Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.

Factors that Influence the Direction and Rate of Osmosis

1. Solute concentration – higher concentration → more negative Ψ_s → water moves toward the higher solute side.
2. Pressure potential – turgor pressure (positive Ψ_p) can oppose water entry; tension (negative Ψ_p) can enhance water loss.
3. Temperature – raises kinetic energy of water molecules; effect on Ψ is small and not examined in IGCSE questions.
4. Surface area of the membrane – larger area allows more water to pass per unit time.
5. Distance over which water must travel – greater distance slows the rate (diffusion principle).

Direction of Water Movement

• Water moves into a cell when the external water potential is higher (less negative) than the internal water potential: Ψ_outside > Ψ_inside.
• Water moves out of a cell when the internal water potential is higher (less negative) than the external water potential: Ψ_inside > Ψ_outside.

Osmosis in Plants

• Root water uptake – Root cells accumulate mineral ions, making Ψ_s more negative than the surrounding soil water. Water therefore moves from the soil (higher Ψ) into the root (lower Ψ).
• Turgor pressure – Water entering the vacuole creates a positive Ψ_p. This outward pressure against the cell wall **supports non‑woody tissues** (the syllabus wording: “plants are supported by the pressure of water inside the cells pressing outwards on the cell wall”).
• Transpiration pull – Evaporation of water from leaf mesophyll cells makes Ψ_p strongly negative (tension). The resulting water‑potential gradient (soil → root → xylem → leaf → air) pulls water upward through the xylem.
• Effect of external solutions on plant tissue – Immersing a thin slice of potato (or carrot) in solutions of different concentrations produces three observable states:
  • Turgid (isotonic or hypotonic solution): cells swell, Ψ_outside > Ψ_inside, water enters.
  • Plasmolysed (hypertonic solution): cells shrink, Ψ_inside > Ψ_outside, water leaves.
  • Flaccid (very hypertonic): cells become limp; extreme water loss.

Osmosis in Animals

• Blood plasma – Normally around –0.03 MPa (≈300 mOsm kg⁻¹). A more negative plasma Ψ draws water into cells; a less negative plasma Ψ causes water to leave cells.
• Kidney nephrons – Re‑absorb solutes selectively, altering Ψ_s of the tubular fluid. This controls how much water is re‑absorbed back into the bloodstream.
• Cell‑volume regulation – Animal cells use ion pumps and channels to keep their internal Ψ close to that of the extracellular fluid, preventing swelling (lysis) or shrinking (crenation).

Practical Investigations (Core Experiments)

1. Dialysis‑tubing experiment (required by the syllabus)
   • Materials: dialysis tubing, 0.2 M sucrose solution (inside), beakers containing 0 M (distilled water), 0.2 M, and 0.4 M sucrose (outside).
   • Variables:
     • Independent: external sucrose concentration (hence external Ψ).
     • Dependent: change in mass of the tubing (water movement).
     • Control: the 0.2 M external solution, which is isotonic to the interior.
   • Procedure: seal the tubing, weigh it, place each tube in its beaker for 30 min, blot dry, re‑weigh.
   • Expected outcome: mass increases when external Ψ > internal Ψ (water enters) and decreases when external Ψ < internal Ψ (water leaves).
2. Potato‑slice (or carrot) experiment
   • Materials: potato slices, 0 %, 5 % and 10 % NaCl solutions, balance.
   • Variables:
     • Independent: external NaCl concentration.
     • Dependent: change in mass of each slice.
     • Control: slice kept in distilled water (0 % NaCl).
   • Procedure: record initial mass, immerse slices for 30 min, blot dry, record final mass.
   • Interpretation:
     • 0 % NaCl (hypotonic) → mass gain (turgid cells).
     • 5 % NaCl (moderately hypertonic) → slight mass loss (flaccid cells).
     • 10 % NaCl (strongly hypertonic) → marked mass loss (plasmolysis).

Comparative Table of Typical Water Potentials

Summary Paragraph (Direct answer to the learning objective)

Water‑potential gradients drive osmosis, allowing organisms to take up water where Ψ is higher (e.g., soil → root cells) and lose water where Ψ is lower (e.g., leaf mesophyll → air). In plants, osmotic uptake creates turgor pressure that supports non‑woody tissues and, together with transpiration‑induced tension, powers the continuous upward movement of water. In animals, solute regulation (via kidneys, ion pumps, etc.) keeps cellular Ψ close to that of the extracellular fluid, ensuring that cells neither swell nor shrink. Thus, water potential and osmosis are fundamental to both the acquisition and the controlled loss of water in all living organisms.

Key Points to Remember (Checklist – matches syllabus wording)

• Water always moves from a region of higher water potential (less negative Ψ) to a region of lower water potential (more negative Ψ).
• Solutes lower water potential (more negative Ψ_s); pressure can raise Ψ (positive Ψ_p = turgor) or lower Ψ (negative Ψ_p = tension).
• Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall (turgor pressure).
• Root cells take up water osmotically, creating turgor; transpiration creates a very negative Ψ in leaf mesophyll, producing a continuous pull.
• Animal cells maintain a near‑constant Ψ by regulating solute concentrations (kidney re‑absorption, ion pumps), preventing lysis or crenation.
• Both uptake (roots, kidney re‑absorption) and loss (transpiration, urine formation) of water are controlled by water‑potential gradients.

Sample Exam Question

Explain how a decrease in soil water potential during a drought affects the water‑potential gradient between the soil and a plant’s root cells, and describe the likely physiological response of the plant.