Question

Make a neat and labelled diagram of the alimentary canal of man and explain the physiology of protein and fat digestion.

Answer 1

Hint: The entire process of digestion starts with chewing or mastication of food in our mouths. As we break down the foodstuffs into smaller and smaller particles, various enzymes are released, which act on carbohydrates and fats. The salivary amylase and lipase.

Complete Answer:
- The human GI tract or alimentary canal begins at the mouth and ends at the anus. From the mouth, food enters the oesophagus from where it is taken into the stomach. The pyloric sphincter controls the release of the gastric contents into the duodenum and small intestine, and then into the colon or large intestine, ending at the rectum. Waste products are then excreted through the anus.
- Along with the alimentary canal are associated the pancreas, liver, salivary glands, and gallbladder which produce the required digestive enzymes.
- Proteins are polymers of amino acids. They may be monomeric or multimeric. For them to be absorbed, they need to be broken down into free amino acids and some di- and tripeptides. Protein digestion does not begin in the mouth, but in the stomach.
- Gastric juices which are at a very low pH as well as hydrolytic enzymes which include endo and exopeptidases break down the proteins into smaller chains and free amino acids. Pepsin is the main endopeptidase in the stomach which breaks protein chains into proteoses and peptones.
- In the intestines, pepsin is inactivated and pancreatic enzymes like trypsin, elastase, and carboxypeptidases take over. The di and tripeptides are absorbed by cells of the intestinal lumen, further hydrolysed to free amino acids, and these then enter circulation.
- Fat digestion begins with emulsifying them so that there is a greater surface area for the lipases to act on. A small amount of digestion begins in the mouth, but most digestion of fat is in the intestines.
- Digestion in the stomach is about emulsification, in the intestines, bile acids solubilise lipids in the form of micelles, and enzymes like pancreatic lipase and phospholipase A2 release free fatty acids, monoglycerides and cholesterol from the triglycerides.
- Being fat soluble, these digestion products easily cross the cell walls of the lining epithelium. Lipid digestion products then reform triglycerides, combine with Apo proteins to form chylomicrons, and are released into the bloodstream.

Note: Carbohydrate digestion begins with salivary amylase in the mouth, and is completed in the intestine. Digestion has to result in production of monosaccharides, as only these carbohydrate digestion products are absorbed.