DIGESTION

 

SIMILAR REQUIREMENTS OF FOOD IN ANIMALS

 

All animals have similar requirements. They have to obtain their nutrients directly from their external environment. Some of these nutrients such as oxygen, water and enorganic salts usually readily available in the environment and their molecules and ions are all in a form that can enter the cells and be used. The rest of nutrients, which we commomly call food, are ususally   in the form of proteins, carbohydrates, fats and vitamins, etc, and their molecules (expert for some kind of vitamins) are too large to pass into the cells through their membranes. Therefore, such complex compounds of large molecules must be broken down into simpler compounds of small molecules so that they can get into cells. E.g., protein must be broken down amino-acids, starches into simple sugars, fats into fatty acids and glycerols, because these then can readily pass, through cell membranes.