Active Transport

Marta D. de Jesus mdejesus at
Thu Sep 16 12:39:49 EST 1999

Dear Dr. Goddard

    You wrote:

> In the active transport lab below, isn't an alternative explanation
> possible that "live" plasma membranes are impermeable to the dyes being
> used?

True.  In fact in an old (pre 1970s?) book on microscopy that I have it talks
about the structure and charge on trypan blue and congo red to be reasons for
their exclusion, as well as their tendancies to "floculate".  To be honest,
I'm not sure about the provenance of this exercise; it is one that I
inherited with the lab manual when I was assigned the class.

> 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.

I'll admit to not having tried this.  But will this treatment also do other
damage, i.e., not just changing permeability, so that really we are testing
multiple effects?

> 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?

Is there a cytological dye/agent that can be followed like this?  I wonder if
one of the stains that forms a colored product inside a live cell (aka vital
stains) could be doing this.  Any plant cytologists????

Perhaps the lab I described would be better described as a simulation rather
than an actual exercise in active transport, but it could be there is an
obscure reference out there that actually found that this was a real instance
of active transport.  I must admit I do not know.

Marta D. de Jesus
Ventura College
mdejesus at

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