Diffusion & Osmosis Lab

This week’s lab will be focusing on Diffusion and Osmosis. Before getting started, please read through the entire lab and answer the pre-lab questions on page 45.

Part I. Passive Diffusion

EXERCISE I. OBSERVING BROWNIAN MOTION

Re-read the Brownian Motion section in your lab manual and then watch the following video to see a demonstration.

Next, watch the video below looking at a suspension of carmine dye in water and answer the questions for Exercise I. in your Potato Cell Osmolarity Scientific Report.

EXERCISE II. DIFFUSION AND OSMOSIS THROUGH A SEMI-PERMEABLE PHYSICAL MEMBRANE

This week we will be using dialysis tubing to demonstrate selective permeability. Please re-read the background, materials, and procedure for Exercise II. and fill out the question, hypothesis, and prediction sections in your lab report. Next, watch the video below to see how we would fill a dialysis bag with a glucose and starch solution (simulating the glucose and starch found in the cytoplasm of a cell).

Now that the dialysis tubing has been filled and rinsed, we take the initial weight seen below.

Now it is time to place the filled dialysis tubing into a beaker with iodine. Remember from our biomolecules section, when iodine reacts with starch we will see a dark blue/black color form. We will repeat the dialysis tubing set up and place the filled tubing in a second beaker without iodine so that we can perform a Benedict’s test to look for the movement of glucose after the incubation period. You will see these two set ups below and we will follow them over the course of 60 minutes.

At 0 minutes:

After 15 Minutes:

After 60 Minutes:

Completed Experiment:

After the 60 minute incubation time, we re-weight our dialysis tubing, as seen below.

Using the images above, fill out Tables 1 and 2 in your lab report.

The final test that we will need to conduct is a Benedict’s test to look for the movement of glucose. Please see the results of the Benedict’s test below and fill out Table 3. in you lab report. Finish this section with a strong conclusion statement.

Benedict’s Test Post-Incubation:

Part II. Osmosis

EXERCISE I. OSMOTIC BEHAVIOR OF PLANT CELLS

In this week’s lecture we learned about tonicity, the ability of a solution to cause a cell to gain or lose water. Please re-read the background information on Osmosis (under Part II.) and the osmotic behavior of plant cells (Exercise I). Remember that when we are talking about tonicity, we are comparing two adjacent solutions. Let’s imagine that we are comparing the solution inside a cell with a solution outside the cell. If the solution outside the cell was isotonic, the solute concentration is the same as that inside the cell and we would see no net water movement across the plasma membrane. Now let’s imagine that the solution outside the cells was hypertonic. In this case, the solute concentration is greater than that inside the cell, so water would move out of the cell. Lastly, if the solution outside the cell was hypotonic, the solute concentration is less than that inside the cell and so water would move into the cell.

In Exercise I. we use the light microscope to examine Elodea, an aquarium plant, immersed in either an isotonic, hypertonic, or hypotonic solution. Using the descriptions of tonicity above, think about what would happen to the Elodea as it is placed in each of the three different solutions. Would the Elodea cells gain or lose water or would there be no net movement of water? Imagine what this is doing to the central vacuole of the Elodea and how the chloroplasts within the cytoplasm would look as a result. Observe the three images of Elodea below after being placed in solutions labeled A, B, and C. Record your observations in Table 4. of your lab report and determine the tonicity of solutions A, B, and C.