Xylem, Phloem, Water Uptake and Transpiration
BiologyΒ· 8.1, 8.2, 8.3Β· 25 min read
1. Xylem and Phloem Tissue Structure & Functionβ β ββββ± 5 min
Xylem
Dead, hollow, lignified tissue that transports water and dissolved mineral ions from the roots to all above-ground parts of the plant, and provides structural support to stems and leaves.
Phloem
Living tissue made of sieve tube elements (with perforated sieve plates) and companion cells, that transports sucrose and amino acids around the plant in a process called translocation.
State two structural and two functional differences between xylem and phloem tissue.
- 1
- Structural difference 1: Xylem is made of dead lignified cells, phloem is made of living cells.
- 2
- Structural difference 2: Xylem cells have no end walls, phloem cells have perforated sieve plates between cells.
- 3
- Functional difference 1: Xylem transports water/mineral ions, phloem transports sucrose/amino acids.
- 4
- Functional difference 2: Xylem transports substances only upwards, phloem transports substances in both directions.
Exam tip:
You may be asked to label xylem and phloem in stem, root or leaf cross-section diagrams: xylem is usually found in the centre of roots, and in vascular bundles on the inner side of stems.
2. Water Uptake by Root Hair Cellsβ β ββββ± 6 min
Root Hair Cell
Specialised epidermal cell found on the surface of plant roots, adapted for efficient uptake of water and mineral ions from soil.
Long, thin hair-like projection increases surface area by >10x for faster absorption
Thin cellulose cell wall reduces diffusion distance for water and ions
High solute concentration in cytoplasm creates a water gradient for osmosis (Extended only)
Many mitochondria provide energy for active transport of mineral ions into the cell
Explain how one adaptation of a root hair cell allows efficient water uptake from soil.
- 1
- State the adaptation: Root hair cells have a very large surface area due to their long, thin hair extension.
- 2
- Link to process: This large surface area maximises the rate of osmosis, the passive movement of water from the soil into the cell down a water concentration gradient.
What process drives the movement of water into root hair cells from soil?
Active transport
Osmosis
Transpiration
Translocation
Reveal answer
Osmosis βOsmosis is the net movement of water across a partially permeable membrane from a region of higher water concentration to lower water concentration, which powers water uptake by roots.
3. Transpiration and the Transpiration Streamβ β β βββ± 7 min
Transpiration
The loss of water vapour from plant leaves, first via evaporation of water from the surfaces of mesophyll cells into internal air spaces, then via diffusion of water vapour out of open stomata into the atmosphere.
The transpiration stream is the continuous, passive flow of water from the roots, up through xylem vessels in the stem, to the leaves where it is lost as water vapour. This stream also carries dissolved mineral ions to all plant tissues, and cools leaves via evaporative cooling.
Describe the full pathway of a water molecule from soil to the atmosphere through a flowering plant.
- 1
- Water moves into the root hair cell via osmosis, then passes through root cortex cells to reach xylem vessels in the centre of the root.
- 2
- Water is transported upwards through xylem vessels in the stem to the mesophyll tissue of leaves.
- 3
- Water evaporates from the moist surface of mesophyll cells into the air spaces inside the leaf.
- 4
- Water vapour diffuses out of the leaf through open stomata into the surrounding atmosphere.
Exam tip:
Always name all tissues in the pathway for full marks: root hair β root cortex β xylem β mesophyll cells β stomata. Do not skip key steps.
4. Factors Affecting Transpiration Rateβ β β βββ± 6 min
Factor | Effect on Transpiration Rate | Explanation |
|---|---|---|
Temperature | Increases | Higher temperature increases evaporation rate of water from mesophyll cells, and speeds up diffusion of water vapour out of stomata. |
Wind / Air movement | Increases | Wind blows away water vapour that accumulates near stomata, maintaining a steep concentration gradient for water vapour diffusion out of leaves. |
Humidity | Decreases | High humidity means air outside the leaf has high water vapour concentration, reducing the diffusion gradient for water loss. |
Light intensity | Increases (up to a limit) | Brighter light causes stomata to open wider to take in COβ for photosynthesis, allowing more water vapour to escape. |
A student moves a potted plant from a shaded, humid garden into a sunny, windy position. Explain the effect on transpiration rate.
- 1
- State the overall effect: Transpiration rate will increase significantly.
- 2
- Link to light: Higher light intensity causes stomata to open wider, allowing more water vapour to escape.
- 3
- Link to wind: Wind blows away water vapour near stomata, maintaining a steep diffusion gradient for water loss.
- 4
- Link to temperature (if sunny): Higher temperature from sun increases evaporation rate of water from mesophyll cells.
5. Extended Only: Water Potential, Turgor and Plasmolysisβ β β β βExtended onlyβ± 5 min
Water Potential
A measure of the tendency of water to move from one region to another. Water always moves down a water potential gradient, from a region of higher water potential to lower water potential, across a partially permeable membrane.
Turgid
Describes a plant cell placed in pure water: water moves into the cell by osmosis, the cytoplasm swells and presses against the rigid cell wall, making the cell firm. Turgor supports non-woody plant tissues.
