Vitamins and Coenzymes

Vitamins are chemical compounds that are vital to life and indispensable to body functions. They often exist as provitamins, inactive forms that must be converted into active vitamins before they can perform metabolic tasks in the body’s cells. There are thirteen individual vitamins required by the human body for growth and maintenance of good health.

Vitamins are grouped on the basis of their ability to dissolve in water or fat. The fat-soluble Vitamins A, D, E, and K generally are found together in the fats and oils of foods. Once absorbed, they can be stored indefinitely in the liver and fatty tissues of the body. This capacity for storage can lead to unwanted toxic buildup of certain vitamins, A, D, and K in particular, that can cause great harm. Deficiencies of any vitamin can also be harmful. The National Academy of Sciences publishes recommended intake values for all thirteen vitamins.

Each of the fat-soluble vitamins performs unique functions in the body. The three active forms of Vitamin A are required for night and color vi­sion, reproduction, and cell maturation and differentiation, the process by which precursor cells develop into a specific cell type. Vitamin A also plays a role in fighting infections and in the development and maintenance of bone. Beta-carotene and other provitamin forms of vitamin A known as carotenoids are antioxidants, chemicals that block the harmful cancer- causing effects of oxidizing agents (oxygenlike molecules) on cells. This antioxidant property may also play a role in vitamin A’s prevention of heart disease.

The provitamin form of vitamin D is made by skin cells using sunlight and a derivative of cholesterol. It is then converted to its active form in the kidneys and liver. Vitamin D plays a role in the differentiation of cells in the intestines, skin, immune system, and bones. It also regulates blood cal­cium levels, which are important in maintaining proper bone density.

Vitamin E’s major function in the body is as an antioxidant. It inserts itself into cell membranes and protects substances inside the cells, such as deoxyribonucleic acid (DNA), from being chemically modified by oxygen like molecules.

Vitamin K is involved in synthesizing proteins that help blood clot. It is also necessary for making a key protein important in bone formation. In addition to dietary sources of vitamin K, the body can use vitamin K man­ufactured by bacteria that live in the intestines.

The water-soluble vitamins include vitamin C and the group of eight vitamins known collectively as Vitamin B. They are not readily stored and are excreted in urine when consumed in excess of the body’s needs. Vita­min C’s roles include assisting in the production and maintenance of colla­gen, a protein found in bones, skin, teeth, and tendons. Vitamin C also plays roles in supporting the immune system and producing thyroxine, the hor­mone that regulates body temperature and metabolism.

The B vitamins act as part of coenzymes, small molecules that combine with an enzyme to make it active. Enzymes are proteins responsible for cat­alyzing most chemical reactions in the body, such as digesting food and syn­thesizing new compounds. The B vitamins riboflavin, thiamin, niacin, pantothenic acid, and biotin help the body use protein, fat, and carbohy­drate to produce energy for the body’s cells.

Vitamin B₆ assists in the synthesis of new proteins in the cell by as­sembling protein building blocks called amino acids. Folate and Vitamin B₁₂ are required for cell multiplication. In particular, folate is involved in synthesizing DNA for the dividing cells. Vitamin B₁₂ helps folate enter cells. B₁₂ also maintains the protective sheaths that surround nerve fibers.

References

Committee on Dietary Allowances. Recommended Dietary Allowances, 10th ed. Wash­ington, DC: National Academy Press, 1989.

Committee on Dietary Reference Intakes. Dietary Reference Intakes for Calcium, Phos­phorus, Magnesium, Vitamin D and Fluoride. Washington, DC: National Academy Press, 1997.

          . Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vi­tamin B₁₂, Pantothenic Acid, Biotin and Choline. Washington, DC: National Acad­emy Press, 1998.

Sizer, Frances, and Eleanor Whitney. Nutrition Concepts and Controversies, 8th ed. Bel­mont, CA: Wadsworth, 2000.

Smolin, Lori A., and Mary B. Grosvenor. Nutrition Science and Applications, 3rd ed. Fort Worth, TX: Saunders College Publishing, 2000.

Wardlaw, Gordon M. Perspectives in Nutrition, 4th ed. Boston: McGraw-Hill, 1999.

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مملكة البدائيات (Monera)

كيس الصفار ( المح ) Yolk sac

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