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 moleculePotassium iodide KI

Water H2OGlucose 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 watchThistle funnel Marker penGas jar Sugar testape100ml 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.

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?