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Digestion and Types

Updated on November 11, 2017
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The complex and big organic molecules of vegetable and animal origin with are consumed by animals and human beings remain unused by the body as such. These complex molecules are broken down into simpler and smaller molecules before they enter into cells the process of breaking down of complex and large or insoluble molecules under the action of various enzyme is termed digestion.
Digestion can be defined as :
"The process of conversion of undiffusable form of food into diffusible or simpler form with the help of enzymes."
Digestion in usual involve of hydrolysis of nutrition molecules by enzymes call hydrolases. In lower organisms, the food is digested by enzyme inside the cell. Such digestion is known as intracellular digestion. In higher animals, cells usually synthesize one or few specific digestive enzyme and secrete these into the surrounding medium. The food materials are digested outside the cell and products of digestion are then absorbed into cells. Such digestion is called extracellular digestion. In mammals, enzymatic digestion occurs mainly in mouth, stomach and small intestine.

The principal forms of carbohydrates in our food are :
(1) Polymers like starch and glycogens,
(2) Disaccharides like sports like sucrose and lactose, and
(3) Simple sugars like glucose and Fructose.
The digestion of carbohydrates begins in the mouth. In the mouth, food is combined with saliva. Saliva contains enzyme amylases which hydrolyses starch into maltose.
Further digestion of carbohydrates stops in the stomach because of pH is acidic and the amylases become inactive in an acidic pH . When the food passes into the intestine, it gets mixed with amylases produced by the pancreas and small intestine walls. These hydrolyse the polysaccharides completely into monosaccharides, such as glucose, fructose and galactose. This monosaccharides pass into the bloodstream and to the liver through intestinal wall. The amylases present in the liver convert galactose into glucose. Fructose is either converted into glucose or utilized as such for various purposes.
Some polysaccharides are fairly common in our diet but do not provide energy they have no enzymes to digest them. Cellulose is one such example. However cellulosic fiber indigested with the diet helps in normal functioning of the large intestine by providing the necessary roughage. Grazing animals also do not possess a correct digestion enzyme for cellulose but depends on intestinal bacteria for degenerating cellulose.
Glucose absorbed into blood is the major source of energy for cells. Cells of brain and nervous system depends entirely on blood glucose for their function .The concentration of blood sugar is regulated by the hormone insulin. If sugar level is very high, simple sugar are converted into polysaccharide and glycogens in the liver. On the other hand, If the blood sugar level is very low, the storage glycogen gets hydrolyzed to raise the level. Malfunctions of a biological system can lead to too much blood sugar (called hyperglycemia) or too low blood sugar (called hypoglycemia). Either of these two conditions indicate one or the other type diabetes.

Lipids denote a group of compounds which includes fats and oils. The digestion of fats and oils occur.
When fats (or lipids) are hydrolyzed by the enzymes call lipases. Lipases are soluble in water and do not dissolve in fats and oils. The fats (lipids) are on the other hand are insoluble in water. Thus lipase can digest fat only when fat droplets are broken into tiny droplets to form an emulsion of the oil in water type. The bile fluid helps in emulsification of fats and oils.
The fat is greatly digested in small intestine. The pancreatic juice contains pancreatic lipase, which is principal enzyme for fat digestion. The intestinal lipase found mainly in the intestinal epithelial cell. Bile salts are steroids secreted by the liver in the bile. These steroids helps in emulsification of fats and the facilities the lipase activity.
Fatty acids monoglycerides and glycerol are the major end products of fat digestion.

The pH in the stomach is acidic (pH = 22). The digestion of proteins starts in the stomach, and a low pH helps in the denaturation of proteins prior to digestion. Enzymes proteases breakdown large protein molecules to smaller peptides and the enzyme peptidases transform peptides into amino acids.
Protein digestion begins in the stomach, and involves the enzyme pepsin (which functions at the acidic pH). Digestion continue in the intestine in the presence of trypsin, chymotrypsin in and different other peptidases secreted by pancreas and intestine. The dietary proteins are broken down into individual amino acids. Amino acids are absorbed into the bloodstream through intestinal mucosa. These amino acids are used to reassemble into newer proteins or for further breakdown.


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