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david walker david at alegba.demon.co.uk
Sat Mar 25 08:59:49 EST 2000


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
possible?

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
manual.

David Walker





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