Active Transport

Russell_Goddard rgoddard at valdosta.edu
Thu Sep 16 07:24:04 EST 1999


In the active transport lab below, isn't an alternative explanation
possible that "live" plasma membranes are impermeable to the dyes being
used? Boiling yeast cells for 1 min will kill most of those cells but it
also is possible that the plasma membrane is permeabilized by the
perturbation.  A simple test may be to permeabilize the live cell
membranes with a detergent such as saponin, or even Triton X-100 for that
matter.  The trick will be to show that the dye can enter the cell but at
a rate not exceeding what can be transported out.  

So the real question is, "if you are applying dye to the outside of the
cells, how can you assume it gets inside the cell to be transported out
again?"  Is there a reference or other evidence with which I'm not
familiar?

************************************************************************
Russell H. Goddard		Phone:  (912) 249-2642
Valdosta State University	Main Office:  (912) 333-5759
Biology Department		FAX:	(912) 333-7389
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Valdosta, GA  31698-0015     http://www.valdosta.edu/~rgoddard/
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On 15 Sep 1999, Marta D. de Jesus wrote:

> Dear Dr. Perry,
> 
>     Here's another active transport lab:  I'll admit this sounds very
> simplistic, but my college has been doing this with beginning Bio majors
> for years:
> 
> 1. Make a live yeast culture (bakers yeast with a little sugar dissolved
> in water).
> 2. Have the students take 2 samples in test tubes.
> 3. Boil one of the tubes for ~ 1 min.
> 4. Add a small drop of Congo Red to each tube (explain that the dye is
> actively transported; but not the direction of the transport).
> 5. Have the students examine the 2 samples macro- and microscopically
> and then ask them to explain the color differences with regard to active
> transport.
> 
>     Congo Red will be actively transported out of living cells (Trypan
> Blue can also be used), so the uncooked sample is much darker red
> macroscopically and the cells are less colored at the microscopic
> level.  This tends to shake them up a little because while they know
> that cells have to be alive to do active transport (thus non-boiled),
> many of them assume (probably from the standard explanations in many
> texts) that active transport has to be "into" the cell.  So some will
> twist their observations to fit their assumptions (tell you that the
> non-boiled cells are "really" darker red, or that the amount of Congo
> Red added to the 2 samples was very different).
>     I've modified this basic exercise a little by having them take
> sequential samples at different time points so they can see the
> progression in color change and have them use micropipetters so the
> addition of Congo Red is more precise (could do a quantitative version
> with a colorimeter I suppose) .
> 
> Marta D. de Jesus
> Associate Professor
> Ventura College
> mdejesus at ventura.cc.ca.us
> 
> 
> 
> 




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