DIGESTION AND ABSORPTION

         The food (i.e., organic nutrients viz. carbohydrates, proteins and fats) are essentially required for the production of energy. The complete oxidation of food produces energy in the form of ATP (Adenosine Tri Phosphate) utilized for various functions. Bu, naturally, organic nutrients are present in the form of high molecular weight, complex and large-sized organic compounds. Therefore, in order to utilize them, they have to be broken down into low molecular weight, simple and small-sized molecules. This process in physiology is known as digestion. Teeth help in physical digestion whereas enzymes carry out chemical digestion. Digestion is carried out by various kinds of enzymes and acids (see table B.6.3).

 However, digestion without absorption is incomplete, because the digested nutrients have to ultimately reach the cells or tissues, which will oxidize them for their energy requirements. Hence, the digested nutrients must cross the cell barriers, enter the blood stream, and reach their final destination, the cell or tissues. This process is called as absorption. Thus, digestion and absorption are inter-related. In simple organisms, food is captured or engulfed either by pseudopodia, cilia or flagella. This is known as phagocytosis (“phago” = feeding; “cyto”=cell). The phagocytiosed food forms a food vacuole. The enzyme inside the food vacuole digested food is then absorbed by diffusion. In higher organisms, however, the process is complicated.

A summary of various digestive processes operating in the alimentary canal of humans is given in the table.