You design an experiment to test the effect(s) that various compounds have on th

Question

You design an experiment to test the effect(s) that various compounds have on the osmotic potential of a model cell. You fill each of three dialysis bags half full with one of the following substances.

Bag 1. 5% by weight of glucose in distilled water

Bag 2. 5% by weight of egg albumin (protein) in distilled water

Bag 3. 5% by weight of glass beads in distilled water.

The final weight of each bag is 10 g.

You put the dialysis bags into three separate flasks of distilled water so they are completely submerged. After 2 hours, you remove the bags and record these weights:

1. Glucose bag: 13.2 g

2. Albumin bag: 10.1 g

3. Glass bead bag: 10.0 g

Explain the results of each bag. (Consider how much of each substance you are putting in – look for molecular weight of glucose and albumin.)

Explanation / Answer

Dialysis is the technique used to concentrate a molecule of given size (molecular weight) based upon the movement of solvent by osmosis through a semi-permeable membrane. In these experiments, the movement of solvent takes place from a region of high solvent volume to a region of low solvent volume until an equilibrium is reached. Importantly, the dialysis membrane is a semi-permeable membrane which allows movement of some molecules but not all.

According to the information, three bags are filled with different amounts of glucose, albumin and glass beads and allowed to rest in separate flasks with solvent. Consider that each flask has similar solvent volume. Also, the final weight of each bag is 10g. The observations made after two hours can be explained as below:

1. Glucose bag: 13.2 g: A solution containing 1.5% glucose by weight was added to this bag and final weight was 10g. The average molecular weight of glucose is 180 kDa. This means that initial weight of glucose was 1.5*10/100 or 0.15g. Since final weight of the bag after 2 hours is 13.2g, it suggests that solvent freely moved inside this bag and this is why the final weight of the bag increased than initial weight.

2. Albumin bag: 10.1 g: The average molecular weight of albumin is nearly 60 kDa and the bag was initially filled with 2.5% of total 10g i.e. 2.5*10/100 or 0.25g. After 2 hours, the weight of the bag was nearly 10.1g i.e. no significant change occured in total weight of the bag. This observation states that the albumin dialysis bag was already in equilibrium with the solvent and no net movement of solvent occured and no further concentration of albumin took place.

3. Glass bead bag: 10.0 g: Since the glass beads are completely impermeable to semi-permeable membrane of the dialysis bag, no net movement of either solute or solvents will take place in this dialysis bag and hence, no change in net weight was observed.

Thus, this observation suggests that a solute with higher permeability and solubility shows larger change in total weight whereas a solute with least permeability and solubility shows least change in final weight of the dialysis bag.

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