The process of digestion is achieved by chemical and mechanical processes.
Digestion in Mouth and Stomach
Protein digestion does not take place in the mouth. It starts in the stomach. The gastric juices contain HCL and pepsinogen. Pepsinogen is converted into active pepsin in the presence of a strongly acidic pH in the stomach. Pepsin is a very proteolytic enzyme and hydrolyses many proteins into smaller molecules of proteases and peptones. It acts on a variety of proteins. As the food passes to the duodenum, pepsin action is stopped by the alkalinity of intestinal contents.
Pepsin is also hydrolysed soluble casein into paracasein and whey protein. Paracasein is then precipitated spontaneously as calcium paracaseinate to form the solid curd.
As another milk coagulating protease which is called rennin is also found in the gastric juices of human beings during infancy and in the calf. In adults, gastric juice is devoid of rennin. Rennin is secreted inactive prorenin which is converted into rennin in the acidic medium.
Digestion in Intestine
During digestion, when proteins leave the stomach, they are mainly in the form of proteoses, peptones, and polypeptides. The partial breakdown products are attached by the major proteolytic pancreatic enzymes.
Pancreatic proteases are secreted in inactive forms like trypsinogen, chymotrypsinogen and procarboxypeptidase. In the presence of enterokinase inactive trypsinogen is converted into active trypsin. Trypsin activates chymotrypsinogen and procarboxypeptidase into chymotrypsin and carboxypeptidase.
Trypsin hydrolysis proteins into peptides, but has no action on keratins and milk protein–casein. Animals which drink the blood of their prey, trypsin hydrolyses fibrinogen of blood into fibrin, leading to blood coagulation.
Chymotrypsin hydrolyses casein into paracasein. Paracasein is then coagulated into calcium paracaseinate.
Carboxypeptidases hydrolyse polypeptides to simple peptides and amino acids.
The intestinal juice contains three important proteolytic enzyme, enteropeptidase, aminopeptidase and dipeptidases. The intestinal peptidases act best at a pH 7.0-8.0.
The principal action of enterokinase is to convert the inactive proteolytic proenzyme of the pancreatic juice, trypsinogen into trypsin.
Aminopeptidase hydrolyses polypeptides to amino acids and simpler peptides by removing the amino acid containing the free amino group from the polypeptide this progressively shorten the polypeptide chain into dipeptides.
When polypeptide has reached the dipeptide stage, the dipeptide is finally hydrolysed by the activity of dipeptidase.
Jaundice, constipation, diarrhoea, vomiting, PEM and indigestion are common disorders of the digestive system.
It is characterized by yellow skin pigmentation of the skin and sclera of the eye due to bilirubin and excess of bilirubin concentration in blood. Bilirubin is a yellow pigment which is normally produced when old or damaged red blood cells are broken down in the spleen.
Infrequent or difficult defecation, which produces discomfort is known as constipation. It is caused by decreased motility of the intestines. It is caused by improper bowel habits, spasms of the colon, the insufficient fibre in the diet, inadequate fluid intake, lack of exercise and emotions.
The usual treatment is to use a mild laxative, such as milk of magnesia.
Frequent or profuse defecation of liquid faeces is called diarrhoea. It results in frequent dehydration and electrolyte imbalances, abdominal pain, bleeding from the rectum. It may be caused by lactose intolerance, stress and microbe that irritate gastrointestinal mucosa. The usual treatment is the replacement of fluids and electrolytes lost through the bowel.
It is a condition in when the content of the stomach come out through oesophagus into the mouth and out of it. It could be caused by nausea, constipation or the changes in the body’s equilibrium during travel by car, air or sea. Antiemetics are necessary to suppress nausea and vomiting.
Any minor discomfort coming on after meals tend to be called indigestion. Symptoms generally described as indigestion include a feeling of fullness in the stomach, discomfort in the upper abdomen, burping, a hot feeling behind breastbone and water brash. It causes trivial such as eating or drinking too much.