Investigating Diffusion & Osmosis

Before the method, be crystal clear on the two processes the practicals demonstrate:

• Diffusion — the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, i.e. down a concentration gradient. It does not need a membrane.
• Osmosis — the net movement of water molecules from a dilute solution (high water potential) to a more concentrated solution (low water potential) across a partially permeable membrane.

A partially permeable membrane lets small molecules (like water) through but blocks larger ones (like sucrose or starch). Living cell membranes are partially permeable, and so is Visking tubing, which is why it makes such a useful artificial model.

Why we test both living and non-living systems:

• Living (potato cylinders) shows osmosis happening in real cells with real membranes.
• Non-living (Visking tubing, agar cubes) lets us see the same physics with no cells getting in the way — cleaner, faster, easier to control.

This is the practical examiners ask about most. You investigate how the concentration of a sugar (or salt) solution affects the mass of potato tissue.

Method (learn the order):

1. Use a cork borer to cut several potato cylinders of the same diameter, then trim them to the same length with a scalpel and tile.
2. Blot each cylinder with paper towel to remove surface water and find its starting mass on a balance. Record it.
3. Prepare a range of sucrose concentrations (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³). Put the same volume of one solution in each labelled test tube/beaker.
4. Place one cylinder in each solution and leave for a set time (e.g. 30 minutes), at the same temperature.
5. Remove each cylinder, blot it the same way, and find its final mass.
6. Calculate the percentage change in mass for each concentration and plot a graph.

Control variables (the "fair test" marks):

• Same volume of solution, same temperature, same time, same type of potato, cylinders the same size, blotted the same way each time.

What the results mean:

• Mass increases → solution is more dilute than the potato cell sap → water moves into the cells by osmosis.
• Mass decreases → solution is more concentrated than the cell sap → water moves out of the cells.
• No change → the solution concentration equals the concentration inside the cells (the curve crosses the x-axis here).

Raw mass change is misleading because cylinders never start at exactly the same mass. So we always calculate percentage change in mass:

$\text{percentage change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$

A positive value means a gain (water in); a negative value means a loss (water out). Using percentages makes every cylinder comparable on the same axis.

Plotting the graph:

• x-axis: concentration of solution (mol/dm³) — the independent variable.
• y-axis: percentage change in mass (%) — the dependent variable. The y-axis must run below zero so losses can be plotted.
• Draw a line/curve of best fit.
• The point where the line crosses the x-axis (0% change) gives the solution concentration equal to the cell contents — a key result you can be asked to read off.

Visking (dialysis) tubing — a partially permeable membrane model

• Fill a length of Visking tubing with a mixture of starch solution and glucose solution, tie it off and rinse the outside.
• Place it in a beaker of water containing iodine solution (and test the surrounding water for glucose over time).
• Results: the contents go blue-black because iodine is a small molecule and diffuses IN through the tubing to meet the starch. Glucose diffuses OUT into the surrounding water (positive Benedict's test), but starch molecules are too large to pass and stay inside.
• This shows the tubing is partially permeable — exactly like a cell membrane. Water entering by osmosis can also make the tubing swell.

Agar jelly cubes with dye — a diffusion and surface-area model

• Make agar jelly containing an indicator (e.g. coloured with universal indicator, or plain agar with dye diffusing in).
• Cut cubes of different sizes (e.g. 1 cm, 2 cm, 3 cm) and place them in a coloured solution (e.g. dilute acid/dye).
• Measure the time for the colour/dye to diffuse fully to the centre of each cube.
• Results: the smallest cube colours completely fastest because it has the largest surface-area-to-volume ratio — there is more surface per unit of volume for diffusion, and the centre is a shorter distance away.

These non-living models are popular exam contexts because the variables are easy to control and the link to surface area : volume and partial permeability is clear.

Evaluation marks are where many students drop easy points. A strong evaluation has three parts for each issue:

1. The error / limitation — be specific (not "human error").
2. How it affects the result — would it make values too high or too low, or just less reliable?
3. A realistic improvement.

Good evaluation points for the potato practical:

• Drying/blotting was not identical each time → leftover surface water adds mass → blot each cylinder the same number of times with fresh towel.
• Cylinders were not exactly the same size → different surface areas affect the rate → cut with the same cork borer and trim to the same length carefully.
• Only one cylinder per concentration → an anomaly can't be spotted → repeat and take a mean percentage change.
• Temperature drifted → changes the rate of osmosis → use a water bath to keep temperature constant.

Reliability vs validity (for A answers):*

• Repeats and means improve reliability (consistency).
• Controlling variables (volume, temperature, potato type) improves validity (you're really testing concentration and nothing else).

Always point out anomalous results on a graph and say they should be ignored when drawing the line of best fit — but only after checking they aren't genuine.