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Carbohydrate structure and function

Carbohydrates (including sugars) are made of carbon, hydrogen and oxygen. They come in various sizes and chemical arrangements and hence serve multiple functions in biology, including energy storage and structural support.

The smallest units (monomers) of carbohydrates are simple sugars and include trioses, pentoses and hexoses, so named due to the number of carbon atoms present (3, 5 and 6 respectively). They are monosaccharides.

The triose glyceraldehyde for example, is an intermediate in the metabolism of carbohydrates to produce energy during cellular respiration. The pentose deoxyribose is a constituent of no other than DNA itself, while the hexose glucose needs no introduction as the central energy processing molecule created in photosynthesis and expended in respiration.

Glucose is a key carbohydrate in biology because it is the preferred fuel that provides energy during cellular respiration as well as the building block for complex polysaccharides and other compounds that use it such as glycoproteins. As a monomer, it is small and water soluble and thus accessible to cells for respiration. Plants and other organisms can create it through photosynthesis. It can be built into starch or glycogen and stored away for later use. When required, it can be obtained by breaking starch or glycogen down again. This is what maintains blood sugar levels.

The points where the lines intersect each symbolise a carbon (C) atom. You need not show those. The figure above is taken from the specification itself, so take it as a good guide. So the monosaccharide alpha glucose (commonly, just glucose) somehow becomes a polysaccharide, This is achieved by condensation reactions, and the bonds formed are called glycosidic bonds.

You should be able to draw this. The resulting molecule, maltose, is a disaccharide (two monomers). If you keep adding glucose molecules to the chain, you get… *drum roll please* …starch. Starch is made up of multiple (very many indeed) monomers, so it is a polymer i.e. it is made of multiple monosaccharides, so it is a polysaccharide.

The reverse reaction of breaking it down into monomers is hydrolysis.

You also need to know about two other disaccharides and their constituent monosaccharides – sucrose and lactose.

Sucrose is made of glucose and fructose.
Lactose is made of glucose and galactose.
It’s easy enough to remember: they’re both made of glucose, and lactose (MILK) is also made of galactose (galaxy – Milky Way). Both sucrose and lactose are formed similarly by the condensation of their monosaccharides.

Further polymerisation to three or more monosaccharides results in the formation of polysaccharides such as starch, glycogen, cellulose and chitin.

Starch is the energy storage in many plants and cereals that form the basis of our own food e.g. rice and potatoes, while glycogen is our mammalian energy storage e.g. in the liver and muscles. Cellulose is made of beta glucose unlike the others which are made of alpha glucose units. This gives it a different overall structure and makes it great as a strong structural element in cells e.g. plant cell walls. Chitin is similarly strong and used for structural purposes in places such as fungal cell walls and insect exoskeletons.

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