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This lesson covers xylem transport and transpiration as required by the Edexcel A-Level Biology specification (9BI0). You need to understand the structure of xylem vessels, the cohesion-tension theory, factors affecting the rate of transpiration, and the use of potometers.
Plants, like large animals, have a small surface area to volume ratio relative to their metabolic needs. They require transport systems to move:
Plants have two separate transport tissues:
| Tissue | Direction of transport | Substance transported |
|---|---|---|
| Xylem | Roots → leaves (upward) | Water and dissolved mineral ions |
| Phloem | Source → sink (any direction) | Sucrose and amino acids |
Xylem vessels are the main conduits for water transport. They are formed from dead cells arranged end to end in continuous tubes.
| Feature | Function |
|---|---|
| No cell contents | Unobstructed pathway for water flow |
| No end walls | Continuous hollow tube — water flows freely |
| Lignin in cell walls | Provides strength and rigidity; prevents collapse under the tension created by transpiration; waterproof |
| Lignin deposition patterns (spiral, annular, reticulate) | Spiral and annular patterns allow the vessel to stretch in growing regions; reticulate (net-like) patterns provide maximum strength in mature regions |
| Pits (gaps in the lignin) | Allow lateral movement of water between adjacent xylem vessels and into surrounding cells |
| Narrow lumen | Supports the water column by capillarity (adhesion of water to the vessel walls) |
Key Definition: Xylem vessel — a dead, hollow, lignified tube formed from cells arranged end to end with no end walls, providing an unobstructed pathway for the mass flow of water and mineral ions from roots to leaves.
Water follows this route:
The following diagram shows the transpiration stream from soil to atmosphere:
graph BT
A["Soil Water"] --> B["Root Hair Cells<br/>(osmosis)"]
B --> C["Xylem in Root"]
C -->|"Cohesion-Tension"| D["Xylem in Stem"]
D --> E["Xylem in Leaf"]
E --> F["Mesophyll Cells"]
F -->|"Evaporation"| G["Stomata"]
G --> H["Atmosphere"]
The cohesion-tension theory explains how water is pulled up through the xylem from the roots to the leaves. It is the most widely accepted mechanism.
| Force | Explanation |
|---|---|
| Transpiration pull (tension) | Water evaporates from the mesophyll cells into the air spaces of the leaf and exits through the stomata. This creates a tension (negative pressure) that pulls water upwards through the xylem. |
| Cohesion | Water molecules are attracted to each other by hydrogen bonds. This cohesion keeps the water column intact as it is pulled upward — it acts like a continuous chain. |
| Adhesion | Water molecules are attracted to the hydrophilic lignin walls of the xylem vessels. This helps support the water column against gravity and contributes to capillarity. |
Exam Tip: When explaining the cohesion-tension theory, always mention: (1) transpiration creating tension at the top, (2) cohesion keeping the water column continuous, and (3) adhesion supporting the column against gravity. This combination of three forces is the complete explanation.
| Evidence | Explanation |
|---|---|
| Xylem sap is under tension (negative pressure) | If you cut a xylem vessel, air is drawn in (rather than water flowing out), confirming negative pressure inside |
| Trunk diameter decreases during the day | Transpiration is highest during the day; the tension causes the xylem (and the trunk) to narrow slightly |
| Water movement correlates with transpiration rate | When transpiration is high (e.g. on hot, dry days), the rate of water movement through the xylem increases |
| Continuous water column | Removing a ring of bark (which contains phloem but not xylem) does not immediately stop water transport — the water column in the xylem remains intact |
Transpiration is the evaporation of water from the aerial surfaces of a plant, primarily through the stomata in the leaves. Approximately 99% of the water absorbed by the roots is lost through transpiration.
Although transpiration is often described as an "unavoidable consequence" of gas exchange, it also has benefits:
| Factor | Effect | Explanation |
|---|---|---|
| Temperature | Increase → faster transpiration | Higher temperature increases the kinetic energy of water molecules, increasing the rate of evaporation; also increases the water vapour capacity of the air (lowers its water potential) |
| Humidity | Increase → slower transpiration | Higher humidity reduces the water potential gradient between the leaf air spaces and the external air |
| Wind speed | Increase → faster transpiration | Wind removes the layer of saturated air near the leaf surface, maintaining a steep water potential gradient |
| Light intensity | Increase → faster transpiration | Light causes stomata to open (via guard cell mechanism), providing a larger pathway for water vapour loss |
| Soil water availability | Decrease → slower transpiration | Less water is available for absorption; plant may wilt and stomata may close |
Exam Tip: When explaining the effect of each factor on transpiration rate, always refer to the water potential gradient between the inside of the leaf and the external atmosphere. The steeper the gradient, the faster the rate of transpiration.
A potometer measures the rate of water uptake by a leafy shoot, which is closely related to (but not exactly equal to) the rate of transpiration. A small amount of absorbed water is used in photosynthesis and to maintain cell turgor.
| Variable | How to control/measure |
|---|---|
| Distance moved by bubble | Read from the graduated scale at regular time intervals |
| Temperature | Use a water bath or record ambient temperature |
| Wind speed | Use a fan at a set distance or shield with a screen |
| Humidity | Use a damp cloth near the leaves or a humidity meter |
| Light intensity | Use a lamp at set distances or use a light meter |
Exam Tip: A potometer does not directly measure transpiration — it measures water uptake. In exam answers, state this distinction explicitly. However, since the vast majority of water taken up is lost by transpiration, the potometer gives a good approximation of transpiration rate.
