Explain how water is moved through the plant via transpiration pull.

Transport in Plants – Transpiration Pull (Cohesion‑Tension Theory)

Learning Objective

Explain how water is moved through a plant by transpiration pull, using the cohesion‑tension theory and the concept of water potential.

1. What is Transpiration?

• Definition: Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of leaves, but also from stems, flowers and young fruits.
• Path of water vapour in a leaf: mesophyll cells → air spaces → stomatal pores → atmosphere.
• Transpiration serves two essential functions:
  • It creates the negative water potential that drives the upward movement of water (the “pull”).
  • It cools the leaf and maintains the flow of mineral nutrients from the roots.

2. Water Potential (Ψ_w)

Water moves from regions of higher (less negative) water potential to regions of lower (more negative) water potential.

Ψ_w = Ψ_s + Ψ_p + Ψ_g + Ψ_m

3. Structure & Function of Xylem (Supplement 8.3)

• Composed of dead, lignified cells – vessels (angiosperms) and tracheids (gymnosperms).
• Thick secondary walls give rigidity, preventing collapse under tension.
• Absence of transverse (cross‑) walls creates a continuous tube for bulk flow.
• Narrow diameters (10–100 µm) enhance capillary action and surface‑tension effects.
• Hydrophilic lignin and cellulose provide adhesion sites for water molecules.

4. Step‑by‑Step Process of Transpiration Pull

1. Evaporation from mesophyll cells: Water leaves the cytoplasm, diffuses into intercellular air spaces and then out through open stomata, creating a water‑deficit at the leaf surface.
2. Development of a more negative Ψ_w in the leaf: The loss of water makes Ψ_w in mesophyll cells more negative than in the xylem.
3. Cohesive pull: Because water molecules are strongly attracted to each other (hydrogen bonding), the decrease in Ψ_w at the leaf surface pulls on the continuous column of water in the xylem.
4. Adhesion to xylem walls: Water molecules also stick to the hydrophilic walls of vessels and tracheids, preventing the column from breaking under tension.
5. Capillary action: The narrow lumen of xylem vessels generates additional upward force via surface tension.
6. Transmission of tension (negative pressure) down the xylem to the roots.
7. Root uptake: At the root‑soil interface water moves into the root cortex by osmosis (from higher to lower Ψ_w) and enters the xylem, completing the circuit.

5. Factors Influencing the Rate of Transpiration (Core AO2)

6. Investigation: Effect of Temperature on Transpiration (Core AO3)

1. Select three similar healthy leaves (or three potted seedlings).
2. Enclose each leaf in a pre‑weighed airtight container fitted with a small opening for the petiole.
3. Place the containers in water baths set at 15 °C, 25 °C and 35 °C respectively.
4. Record the mass loss (water loss) every 10 minutes for 60 minutes using a balance that logs data.
5. Convert mass loss to volume (1 g ≈ 1 ml) and calculate the rate of transpiration (ml h⁻¹).
6. Plot temperature (°C) on the x‑axis and rate of transpiration on the y‑axis; determine the gradient.
7. Discuss sources of error (leaf damage, imperfect sealing, air currents) and suggest improvements (use silicone grease, repeat trials, minimise light differences).

7. Why the Cohesion‑Tension Theory Works

• Cohesion: Hydrogen bonds between water molecules give the column high tensile strength, allowing it to withstand negative pressures of up to –2 MPa in tall trees.
• Adhesion: Attraction of water to the lignified walls counteracts gravity and prevents cavitation.
• Capillarity: Narrow xylem lumens generate additional upward force, especially in herbaceous plants.
• Dead, lignified cells: Lack of metabolic activity means the cells can sustain large tensile stresses without collapsing.

8. Common Misconceptions

• “Plants pump water like a heart.” – There is no active pumping; the pull is generated by evaporation.
• “Transpiration only cools the plant.” – Cooling is a side‑effect; the primary role is to create the negative Ψ_w that drives water and mineral transport.
• “All water moves upward as bulk flow.” – Bulk flow occurs in the xylem, but water also moves laterally through cortex cells by diffusion and osmosis.
• “Capillary action alone lifts water to the top of a tree.” – Capillarity contributes, but the dominant force is the tension generated by transpiration.

9. Links to Other Syllabus Topics

10. Summary

Transpiration pull, explained by the cohesion‑tension theory, is the main mechanism that moves water from roots to leaves in most vascular plants. Evaporation from stomata creates a strong negative water potential in the leaf. Cohesive forces between water molecules transmit this tension down a continuous column of water in the xylem, while adhesive forces to the vessel walls and capillary action help counteract gravity. The specialised structure of xylem (lignified, dead, narrow tubes) enables the column to withstand the resulting tension. Environmental factors such as stomatal aperture, temperature, humidity, wind and light modulate the rate of transpiration, and therefore the speed of water movement.

11. Exam‑Style Practice Questions (Core AO2 & AO3)

1. Define transpiration and state the main pathway by which water vapour leaves a leaf.
2. Explain how a decrease in leaf water potential leads to upward movement of water in the xylem.
3. List two structural features of xylem that allow it to sustain the negative pressures generated during transpiration.
4. Predict the effect on the rate of transpiration if a plant is moved from a humid greenhouse (80 % RH) to a dry greenhouse (30 % RH). Justify your answer using water‑potential concepts.
5. Design a simple experiment to investigate the effect of wind speed on transpiration rate. Include a hypothesis, variables, method and how you would analyse the data.