explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells (knowledge of solute potential and pressure potential is not expected)

Movement of Water Between Cells and Solutions

Water moves across cell membranes by osmosis – the diffusion of water through a semi‑permeable membrane. The net direction of water flow is governed by the water potential (Ψ_w) of the two compartments.

1. Water potential

• Definition: Ψ_w is the tendency of water to move. It combines the effects of dissolved solutes (solute potential, Ψ_s) and pressure (pressure potential, Ψ_p):

  
  Ψ_w = Ψ_s + Ψ_p

• Sign convention

  • Pure water has Ψ_w = 0 MPa (reference state).
  • Solutes make Ψ_s negative → Ψ_w becomes more negative.
  • Positive pressure (e.g., turgor in a plant cell) makes Ψ_p positive → Ψ_w becomes less negative.

• Direction of flow: Water always moves from a region of higher (less negative) Ψ_w to a region of lower (more negative) Ψ_w until Ψ_w is equal on both sides.
• Exam‑relevant note: The Cambridge IGCSE/A‑Level syllabus does not require you to calculate Ψ_s or Ψ_p numerically. Understanding the qualitative effect of solutes (lowering Ψ_w) and pressure (raising Ψ_w) is sufficient.

2. Factors that influence membrane permeability

• Cell membranes are selectively permeable – they allow water to pass but restrict most solutes.
• Specialised channel proteins called aquaporins increase the rate of water movement. Their number and opening can be regulated by the cell.

3. Typical water‑potential values (illustrative only)

These numbers are only illustrative; actual values depend on temperature, the specific solute, and the exact concentration.

4. Worked example (qualitative use only)

Problem: Estimate the water potential of a 0.2 M sucrose solution at 25 °C, assuming the pressure potential is zero.

1. For a non‑electrolyte, the solute potential can be approximated by the formula

   
   Ψ_s ≈ –i CRT

   where i = 1 (no dissociation), C = 0.2 mol L⁻¹, R = 0.0831 L·MPa mol⁻¹ K⁻¹, T = 298 K.

2. Calculate: Ψ_s ≈ –(1)(0.2)(0.0831)(298) ≈ –4.95 MPa. (In the exam you only need to know that the value is negative and larger in magnitude than that for a 0.1 M solution.)
3. Since Ψ_p = 0, Ψ_w = Ψ_s ≈ –5 MPa (very low water potential).
4. Interpretation: Water would move into this solution from any compartment with a higher (less negative) Ψ_w (e.g., pure water).

Remember, you are not required to perform this calculation in the exam; the example simply shows why a higher solute concentration means a more negative Ψ_w.

5. Effects of water movement on plant cells

• Turgid (full) cell – hypotonic external solution

  • Water enters, the central vacuole expands.
  • The plasma membrane pushes against the rigid cell wall.
  • The wall resists further expansion, generating turgor pressure (Ψ_p > 0) that keeps the plant upright.

• Plasmolysed cell – hypertonic external solution

  • Water leaves the cell, the vacuole shrinks.
  • The plasma membrane pulls away from the cell wall.
  • The cell becomes flaccid, leading to wilting.

6. Effects of water movement on animal cells

• Swelling and possible lysis – hypotonic external solution

  • Water enters, the cell swells.
  • Because there is no rigid cell wall, the rising internal pressure can cause the membrane to burst (lysis).

• Crenation (shrinking) – hypertonic external solution

  • Water leaves the cell, the cytoplasm contracts.
  • The cell becomes irregularly shaped (crenated).

7. Comparison of plant and animal cell responses

8. Practical investigations (syllabus‑required)

Leaf‑disc assay (water‑potential estimation)

1. Place equal‑sized leaf discs in a series of sucrose solutions of known concentration (hence known Ψ_w).
2. Record the time taken for each disc to become fully turgid (or to plasmolyse).
3. Plot time against solution Ψ_w. The point where the curve levels off gives an estimate of the leaf’s own water potential.

Onion‑epidermis experiment (cell‑shape observation)

• Mount a thin strip of onion epidermis in a drop of distilled water, isotonic saline, or hypertonic sucrose solution.
• Observe under a light microscope and note swelling, lysis, or plasmolysis.

9. Link to other topics

10. Summary

• Water moves from higher to lower Ψ_w; Ψ_w = Ψ_s + Ψ_p.
• Solutes lower Ψ_w (more negative); pressure (turgor) raises Ψ_w (less negative).
• Plant cells: a rigid cell wall converts excess water influx into turgor pressure, preventing bursting; loss of water causes plasmolysis.
• Animal cells: no cell wall, so they must stay isotonic to avoid lysis (hypotonic) or crenation (hypertonic).
• Practical investigations such as the leaf‑disc assay and onion‑epidermis test link theory to observable behaviour and satisfy the syllabus requirement to *investigate* water movement.