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Carbohydrates - Coggle Diagram
Carbohydrates
Monosaccharides
In carbohydrates, the basic monomer unit is a sugar, otherwise known as a saccharide. A single monomer is therefore called a monosaccharide. A pair of monosaccharides can be combined to form a disaccharide. Monosaccharides can also be combined in much larger numbers to form polysaccharides.
Monosaccharides are sweet-tasting, soluble substances that have the general formula (CH2O)n where n can be any number from three to seven.
Example of monosaccharides include glucose, galactose and fructose. Glucose is a hexose (6 carbon) sugar and has the formula C6H12O6. However, the atoms of carbon, hydrogen and oxygen can be arranged in many different ways. For example, glucose has two isomers - alpha glucose and beta glucose.
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Disaccharides
When combined in pairs, monosaccharides form a disaccharide. For example:
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When the monosaccharides join, a molecule of water is removed and the reaction is therefore called a condensation reaction. The bond that is formed is called a glycosidic bond.
When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond releasing the constituent monosaccharides. This is called hydrolysis (addition of water that causes break down).
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Polysaccharides
Polysaccharides are polymers, formed by combining together many monosaccharide molecules. The monosaccharides are joined by glycosidic bonds that were formed by condensation reactions.
As polysaccharides are very large molecules, they are insoluble. This feature makes them suitable for storage. When they are hydrolysed, polysaccharides break down into disaccharides or monosaccharides. Some polysaccharides, such as cellulose, are not used for storage by give structural support to plants.
Starch is a polysaccharide that is found in many parts of plants in the form of small granules or grains, for example starch grain in chloroplasts. It is formed by the joining of between 200 and 100,000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions.
Test for starch
Starch is easily detected by its ability to change the colour of the iodine in potassium iodide solution from yellow to blue-black. The test is carried out as room temperature. The test is carried out as follows:
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