Методические указания по английскому языку по дисциплине английский язык предназначены для студентов изоп нгау всех направлений подготовки.
Скачать 1.3 Mb.
|
THE GENERAL CHEMISTRY OF THE CELL / PART 2 /All the substances present in protoplasm are known to fall into two great classes: inorganic and organic substances. The most important inorganic compound in protoplasm is water. The movement of substances in or out of cells is dependent on their being dissolved in water. Water protects against extremes of temperature. Its ability to conduct heat well is useful in equalizing temperatures throughout a living body. Organic compounds occur only in living bodies or in their products or remains. The element carbon (С) is present in all organic compounds. Organic compounds are somewhat unique since the atoms of carbon can occur as chains or join to form rings. The organic compounds are known to be divided into three main classes -the carbohydrates, the lipids and the proteins. Proteins are all-important structural components in every cell amounting to 15% in protoplasm. A protein is built of sequences of amino acids linked together by means of peptide bonds to form a chain. Each amino acid is composed of the carboxyl group (-COOH) and the amino acid group (-NH2), attached to the carbon atom next to the carboxyl group. The general formula is R.CH(NH2)COOH where R is a variable grouping of atoms. Every organism maintains a supply of amino acids in a definite proportion of the 20 common and 2 rare amino acids, peculiar to each species of organism. The particular properties of the protein depend upon the order in which the 20 amino acids are arranged' and repeated in the chain. One of the simplest of the proteins is insulin. Its molecule contains 51 amino acids representing 15 different kinds. Finally, there are the instructional or information-carrying molecules in cells, nucleic acids. These are composed of five main nitrogen-containing purine and pyrimidine bases, ADENINE (A), GUANINE (G), CYTOSINE (C), THYMINE (T) and URACIL (U), combined with phosphoric acid and ribose or the related deoxyribose. They make up the giant molecules1 of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) which are responsible for arranging the amino acids in proper sequences in the proteins. Whereas the units of a protein are amino acids, the units of a nucleic acid are nucleotides. On the basis of results of X-ray crystallography on DNA, James Watson and Francis Crick proposed in 1953 their now-famous model, which shows DNA as composed of two strands. Usually two strands are coiled round each other in the form of a double spiral, with the strands connected to each other through a pair of bases. The bases can only be paired in two ways. Adenine pairs with thymine or uracil, and cytosine pairs with guanine. DNA is found only in the chromosomes of plants and animals and the corresponding structures in bacteria and viruses. RNA is found in the nuclei, the ribosomes and the cytoplasm of all cells of all organisms. A single strand of RNA produced from a lengh of DNA (a cistron) by the enzyme transcriptase is called messenger RNA (mRNA). A molecule of mRNA contains the genetic code to be translated into a particular polypeptide molecule. The sequence of amino acids in a protein is determined by the sequence of the nucleotides in a molecule of messenger RNA. Enormous advances have been made in the last twenty years in giving precise chemical knowledge on these matters. We expect the researchers to make new discoveries in the structure and chemistry of the cell. WHAT SHAPES A CREATURE'S LIFE COURSE AND BEHAVIOR?More than 130 years ago, Charles Darwin laid out his elegant and then shocking theory that it was natural selection, the survival of individuals most suited to the environment, that molded species over evolutionary time. But until very recently, Darwin's ideas remained unproved. Now, for the first time, studies in the wild are rigorously demonstrating the particulars of how evolution works. Last month, David Reznick, an evolutionary ecologist at the University of California at Riverside, published the results of an 11-year experiment involving guppies living in the Aripo River of Trinidad. The experiment proved that predators are among the principal forces driving the evolution of species, just as predicted by a mathematical model that modern biologists had formulated to refine Darwin's theme. According to the model, animals that are preyed on as adults will evolve to produce as many babies as they can, as early in life as possible. «If your chances of dying young are good, then having babies yearly is important», says Reznick. However, the earlier a species reproduces, the sooner it burns out, so to speak, and the shorter its life span. Conversely, whose juveniles bear the brunt of predatory attacks tend to have their young later in life, in effect choosing to bear hardier offspring over a longer adult life. The experiment that Heznick devised, along with his colleagues, was delightfully uncomplicated. The researchers tested the mathematical model simply moving 200 guppies from the base of a 20-foot waterfall in the Aripo River, where predatory fish eat only adult guppies, to the top of the waterfall. There, the single predator is a killfish, a species that devours only young guppies. After 60 generations, the experimental guppies had evolved in their new environment just as the model predicted. The fish now reach sexual maturity nine days later when they first give birth. What's more, they have fewer offspring in their first brood, apparently saving themselves for future breeding opportunities. Reznick's is only one of several recent studies verifying Darwin's grand theory in nature. In others, for instance, parasites are proving to be an even more potent agent of evolutionary change than predators. Marlene Zuk, a colleague of Reznick's at Riverside, recently completed a study demonstrating that parasites affect how female jungle fowl, the wild ancestors of, barnyard chickens, choose their mates. She found that rematode parasite, which lives in the gut, renders the eye and comb of the male fowl dull and consequently unattractive to females. The females prefer instead cocks bright of eye and comb, who will bestow their offspring with genes that will make them resistant to parasites. Darwin's theory is finally being put to the test, as it is coming through with flying colors. |
Похожие:
 | Методические указания предназначены для студентов 2 курса всех специальностей заочной формы обучения |  | Методические указания предназначены для выполнения практических работ учебной дисциплины «Английский язык» для студентов 1, 2 и 3... |
| Методические указания предназначены для самостоятельного изучения студентами дисциплины и их подготовки к зачётам | | Методические указания предназначены для студентов II-IV курсов очной формы получения образования спо всех профилей |
| Методические указания предназначены для студентов I курса всех технических специальностей нгту, изучающих английский язык (уровень... | | Методические указания предназначены для студентов I курса всех технических специальностей нгту, изучающих английский язык (уровень... |
| Методические указания предназначены для студентов 3 курса заочной формы обучения специальности 15. 02. 08 «Судостроение» | | Методические указания предназначены для студентов 3 курса заочной формы обучения специальности 26. 02. 06 «Эксплуатация судового... |
| Методические указания предназначены для студентов 3 курса заочной формы обучения специальности 26. 02. 06 «Эксплуатация судового... | | Апалькова В. Г. Английский язык. Рабочие программы. Предметная линия учебников «Английский язык в фокусе» 5-9 классы. Просвещение,,... |