 In this module, we will talk about carbohydrates. Carbohydrates are polymers of sugar molecules. In this module, we will look at the basic structure and what contributions these molecules have in cellular chemistry, cellular physiology. Carbohydrates play a very significant role in storing energy. We are familiar with a simple carbohydrate, glucose. Glucose is the fuel which is oxidized in cell and provides energy for other cellular processes that require energy. Carbohydrates also have a structured role. I'll give you an example of that later in this module. For now, I would like to mention that carbohydrates can be classified into four different groups. The monosecorides, in which the monomer, the basic unit of carbohydrates, exist in its own form, on its own. The disaccharides, the simple example of a disaccharide is table sugar, which we use. It is a molecule in which two monomers are linked together and we use that in our daily routine. Oligosecorides are molecules in which three or up to 20 monomers are linked together to a covalent bond. We look at the nature of the spawn also. These molecules, oligosaccharides, are small molecules, small carbohydrate molecules which can be used to decorate proteins. They can be attached to protein molecules, which we talked about earlier, and they can modify the function of these proteins. Polysaccharides are very large polymers of these smaller units, monomers. These polysaccharides have two roles. Polysaccharides have a structured role. For example, wood is a type of a polysaccharide and also these polysaccharides can have a role in storing energy. For example, the rice, the wheat, they all are examples of polysaccharides. Monosaccharides that I'm going to talk about are glucose, fructose, and ribose. The structures are on the screen. You will notice that basic formula of these carbohydrates is a carbon atom attached to a hydroxyl group and a hydrogen atom. This is the basic formula. So for a monomer like glucose, fructose, and ribose, the formula is CH2O. So these are the linear forms of these molecules. However, in the cell, these molecules can become circular and they exist predominantly in a circular form. We'll look at that too. Glucose, for example, can exist in two forms, linear form, which I showed you earlier, and a circular form. When glucose molecule becomes a circular molecule, it has two choices. Either it can adapt an alpha configuration or a beta configuration, depending upon the orientation of aldehyde group when the ring closes. You will see the difference between these two molecules in alpha glucose, the hydroxyl group is pointing downwards, whereas in beta glucose, hydroxyl group is pointing upwards. The consequence of this chemistry or this organization of atoms in this molecule is very significant, which will become clear in subsequent modules. One of the monosecrites that I would like to talk about, we will talk about this monosecrite in significant detail when we talk about DNA or ribonucleic acids, that this molecule has five carbon atoms. You can see they have been numbered 1, 2, 3, 4, and 5. When this molecule circulizes, becomes circular, look at, please pay attention to the arrangement of these carbon, the position of these carbon atoms. There is a carbon number one with a hydroxyl group. There's a carbon number two with a hydroxyl group. Carbon three has a hydroxyl group, and then carbon number five has a hydroxyl group. Carbon atom number five and carbon atom number three, they are very important carbon, carbon atoms in this molecule when we start talking about the DNA polymer.