Monosaccharides are the simplest carbohydrates. There are two main categories for monosaccharides, pentoses and hexoses. Glucose is a hexose, the general formula for Glucose is C6 H12 O6. Glucose is a six-membered ring consisting of 5 carbons, 1 oxygen. Glucose can exist in two ring forms, Alpha and Beta. In Alpha Glucose the OH group is below the ring on carbon 1, however in Beta Glucose the OH group is below. Glucose is a monomer, monomers are used to make polymers. Glucose is used to make disaccharides and polysaccharides (polymer).
Disaccharides are formed when two monosaccharides are condensed together. One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond.The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of a 1,4-glycosidic bond. The reverse of this reaction, the formation of two monosaccharides from one disaccharide, is called a hydrolysis reaction and requires one water molecule to supply the H and OH to the sugars formed. Some examples of disaccharide are Sucrose (Glucose + Fructose), Lactose (Glucose + Galactose), and Maltose (Glucose + Glucose).
olysaccharides are carbohydrates whose molecules are polymers - that is, they are made from many monosaccharides monomers linked together. They are very large molecules, and so are not soluble in water.
Starch is a mixture of two polysaccharides, amylose and amylopectin. Amylose is made of many Alpha Glucose units, linked with glycosidic bonds between carbon 1 on one glucose and carbon 4 on the next. Amylopectin is also formed of Alpha Glucose, but has branches where the linkages are on carbon 1 and carbon 6. Starch molecules curl up into spirals. Starch is the main storage carbohydrate in plant cells, where it forms grains inside chloroplasts. It is very compact and, because it is insoluble, it does not affect the water potential of the cell.
Glycogen has similar structure to amylopectin. It is the storage carbohydrate of animals, and is found in lover and muscle cells.
Cellulose is a polmer of Beta Glucose, joined with (1-4) links. Its molecules do not curl up, but form long straight chains. These chains tend to lie parallel to each other, forming a bundle of molecules called a microfibril. They are very insoluble, and difficult to digest, because few organisms make an enzyme that can break the Beta (1-4) linkages.
The main functions of carbohydrates as a whole are:
- Substrate for respiration (glucose is essential for cardiac tissues)
- Intermediate in respiration (e.g. glyceraldehydes)
- Energy stores (e.g. starch, glycogen)
- Structural (e.g. cellulose, chitin in arthropod exoskeletons and fungal walls)
- Transport (e.g. sucrose is transported in the phloem of a plant)
- Recognition of molecules outside a cell (e.g. attached to proteins or lipids on cell surface membrane).
