Complex molecules need to be converted to simple molecules for oxidation:
You should remember that carbohydrates like starch and cellulose are complex molecules and are made up of several molecules of simple sugars like glucose. Even common sugar is made up of two different simple sugar molecules like glucose and fructose. Such complex molecules are not oxidised directly to produce energy. They are first converted to simple sugars like glucose and then they are oxidised. Breakdown of complex carbohydrates to simple sugars like glucose occurs during digestion of food materials in the digestive system. In this process, energy is not released and carbon dioxide and water are not produced. Similarly, the fats present in the food arr complex molecules made up of several fatty acids. These complex fat molecules are broken down and the fatty acids formed from them are used for the production of energy.
Energy is released from glucose and fatty acids in several stages :
Energy is produced only when glucose and fatty acids are transported from the digestive system to all the other cells in the body of the animals. You have learnt in the previous chapter that blood carries these substances from the digestive system to the cells. In plants, glucose produced during photosynthesis is carried to all the cells through phloem. In unicellular organisms, there is no digestive system and glucose and fatty acids are produced in the same place where they are oxidised. In other words, during respiration, energy is producechvhen the glucose or fatty acids are oxidised in the cells. As this process occurs in the cells, this is called CELLULAR RESPIRATION.
In the cells, glucose or fatty acids are oxidised and energy is produced in several stages. Some of these stages require oxygen while some may not require oxygen. Each of these stages may have one or several reactions. In each of these reactions, a new product is formed. The product produced in one reaction may be used up in the next reaction and another new product may be ! formed. Therefore, several products are formed before glucose or fatty acids are oxidised. As these products are produced during the metabolism they are called METABOLIC INTERMEDIATES. Different stages and reactions in which glucose and fatty acids are oxidised and the intermediates formed are similar in almost all the organisms - microbes, plants and animals including man.
Energy is not produced in each and every reaction during the oxidation of glucose or fatty rids. Small amounts of energy is produced, that too only in some of these reactions. This energy is converted to chemical energy and is stored in adenosine triphosphate (ATP). Some of the reactions involved in the oxidation of glucose or fatty acids may require energy. In such cases energy present in ATP is provided to the reaction and ATP is converted to adenosine diphosphate (ADP). This ADP is once again converted to ATP in some othef energy producing reaction.
Points to remember:
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Complex molecules are not oxidised directly to produce energy.
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Energy is produced when gludose or fatty acids are oxidised in the cells in a process called as cellular respiration.
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Energy is released from glucose or fatty acids in several stages.
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Energy is not produced in each and every reaction during the oxidation of glucose or fatty acids.
