II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

Cells are made up of many substances, especially carbohydrates, lipids, proteins, and nucleic acids. Each of these substances plays a vital role in the processes occurring in living structures. In fact, life on the Earth in its present form would be impossible without any of these substances.

Carbohydratesare a large group of compounds in which hydrogen and oxygen, in the proportions in which they exist in water, are combined with carbon. As a class, carbohydrates, are the most abundant organic compounds found in nature. They are produced by green plants and by some bacteria using the process of photosynthesis.

The carbohydrate group consists principally of sugar, starch, dextrin, cellulose, and glycogen, substances that constitute an important part of the human diet and that of many animals. The simplest of them are the simple sugars, or monosaccharides. The most important of them is glucose. Two monosaccharide molecules joined together by an oxygen atom, with the elimination of a molecule of water, yield a disaccharide, of which the most important are sucrose, lactose, and maltose. Polysaccharides have enormous molecules made up of one type or several types of monosaccharide units.

Within living organisms, carbohydrates serve both essential structural and energy-storage functions. In plants, cellulose and hemicellulose are the main structural elements. In invertebrate animals, the polysaccharide chitin is the main component of the exoskeletons of arthropods. In vertebrate animals, the cell coatings of connective tissues contain carbohydrates. Cell membranes are rich in glycoproteins, and so forth. Plants use starch and animals use glycogen to store energy; when the energy is needed, the carbohydrates are broken down by enzymes.

Lipidsare a diverse group of fatty substances found in all living organisms. Lipids are distinguished from other classes of organic compounds in that they do not dissolve in water but are soluble in alcohol, ether, or other organic solvents. Among the most important lipids are the phospholipids, which are major components of the cell membrane. Phospholipids limit the passage of water and water-soluble compounds through the membrane, enabling the cell to keep its contents separate from the outside environment.

Fats and oils, which are composed of triglycerides, serve as stored energy reserves in plant and animal cells. Each triglyceride is composed of three fatty acid molecules bonded to one glycerol molecule. When an organism has excess energy available from food or from photosynthesis, it may use that energy to form stores of triglycerides. These can later be broken down to yield energy when the organism needs it. Fats and oils contain twice as much stored energy, per unit of weight, as carbohydrates or proteins. Other important lipids are the waxes, which form protective coatings on the leaves of plants and the skins of animals, and the steroids, which include vitamin D and several key hormones.

Proteinsconstitute a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as the predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteios, or “primary.”

Protein molecules range from the long, insoluble fibers that make up connective tissue and hair to the compact, soluble globules that can pass through cell membranes and set off metabolic reactions. They are all large molecules, ranging in molecular weight from a few thousand to more than a million, and they are specific for each species and for each organ of each species. Humans have an estimated 30,000 different proteins, of which only about 2 percent have been adequately described. Proteins in the diet serve primarily to build and maintain cells, but their chemical breakdown also provides energy, yielding close to the same 4 calories per gram as do carbohydrates.

Besides their function in growth and cell maintenance, proteins are also responsible for muscle contraction. The digestive enzymes are proteins, as are insulin and most other hormones. The antibodies of the immune system are proteins, and proteins such as hemoglobin carry vital substances throughout the body.

Nucleic Acids are extremely complex molecules produced by living cells and viruses. Their name comes from their initial isolation from the nuclei of living cells. Certain nucleic acids, however, are found not in the cell nucleus but in cell cytoplasm. Nucleic acids have at least two functions: to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of specific proteins. How nucleic acids accomplish these functions is the object of some of the most intense and promising research currently under way. The nucleic acids are the fundamental substances of living things, believed by researchers to have first been formed about 3 billion years ago, when the most elementary forms of life began on earth.

The two classes of nucleic acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA). The backbones of both DNA and RNA molecules are shaped like helical strands. Their molecular weights are in the millions. To the backbones are connected a great number of smaller molecules (side groups) of four different types. The sequence of these molecules on the strand determines the code of the particular nucleic acid. This code, in turn, signals the cell how to reproduce either a duplicate of itself or the proteins it requires for survival.

Читайте також:

<== попередня сторінка	\|	наступна сторінка ==>
VII. Using additional sources of information prepare a report on one of the following topics and present it to the class	\|	Vocabulary notes

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.005 сек.