Carbohydrates

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Starch

Basic unitα glucose


Function: the main storage molecule in plants

Structure: starch is made of two compounds - amylose and amylopectin. Both are, of course, made of α glucose, but their overall shapes differ. Amylose is a spiral, while amylopectin has branches. Combined, they give starch the appearance of a tightly wound molecule like a brush.

Crucially, starch is an excellent storage compound, so must satisfy certain requirements. Its size must be relatively big so that it is not soluble. This prevents it from causing an osmotic effect in cells whereby water floods in. The molecule must be compact in order to take up little space rather than a lot. This is achieved by the branches and spirals within starch. Finally, the branches also contribute to the readiness of the glucose molecules of being "nipped off" and quickly usable. This is because only glucose molecules at the ends of starch can be used in that way.
Glycogen

α glucose


the main storage carbohydrate in mammals

The structure of glycogen is essentially the same as that of amylopectin i.e. branched structure. The difference is that glycogen is even further branched compared to amylopectin. This enables a quicker build-up and breakdown of glycogen, hence meeting the superior energy demand of animals as compared with plants.
Cellulose

β glucose


confers structural strength in plant cell walls

Cellulose is the only molecule in this list which is solely made of β glucose. Despite beta glucose only having one chemical group different from alpha glucose, the result is a significant overall structural difference in cellulose. Unlike amylopectin and glycogen, cellulose has a structure based upon straight chains of beta glucose units, rather than spiraling chains of alpha glucose molecules.

These straight chains laid next to one another form hydrogen bonds which strengthen them into larger sub-units called microfibrils. Microfibrils are what cellulose is made of, and what gives plant cell walls their great strength.
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