How does digestion takes place?

How does digestion takes place?Food is the basic and essential requirement of man for his very existence .

The food we eat consists of carbohydrates, proteins, lipids, vitamins and minerals. The bulk of food ingested is mostly in a complex macromolecular form which cannot, as such, be absorbed by the body. Digestion is a process involving the hydrolysis of large and complex organic molecules of food stuffs into smaller and preferably water soluble molecules which can be easily absorbed by the gastrointestinal tract for utilization by the organism. Digestion of macromolecules also promotes the absorption of fat soluble vitamins and certain minerals .Cooking of the food ,and mastification (in the mouth )significantly improve the digestibility of foodstuffs by the enzymes. as well as absorption are complicated processes that occur in gastrointestinal tract (GIT) involving many organs .The digestive organs possess a large reserve capacity. For instance, pancreas secrete enzymes 5-10 fold higher than required for digestion of foods normally ingested. The digestion and absorption of individual foods, namely carbohydrates, proteins, lipids and nucleic acids, is described here.
Carbohydrates: The principal dietary carbohydrates are polysaccharides(starch, glycogen) ,disaccharides (lactose, sucrose), and , to a minor extent ,monosaccharide’s (glucose, fructose).
Digestion: The digestion of carbohydrates occur briefly in mouth and largely in intestine .The polysaccharides gets hydrated during heating which is essential for their efficient digestion Carbohydrates are the only nutrients for which the digestion begins in the mouth to a significant extent.
Proteins: The proteins subjected to digestion and absorption are obtained from two sources –dietary and endogenous. Proteins are degraded by a class of enzymes namely hydrolases-which specifically cleave the peptide bonds, hence known as peptidases. They are divided into two groups
Endopeptidases: which attack the internal; peptide bonds and release peptide fragments, e.g. pepsin, trypsin.
Exopepeptidases: which act on peptide bonds of terminal amino acids.
Protein digestion begins in the stomach .Gastric juice produced by the stomach contains hydrochloric acid and a protease proenzyme namely pepsinogen.
Lipids: Lipids are insoluble or sparingly soluble in aqueous solution. The digestive enzymes, however, are present in the aqueous medium. This poses certain problems for the digestion and absorption of lipids .Fortunately, the digestive tract possesses specialized machinery to, increase the surface area of the lipids for digestion, solubilize the digested products for absorption.
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