SUBJECT: Elodea diffusion lab

DATE: 4/97

 

I was interested in the comments by SDSCHNELL (no sig) about using Elodea

for a diffusion lab. I'm wondering if it could be made a bit more

investigative in which students would determine the isotonic concentration.

I'm thinking they could be given a strong salt solution to dilute into a

series of tubes. Place an elodea leaf in each tube and after a time, look

at them under the scope.

 

Any thoughts about this?

 

My colleague says he has had problems with Elodea not shrinking like it

should, and neither of us could figure why that should be so.

 

Lane

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Lane Lester / llester@athens.net / Athens, Georgia USA

 

 

Time seems to be a factor. If you are wicking the salt under the

coverslip, watch the side closest to the addition. It can take what seems

to be a few minutes. I've never tried to time it. I'm sure there is a

dilution factor involved in this method as well. The highest concentration

will be near the source initially. --cmw

 

Charlene M. Waggoner, Ph.D. "Great art is eternal;

Department of Biological Sciences great science tends to be

Bowling Green, State University replaced by greater science."

Bowling Green, OH 43403

-- John A. Moore

cwaggon@bgnet.bgsu.edu

 

 

We do this in one of our intro courses and it works best with tips of young

leaves. We usually get plasmolysis in the range of 1-1.5% NaCl. Finer

concentrations can pinpoint it better.

 

Greg Anderson

Department of Biology "Dance you buggers, dance!

44 Campus Ave Or you'll never get to Heaven!"

Bates College

Lewiston, ME 04240 God, speaking to the Methodists,

from the song "The Last Dance"

by Peter Coe

ganderso@abacus.bates.edu

(207)786-6110

 

 

 

Most lab manuals insgtruct us to use a 1% NaCl solution. I use a 25%

solution and it does work.

 

Someone wrote about obtaining sheep blood since we can no longer use human

blood in public secondary schools.

 

I will look into this.

 

Stuart D. Schnell

Science Department

John C. Fremont High School

7676 South San Pedro Street

Los Angeles, CA 90003

 

 

>I'm wondering if it could be made a bit more

>investigative in which students would determine the isotonic concentration.

 

You can do this by looking for incipient plasmolysis (ie., inferred by a series of

dilutions whose % plasmolyses are graphed to determine 50% point). Have

students generate a series of concentrations from a 1M STK (0.1M, 0.2M, 0.3M, etc.) and

determine % plasmolysis for each concentration, then graph as indicated above.

 

A nice extension to this idea is to try salts of different valences (e.g, divalent = NaCl, KNO3;

trivalent = MgCl2) and even molecular compounds with different MW values

(e.g., glucose, glycerol). Students can compare incipient plasmolysis points and look for

relationships (e.g., concentration of trivalent < concentration of divalent @ incipient

plasmolysis, also lower MW has higher concentration @ incip. plasmolysis than higher MW

compd., all other things being equal).

 

Todd

Todd Bennethum, Science Teacher

Arapahoe High School

2201 E. Dry Creek Road

Littleton, CO 80122

 

e-mail: tbenneth@model.cudenver.edu

WWW: http://bilbo.bio.purdue.edu/~tbenneth

 

 

If anyone is looking for a good source for sheep blood, try Colorado Serum

Company (1-303-295-7527). They are reliable. I purchase the Alsiever's (sp?)

prep. It costs about $30-40 for a vial but this is more than enough for your

classes. The cells work wonderfully in a simple experiment using pure water,

3%NaCl and 0.9% NaCl. Just keep the blood refridgerated the week you want to

use it.

Sandy Buckles

Lab Instructor

John Carroll University

Cleveland, Ohio

(216)397-4486

sbuckles@jcvaxa.jcu.edu

 

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