Diffusion and Non-Living Membrane Practicals

Explained

Two model experiments, and what they stand in for

Both of these practicals use non living materials to show something that happens in cells. Agar cubes stand in for cells of different sizes; Visking tubing stands in for a partially permeable cell membrane.

Using models rather than cells is deliberate. You can cut agar to whatever size you like and see straight through it, and neither of those is true of a cell.

Agar cubes and surface area to volume ratio

Make agar containing an indicator, usually one that changes colour in acid, and cut it into cubes of different sizes: 1 cm, 2 cm and 3 cm along each edge.

Put them all into the same acid at the same time. The acid diffuses in from every face, and the colour changes behind it, so you can watch the change move inwards and time how long each cube takes to change completely.

The small cube changes throughout first. That is the result, and the explanation is a ratio rather than a size.

A 1 cm cube has a surface area of 6 square centimetres and a volume of 1 cubic centimetre, so its ratio is 6 to 1. A 2 cm cube has 24 and 8, a ratio of 3 to 1. A 3 cm cube has 54 and 27, a ratio of 2 to 1.

So as an object grows, its volume grows faster than its surface area, and the surface area to volume ratio falls. Every cubic centimetre of the large cube has less surface serving it, and the acid has further to travel to reach the centre.

That is why cells are small, why large organisms need transport systems and specialised exchange surfaces, and why the alveoli and the villi are shaped as they are. This practical is the reason behind several other topics.

Visking tubing

Visking tubing is partially permeable: small molecules such as water and glucose pass through it, and large ones such as starch and protein do not.

Fill a length of it with a starch and glucose solution, knot both ends, rinse the outside thoroughly, and suspend it in a beaker of water. Test the surrounding water at intervals.

After a while the water outside tests positive for glucose with Benedict's solution and negative for starch with iodine. The glucose molecules were small enough to pass through; the starch molecules were not.

Rinsing the outside before you start is essential. Any solution spilled on the tubing during filling would appear in the water immediately and would look exactly like a positive result.

Using it for osmosis

Fill the tubing with concentrated sugar solution and place it in distilled water, or the reverse, and measure the change in mass.

Blot the tubing dry before every weighing. Water clinging to the outside adds mass that has nothing to do with what moved through the membrane, and it is the largest avoidable error in this experiment.

Calculate the percentage change in mass rather than the change in grams. Percentage change equals the change divided by the starting mass, multiplied by 100.

The reason is comparability. Two pieces of tubing are never exactly the same starting mass, so a gain of 0.5 g means something different for each. A percentage removes the starting mass from the comparison, and it lets you plot results from different samples on the same axes.

Variables and improvements

Change one thing: the cube size, or the concentration outside the tubing.

Keep the same temperature, the same volume of solution, the same time, the same concentration of acid or sugar, and the same type of agar or tubing.

Repeat each condition at least three times and take a mean, and identify anomalous results rather than averaging them in.

Temperature is worth stating explicitly. Diffusion and osmosis both speed up when it rises, because the particles have more kinetic energy and move faster, so an uncontrolled temperature affects the result directly.

What the models cannot show

An agar cube has no living membrane, no active transport and no metabolism, and the acid moves through it by diffusion alone.

Visking tubing lets molecules through by size alone, whereas a cell membrane also controls what crosses it using protein channels and pumps.

Both limitations are worth mentioning if a question asks you to evaluate the model. The experiment demonstrates one mechanism cleanly, and a real cell is doing several things at once.