The basic structure and functions of starch, glycogen and cellulose and the relationship of structure to function of these substances in animals and plants.

Starch is the form of carbohydrate which plants store energy as: in small grains especially in the seeds and storage organs.
It is a polysaccharide made up of α-glucose to make a long straight chain which then winds up tight (unbranched helical chain).
Being wound up so tight means you can fit a lot of it in a small space and therefore a convenient way to store energy.
It is also a positive that it is made up of α-glucose, because this means when it is hydrolysed (broken down) that will be the molecule produced and it is easy to transport and use in respiration reactions.
Starch is also insoluble, this is good for two reasons: one, it does not tend to diffuse out of cells; two, it doesn't tend to draw water into cells by osmosis.

Glycogen is the form of carbohydrate which animals store energy as: in small granules especially in the muscles and liver.
It is a polysaccharide made up of α-glucose to make a short and very branched chain which winds up tight (branched helical chain).
Like starch it is insoluble, fits a lot of energy into a small space and makes α-glucose when hydrolysed, but because it is shorter is is hydrolysed more quickly.

Cellulose is found in plant cell walls.
It is a polysaccharide made up of β-glucose. If you have two β-glucose molecules and perform a condensation reaction, one of the molecules will have to turn up side down; this is because the order of the OH and the H is reversed on one side, so to match up it has to be turned round. This fact means that in a chain of β-glucose the 'CH2OH' group will alternate between being at the top and the bottom of the chain. The importance of this is that it can't coil up.
So, the chain is straight and unbranched which means several chains can lie next to each other; hydrogen bonds will form between these chains creating a strong 'microfibril' (what fibres are made of).

Beta glucose forming a glycosidic bond

Chain of cellulose

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