Carbohydrates are organic compounds consisting of carbon, hydrogen and oxygen atoms. They are the main energy source of organisms making them vital for life. Large carbohydrates are complex molecules composed of small, repeating units of monosaccharides.
Monosaccharides
Monosaccharides are single sugar units and are the simplest form of carbohydrates, they are also called simple sugars. They have the following general formula:
Where n is greater than 3, usually between three and seven. The most common monosaccharides have 5 or 6 carbon atoms (so n is usually equal to 5 or 6) such as glucose with the molecular formula:
The structure for glucose (the main form of sugar stored in the body for energy) is:
The numbers in red are number each of the carbon atoms, which becomes important when monosaccharides bond together. This is the structure of α-glucose. There is also a β-glucose, the only difference being the direction of the hydroxyl group and the hydrogen atom on carbon 1:
The positions of atoms in the structure are very important. If we look at the molecular formula of fructose (the main sugar found in fruits) it is exactly the same as glucose:
However the structure of fructose looks like this:
Disaccharides
If two monosaccharides bond together they form a disaccharide (also a simple sugar). Similar to lipids, disaccharides are formed by condensation reactions and the bond can be broken by hydrolysis reactions.
The bond formed between two monosaccharides is called a glycosidic bond which is a type of covalent bond.
Below is an example in which two α-glucose molecules join together to form maltose (a disaccharide):
The position of a glycosidic bond is important. To help identify the position we number the bond using the carbon atoms the bond forms between. If you look back at the structure of α-glucose you can see that the carbon atoms involved in the bonding to from maltose are carbon 1 and carbon 4. Therefore we would call this a 1-4 glycosidic bond.
Another example of a disaccharide is sucrose. This is formed by a 1-2 glycosidic bond between α-glucose and fructose. See if you can draw it!
Polysaccharides
A carbohydrate containing a chain of more than two monosaccharides is called a polysaccharide. They are long chained and complex, potentially consisting of thousands of monosaccharides.
Here are two examples of a polysaccharide:
- Starch
- Glycogen
Glycogen
Glycogen is used to store glucose in humans and animals. It is composed of α-glucose with both 1-4 and 1-6 glycosidic bonds.
Despite being a large molecule it is very compact making it suitable for storage and it is also insoluble. There are many branches, making it easy for glucose to be released when required.
Starch
Starch is used to store glucose in plants. It too is composed of α-glucose, however, it is composed of a mixture of two different polysaccharides called amylose and amylopectin.
Amylose has 1-4 glycosidic bonds and is unbranched. Whereas amylopectin consists of both 1-4 and 1-6 glycosidic bonds and is branched (it is similar in structure to glycogen). Like glycogen starch is also insoluble in water.
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