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Proteins are veryimportant for our body, as they determine the structure and function ofall cells, and thus the tissues and organs. They are a necessary partof our diet. However, proteins cannot be absorbed into our body assuch, because proteins in foods are too complex and too large totravel through the wall of our intestines. They need to be brokendown by the process of digestion. Let us see how a complex molecule ofprotein is digested.

The mouth

Proteins are not digestedin the mouth. What happens to almost all foods (save starch) in themouth is just the chewing, which serves to turn the food into a well- ground pulp. Molecular structure of proteins remains unchanged inthe mouth. Ground food is swallowed and ten it goes to the stomach.

The stomach

Stomach is basically anacid bath blender. It is a muscular sack whose inner lining secretesa powerful acid known as hydrochloric acid or stomach acid. pH of thesecreted acid can be as low as 1. The food, soaked in this acid,stays in the stomach for a while. Contractions of the stomach (thereare three contractions per minute on average) serve to mix itscontents so that the entire quantity of food is evenly exposed toacid. Other types of stomach glands secrete enzymes, which serve tobreak up food ingredients chemically, and mucus, whose role is toprotect the stomach itself from the hydrochloric acid by creating aprotective layer over the stomach's walls lining. Despite this, theacid gets to the lining and destroys it, so it needs to be replacedall the time. It is thought that the entire stomach lining isreplaced in just three days.

Enzymes that we mentionedare proteins whose role is to chop molecules in the food into simplerforms. A protein molecule can consist of many hundreds of aminoacids. You can imagine a protein molecule as a string of amino acidssimilar to a pearl necklace, which has been bent and twisted andeventually combined with more strings to form an adequate shape,according to its purpose.

Enzymes in the stomach cutup this string into smaller pieces, more suited for the final stageof digestion. How fast the cutting (breaking) of the protein intosmaller pieces (known as peptones and proteose) lasts depends on anumber of factors, such as quantity of the enzymes, amount of proteinin the food, temperature and acidity of the food, acidity of thestomach and presence of factors that inhibit digestion. In general,protein digestion in the stomach lasts for some four hours.

The small intestine

The food that was digestedin the stomach, known as chyme, is moved into the small intestine byrhythmic contractions of the stomach that squeeze the chyme out. Thefirst part of the small intestine is known as the duodenum. In it,the highly acidic chyme is neutralized (the acid is destroyed) bymixing with alkaline secretions.

Enzymes produced by the pancreas,trypsin and chymotrypsin break the proteins, peptones and proteosemolecules into amino acids. Walls of the small intestine are coveredwith villi, microscopic finger-like protrusions, which increase thesurface of the intestine walls. The walls of the villi are very thinand blood capillaries and lymph vessels are just beneath. Amino acidspass through the thin walls and through the capillary walls, andenter the bloodstream, which will carry them through the entire body,distributing them wherever they are needed for synthesis of proteinsadequate to our organism's needs.

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