Yet more glucose

David R.Hershey dh321 at excite.com
Tue Jan 25 18:20:50 EST 2005


Sucrose is certainly a good choice as a major product of photosynthesis
for many species. Starch is probably an even better choice. Even better
is to mention both because some species produce little starch in their
leaves. I think a C3 product is at least better than saying glucose is
the product.

Historically, starch was recognized as the first visible product of
photosynthesis by Julius Sachs in the 1860s. Starch grains accumulate
in chloroplasts of many species during photosynthesis. Other good
reasons for using starch as the major product of photosynthesis is that
students can easily identify starch in leaves using the iodine test. It
really helps students remember concepts if they demonstrate the concept
to themselves, and they can easily test for starch in leaves. They
cannot easily test for C3 products of photosynthesis.

I believe that the reasons why glucose is so often used as the major
product of photosynthesis in textbooks is to make the summary equation
for photosynthesis simpler and the exact reverse of the summary
equation for cellular respiration. C6H12O6 as the product makes the
summary equation easier to balance than with starch or sucrose. It is
an oversimplication without telling students that it is an
oversimplication or explaining why.

Ganong (1908) does not have the term glucose in his index or mention it
in his lengthy discussion on photosynthesis. He refers to C6H12O6 in
his photosynthesis summary equation as "photosynthate" and states "the
chemical composition of the photosynthate approximates to C6H12O6,
which may be taken as a conventional formula for the photosynthate in
general." Even he, the leading botany teacher of his era, didn't fully
explain the apparent contradiction that he wrote C6H12O6 in the summary
equation but emphasized starch formation during photosynthesis:

"The student is here in contact with one of the most significant facts
in all organic nature, viz., the appearance of starch in lighted green
leaves."

David R. Hershey
dh321 at excite.com


Reference


Ganong, W.F. 1908. A Laboratory Course in Plant Physiology. New York:
Henry Holt.




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