Laboratory #4 - DIFFUSION & OSMOSIS

Your body is composed of trillions of cells, and most of these contain between 75-85% water. Therefore, an understanding of diffusion (movement of solutes from high concentration to low concentration) and osmosis (the movement of water across a differentially permeable membrane) is crucial to the study of biology. After completing this lab, you should be familiar with the following terms:

DIffusion and Molecular Weight(procedure 4.2):

The picture below shows a petri dish that was filled with agar (kind of has the consistency of jello). In one region of the plate (upper portion) a few drops of green stain was added (molecular weight = 150 g/mole). In the opposite region, a few drops of red stain was added (molecular weight = 400 g/mole). Based on these results, what can you conclude in regards to the relationship between molecular weight and rate of diffusion?

Diffusion across a differentially permeable membrane(procedure 4.3):
To study diffusion and osmosis, dialysis bags were filled with several different types of solutions. Dialysis membranes are differentially permeable (only molecules that are small enough can pass through the membrane). The basic setup is shown below.

For this portion of the exercise, two different experimental treatments were setup.
In treatment #1, a dialysis bag was filled with phenolphthalein (a pH indicator that turns red in basic/alkaline conditions). The bag was then submerged into a beaker containing sodium hydorxide (NaOH- a very alkaline solution).

In treatment #2, a dialysis bag was filled with starch. The bag was then submerged into a beaker containing iodine (iodine turns dark brown when it reacts with starch). The results of these treatments are shown below.


Based on these results, which if the following substances were able to cross the dialysis membrane?


Four dialysis bags were setup according to the treatment shown in the following diagram.

At 15 minute intervals, the bags were removed and weighed. The following graph shows the results of the weight changes for each bag over a 1 hour period.

Based on the above results, what can you conclude about the relationship between concentration of solutes and rate of osmosis?

PLASMOLYSIS(procedure 4.8):

Plasmolysis refers to the shrinking of the cytoplasm of a cell in response to diffusion of water out of the cell. This may occur when cells are placed in a solution containing a hight concentration of solutes. Observe the pictures below which illustrate this phenomenon.

Elodea plasmolyzed

Potato cores in 0%, 5%, and 15% saltwater

plain water
salt water

This concludes the review for Lab 4 - Diffusion & Osmosis.

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