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Carbohydrates are the primary source of energy for living organisms. Some are stored as food, starch being the vegetable kingdom's major food storage product. Some form the structural frame work, cellulose being the major structural component in the walls of the plant cells.
3. Lipids (Fats) are the compounds made up of carbon, hydrogen and oxygen but unlike carbohydrates, hydrogen and oxygen are not in the ratio of 2 : 1. They are insoluble in water.
Fats are extremely good sources of energy; in animals they form reserve food besides glycogen; hormones which are steroids (a type of fat), regulate several physiological activities of animals.
ENZYMES AND THEIR ACTIONS
In plants as well as animals, thousands of chemical reactions taking place at a relatively low temperature level and enzymes are the substances which take the place of high temperatures. Enzymes are the most important chemicals in the cells which are synthesized by them as ordered by its nucleus. Even though they catalyse the production of so many substances, they cannot make themselves.
They consist of proteins with or without some other compound. They are effective in minute concentration and they act as catalysts, i.e., bring about chemical changes without themselves undergoing any change. They are specific in action. Many enzymes catalyse only a single chemical reaction with a particular set of reactants and do not act even on similar compounds—For example, the carbohydrates work on carbohydrates only, the lipases react with the fats only and the proteases react with the proteins only. This specificity is related to the configuration of both the enzyme itself and the substance upon which the enzyme works, in much the same way as the indentations of a specific key fit into the lock for which it is made. Some act only in alkaline medium, some in acidic and others in neutral medium; most of them are intracellular; some digestive enzymes are extracellular; they are destroyed at temperatures above 70°C.
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