The Role of Carbohydrates

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The Role of Carbohydrates

Carbohydrates are a very large group of molecules that can be synthesised by plants. They are molecules which contain carbon, hydrogen and oxygen atoms. Usually there is a ratio of 2:1 of hydrogen to oxygen atoms in a carbohydrate molecule. Carbohydrates are very common constituents of plants. They make up to around 90% of the dry mass of plants. Carbohydrates are also an essential part of the animal diet and they are usually obtained directly or indirectly from plants. The functions of carbohydrates vary greatly. There are many different carbohydrates with different sizes and structures, all of which perform a different task in plants or animals. Functions range from being an energy store to providing structural support and strength.

There are three types of carbohydrates; they are the monosaccharides, disaccharides and polysaccharides. This division is based on the fact that polysaccharides are non-sugars whereas monosaccharides are simple sugars and disaccharides are compound sugars. Each group has its own distinctive properties and for every carbohydrate that falls under one group, they share a general formula.

Monosaccharides are simple sugars and contain carbon, hydrogen and oxygen in the ratio 1:2:1 so every monosaccharide has the general formula (CH2O)n, where n can be any number between 3 and 9. They are small molecules with low molecular masses. They are sweet tasting, crystalline, soluble in water and are all redusing sugars. They also all contain at least one carbonyl group and at least two hydroxyl groups. These two groups are an important part of the carbohydrate molecule because they are the reactive groups that have significant roles in the reactions that take place within cells. The simplest monosaccharides are called trioses because they have got 3 carbon atoms in a molecule. There are two main trioses; one called glyceraldehyde and the other called dihydroxyacetone. Both are structural isomers of each other so both have the formula C3H6O3 but the atoms are arranged differently so giving them different characteristic properties. Below are the structures of the two:

Glyceraldehyde is an important carbohydrate formed as an intermediate in the metabolic pathways of respiration and photosynthesis. In respiration the glucose reactant is split in half to make this and in photosynthesis, two of these molecules join to make one glucose molecule. It is known as an aldose sugar because it has an aldehyde group at the end H-C=O. The dihydroxyacetone molecule has the same atoms but arranged differently, this time without an aldehyde group so the molecule is known as a ketose because it possess a keto group C=O. All the sugars that occur naturally derive from either one of these two trioses. All the aldoses are formed from glyceraldehyde and all the ketoses are formed from dihydroxyacetone.
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Pentoses are monosaccharides too but instead have 5 carbon atoms in a molecule with the general formula C5H10O5. Like the trioses, pentoses have a carbonyl group and at least two hydroxyl groups. Ribose and deoxyribose are two such pentoses which are also important constituents of RNA (ribonucleic acid)and DNA (deoxyribonucleic acid) respectively, are aldoses and can exist either as chain or ring forms:

One other main type of monosaccharide are hexoses which have 6 carbon atoms in a molecule and have the general formula C6H12O6. Glucose and fructose are examples of hexoses.

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