8.1.3B3. Plant Nutrition and Transport
Pathway Taken by Water — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 11 Unit quiz
Part 1 of 4The route in order
1Root hair cell
- Root hair cell Water from the soil enters the long, thin extension of a root hair cell.
2Root cortex cells
- Root cortex cells It passes across the cortex, moving from cell to cell towards the centre of the root.
3Xylem vessels
- Xylem vessels It enters the xylem in the middle of the root and is carried up the stem to the leaves.
4Leaf mesophyll cells
- Leaf mesophyll cells In the leaf it leaves the xylem and enters the mesophyll cells, where some is used and some evaporates.
Part 2 of 4Why water moves into the root: osmosis
Soil water is dilute
- Soil water is dilute Soil water contains few dissolved substances, so it has a high water potential.
Root hair cell sap is concentrated
- Root hair cell sap is concentrated The cytoplasm and vacuole of a root hair cell contain dissolved sugars and ions,
- so water potential is lower inside the cell.
Net movement
- Net movement Water moves by osmosis across the partially permeable cell membrane, from the soil into the root hair cell.
Gradient continues inward
- Gradient continues inward Each cell further in has a slightly lower water potential than the one before,
- so water keeps moving across the cortex towards the xylem.
Part 3 of 4Structures along the pathway
| Structure | Adaptation or feature | Role in the pathway |
|---|---|---|
| Root hair cell | Long, narrow extension that gives a large surface area | Absorbs water from the soil by osmosis |
| Cortex cells | Many thin-walled cells in contact with each other | Water passes through them from the outside of the root towards the centre |
| Xylem vessel | Hollow tubes of dead cells with no end walls and strengthened walls | Carries water and dissolved minerals up to the leaves |
| Mesophyll cells | Moist cell walls exposed to air spaces | Receive water; some is used in photosynthesis and the rest evaporates |
Part 4 of 4Checking your understanding of the sequence
1
Start by naming the cell at the soil boundary: the root hair cell.
2
State the process that moves the water in: osmosis, down a water potential gradient.
3
Name the tissue that carries water over long distances: the xylem.
4
Finish with where the water arrives in the leaf: the mesophyll cells.
5
In any written answer, use the words 'root hair cell → root cortex cells → xylem → leaf mesophyll cells' in that order.
Key terms
| Term | Meaning |
|---|---|
| Osmosis | The net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. |
| Root hair cell | A root cell with a long extension that absorbs water and mineral ions from the soil. |
| Cortex | The layer of cells between the outside of the root and the xylem. |
| Xylem | Tissue made of hollow tubes that carries water and minerals from roots to leaves. |
| Mesophyll | The tissue in the middle of a leaf where photosynthesis takes place. |
| Water potential | A measure of the tendency of water to move; dilute solutions have a high water potential. |
Avoid these mistakes
1
Saying water enters root hair cells by active transport. Water moves by osmosis; it is mineral ions that may be taken in by active transport.
2
Leaving out the cortex when listing the pathway.
3
Confusing the xylem (water up) with the phloem (sugars around).
4
Describing osmosis as movement from 'weak to strong solution' without saying it is the movement of water through a partially permeable membrane.
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