Polysaccharides – Complex Carbohydrates

by Kevin Ahern, PhD

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    00:00 of the molecule is unaltered in maltose.

    00:00 Now polysaccharides are carbohydrates that contain multiple sugars in a polymeric form.

    00:07 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.

    01:00 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.

    01:24 We as humans cannot break beta-1,4 linkages whereas we can break alpha-1,4 linkages.

    01:31 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.

    02:01 And cellulase has the property that it will break beta-1,4 linkages and release glucose.

    02:08 That means that the plant that's eating grass out in the field is deriving glucose from what it's eating.

    02:15 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.

    02:58 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.

    03:19 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.

    About the Lecture

    The lecture Polysaccharides – Complex Carbohydrates by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. Amylose has α-1,4 linkages and cellulose has β-1,4 linkages
    2. Cellulose is more branched than amylose
    3. Amylose contains α-1,6 linkages and cellulose contains α-1,4 linkages
    4. Cellulose does not contain glucose
    1. It is a branched polymer containing only glucose
    2. It is an oligosaccharide
    3. It is a polymer of glucose with only α-1,4 linkages
    4. It is the same as amylopectin

    Author of lecture Polysaccharides – Complex Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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