Osmosis
Osmosis is the passive movement of water across a selectively permeable membrane. The motive force driving water movement is generated by the presence of a concentration (or energy) difference operating in the following manner: water always moves (unless there is an obstruction) from a region of higher concentration (or higher energy) to a region of lower concentration (or lower energy) 1. Although it may seem odd, the effective concentration of water is lowered if solutes are added to it. Put another way, the concentration and available energy for movement in a sample of pure water is greater than the same amount of water with solutes added. The more solutes that are added to water, the lower the energy and the lower the water concentration gets.
This lab will allow you to investigate the effects of solute concentration on osmosis rate using two plexi-glass chambers separated by a semi-permeable membrane: one chamber to mimic the shape of a cell and the other, its external environment. The object of the experiments will be to place solutions of glucose and buffer in the cell chamber and buffer only in the environment chamber and then measure the rates of water movement into the cell chamber.
What
is osmosis rate?
A rate is a quantity expressed per unit of time. For example, the speed you travel in your automobile is a rate, e.g., 80 miles/hour. So, osmosis rate is expressed as the amount of water moved per time, e.g., 0.5 mL/minute.
How
do you determine osmosis rates?
In this experiment, osmosis rates are determined by measuring the increase in the volume of water in the cell chamber over a fixed time period. If the increasing volumes of water (dependent variable) are plotted against the time they were measured (independent variable), the slope of the resulting line is the osmosis rate.
How
do you measure water volume?
A one-holed stopper with a graduated pipet attached to it will be fitted into an opening in the cell chamber. As water travels into the chamber it will force water into the pipet where its volume can be measured.
What to do today:
This
is the research question that we are considering today: Does the
Experimental Set-up:
Refer to online laboratory manual
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-
|
Sugar Conc. ____ M |
Sugar Conc. ____ M |
Sugar Conc. ____ M |
Sugar Conc. ____ M |
||||
|
time |
mL |
time |
mL |
time |
mL |
time |
mL |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Rate, mL/min |
|
Rate, mL/min |
|
Rate, mL/min |
|
Rate, mL/min |
|
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?