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N. Harrison vulpia at sonic.net
Sun Mar 26 11:28:06 EST 2000

On 25 Mar 2000 13:59:49 -0000, david at alegba.demon.co.uk (david walker)

>Re: photosynthesis lab
>The recent  attention given to 'photosynthesis lab'  gave me pause for
>thought not least because of the emphasis put on starch pictures, an old
>favourite of mine, but also because one underlying fundamental question was
>not mentioned, thereby posing a couple more. Is it enough in such class
>experiments to demonstrate that starch only forms in an illuminated part of
>the leaf? Should the question then be asked that, since it is entirely
>possible to produce starch pictures of definition which equals the best done
>by conventional photography,  why is this possible at all?  Chloroplasts
>export triosephosphates to the cytoplasm where sucrose is then made. Sucrose
>then seemingly  migrates through the leaf without causing high definition
>starch pictures to become blurred. Conversely, if leaf discs  are floated on
>a sucrose they make starch very happily in total darkness. How is this
>I have, as you would guess, a reason for asking about this. I am writing a
>book. It is half done. It was not my intention to include a chapter on
>practical stuff for classes but maybe I should? For example one very simple
>but totally arresting experiment is to prepare a dense but clear solution of
>chlorophyll in alcohol, place it in a rectangular cuvette and illuminate it
>strongly from one side (preferably using a light 'pipe'). Apart from
>anything else, the contrast between the green light that exits in one plane
>and the deep red fluorescence that emerges from an other is very beautiful.
>The real interest then comes from the attempts of students to explain what
>they see. Would this be regarded as a sensible and realistic approach or are
>we simply wishing to share our experience of (and pleasure in) phenomena
>without inviting explanation?
>I shall be happy (should they so wish) to send  a picture (in PDF) of such
>fluorescence to anyone who cares to Email me with their views on the
>desirability, or otherwise, of including one or two simple demonstrations of
>this sort in a book which is not intended, in any way, as a practical
>David Walker
David, I looked at your work on this book which you put on the web,
and I think an "Appendix" or "Supplement"  at the end
would be a great addition. I picture your charming book being
used in a teaching environment - why not add some simple
experiments to further illustrate the principles/
  We set up an old clunky spectrocope with chlorophyll extract
on one ringstand and acetone on the other (in tubes, naturally)
and the students look at both, swinging the spectrocope around
from one to the other. We have a dissecting miccrosope lamp
aimed at the back of the tube with the chlorophyll extract, whiich
nicely shows the fluorescence. The students are always baffled
by this, and we try to lead them, with hints, to understand what
is happening, because we believe it is of great importance that
they "picture" the action of the electrons, in their minds, so they
can see the importance of the electron acceptor in intact leaves.
   I put the geranium plant in the dark today, because on Monday
the non-majors class is doing PSN - and the following week, it
will be the majors class's turn (I hope the geranium holds up for
two sessions in the dark!) - NH

Nancy Harrison
T.A.Botany at Santa Rosa Jr.College
Milo Baker Chapter, California Native Plant Society

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