Polysaccharides – Biological Molecules Ep 8

Last week we looked at glucose and disaccharides, which are simple carbohydrates. However, many carbohydrate molecules are made up of long chains of monosaccharides joined together to form polysaccharides. Today we will look at polysaccharides made from glucose.

We saw in the last article that to form the disaccharide maltose, two glucose molecules are joined together in a condensation reaction to form a glycosidic bond. This is exactly what happens to form a glucose polysaccharide, there’s just a lot more glucose molecules joined together. Polymers of glucose can be stored and broken down by hydrolysing the glycosidic bonds when glucose is needed.

A polymer of glucose (a polysaccharide)

Starch

Plants and animals have different ways of storing glucose. In plants, excess glucose is stored as starch. Starch is a mix of two polysaccharides called amylose and amylopectin, both of which are polymers of α-glucose (if you remember, glucose has two isotopes: α-glucose and β-glucose). These two polysaccharides have different structures:

  • Amylose is an unbranched polymer of α-glucose. It coils up, making it nice and compact to optimise storage.
  • Amylopectin is a branched polymer of α-glucose. The branches mean there are more ‘ends’ for enzymes to access and hydrolyse the glycosidic bonds, so the glucose can be released more quickly when needed as an energy source.
Two types of starch

These large starch molecules are insoluble (they don’t dissolve in water) which is another property making them good for storage. Insoluble molecules do not affect the water potential, so don’t cause water to be drawn into cells by osmosis. This can only be a good thing, otherwise the cells would swell up and possibly burst.

Cellulose

Another polysaccharide found in plants is cellulose. Cellulose is an important part of the cell wall and provides strong structural support for the plant. The key difference about cellulose it is a polymer of β-glucose. This means that the chains are straight instead of coiled, and hydrogen bonds can form between the chains which creates strong microfibrils (fibres).

Three molecules of cellulose joined with hydrogen bonds

Glycogen and branched polysaccharide chains

Animals store glucose a little differently. In animals, a polysaccharide called glycogen is made from α-glucose. It is quite similar to amylopectin but has many more branches. The branches mean that the glucose can be released more quickly and makes it a compact molecule. Again, glycogen is insoluble in water so doesn’t draw water into cells by osmosis.

Lastly, let’s take a look at how chains of glucose can branch. It all depends on where the glycosidic bonds form. We can number the carbons in a glucose molecule from one to six (see diagram). The glycosidic bond shown in the previous article is a 1-4 glycosidic bond because carbon 1 is joined to carbon 4 with by the glycosidic bond. To create a branch, a glycosidic bond can form using carbon 1 and carbon 6 instead to get a 1-6 glycosidic bond.

An α-glucose molecule with the carbon atoms numbered

So any polysaccharides with branches (glycogen and amylopectin) have 1-6 glycosidic bonds as well as 1-4 glycosidic bonds.

Summary

  • Glucose monosaccharides are joined with glycosidic bonds to form a polysaccharide chain.
  • In plants, glucose is stored as starch (amylose and amylopectin – chains of α-glucose). Amylose is coiled, amylopectin is branched.
  • In animals, glucose is stored as glycogen which is highly branched.
  • Cellulose is a polysaccharide found in plant cell walls and gives structural support. Chains are held together with hydrogen bonds
  • Glucose storage molecules are compact and insoluble, and the branches enable quick hydrolysis of the glycosidic bonds to release glucose.

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