III. Read and translate the text. Organic chemistry is the chemistry of the compounds of carbon

ORGANIC CHEMISTRY

Organic chemistry is the chemistry of the compounds of carbon. There are many such compounds - over a half million different or­ganic compounds have been described in the chemical literature. The simplest, organic compounds are the hydrocarbons. The carbon atom possesses the property of being able to form either chains or rings by joining up a number of similar carbon atoms together. The carbon atom is tetravalent (lour bonds) whereby other atoms can be joined to it. A feature of organic chemistry is that the number of other elements involved is small: hydrogen, oxygen and nitrogen are by far the most common.

The three biggest classes of the organic compounds are proteins, carbohydrates and lipids.

Proteins are made of many amino acids linked together. Amino acids are compounds having both the properties of amines and acids. There ate many naturally occurring amino acids. The substances are found either free as components of plant or animal tissues or as a product of protein hydrolysis. Several of them are essential in human nutrition. The amino acids essential for human equilibrium are: isoleucine, leucine, lysine, methionine, phenylanine, iheonine, and valine. Histidine is required by infants.

Proteins are among the.most important substances in plants and animals. Proteins occur as separate molecules or as reticular constitu­ents of cells. Proteins are separated into groups based chiefly on physi­cal properties (solubility, coagulation, precipitation). These groups are: albumins, globulins, prolamines, nucleo-proteins, phosphoropro-teins (casein in milk), albuminoids, chromoproteins (color of hemo­globin).

The human body contains different proteins. The blood, hair fin­gernails, skin, tendon and muscle fibers consist mostly of protein. Plant and animal food contains proteins (meat, eggs, milk and cheese, cereals and nuts). Special enzymes convert proteins into simple amino acids so that the body can use them.

Carbohydrates occur widely in nature. The most important carbohydrates are sugars, starches, and celluloses. The carbohydrates are classified according to the number of carbon, atoms in the molecule: monosaccharides-pentoses (xylose, ribose), hexoses (glucose, galactose, fructose), disaccharides (sucrose, maltose and lactose) and polysaccharides (cellulose, starch, glycogen and inulin).

A common simple sugar is glucose, a primary product of photosynthesis, present in every plant cell. Jt occurs in many fruits and is present in the blood of animals. Fructose, the so-called fruit sugar, is present in fruits. Fructose1 condenses to form inulin. Inulin is an important food reserve in some plants. Ordinary sugar (cane-sugar, beet-sugar) is sucrose. The molecules of it are found in plants (wheat, maize, barley, carrots, sugar beet), in most fruits and leaves of many plants.

Important polysaccharides include, starch, glycogen and cellulose. Starch is the most common food reserve material in plants (in the seeds, tubers, and in the leaves, as a temporary product of photosynthesis). When a drop of potassium iodide solution is placed on a part of the plant, the granules of starch become blue. Starch is an important constituent of foods (potatoes, wheat and others). Glycogen (animal starch) is a, substance similar to starch occurring in the blood and internal organs, especially liver, of animals. It serves as a reservoir of carbohydrates. Whenever the.concentration of glucose in blood becomes low, glycogen is rapidly hydrolyzed into glucose.

Lipids are any of a diverse class.of organic compounds, found, in all living things that are greasy and insoluble in water. One of the three large classes of substances hi foods and living ceils, lipids contain more than twice as much energy (calories) per unit of weight as the other two (proteins and carbohydrates). They include the fats and edible oils (e.g. butter, olive oil, corn oil), which are primarily triglycerides; phospholipids (e.g. lecithin), which are important in cell structure and metabolism; waxes of animal or plant origin; and sphingolipids, complex substances found in various tissues of the brain and nervous system.

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