Plasmolysed
Describes a plant cell placed in a concentrated solution: water moves out of the cell by osmosis, the cytoplasm shrinks and pulls away from the cell wall, making the cell flaccid.
A piece of potato is placed in a concentrated sucrose solution. Explain why it decreases in mass.
- 1
- Compare water potentials: The concentrated sucrose solution has a lower water potential than the cytoplasm of potato cells.
- 2
- Describe water movement: Water moves out of the potato cells by osmosis, down the water potential gradient.
- 3
- Link to mass change: Loss of water from cells causes the potato tissue to shrink, reducing its overall mass.
6. Extended Only: How Water Rises in the Xylemβ β β β βExtended onlyβ± 5 min
Extended candidates must be able to explain how water is pulled up the xylem, and how the structure of the leaf affects how quickly water is lost.
Transpiration pull
As water evaporates from the mesophyll cells and is lost from the leaf, more water is drawn up the xylem to replace it. This pulling force is called the transpiration pull.
The water in the xylem forms a continuous column from the roots to the leaves. Water molecules are attracted to one another (this attraction is called cohesion), so they stick together. Because of cohesion, when water is pulled out at the top of the column by the transpiration pull, the whole column of water is drawn up the xylem together, without breaking.
Explain how water moves up the xylem from the roots to the leaves.
- 1
- Water evaporates from the surfaces of the mesophyll cells and is lost from the leaf by transpiration.
- 2
- This creates a transpiration pull that draws water up the xylem to replace the water lost.
- 3
- The water in the xylem forms a continuous column; the water molecules are held together by cohesion (forces of attraction between water molecules).
- 4
- Because the molecules stick together, the whole column of water is pulled up the xylem as one, from the roots to the leaves.
Exam tip:
For full marks, use the exact ideas the mark scheme wants: a transpiration pull draws up a continuous column of water, and the column is held together by cohesion (forces of attraction between water molecules).
7. Common Pitfalls
Wrong move:
Stating phloem only transports nutrients downwards from leaves to roots
Why:
Phloem transports sucrose and amino acids in both directions, to growing shoots, fruits, seeds and storage organs as well as roots
Correct move:
State that phloem transports dissolved nutrients in both directions around the plant via translocation
Wrong move:
Confusing transpiration and translocation
Why:
Transpiration is water loss from leaves via xylem, while translocation is transport of food via phloem
Correct move:
Explicitly link transpiration to xylem/water, and translocation to phloem/sucrose/amino acids
Wrong move:
Claiming active transport moves water into root hair cells
Why:
Water moves via passive osmosis; active transport is only used to uptake mineral ions against a concentration gradient
Correct move:
Link water uptake to osmosis, and mineral ion uptake to active transport
Wrong move:
Stating stomata open to allow transpiration
Why:
Stomata open to take in COβ for photosynthesis; water loss via transpiration is an unavoidable side effect
Correct move:
Explain transpiration is a consequence of stomata being open for gas exchange for photosynthesis
Wrong move:
(Extended) Describing plasmolysis as plant cell bursting
Why:
Plant cells have a rigid cell wall that prevents bursting; plasmolysis is only shrinkage of cytoplasm away from the cell wall
Correct move:
State plant cells become turgid in pure water, and plasmolysed in concentrated solutions; only animal cells burst in pure water
8. Quick Reference Cheatsheet
Term | Core Function/Feature |
|---|---|
Xylem | Transports water/minerals upwards, dead lignified cells |
Phloem | Transports sucrose/amino acids both directions, living cells |
Root Hair Cell | Large surface area for osmosis of water from soil |
Transpiration | Loss of water vapour from leaves via stomata |
Transpiration Rate Drivers | Increases with βtemp, βwind, βlight, βhumidity |
(Extended) Turgid | Firm plant cell full of water, supports non-woody tissue |
(Extended) Plasmolysed | Flaccid plant cell, cytoplasm shrunk from cell wall |
(Extended) Transpiration pull | Draws a continuous water column up the xylem; column held together by cohesion (attraction between water molecules) |
9. Frequently Asked
What is the key difference between xylem and phloem transport direction?
Xylem transports water and mineral ions only upwards from roots to leaves. Phloem transports sucrose and amino acids both upwards and downwards to all growing and storage regions of the plant, in a process called translocation.
What factors increase transpiration rate?
Four main factors increase transpiration rate: higher temperature, increased wind/air movement, brighter light (stomata open wider for photosynthesis), and lower humidity. All of these increase the concentration gradient for water vapour to diffuse out of leaves.
Going deeper
What's Next
Now that you have mastered xylem, phloem, water uptake and transpiration, you are ready to progress to the remaining subtopics in the CIE IGCSE Biology 0610 Transport in Plants unit, including translocation and practical investigations of transpiration rate using a potometer. This topic is frequently tested in both Core (Paper 1/3) and Extended (Paper 2/4) exams, so practice drawing labelled diagrams of root hair cells and vascular tissue cross-sections, and answering extended response questions about transpiration factors to maximise your marks. You should also work through past paper questions for this topic to familiarise yourself with exam phrasing and mark scheme requirements.