DIGESTION OF FOOD
Digestion in Mouth: The teeth facilitate mastication of food during which food is broken into small parts. Mastication makes it easier to mix saliva in the food and swallowing also becomes easier. The tongue also helps in mixing the saliva in the food. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
Action of Saliva: The saliva contains electrolytes (Na+, K+, Cl-, HCO-) and enzymes (salivary amylase and lysozyme). Salivary amylase initiates the chemical process of digestion. About 30% of starch is hydrolysed by salivary amylase (optimum pH 6.8); into a disaccharide, i.e. maltose. Lysozyme acts as an antibacterial agent and thus prevents infections.
Starch + Salivary amylase → Maltose
The bolus is then sent into the pharynx and then into the oesophagus by swallowing or deglutition. The bolus passes down through the oesophagus due to peristalsis. Successive waves of muscular contractions in the alimentary canal are called peristalsis. The gastro-oesophageal sphincter controls the passage of food into the stomach.
Gastric glands are present in the mucosa of stomach. There are three major types of gastric glands which are as follows:
- Mucus Neck Cells: They secrete mucus.
- Peptic or Chief Cells: They secrete the proenzyme pepsinogen.
- Parietal or Oxyntic Cells: They secrete HCl (Hydrochloric acid) and intrinsic factor. The intrinsic factor is essential for absorption of vitamin B12.
Digestion in Stomach
The food stays in stomach for 4-5 hours. The churning movements of muscular wall of stomach mix the food thoroughly with the acidic gastric juice. Food is now called the chime.
The proenzyme pepsinogen gets converted into the active enzyme pepsin; on exposure to hydrochloric acid. Pepsin converts proteins into proteoses and peptones (peptides).
Mucus and bicarbonates play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.
Pepsinogen + HCl → Pepsin + Protein → Proteose + Peptone
HCl provides the acidic pH (1.8) which is optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.
Digestion in Small Intestine:
The muscularis layer of small intestine generates various types of movements. These movements help in thorough mixing of food with various secretions in the intestine. The secretions released in small intestine are; bile, pancreatic juice and intestinal juice.
Pancreatic Juice: The pancreatic juice contains inactive enzymes; trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by an enzyme, enterokinase. Enterokinase is secreted by the intestinal mucosa. Trypsinogen becomes active trypsin and activates the other enzymes in the pancreatic juice.
Bile: Bile contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids. Bile does not contain enzymes. Bile helps in emulsification of fats and also activates lipases.
Intestinal Juice: Goblet cells are present in the intestinal mucosal epithelium. These cells secrete mucus. The secretions of the brush border cells of the mucosa; along with the secretions of the goblet cells, constitute the intestinal juice or succus entericus. The intestinal juice contains various enzymes; like disachharidases (e.g. maltase), dipeptidases, lipases, nucleosidases, etc. The mucus; along with the bicarbonates from the pancreas; protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8). Alkaline medium is necessary for action of enzymes in small intestine. Sub-mucosal glands (Brunner’s glands) also help in this.
Proteins, proteoses and peptones are acted upon by the proteolytic enzymes of pancreatic juices; as given below:
Proteins/Peptones/Proteoses + Trypsin/Chymotrypsin/Carboxypeptidase → Dipeptides
Carbohydrates are hydrolysed by pancreatic amylase into disaccharides:
Polysachharides + Amylase → Disachharides
Fats are broken down by lipases with the help of bile into di- and monoglycerides:
Fats + Lipases → Diglycerides → Monoglycerides
Nucleases in the pancreatic juice act on nucleic acids to form nucleotides and nucleosides:
Nucleic acids + Nucleases → Nucleotides → Nucleosides
The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms.
Dipeptides + Dipeptidases → Amino Acids
Maltose + Maltase → Glucose + Glucose
Lactose + Lactase → Glucose + Galactose
Sucrose + Sucrase → Glucose + Fructose
Nucleotides + Nucleotidases → Nucleosides + Nucleosidases → Sugars + Bases
Di and monoglyceridses + Lipases → Fatty acids + Glycerol