The living cells contain a large number of molecules. Such molecules are generally classified into carbohydrates, proteins and lipids. These molecules are important to sustain life and almost every molecule has a defined function. Some of them have structural functions. For example, several lipid molecules are integral components of membranes. There are also structural proteins like collagen. Keratin which is present in nails is another structural protein. Some molecules have transport functions and some of them act as energy carriers. Such energy yielding molecules are called high energy compounds. ATP is the classical example of a high energy compound. Several molecules are complex. Thus, you can have a combination of proteins and carbohydrates. Thus, one gets glycoproteins. There are also glycolipids.
Some molecules act as catalysts in biochemical reactions. Catalysts are compounds which do not undergo permanent chemical change at the end of a reaction and are regenerated in the intact form. Enzymes are examples of catalysts. Almost all the reactions in the living organisms are brought about enzymes. There are thousands of reactions taking place in living organisms. It is a mind boggling feature that there are as many enzymes present in living organisms. Most of the enzymes are proteins. The most interesting feature about enzymes is that they exhibit a high degree of specificity. Thus, some enzymes act only on carbohydrates while some enzymes act only on lipids.
Some molecules act as carriers. For example, lipoproteins are molecules capable of transporting lipids. LDL, VLDL and HDL are good examples. Hemoglobin is an important protein and has the important function of transporting oxygen. Some molecules transport metals. Transferrin is a protein which transports iron while another protein called ceruloplasmin transports copper. There are molecules which carry information. DNA is a molecule which carries genetic information. RNA is involved in the process of implementing this information. Neurotransmitters are molecules of the nervous system and also carry information. Acetylcholine and noradrenaline are typical neurotransmitters. Some of the molecules play a regulatory role. Muscle is built of proteins while the fat present in our body is a lipid called triglyceride. Glucose and triglyceride are efficient fuel molecules. Thus, it is clear that life is not possible without molecules.