Investigate osmosis using materials such as dialysis tubing.

3.2 Osmosis

Learning Objective

Investigate osmosis using dialysis tubing and understand the factors that affect its rate, linking the phenomenon to the biological role of water in living organisms.

Key Concepts

• Water potential (ψ_w) – the potential energy of water per unit volume. Water moves from a region of higher ψ_w (higher water potential) to a region of lower ψ_w (lower water potential).
• Definition of osmosis – the diffusion of water molecules through a semi‑permeable membrane from a region of higher ψ_w to a region of lower ψ_w.
• Semi‑permeable membrane – a membrane that allows water to pass but restricts most solutes.
  Examples: dialysis tubing, animal cell plasma membrane, plant cell wall & plasma membrane, root‑hair membranes.
• Biological importance of water as a solvent
  • Digestion – water dissolves nutrients, allowing them to be absorbed.
  • Excretion – kidneys filter blood; water movement by osmosis concentrates waste in urine.
  • Transport – blood plasma carries dissolved gases, ions and metabolites; osmosis maintains plasma volume.
• Factors influencing the rate of osmosis (Cambridge syllabus + one extra)
  • Concentration (or water‑potential) gradient – larger Δψ_w → faster flow.
  • Temperature – higher temperature increases kinetic energy; a 10 °C rise roughly doubles the diffusion coefficient.
  • Surface area of the membrane – more area provides more pathways for water.
  • Pressure – external pressure can oppose or enhance osmotic flow (e.g., turgor pressure, osmotic pressure).
  • Membrane thickness (diffusion distance) – thinner membranes give a higher rate (rate ∝ 1/thickness).
  • Additional factor: Agitation of the external solution – stirring reduces the boundary layer and can increase the rate.

Materials Required

Method

1. Preparation of the dialysis tube

1. Seal one end of a 5 cm piece of dialysis tubing with a rubber band.
2. Using a pipette, fill the tube with 10 mL of distilled water (or the required internal solution for a particular trial).
3. Seal the opposite end with a second rubber band, making sure there are no leaks.
4. Gently blot the outside of the tube with a paper towel and weigh it on the balance. Record this as the initial mass (m_i).

2. Setting up the external solution

5. Label a beaker with the sucrose concentration to be used, add 100 mL of that solution, and record its temperature.
6. Cover the beaker with foil or a watch glass to minimise evaporation.
7. If investigating the agitation factor, place a stirring rod in the beaker but do not start stirring yet.

3. Running the experiment

8. Place the pre‑weighed dialysis tube gently into the beaker, ensuring it is fully immersed and not touching the bottom.
9. Start the stopwatch. For the standard investigation keep the tube undisturbed for 30 minutes. If testing agitation, stir the solution continuously at a gentle speed.
10. Maintain a constant room temperature (≈20 °C) and note any temperature change of the bath.
11. After the set time, remove the tube, blot the exterior quickly, and weigh it again. Record this as the final mass (m_f).
12. Calculate:
    • Mass change: Δm = m_f – m_i (g)
    • Percentage change: %Δm = (Δm / m_i) × 100 %
13. Rinse the tubing thoroughly with distilled water, dry the exterior, and repeat the procedure with a different external concentration. Perform at least three trials per concentration for reliability.

Observations – Data Table

Graphical Representation

Plot %Δm (y‑axis) against sucrose concentration (M) (x‑axis). A straight‑line increase demonstrates that the rate of osmosis is proportional to the water‑potential gradient, fulfilling AO2 requirements for data presentation.

Explanation of Results

• Direction of flow – Water moves from the region of higher ψ_w (outside the tube when the external solution is more concentrated) to the region of lower ψ_w (inside the tube). This increases the tube’s mass.
• Magnitude of change – The larger the concentration (or ψ_w) difference, the larger the Δψ_w and therefore the larger the %Δm observed.
• Hydrostatic pressure – As water enters, an internal pressure builds (analogous to turgor pressure). In a flexible dialysis tube this pressure is small, so the mass continues to rise over the 30‑minute period.
• Trial 1 (distilled water outside) – ψ_w outside > ψ_w inside, so water leaves the tube, giving a negative Δm. This confirms that osmosis always proceeds from higher to lower water potential.

Quantitative Hints for the Factors

• Concentration gradient – Rate ∝ Δψ_w. Doubling the external sucrose concentration roughly doubles %Δm.
• Temperature – An increase of 10 °C typically doubles the diffusion coefficient (D). Expect roughly twice the %Δm at 30 °C compared with 20 °C, assuming other conditions unchanged.
• Surface area – If the length of tubing is doubled (≈ double surface area), %Δm also roughly doubles.
• Pressure – Applying external pressure (e.g., pressing the beaker) reduces the net water influx; sufficient pressure can reverse the flow, illustrating osmotic pressure.
• Membrane thickness – Using a thinner piece of dialysis tubing (or a membrane with a shorter diffusion path) increases the rate; rate ∝ 1/thickness.
• Agitation – Stirring the external solution reduces the stagnant layer, increasing the effective concentration gradient and thus the rate of osmosis.

Safety Precautions

• Handle glassware carefully to avoid breakage.
• Wear gloves when handling sucrose solutions and any chemicals.
• Dispose of solutions according to school/environmental guidelines.
• Do not ingest any solutions.
• Rinse the dialysis membrane thoroughly between trials; discard any tube that is torn or shows leaks.

Extension (Higher‑Order Thinking) Questions

1. Predict the outcome if the dialysis tubing contains 0.5 M sucrose and is placed in distilled water. Explain using water‑potential concepts.
2. How would raising the temperature from 20 °C to 35 °C affect the % mass change? Provide a kinetic‑theory rationale.
3. Explain how osmosis underlies water uptake by plant roots and the maintenance of cell turgor pressure.
4. Design a variation of the experiment to investigate the effect of external pressure on the direction of water movement.

Suggested Mark Scheme (5 marks each)

Summary

Osmosis is the movement of water across a semi‑permeable membrane driven by a water‑potential gradient (higher ψ_w → lower ψ_w). The dialysis‑tube experiment allows students to quantify the direction and magnitude of water movement, relate the observations to the concentration gradient, temperature, surface area, pressure, membrane thickness, and agitation, and connect these ideas to real‑world biological processes such as digestion, kidney filtration, blood transport, and plant water uptake.