Principle

Diffusion is the net movement of matter along a concentration or energy gradient.

Osmosis is a special form of diffusion involving the movement of water.

Objectives

1. Investigate the factors that effect the rates of diffusion  for water (osmosis) and solutes (dialysis).

2. Investigate the role of membrane structure can play in permeability.

3. Learn to use a computer assisted graphing program and a data acquisition system for data analysis.

Research Questions

What would happen to a cell if water began to diffuse:

• Into the cell?  How would cell volume change?  Would its weight change, too?
• Out of the cell? How would cell volume change?  Would its weight change, too?

What conditions would cause water to diffuse across the cell membrane into the cell?  Which would cause it to diffuse out of the cell?

How could you design an experiment to measure the flow of water across a membrane?  What data would you need?

Internet
Experimental Set-up
Osmosis Tutorial
Diffusion Tutorial

Students need

• graph paper
• metric ruler
• calculator
• marking pen

Objectives
1. Investigate the responses of an artificial cell to changes in its water status by creating different concentration gradients across a artificial cell membrane.

2. Calculate osmotic rates of water entering a cell by graphing changes in volume over time.

3. Examine the relationship between solute concentration in the cytosol with the rate of osmosis.

Hypothesis formulation
Construct a hypothesis to predict the effect of sugar concentration on the rate of osmosis.
Note:  The sugar (Glucose or mannitol) has a mole. wt. = 180g/mole.  The membrane has a molecular weight cut off (MWCO = 100 g/mole).

What would be a suitable control?  What rate of osmosis would you expect in the control?

Experimental Design
Treatments:

The stock concentration of glucose (sugar) is 1.20 M in phosphate buffer.  Your class collectively should select 4 concentrations to use within the range of  0 to 1.20 M.

• What concentration would serve as a control?

Replications = Each lab group will serve as a replication.

Dependent variable = change in volume over time
The unit of rate = mL/min. 

Procedure  Experimental Set-up

1.  Fill the short (cell) side of the chamber with a known concentration of sugar in phosphate buffer.  Using the syringe with an attached piece of tubing. Be careful placing the tubing into the chamber so you DO NOT contact the membrane.

Note: Your lab instructor will expect you to know how to use the mass-balance equation to make the desired concentration.

2. Fill the tall (environment) side of the chamber with only phosphate buffer using the syringe.

3.  Place a stir bar in the cell side and position the chamber on a stir plate, so the cell side is at the center.  Turn the stir plate on and adjust the speed so the stir bar is rotating moderately slowly.

4.  Position the stopper with the pipette into the hole over the cell side of the chamber containing the sugar solution.  Apply slight pressure only; just enough to seal the chamber.  Check to be sure there are no trapped air bubbles.

5.  Wait a few minutes until the sugar "cytosol" begins to move into the pipette.  Lightly mark the pipette at the starting volume and record it on your data collection table.

6.  Measure the flow of water into a cell over time by observing the change in volume in a pipette attached to an artificial cell.  Construct a graph and plot the data.

Data Collection Table- 

Questions
1.  How much did the rates change?  Why did it change?
2.  Did the flow of water reach equilibrium?
3.  Is there a relationship between rate of osmosis and concentration?  What is it?