Carbohydrates Part 2: Polysaccharides


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. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me:

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42 thoughts on “Carbohydrates Part 2: Polysaccharides

  1. I love your videos… Totally helping me understand. Just enough information then I look up the rest. Great intro too. 🙂

  2. Your videos are really helpful.. I watch them daily as a quick revision.. Huge thanks from my side.. make more videos on biochem.. love from India

  3. You are such a great teacher! Everything starts getting put together when I listen to your lectures. Thank you so much.

  4. Professor Dave:

    You have given me a sense of clarity and understanding in my organic and biochemistry classes that I will forever be thankful for you. YOU ARE AWESOME!

  5. i love your theme. makes me want to listen better to your tutorials. please make more video about alpha and beta projecting units. love from India. 🙂

  6. Interesting.. but I find that the Gammabetamigratinghydroxlycaratinealcholholicacetalcoline1b. enzyme to be conflicting with your diagram.

  7. congratulations!!! that was amazing.I'm looking forward to becoming a biochemist myself.I just want to ask you a question professor Dave.Should i study biology or chemistry first before going into biochemistry????I'm asking this because in my country you can't study biochemistry without having a biology or chemistry degree.Anyone is of course free to leave an answer.It would help me a lot!!

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