9.2 - Digestion

Types of Digestion

Digestion is the process of breaking down macromolecules into smaller and simpler molecules that are readily absorbed by the body cells. In humans and other mammals, digestion involves two processes which are physical and chemical digestions.

• Physical Digestion
  • Breakdown of large food pieces by mechanical means
  • Includes chewing and slicing action of the mouth; churning action of the stomach
• Chemical Digestion
  • Breakdown of complex moleculers to simple soluble molecules
  • Involves the use of digestive enzymes

Digestion of Carbohydrates in the Mouth

1. Digestion process begins in the mouth.
2. The presence of food triggers the secretion of saliva from the salivary glands.
3. Saliva contains salivary amylase which begins the hydrolysis of starch to maltose.
4. The optimum pH between 6.5 - 7.5 in the mouth allows salivary amylase to function optimally.

5. Chewed food is rolled into a mass called a bolus for easier swallowing.

Figure 2 - Peristalsis is an involuntary muscle movement. The rhythmic contraction and relaxation are triggered by bolus in the lumen.

6. During swallowing, a cartilage flap called the epiglottis closes the airway temporarily to prevent food from entering the trachea.
7. Movement of bolus along the esophagus is aided by peristalsis process until it reaches the stomach. Peristalsis is a continuous wave-like muscular contractions and relaxation along the alimentary canal.

Digestion of Protein in the Stomach

The epithelial lining of the stomach contains gastric glands that secrete gastric juices. There are 3 main cells involved in the digestion process in the stomach.

• Chief cells produce pepsinogen
• Parietal cells secrete hydrochloric acid
• Mucous cells release mucus

Figure 3 - Secretion of gastric juices contain mucus, hydrochloric acid and enzymes pepsin and renin.

Pepsinogen is an inactive enzyme which will be activated by hydrochloric acid to form pepsin. Pepsin can then start the hydrolysis of proteins into polypeptides.

Hydrochloric acid in the stomach has 3 important functions:

• Gives an acidic condition (pH 1.5 -2.0) for action of pepsin
• Stops the activity of salivary amylase
• Kill bacteria present in the food

Mucus protects the stomach wall from the action of hydrochloric acid and digestive enzymes. Food is thoroughly churned and mixed with the gastric juices by peristaltic action of the stomach wall for a number of hours. Eventually, the content of the stomach becomes a semi-fluid called chyme. Relaxation of the pyloric sphincter allows the chyme to enter the duodenum slowly.

Digestions of Carbohydrates, Proteins and Lipids in the Small Intestine

The small intestine consists of duodenum, jejunum and highly coiled ileum. Duodenum, the first part of the small intestine, receives chyme from the stomach. At the same time, secretions from the gall bladder and pancreas.

• Liver

  • Secretes bile which is stored in the gall bladder.

• Bile

  • Reduces the acidity of the chyme
  • Gives alkaline condition (pH 7.6 – 8.6) for enzyme action
  • Emulsifies lipids into tiny droplets to provide bigger surface area for enzyme digestion.

• Pancreas

  • Secretes pancreatic juices which contains pancreatic amylase, trypsin and lipase.

Glands in the wall of the ileum secrete intestinal juice which contains digestive enzymes to complete the digestion of peptides and disaccharides.