1. Diffusion experiment
Introduction
The atoms of various elements are not all equal in size, but for the purposes of this practical, we will
consider them to be so. So we will compare the relative sizes of the substances with which we are
dealing via the number of atoms within each molecule. Please complete the following table to ascertain
the relative sizes:
Substance Molecular formula No. of atoms per molecule
Potassium iodide KI
Water H2O
Glucose C6H12O6
Starch (C6H10O5)500
In this experiment we will be observing the movement of the aforementioned substances over a semi-
permeable membrane. Dialysis tubing is essentially a very tightly woven mesh, with gaps of a fairly
regular size.
Diffusion in the process by which a substance will cross a membrane in a singular direction, based on
the concentration gradient, until equilibrium is reached.
Osmosis is a special form of diffusion. In this case the substance is unable to pass through the
membrane so, instead, water will cross the membrane until equilibrium is reached.
Materials
Retort stand and clamp Stop watch
Thistle funnel Marker pen
Gas jar Sugar testape
100ml beaker Soluble starch solution (5%)
Rubber band Glucose solution (5%)
Dialysis tubing Potassium iodide solution
Method
Group A will be using a 5% glucose solution. Group B will be using a 5% soluble starch solution.
A. B.

2. 1. Prepare your retort stand and clamp. Fill gas jar ¾ full with water. If you are in a group that
will be using the starch solution, add a few drops of potassium iodide to the water in the gas
jar, just enough to permanently discolour the water.
2. Half fill a 100ml beaker with your allocated solution
3. Wet a length of dialysis tubing under a tap for 1 minute, massaging it until soft and pliable
4. Tie a knot in one end of the tubing. Massage open the other end of the tubing and insert the
narrow end of the thistle funnel. Secure the bag by repeatedly wrapping a rubber band until
very tight. Make sure the dialysis tubing stays moist the entire time.
5. Tilt the funnel on an angle and very slowly fill the tubing with your solution, you must avoid
any air bubbles. If any appear, immediately squeeze them out the top of the tubing.
6. Hold funnel upright and top up with solution until it comes half way up the stem of the
funnel.
7. Once the tubing is full of solution, attach it to the clamp and immerse in to the gas jar full of
water. Make sure the entire tubing is covered by water. Mark the point on the thistle funnel to
which the solution reaches.
8. Start the stop watch and every 10 minutes record your observations.
- Obs. 1: Has the solution height changed since your last reading
- Obs. 2a: Has the constitution of the water in the gas jar changed (testape can be used
to check for the presence of glucose).
- Obs 2b: Has the constitution of the solution in the tubing changes (stach stains
blue/black in the presence of iodine).
Write-up
Please complete a full prac write-up including AIM, HYPOTHESIS, RESULTS and CONCLUSION.
You may cut out the materials and method and paste in to your write-up.
You will also need to obtain results from another group that did the alternate experiment to you
Please include the following questions in your conclusion:
Q1. Based on the results of the experiment, can you make an estimation of the size of molecules that
are able to pass through the dialysis tubing? What evidence do you have for this assumption?
Q2. In the case of the glucose solution, which direction did substances diffuse and what evidence was
there for this?
Q3. In the case of the starch solution, which direction did substances diffuse and what evidence was
there for this?
Q4. Explain the difference in structure between an actual plasma membrane and the dialysis tubing.
Q5. Did your experiment create any results that cannot be explained by the processes of diffusion or
osmosis?
Q6. If you listed any anomalous results in answer to the above question, what does this indicate about
this experiment as a model for action across a plasma membrane? How could it have been improved?