<no subject>

John E. Silvius SILVIUSJ at cedarnet.cedarville.edu
Sat Mar 25 13:13:16 EST 2000


>>> david walker <david at alegba.demon.co.uk> 03/25/00 08:59AM >>>
wrote:

>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

>>>>>
With regard to David's suggestion of including a chapter in his book
on "practical stuff", I would certainly encourage him to do so.   I
also agree that there are additional opportunities for developing
critical reasoning in experiments such as the starch prints.    For
example, when I do this experiment as a demo in my botany classes, I
use a message printed in black letters on a framed acetate about the
size of a 2x2 slide and remove it, of course, prior to students seeing
the leaf (sunflower).    

We approach the demonstration asking about solubility of leaf
pigments rather than focusing on "starch",  and I use boiling water
and boiling 80% ethanol as two likely solvents.   Before the leaf is
exposed to boiling water I ask students what is on the inside of
leaves and "air" is one of the correct responses.    Then,  "What
would you expect to happen when a leaf is heated?" is a question that
prepares them to interpret the bubbles of air that appear on the
epidermis (lower epidermis shows more clearly...wonder why??) when it
is submerged in the hot water.

The water of course does not bring out the pigments, but we can
discuss loss of turgor that is evident when the leaf is "brought up
for air"  (held by petiole with forceps).    This opens to a
discussion of denaturation of membranes and the possibility that water
may actually pave the way for a better solvent, ethanol, to bring out
the pigments.    They see support for this in the ethanol exposure to
follow.

The starch print development then follows, and at least part of
David's point about carbohydrate mobility can be addressed.   My
interpretation is that unless a given mesophyll cell is illuminated it
will not store starch, and even if soluble carbohydrates are
translocated to darkened regions (?), the experiment suggests that
will no starches are formed there; at least under the circumstances in
which the leaf has been attached as a "source" to the intact plants
with "sinks" to which sucrose can be exported.    

Thanks for the discussion of this laboratory, and I encourage David
to add the chapter and to send a PDF of fluorescence.







---

John E. Silvius
Professor of Biology
251 N. Main St.
Cedarville College
Cedarville, Ohio   45314
E-mail:   silviusj at cedarville.edu 
http://www.cedarville.edu/dept/sm/jes_www.htm
---




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