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Carbohydrates - Biology - Part 2

Updated on September 11, 2015

Note: this is part two of a hub series about carbohydrates.

For the first part, click here.

Amylose and Amylopectin
Amylose and Amylopectin | Source


Starch is the storage carbohydrate for plants. It is made up of the two molecules: amylose and amylopectin.

  • Amylose is a straight chain of between 2000 and 5000 glucose molecules. It has 1,4 glycosidic bond (its 1st and 4th carbons attach to other 1st and 4th carbons in order to form bonds).
  • The position of these 1,4 bonds causes the long chain to coil into a spiral shape.
  • Amylopectin – Is a polymer of glucose but has side branches stemming out of it. 1,6 bonds hold the side branches onto the main chain.

Most plants contain starch grains (stores of starch) that consist of 70-80% amylopectin and 20-30% amylose.

Because starch is insoluble, it has little to no effect on osmosis (net movement of water from high concentrations to low concentrations) in plant cells. This makes it very good for storage purposes. Furthermore, starch is very compact due to its spiral shape and thus can densely store glucose molecules in small spaces.

Cellulose Microfibrils - Cellulose makes up a very large part of plant matter.
Cellulose Microfibrils - Cellulose makes up a very large part of plant matter.


Also known as ‘dietary fibre’ and the ‘non-starch’ polysaccharide.

  • It contains between 1000 and 10,000 glucose molecules in a long chain with no branches.
  • It is a polymer of beta glucose (as opposed to alpha glucose in starch and glycogen).
  • It consists of 1,4 beta glucose glycosidic bonds.
  • Cellulose forms microfibrils: each long straight cellulose chain forms hydrogen bonds between every single -OH group of adjacent cellulose chains to form a very strong structure. This is called a microfibril.

Diagram of the Uses of Glycogen
Diagram of the Uses of Glycogen


Glycogen is the animal equivalent of starch. It is a molecule used for storage in animals, fungi, and bacteria.

Glycogen has a similar structure to Amylopectin. It like starch, has many side branches giving it a high surface area. Glycogen has a similar structure to Amylopectin. This makes the molecule easily hydrolysed (broken down by adding water) and so the glucose monomers are very readily accessible. Humans store glycogen in their muscles and livers.

Carbohydrate Structure Explained by Video :)


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