entering the xylem
Water
- Water after the endodermis and pericycle, water enters the xylem vessels and is moved upward by the transpiration stream.
Mineral ions
- Mineral ions they move in the xylem dissolved in the water, and also travel in the apoplast and in the symplast.
- (by diffusion and water flow)
Oxygen needs
- Oxygen needs active transport requires oxygen and ATP, so waterlogged soils reduce ion uptake and plants show deficiency symptoms.
Fungal hyphae
- Fungal hyphae those associated with roots increase the surface area for uptake of phosphate and water, in return for sugars from the plant.
Part 5 of 6Evidence and investigation
Respiratory inhibitors
- Respiratory inhibitors cyanide reduces uptake of mineral ions, which is evidence for active transport.
Tracers
- Tracers a dye or radioactive tracer placed in the soil can be followed through sections to show the route.
Water uptake
- Water uptake it is measured with a potometer.
Hydroponics
- Hydroponics plants grown without nitrate show stunted growth and yellow older leaves because nitrate is needed for amino acids and chlorophyll,
- and lack of magnesium causes yellowing between the veins.
Part 6 of 6More from the specification
1
Transpiration Pull and Stomatal Regulation
- Evaporation of water molecules from mesophyll cell walls followed by diffusion through open stomata creates negative hydrostatic pressure (tension) in the xylem, pulling continuous water columns upwards by cohesion and adhesion.
2
Translocation of Assimilates in Phloem Tissue
- Photosynthetic assimilates, primarily sucrose, are actively loaded into sieve tube elements in phloem tissue at the source and transported to metabolic sinks (such as roots and growing fruits).
- (such as mature leaves)
3
Mass Flow Hypothesis in Sieve Tubes
- High sucrose concentration at the source lowers water potential,
- causing water to enter sieve tubes by osmosis;
- the resulting hydrostatic pressure gradient drives mass flow of sap towards sinks where sucrose is unloaded.
4
Companion Cell Symport and Phloem Loading
- Companion cells actively pump protons into cell walls, establishing an electrochemical gradient that drives the movement of sucrose molecules into the phloem via proton-sucrose cotransporter proteins.
5
Transpiration Pull and Stomatal Regulation
- Evaporation of water molecules from mesophyll cell walls followed by diffusion through open stomata creates negative hydrostatic pressure (tension) in the xylem, pulling continuous water columns upwards by cohesion and adhesion.
6
Translocation of Assimilates in Phloem Tissue
- Photosynthetic assimilates, primarily sucrose, are actively loaded into sieve tube elements in phloem tissue at the source and transported to metabolic sinks (such as roots and growing fruits).
- (such as mature leaves)
7
Mass Flow Hypothesis in Sieve Tubes
- High sucrose concentration at the source lowers water potential,
- causing water to enter sieve tubes by osmosis;
- the resulting hydrostatic pressure gradient drives mass flow of sap towards sinks where sucrose is unloaded.
8
Companion Cell Symport and Phloem Loading
- Companion cells actively pump protons into cell walls, establishing an electrochemical gradient that drives the movement of sucrose molecules into the phloem via proton-sucrose cotransporter proteins.
9
Loading
- Movement of sucrose into the sieve tubes at a source.
10
Hydrostatic pressure
- The pressure a fluid exerts on the walls of its container.
11
Sieve tube
- A living tube in phloem that carries sap.
12
Companion cell
- A cell that supplies energy to a sieve tube.
13
Sucrose
- The sugar mainly transported in phloem.
14
Phloem
- Tissue carrying sucrose and amino acids.
15
Movement
- An action by an organism or part of it causing a change of position or place.
16
Stomata
- Pores in the leaf through which water vapour leaves.
17
Tissue
- A group of cells of similar structure that perform the same function.
18
Assimilates
- Organic substances such as sucrose and amino acids transported in the phloem.
19
Source
- A part of a plant that releases sucrose or amino acids.
20
Mesophyll
- The tissue in the middle of a leaf where photosynthesis takes place.
Key terms
| Term | Meaning |
|---|---|
| Root hair cell | An epidermal cell with a long extension that increases surface area for absorption. |
| Apoplast pathway | Movement of water through cell walls and intercellular spaces. |
| Symplast pathway | Movement of water through cytoplasm connected by plasmodesmata. |
| Casparian strip | A band of suberin in endodermal cell walls that blocks the apoplast pathway. |
| Endodermis | The layer of cells surrounding the stele in a root. |
| Root pressure | Pressure that pushes water up the xylem, caused by active transport of ions into it. |
| Water potential | A measure of the tendency of water to move; pure water is 0 and solutions are negative. |
Avoid these mistakes
1
Saying water is taken up by active transport — water moves by osmosis and ions are actively transported.
2
Saying the Casparian strip blocks the symplast pathway — it blocks the apoplast pathway.
3
Forgetting that the apoplast pathway carries most of the water until the endodermis.
4
Saying root hairs absorb water through pores — water crosses membranes by osmosis, with aquaporins helping.
5
Thinking root pressure pulls water to the top of a tree.
Last updated: