Download Osmosis in Elodea (SOL #4)

Osmosis in Elodea
Introduction:
In this activity you will be observing osmosis in living plant tissue – again, Elodea densa will be the
organism of study. Osmosis is a special form of diffusion – the diffusion of water through a semipermeable membrane in response to a concentration gradient. A 10% saline solution (10parts salt, NaCl,
to 90parts water) is hypertonic to the cytoplasm and vacuole within the Elodea cells which are hypotonic.
Remember, when dealing with tonic terms, the hypo- (lower) and hyper- (over) refer to the solute (salt
or other substances) of the solution and NOT THE WATER (or solvent). The mantra for osmosis is
‘water always moves from the hypotonic solution to the hypertonic solution’.
Objectives - TLWBAT:
1. differentiate between osmosis and diffusion.
2. explain how freshwater protozoa deal with water balance.
3. define turgor pressure.
4. observe osmosis in plant tissue.
5. determine the percentage of solute in an aqueous solution given the amount of solute and
solvent.
6. refine microscopy techniques.
Procedure:
1. Make a wet mount of an Elodea leaflet using a 10% saline solution instead of water.
2. Observe under low power of a compound microscope and then under high power. You’ve already
created a micrograph of this in a previous lab – find it or create a new one.
3. Make observations for approximately 15min. During this time note any changes you observe
WITHIN the cells. Create a micrograph to illustrate these changes and record appropriate notations of
them.
Analysis Questions:
1. Describe what happened to the Elodea cells when they were bathed in saline solution.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
2. Why did plasmolysis occur? Use the terms hypotonic and hypertonic to describe the direction of
osmosis in this example.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
3. Explain why your observation of Elodea cells bathed in a hypertonic saline solution illustrates osmosis.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
4. Paramecia and other freshwater protozoa always exist in a hypotonic medium – since they don’t have a
cell wall, how do they prevent cytolysis (the cell bursting!) from occurring?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
5. Define turgor pressure.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
6. Are osmosis and diffusion the same process? Explain!
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________