4.6.5Additional lessons
Water Movement in Plant Tissue — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 54 of 101
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.
Section 1 of 6Water Potential
Definition
- Definition Water potential (ψ) measures the tendency of water to move from one place to another, in units of pressure such as kPa.
Pure water
- Pure water Pure water at standard pressure has a water potential of zero, which is the highest possible value.
Direction
- Direction Water moves from a region of higher (less negative) water potential to lower (more negative) water potential.
Equilibrium
- Equilibrium When two cells have the same water potential, there is no net movement.
Section 2 of 6Components of ψ
Solute potential (ψs)
- Solute potential (ψs) Dissolved solutes lower water potential, so ψs is zero or negative, and more solute means a more negative value.
Pressure potential (ψp)
- Pressure potential (ψp) Pressure from the cell wall pushing against the contents raises the water potential and is positive in turgid cells.
Equation
- Equation ψcell = ψs + ψp, so a rise in turgor pressure makes water potential less negative.
Flaccid cells
- Flaccid cells When a cell is flaccid, ψp is about zero, so ψcell equals ψs.
| Cell state | ψs (kPa) | ψp (kPa) | ψ (kPa) |
|---|---|---|---|
| Turgid | -600 | +400 | -200 |
| Flaccid | -600 | 0 | -600 |
Section 3 of 6From Soil to Leaf
1
Soil water has a higher water potential than the root cells, because root hair cells contain more dissolved solutes.
2
Water moves across the root cortex to the xylem, down a gradient of falling water potential.
3
Evaporation from the leaf mesophyll lowers the water potential there, and transpiration pulls water up the xylem.
4
Water potential drops steadily from soil to root to stem to leaf and finally to the very negative water potential of dry air.
Section 4 of 6Between Neighbouring Cells
Cell to cell
- Cell to cell Water moves from the cell with the higher ψ to the neighbour with the lower ψ, through cell walls and plasmodesmata.
Plasmolysis point
- Plasmolysis point As the cell loses water, ψp falls to zero and ψ becomes equal to ψs.
Tissue support
- Tissue support Turgid cells give herbaceous plants their rigidity, so loss of water potential causes wilting.
Section 5 of 6Worked Example
Question
- Cell A has ψs = -500 kPa and ψp = +200 kPa.
- Neighbouring cell B has ψs = -700 kPa and ψp = +150 kPa.
- In which direction does water move?
- Cell Aψ = -500 + 200 = -300 kPa.
- Cell Bψ = -700 + 150 = -550 kPa.
- -300 kPa is higher than -550 kPa, so A has the higher water potential.
Answer Water moves from cell A to cell B, because A (-300 kPa) has a higher water potential than B (-550 kPa).
Section 6 of 6Exam Checklist & Tips
4.6.5 – Key Points for the Exam
- Water potential A measure of the tendency of water to move, with pure water at 0 kPa.
- Solute potential The effect of dissolved solutes on water potential, always zero or negative.
- Pressure potential The effect of physical pressure on water potential, positive in turgid cells.
- Transpiration Loss of water vapour from leaves, which draws water up the plant.
Common Mistakes
- Saying water moves from low to high water potential.
- Treating solute potential as positive.
- Forgetting to add pressure potential when calculating ψ.
- Thinking more negative values mean higher water potential.
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