Release of Energy from Food

When food is burnt in the laboratory , the following reaction occurs:

Food + oxygen ---> carbon dioxide + energy

Oxygen is necessary for food to burn. It is, therefore, an oxidation reaction. The complex food substance is broken down to liberate the simple molecules, carbon dioxide and water, and energy. The reaction is rapid and heat is given off larges amounts in one big step.

    Scientists have shown that food is also burnt in a similar wag in living CLS. The main food that is burnt in cells is glucose. The oxidation process which produces energy, carbon dioxide a d water is summarized below :

C6 H12 O6 + 6O2 ---> enzymes --> 6CO2 + 6H2O + energy

   However , the energy is not released in one big step as shown in the above reaction, but in a series of small steps which are catalysed by enzymes. The energy that is released, but by but, is  stored in adenosine triphosphate (ATP) molecules. The oxidation of glucose to release energy in this manner is known ad cellular reputation. It occurs in the Mitochondria of all living cells.

   One molecule of the 6 carbon glucose is broken down into molecules of the 3 carbon pyruvic acid by enzymes in the cytoplasm of the cell. These reactions do not require oxygen. Each molecule of pyruvic acid is oxidized completely to carbon dioxide and water I. the Mitochondron. This latter series of reactions is known as the Kreb's cycle. Most of the  ATP molecules is formed in this circle. A total of thirty eight ATP molecules are formed when one molecule of glucose is completely oxidized. 

    When needed, lipids and proteins are broken down to small molecules and enter the Kreb's cycles at various points.

ATP and glucose

Glucose , the substrate in cellular respiration, and ATP, the final essential end product, are both molecules that act ad energy stores. So, why does the cell go through the elaborate process of cellular respiration? Glucose is a stable compound which will not release energy quickly and easily as and when it is needed for the cell activities. ATP, however, is an extremely reactive compound, which will release its energy quickly when it is needed as shown :

ATP + H2O 

   

      |    

      |   Enzymes  

     V

ADP + H3PO4 + energy

Energy is released simply by breaking off a phosphate group from ATP. Thus, ATP is the cell immediate energy store.

    Glucose is an idea substance for transferring energy from the organism energy stories to the individual cells. Other larger molecules like starch, glycogen and fats are idea for

• acting as long term energy stores within an organism; and 

• transferring energy from one organism to another ( in the food of heterotrophs).