TESC – fall 2002HUMAN BIOLOGY 

LAB # 3

THE CELL: Diffusion & Osmosis

Purpose

Diffusion is the net movement of molecules from an area of high molecular concentration to an area of low molecular concentration.Osmosis is the diffusion of water through a selectively permeable membrane.The purpose of this exercise is to observe these mechanisms of exchange as well as to discover the behavior of cells in isotonic, hypertonic & hypotonic solutions.

Lab Objective - By completing this exercise, students will learn to:

•Describe the effect of a solute's molecular weight on its rate of diffusion

•Differentiate diffusion & osmosis

•Define isotonic, hypertonic & hypotonic solutions by their effect on cells

A. Simple Diffusion

To observe the diffusion of a solute through a gel and the effect of a solute's molecular weight on it's rate of diffusion, perform the following test:

• Obtain an agar plate. Place it over a piece of paper on which you have drawn two small circles
• Place a small amount (one to three crystals) of potassium permanganate [ MW = 158 ] on the agar surface over one circle and an equal amount of methylene blue [ MW = 374] over the other circle
• Measure the diameter of the diffusion ring, to the nearest millimeter, at 3 minute intervals and record your data in a table

Questions:

1. Prepare a line graph for the two substances by plotting diameter of diffusion ring [in millimeters], against time, [in minutes]

2. Write a conclusion about the relationship between molecular weight and rate of diffusion

B. Osmosis

For this lab, work in grouped pairs 

oObtain an egg, which had been soaked in vinegar to dissolve the shell.

oWeigh the egg – one pair will work with corn syrup and the other pair will work with distilled water

oPlace it in a beaker with correct solution for 30 minutes.

oRemove & gently roll dry on a paper towel. Weigh again. Share and compare weights. 

Question

1. State the solution the egg was soaked in - did the egg gain or lose weight.Provide weights before and after soaking.Compare results of eggs in the room. Explain the results of both solutions.

2. Define - isotonic, hypotonic & hypertonic.Identify which type of solution the corn syrup and the water are.