Hey guys, it's professor Dave. Let's discuss polysaccharides.
Carbohydrates include simple sugars like monosaccharides, but they
also include polysaccharides, which are long polymers of these simple sugars, so
we need to understand the way they polymerize, as well as certain
characteristics of the resulting polymers. First let's return to our
Haworth projection for beta-glucose. In the cyclic form we can see there is a
hemiacetal group, and if we remember the mechanism by which a hemiacetal will
become an acetal, that is essentially what will happen here. We know that if
this hydroxyl is protonated it could leave, and then this carbon could be
attacked by an alcohol, forming what we call a glycosidic bond, as well as an
acetal functional group in the process. But if the alcohol that attacks is one
of the hydroxyl groups on another monosaccharide, we will get a
disaccharide. This is still an acetal functional group but it links two
monosaccharides together via a glycosidic bond. Here we have a beta
glucose linked with another beta glucose which makes cellobiose. Other common
disaccharides are lactose, which is beta-galactose and alpha-glucose, and sucrose
or table sugar, which is alpha-glucose and beta-fructose. We could continue to
polymerize further, making an oligosaccharide, which will have three to
ten saccharide units, or a polysaccharide. Polysaccharides can have thousands of
monomeric units, and nature uses these quite frequently. For example, cellulose
which is abundant in plants, consists entirely of beta glucose units connected
via 1,4-glycosidic bonds.
This means that carbon one, the anomeric carbon of one sugar, is
connected to the hydroxyl on carbon four of another sugar, and it will extend in
beta fashion from the anomeric carbon.
This pattern can extend indefinitely, with beta-glucose monomers connected in
this way. Starch, which is abundant in potatoes and rice, is another
polysaccharide made from glucose, and it only differs from cellulose in that all
the glycosidic linkages are alpha instead of beta. This one difference in
stereochemistry gives the molecule different properties. In addition, starch
is comprised of amylose, starch molecules that are linear, and amylopectin, which is
the same as amylose except it has branching every 20 to 25 glucose units.
The branching will occur between carbon six of one glucose and carbon one of
another, and another chain initiates from there. Similar to amylopectin is glycogen,
which also has alpha 1,4-linkages between glucose molecules. But glycogen
has even more branching, occurring every 10 to 12 glucose units. Glycogen is an
important way that we human store energy inside our cells, and enzymes will break
down some of this glycogen into individual glucose molecules in times of
strenuous activity, or in between meals, while other enzymes can add glucose
units back to the chain to store them for later. So we now have a clearer
picture of what carbohydrates are, from simple sugars like monosaccharides and
disaccharides, to huge polysaccharides with thousands of repeating units.
These will come back later when we learn about cellular respiration, but for now let's
move on to another type of biomolecule: lipids.
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