Cambridge A-Level Biology 9700 – Movement into and out of Cells
Movement into and out of Cells
Learning Objective
Investigate simple diffusion and osmosis using plant tissue and non‑living materials (dialysis/Visking tubing and agar).
Key Concepts
Simple diffusion – passive movement of solute molecules from an area of higher concentration to an area of lower concentration.
Osmosis – diffusion of water across a semi‑permeable membrane driven by a difference in water potential.
Semi‑permeable membrane – allows certain molecules (e.g., water) to pass while restricting others (e.g., large solutes).
Theoretical Background
Simple Diffusion
The rate of diffusion (\$J\$) can be expressed by Fick’s first law:
\$\$
J = -D \frac{\Delta C}{\Delta x}
\$\$
where \$D\$ is the diffusion coefficient, \$\Delta C\$ the concentration gradient and \$\Delta x\$ the distance travelled.
Osmosis
Water moves from a region of higher water potential (\$\Psi_w\$) to lower water potential. The water potential of a solution is given by:
\$\$
\Psiw = \Psis + \Psi_p
\$\$
where \$\Psis\$ (solute potential) is negative and \$\Psip\$ (pressure potential) may be positive or zero.
Factors Influencing Diffusion and Osmosis
Factor
Effect on Diffusion
Effect on Osmosis
Concentration (or water potential) gradient
Greater gradient → faster diffusion
Greater gradient → faster water movement
Temperature
Higher temperature increases kinetic energy → faster diffusion
Higher temperature increases kinetic energy → faster osmosis
Surface area of membrane
Larger area → more molecules can pass per unit time
Larger area → greater water flux
Thickness of membrane
Thicker membrane → slower diffusion
Thicker membrane → slower water movement
Nature of solute (size, polarity)
Smaller, non‑polar molecules diffuse more readily
Only water and very small uncharged particles pass freely
Experimental Design
Materials (Non‑living)
Dialysis (Visking) tubing (cut to 5 cm lengths)
Prepared agar slabs (1 cm thick, 5 cm diameter)
Sucrose solutions of known concentrations (0 %, 5 %, 10 %, 15 % w/v)
Distilled water
Beakers (250 mL)
Balance (±0.01 g)
Timer or stopwatch
Thermometer
Materials (Plant tissue)
Fresh potato strips (1 cm × 1 cm × 2 cm)
Same sucrose solutions as above
Beakers, balance, timer, thermometer
Method – Simple Diffusion (Dialysis Tubing)
Fill each piece of dialysis tubing with 5 mL of 10 % sucrose solution and seal both ends.
Place each sealed tube into a beaker containing 100 mL of distilled water.
Record the initial mass of each tube (including contents).
Allow the system to stand at room temperature (≈22 °C) for 30 minutes.
Remove the tubes, gently blot dry, and record the final mass.
Calculate the change in mass to determine net water movement.
Method – Osmosis (Potato Strips)
Weigh and record the initial mass of each potato strip.
Place each strip into separate beakers containing 100 mL of the different sucrose solutions.
Maintain the beakers at a constant temperature (use a water bath if needed).
After 30 minutes, remove the strips, gently blot to remove surface solution, and weigh again.
Calculate the percentage change in mass:
\$\% \Delta m = \frac{m{\text{final}}-m{\text{initial}}}{m_{\text{initial}}}\times100\$
Observations & Data Recording
Use a table similar to the one below to record results.
Sample
Initial Mass (g)
Final Mass (g)
Mass Change (g)
% Change
Dialysis tube – distilled water
Dialysis tube – 5 % sucrose
Potato – 0 % sucrose
Potato – 10 % sucrose
Analysis
Plot % mass change against external sucrose concentration for the potato strips. The point where % change = 0 corresponds to the isotonic concentration.
For dialysis tubing, a gain in mass indicates water entering the tube (osmosis), while a loss indicates water leaving.
Discuss how the observed trends relate to the theoretical equations presented earlier.
Safety Considerations
Handle sharp scissors when cutting dialysis tubing with care.
Wear gloves and goggles when working with concentrated sucrose solutions.
Dispose of solutions according to school laboratory waste protocols.
Conclusion Points
Simple diffusion occurs down a concentration gradient without energy input.
Osmosis is driven by differences in water potential and can be demonstrated with both living (potato) and non‑living (dialysis tubing, agar) systems.
Factors such as concentration gradient, temperature, surface area, and membrane thickness quantitatively affect the rate of both processes.
Extension Activities
Replace agar with gelatin of varying concentrations to explore the effect of gel density on diffusion rates.
Measure the diffusion coefficient (\$D\$) for a coloured solute (e.g., methylene blue) using the time taken to colour a fixed distance in agar.
Investigate the effect of temperature by repeating the experiments at 5 °C, 22 °C and 35 °C.
Suggested diagram: Cross‑section of a dialysis tube showing solute and water movement; and a potato cell illustrating plasmolysis in hypertonic solution.