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2.7.5Additional lessons

The Transpiration Stream — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 38 of 142

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Pathway of Water

1
Water enters root hair cells from the soil by osmosis because the cell sap has lower water potential.
2
It crosses the root cortex to the xylem, travelling in cell walls and through cytoplasm.
3
Water rises in the xylem vessels of the root, stem and leaf veins.
4
It leaves the xylem and spreads through the cell walls of mesophyll cells in the leaf.
5
Water evaporates from the wet cell walls into the air spaces and diffuses out through open stomata.

Part 2 of 6The Driving Forces

Evaporation
  • Evaporation Loss of water at the leaf lowers pressure there, creating a pull called transpiration pull.
Cohesion
  • Cohesion Hydrogen bonds between water molecules hold them together, so a continuous column is pulled up under tension.
Adhesion
  • Adhesion Water molecules are attracted to the hydrophilic walls of xylem, which helps to support the water column.
Tension
  • Tension The column is under negative pressure, so xylem walls must be strong enough not to collapse.
Mineral ions
  • Mineral ions Ions are taken into the root by active transport and are carried upwards dissolved in the water.

Part 3 of 6Factors and Measurement

Factors and Measurement
Light
  • Light Light causes stomata to open, so the rate of transpiration is higher in the light.
Temperature
  • Temperature A higher temperature increases evaporation and the kinetic energy of water molecules.
Humidity
  • Humidity Humid air reduces the concentration gradient from leaf to air, which slows transpiration.
Wind
  • Wind Moving air removes water vapour from the leaf surface and steepens the gradient.
Potometer
  • Potometer A potometer measures water uptake as the movement of an air bubble along a capillary tube.

Part 4 of 6Transpiration and the Plant

Transpiration and the Plant
Gas exchange
  • Gas exchange Stomata must open to let carbon dioxide in, so water loss by transpiration is an unavoidable cost.
Benefits
  • Benefits The flow of water brings mineral ions to the leaves and helps to cool the leaf by evaporation.
Guard cells
  • Guard cells Guard cells change shape to open and close stomata, so the plant can reduce water loss when short of water.

Part 5 of 6Worked Example

Question

  • A potometer bubble moves 24 mm in 5 minutes.
  • The capillary tube has a radius of 0.5 mm.
  • Calculate the rate of water uptake.

  1. Volume = πr²d = π × 0.5² × 24 = 18.85 mm³.
  2. Rate = 18.85 ÷ 5 = 3.77 mm³ per minute.

Answer About 3.8 mm³ per minute.

Part 6 of 6Exam Checklist & Tips

2.7.5 – Key Points for the Exam

  • Transpiration Loss of water vapour from the aerial parts of a plant, mainly through stomata.
  • Cohesion Attraction between molecules of the same substance, such as water.
  • Adhesion Attraction between water and the walls of xylem.
  • Potometer Apparatus used to estimate the rate of water uptake by a shoot.

Common Mistakes

  • Saying that transpiration is the movement of sugars; sugars are transported in phloem.
  • Writing that the water is pushed up by the root only, ignoring the pull from evaporation.
  • Confusing cohesion (water to water) with adhesion (water to wall).
  • Stating that a potometer measures transpiration exactly, when it measures water uptake.

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