transpiration water potential
Overview
# Transpiration and Water Potential - A-Level Biology Summary ## Key Learning Outcomes This lesson examines the movement of water through plants via transpiration, driven by water potential gradients from soil (-0.5 MPa) through roots, xylem, and mesophyll cells to atmosphere (-30 MPa). Students must understand that water potential (ψ = ψs + ψp) determines water movement direction, with water moving from higher (less negative) to lower (more negative) values. Critical exam topics include calculating water potential, explaining the cohesion-tension theory of xylem transport, and analysing how environmental factors (humidity, temperature, light intensity, wind) affect transpiration rates through their effects on the water potential gradient and stomatal aperture. ## Exam Relevance This topic frequently appears in Paper 2 and Paper 4 practical assessments, particularly questions requiring students to explain
Core Concepts & Theory
Transpiration is the loss of water vapour from the aerial parts of plants, primarily through stomata (small pores in leaves). This process drives the transpiration stream, moving water and dissolved minerals from roots to leaves.
Water Potential (Ψ) measures the tendency of water to move from one area to another. Pure water at atmospheric pressure has Ψ = 0 kPa (the reference point). Adding solutes or applying pressure changes water potential:
Ψ = Ψs + Ψp
Where:
- Ψs (solute potential) = always negative; becomes more negative as solute concentration increases
- Ψp (pressure potential) = turgor pressure in plant cells; usually positive in turgid cells
Key Principle: Water moves from regions of higher (less negative) water potential to lower (more negative) water potential by osmosis.
In plant cells:
- Turgid cells: Ψp is high and positive; cell pushed against cell wall
- Flaccid cells: Ψp = 0; cell loses contact with wall
- Plasmolysed cells: Ψp = 0; cytoplasm pulls away from cell wall
Cohesion-Tension Theory explains water movement:
- Transpiration creates tension (negative pressure) in leaf xylem
- Water molecules cohere (stick together via hydrogen bonds)
- Water column is pulled up xylem from roots
- Root pressure and capillary action provide additional support
Factors Affecting Transpiration Rate:
- Light intensity: Opens stomata
- Temperature: Increases kinetic energy
- Humidity: Reduces diffusion gradient
- Wind speed: Removes saturated air
Remember: LTHW (Light, Temperature, Humidity, Wind)
Detailed Explanation with Real-World Examples
Think of water potential like gravitational potential energy—water 'flows downhill' from high to low Ψ. A concentrated sugar solution has very negative Ψ (like a deep valley), attracting water from surrounding areas.
Real-World Application: Wilting Crops During drought, soil water potential becomes very negative. When soil Ψ < root cell Ψ, water cannot enter roots. Cells lose turgor, and plants wilt. Farmers irrigate to raise soil Ψ, allowing water uptake. Commercial greenhouses control humidity (reducing transpiration) and use drip irrigation to maintain optimal soil Ψ.
Xerophyte Adaptations: Desert plants minimize water loss:
- Cacti: Thick waxy cuticle, sunken stomata (trap humid air), CAM photosynthesis (stomata open at night)
- Marram grass: Rolled leaves with stomata inside, creating humid microclimate; hairs trap moist air
These adaptations reduce the water potential gradient between leaf and atmosphere.
Urban Trees and Transpiration: A mature oak transpires ~150 litres daily. Urban planners use trees for cooling—transpiration absorbs latent heat, reducing city temperatures by 2-5°C. This 'natural air conditioning' demonstrates transpiration's role beyond plant physiology.
Salinity and Agriculture: Coastal soil has high salt content (very negative Ψ). Plants struggle because root Ψ must be even more negative to absorb water. Salt-tolerant crops (halophytes) accumulate compatible solutes to lower their Ψ without damaging proteins.
Analogy: Water potential is like a bank account balance. Pure water = £0. Adding solutes = going into debt (negative values). Water moves toward the 'bigger debt' (more negative Ψ).
Worked Examples & Step-by-Step Solutions
**Example 1: Calculating Water Potential** *Question*: A plant cell has Ψs = -800 kPa and Ψp = +600 kPa. Calculate water potential. If placed in a solution with Ψ = -300 kPa, will the cell gain or lose water? *Solution*: Step 1: Apply formula Ψ = Ψs + Ψp Ψ = (-800) + (+600) = **-200 kPa** Step 2:...
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Key Concepts
- Transpiration: The process of water movement through a plant and its evaporation from aerial parts, primarily leaves, through stomata.
- Water Potential (Ψ): The potential energy of water per unit volume relative to pure water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects.
- 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.
- Stomata: Pores, primarily on the underside of leaves, that regulate gas exchange and water transpiration.
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Exam Tips
- →Clearly distinguish between cohesion and adhesion, and explain how both contribute to the cohesion-tension theory.
- →Remember that water potential is always relative to pure water (Ψ=0). Understand how solutes and pressure affect its value (Ψ = Ψs + Ψp).
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