What are Digestive Glands and explain its role in Digestion ?

The essential functions of Digestive Glands are:-

Salivary Glands:

In man there are three pairs of salivary glands (sub maxillary, sublingual, and parotid) which secrete saliva. Saliva contains an enzyme called salivary amylase which breaks down starch (complex substance) of the food into maltose (a simpler sugar). Thus, in the mouth cavity saliva moistens the masticated food and starts digestion of carbohydrate.

Gastric Gland:

The gastric glands secrete hydrochloric acid and gastric juice which help in digestion of food. The enzyme pepsin present in the gastric juice acts on the proteins of the food and breaks them into smaller units called peptones and proteoses. The food then passes into the small intestine.

Liver:

It is the largest gland of the body. It weighs 1.5 kg in man. Liver performs many functions. As far as digestion is concerned, it secretes a fluid called bile.

Bile juice produced by the liver is stored in the gall bladder. Gall stones which are found in the gall bladder of about 8% of the people are chiefly the concretions (depositions) of cholesterol, bile pigments and calcium salts. Bile is yellowish greenish, alkaline fluid. Bile emulsifies fats which help in breaking them into small globules. In this way, fat globules are easily exposed to the action of fat-hydrolyzing, enzymes. The acidic food coming from the stomach becomes alkaline, when it is mixed with the bile. It is an extremely important step which ensures further digestion of the food. The digestive enzymes that are brought in the duodenum and ileum can catalyse the breakdown of food only in alkaline medium.

Pancreas:

It is the second largest gland of the body. It lies in the fold of duodenum. It is yellowish. It secretes pancreatic juice.

Pancreatic duct pours pancreatic juice into the duodenum. Pancreatic juice contains a number of digestive enzymes. These include trypsin and chymotrypsin for the breakdown of proteins; amylase for the splitting of polysaccharides; lipase for the breakdown of fats and nuclease for the breakdown of nucleic acids. These enzymes catalyse the breakdown of different constituents of food but not sufficiently enough to break all of them into their units.

Intestinal Glands:

The final step of digestion takes place in the ileum. There are numerous smaller glands occupying the walls of the small intestinal tract. These glands secrete what is termed intestinal juice or succus entericus. The intestinal glands are in the form of sunken pits or crypts which are interspersed among the finger-like villi. The digestive enzymes in the intestinal juice include carboxypeptidase and aminopeptidase which break small peptides into amino acids; sucrose, maltase and lactase which brak disaccharides into respective monosaccharides; lipase which breaks lipids into fatty acid and glycerol; and nuclease which breaks nucleic acids into nucleotides.

Absorption of Digested Food

Absorption of completely digested food taken place in the ileum. There are absorptive cells lining the finger-like projections, the villi, of the ileum. These absorptive cells of the villi absorb the units of food by a process involving the expenditure of energy. This process is known as the active transport. The absorbed food is then brought into the blood vessel. The products of lipid digestion are brought into the lymphatic vessel. From here the digested food materials are transported to different parts of the body through circulation.

Assimilation of Digested Food

The process whereby digested food is absorbed and utilized is termed assimilation. One of the ways by which digested food can be utilized is to obtain energy from it by the process of respiration. The excess of amonosaccharides is joined to form glycogen by the enzymes of liver and stored as such. The amino acids may be used in the synthesis of a variety of structural and functional proteins. Ammonia is produced by the removal of amino group of amino acids which gets converted into less toxic urea (nitrogenous waste) in the liver. Urea is removed from the blood through the kidneys. The glycerol and fatty acids either provide energy or get reconverted into fats. These fats are accumulated in different organs below the skin layer. The absorbed food is also utilized for the formation of new cells and tissues, leading to growth and development of the body.

Metabolism and Release of Energy

The sum of all biochemical reactions occurring within the living organisms is called metabolism. They are of two general types:

(i) Catabolism:

Catabolism involved the breakdown of complex molecules into simpler ones. This reaction release energy mainly in the form of heat and is known as exergonic reaction. The examples are the processes of respiration, digestion, etc.

(ii) Anabolism:

This involves the biochemical reactions which lead to the formation or synthesis of complex molecules from simpler ones. In this constructive process, energy is required and, therefore, the process is called endergonic reaction. Photosynthesis, protein synthesis are anabolic processes.

Living organism grow if anabolic rate is higher than the catabolic rate. As an organism approaches old age, the catabolic becomes higher than the anabolic rate.

Two stages are involves in the liberation of energy from food. The first stage involves the breaking down of complex molecules into simpler forms. In the second stage oxygen is required for oxidation of simpler molecules. Along with the liberation of energy CO2 and water are required. For expulsion of CO2 and intake of oxygen animals breath or respire. The chemical reaction taking places I this process remain the same in every organism, whether it is a frog, a bacterium, a bird or a man. This suggests a common ancestry of all organisms.