The food is digested in the stomach by metabolism, as told already. However, the process of digestion starts in the mouth itself, when it is eaten. In the mouth it is masticated, that is, mechanically broken by teeth into smaller particles before being swallowed.
The food, that is vegetarian, is ground by premolar, or molar teeth of dull ends, while for the non-vegetarian food, the sharp canine teeth tear the fleshy part of the food. A dental set has 32 teeth, sixteen in each jaw, which on each side has two incisors, one canine, two premolars and three molars.
The tongue has a rich supply of blood vessels and nerves, which make it sensitive to taste—sweet, saltish, sour, bitter or whatever. The salivary glands in the mouth release saliva, a watery and alkaline fluid.
The saliva (or the salivary enzymes) moistens the food and causes chemical digestion, or decomposition of starch into smaller molecules, so as to make it digestable.
From the mouth, the chewed and saliva-mixed food is swallowed voluntarily down into the elementary canal, which takes the food further down along the canal to the stomach.
The masticated and partially digested food, when arriving in the stomach, causes the stomach to release its gastric juices. The acidic secretions of the juices get mixed up with the alkaline content of the food, and the digestion progresses.
Due to contraction of the muscles of the stomach, the food gets churned up in the stomach. It normally takes three hours to be well mixed up. The enzymes of the stomach break down the proteins into a mixed fluid, called the CHYME, which passes on to the small intestine in smaller molecules.
The first part of the intestine is called the DEODENUM. In the Deodenum, the mixed food (the chyme) is acted upon by the BILE JUICE coming from the liver and also the pancreatic juice coming from the pancreas.
In addition to these, the walls of a part of the small intestine (called ILEUM) also release and pour into them some enzymes. All these—the bile, the pancreatic juice and the enzymes—digest the carbohydrates of the food (into GLUCOSE) the proteins (into AMINO ACIDS) and fats (into fatty acids etc).
The bile helps in making the food alkaline, necessary for the proper enzyme action and also the emulsification of fats, so as to be fully absorbed.
In the above lines, you would have got an idea of what digestion is. Now we shall describe how this digested food is utilized in the cells of the body.