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Hi Randy & All, Jan 13, 2008
We were warned early in general biology that there is no such thing
as a typical plant or animal and the same goes for metabolic pathways.
So broad generalizations are iffy. And my sources are about 40 years old
so they may be current or perhaps not.
But Sucrose is usually (always ?) the first free sugar formed during
photosynthesis (and depending upon demand a whole host of
non-carbohydrates may be manufactured in chloroplasts during
photosynthesis but that is a different story). Phosphate compounds of
glucose and fructose are typically among the initial products of
photosynthesis and, in essence, act as carriers of reactive potential,
being able to form Sucrose by a downhill (i.e. cost-free) reaction.
In some plants (e.g. most grasses & Sugar Maple) carbohydrate is
translocated mostly as Sucrose. And in such situations starch, by some
bio-alchemy, can be formed directly from Sucrose.
In the enzyme-free hydrolysis of Sucrose, hydrogen ion is a
catalyst, i.e. it is not consumed. The reaction is called inversion and
the end products invert sugar because optical rotation is changed from
positive to negative; thus Invertase.
I suspect many Physical Chemists and Chemists in general cut their
teeth from ~1840-1940 using a polarimeter and variations of this
reaction because the reaction, being first-order (affected over time
only by the remaining Sucrose), is ideally suited to investigating a
host of questions under controlled conditions at the molecular level;
e.g. effects of ionic strength on activity coefficients, ionization
constants of weak acids as affected by whatever, effect of temperature
on reaction rate... and all one needs is grade school lab apparatus, a
few chemicals and a polarimeter.
My P.Chem Lab book calls for 4 N HCl, higher than stomach contents I
would guess, and I gather that the reaction rate constant (other things
being equal) is linear on Hydrogen ion activity..
My original point with respect to hydrolysis was that the reaction
was spontaneous under suitable conditions (I just happened to be more
familiar with the non-enzymatic system), i.e. not energy dependent. I
expect Invertase mediated hydrolysis is more rapid but don't know this
for a fact and I think Invertase is secreted into the digestive system
of vertebrates. The Honey Bee as I recall uses Invertase to hydrolyze
the Sucrose of nectars.
Just as a side comment, it is interesting and unusual that both the
initial formation of Sucrose and the subsequent hydrolysis of Sucrose is
apparently cost-free (from the platform of high-energy bonds provided
in any case during photosynthesis of course). So a mechanism that is
cost free and allows the osmotic concentration due to sugars to be
halved or doubled (within the simple Sucrose, Glucose, Fructose system)
might have some practical implications. But after these first two free
transactions, courtesy of light, you have to start burning metabolic energy.
Yt, DW, Kentville
Randy Lauff wrote:
>
>
> On 12/01/2008, David & Alison Webster <dwebster@glinx.com
> <mailto:dwebster@glinx.com> > wrote:
>
> Hi Randy & All, Jan 12, 2008
> While it is true that starch is a common storage carbohydrate,
> soluble sugars are the building blocks and are present especially when
> starches are being formed or used.
>
>
>
> True, but the dominant soluble sugars would likely be the simple ones
> such as glucose (the building block of starch) and fructose.
>
>
>
>
> With respect to sucrose, Bonner and Varner (Plant Biochemistry,
> 1965, 1054 p.) observe that "Sucrose... is by far the most commonly
> found disaccharide in nature, its distribution being universal among
> photosynthetic plants."
>
>
>
> No question. But "most common" is a relative term - the Common
> Nighthawk is the most common goatsucker in the province, but that
> doesn't mean it's common.
>
>
> Hydrolysis of sucrose is spontaneous in the presence of weak acid
>
>
>
> I didn't know that. I always assumed the enzyme sucrase did that job
> exclusively. Does the process also work in the presence of strong
> acids, as found in the stomachs (or stomach-equivalents) of animals?
>
>
> so, although the process would take time, the formation of
> monosaccharides from sucrose would not consume metabolic energy.
>
>
>
> Except of course for the creation and likely the secretion of the acid.
>
>
> I dare
> say birds and ancestors, over evolutionary time, would have
> encountered
> sucrose and common salt on a more regular basis than they would have
> encountered suet.
>
>
>
> It depends which birds of course. Carnivorous birds (including
> insectivores) would have rarely encountered sucrose, unless it was in
> part of the digestive system of the prey (mouth to small intestine),
> as part of the prey's last meal. Suet, which is simply fat, would be
> encountered by many species (albeit not encountered by them in cages
> or logs). I assume that there is some difference in the composition of
> fats in plants and animals (biochemistry was never a strong point of
> mine), though metabolically, there are a lot more similarities than
> differences.
>
>
> Randy
> _________________________________
> RF Lauff
> Way in the boonies of
> Antigonish County, NS.
>
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