In addition to transpiration pull, some plants generate root pressure — a positive pressure in the xylem at the root caused by the active transport of mineral ions into the root xylem. This lowers the water potential inside the xylem, causing water to enter by osmosis. Root pressure can push water a short distance up the stem and contributes to guttation (the exudation of water droplets from leaf tips, especially at night when transpiration is low).
However, root pressure alone is insufficient to transport water to the top of a tall tree — the cohesion-tension mechanism is the primary driver.
| Feature | Detail |
|---|---|
| Xylem structure | Dead, hollow, lignified vessels with no end walls; pits allow lateral movement |
| Cohesion-tension theory | Transpiration pull + cohesion + adhesion drive water upward |
| Transpiration | Evaporation of water from leaves through stomata |
| Factors affecting transpiration | Temperature, humidity, wind speed, light intensity, water availability |
| Potometer | Measures rate of water uptake (approximation of transpiration rate) |
| Root pressure | Minor additional force; causes guttation |
Understanding xylem transport and transpiration is essential — expect questions on the cohesion-tension theory, potometer experiments, and factors affecting transpiration rate.
This material sits in Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport) at the plant-transport pole, paired with phloem (lesson 9) and root water uptake (lesson 10). Candidates must (i) describe xylem-vessel structure (dead, lignified, end-walls dissolved in vessel elements; pits; spiral/annular/reticulate lignin patterns), (ii) explain the cohesion-tension theory of water movement (transpiration → tension → cohesion via H-bonds → adhesion to vessel walls → mass flow), (iii) state and quantify the factors affecting transpiration (light, temperature, humidity, wind, soil water), and (iv) describe potometer methodology and its assumption (water uptake ≈ transpiration). Synoptic with lesson 1 (SA:V — leaf is the high-SA gas-exchange surface that simultaneously loses water), lesson 3 (gas exchange in plants — stomatal aperture as the tradeoff between CO2 uptake and H2O loss), lesson 9 (phloem translocation by mass flow — the symplast counterpart to xylem's apoplast/lumen highway), lesson 10 (root-hair osmosis and the apoplast/symplast/vacuolar pathways through the cortex), Topic 5 (drought-induced stomatal closure cuts CO2 supply and lowers photosynthesis), and Topic 5/8 (CAM and C4 as adaptations to water-limited environments). Refer to the official Pearson Edexcel 9BI0 specification for exact wording.
Question (8 marks):
A student sets up a potometer with a freshly cut leafy shoot of Ligustrum (privet). The capillary tube has a uniform internal diameter of 1.0 mm. Under standard laboratory conditions (still air, 20 °C, ambient humidity), the air bubble travels 60 mm in 10 minutes. The student then directs a fan at the shoot at the same temperature and re-runs the experiment; the bubble travels 180 mm in 10 minutes.
(a) Calculate the rate of water uptake in mm3 min−1 under each condition. (3)
(b) Explain in terms of water potential why the rate increases when the fan is applied. (3)
(c) State two precautions the student must take to ensure the rate of water uptake is a valid approximation of the rate of transpiration. (2)
Solution with mark scheme:
(a) M1 (AO2) — Cross-sectional area of capillary =pir2=pi(0.5)2=0.785 mm2.
M1 (AO2) — Still air: volume =60imes0.785=47.1 mm3 in 10 min, so rate =4.71 mm3 min−1.
A1 (AO2) — Fan: volume =180imes0.785=141.4 mm3 in 10 min, so rate =14.14 mm3 min−1 (a 3× increase). Many candidates lose marks here by quoting the bubble distance as the rate without converting to a volume.
(b) M1 (AO1) — In still air, water vapour accumulates in the boundary layer of saturated air immediately adjacent to the stomatal pores, raising the local water potential of that air.
M1 (AO2) — The water-potential gradient between the leaf air spaces (high Psi, near saturation) and the external air (lower Psi) is therefore shallow, so diffusion of water vapour out of the stomata is slow.
A1 (AO3) — The fan disrupts the boundary layer, replacing saturated air with drier ambient air. The water-potential gradient steepens, so the rate of evaporation (and therefore the tension transmitted down the xylem and the rate of water uptake at the roots) rises.
(c) M1 (AO1) — Cut the shoot under water to prevent air entering the xylem and breaking the continuous water column.
A1 (AO1) — Seal all joints (e.g. with petroleum jelly) to ensure the apparatus is airtight, so that water uptake reflects transpiration loss rather than leakage.
Total: 8 marks (M5 A3).
Question (6 marks): Explain how the cohesion-tension theory accounts for the upward movement of water in a tall tree against gravity.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that water evaporates from the surfaces of mesophyll cells into the leaf air spaces and diffuses out through the stomata (transpiration) |
| 2 | AO1.2 | Stating that loss of water from mesophyll cells lowers their water potential, drawing water out of the leaf xylem by osmosis and creating a tension (negative pressure) at the top of the xylem column |
| 3 | AO2.1 | Linking cohesion: water molecules are attracted to one another by hydrogen bonds, so the water column behaves as a continuous, unbroken thread |
| 4 | AO2.1 | Linking transmission of tension: because the column is continuous, the tension generated at the leaf is transmitted all the way down through the xylem to the roots, drawing water up |
| 5 | AO2.7 | Linking adhesion: water molecules are attracted to the hydrophilic, lignified vessel walls, supporting the column against gravity (capillarity) and preventing the column from breaking |
| 6 | AO3.1 | Synthesis: the plant supplies no metabolic energy to lift water — energy comes from the sun, evaporating water from the leaf and pulling the column up; the lignin secondary walls of the xylem prevent inward collapse under the negative pressure |
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