of the molecule is unaltered in maltose.
Now polysaccharides are carbohydrates that
contain multiple sugars in a polymeric form.
These sugars are usually identical for the
molecules that we call polysaccharides, now
a really good example is amylose. Amylose
is a polysaccharide that's found in plants,
it contains hundreds of individual glucose
residues, each one of them joined by alpha-1,4
linkages. Now amylose is an important sugar
because amylose is a way for the plant to
store glucose and then be able to use that glucose
for energy. Indeed many of the polysaccharides
that exist are there for storage purposes.
Plants get sunlight during the day and use
that sunlight for their energy needs, some of
the energy is stored in the form of glucose
and the glucose is stored in the form of amylose.
When the sunlight goes away, glucose is needed,
amylose is broken down.
Another polysaccharide is cellulose. Cellulose
is similar to amylose in the sense that it's
a polymer of glucose. But the difference between
cellulose and amylose is that cellulose has
the glucose units linked in beta-1,4 linkages
as can be seen here. Now this seemingly minor
modification has some significant implications,
particular from a dietary perspective.
We as humans cannot break beta-1,4 linkages
whereas we can break alpha-1,4 linkages.
We can eat amylose and get glucose out of
it in our digestive process. We cannot eat
cellulose and have the same thing occur, because
we don't have the enzymes necessary to break
beta 1,4 bonds. Now there are some organisms
that can in fact break down cellulose. The
most common ones are ruminants. Cows, for
example are out in the pasture eating grass
because within their rumen, they contain a
bacterium that makes an enzyme known as cellulase.
And cellulase has the property that it will
break beta-1,4 linkages and release glucose.
That means that the plant that's eating grass
out in the field is deriving glucose from
what it's eating.
Now humans do store glucose but they don't
store it in the form of amylose, and there's
a very good reason why as we shall see. Now
glycogen is the storage form of glucose in
humans and in most animals. In fact glycogen
is also a polymer of glucose like amylose
is, and glycogen has alpha-1,4 linkages between
the glucose like amylose does. But glycogen
also has in addition to the 1,4 linkages
between the individual glucose units, it has
branches that are shown as 1,6 as you can
see on the glucoses above the bottom chain.
These branches of glycogen occur fairly frequently
within a glycogen molecule. About every 10
glucoses or so, an individual glucose is branched
off as a 1,6. That branch will then go off
for a long way and within it, about every
10 residues, another branch will occur.
So we could imagine then for a glycogen molecule
that contains thousands of glucoses that there
could be hundreds or thousands of branches that
exist. That's a very important consideration
because the way we break down glycogen is
by starting at the ends and moving inwards,
so the more ends there are, the more glucose
that can be released very quickly. Glycogen is
an energy source force and glycogen is stored
in our liver where it can be released
for our body as needed and also in our muscles
where it can be used as a source of glucose
very quickly, the branched form of glycogen
is important for that reason.