TEXT 4. THE CHEMICAL ELEMENTS ESSENTIAL TO LIFE

 

How many of the naturally occurring elements are essential to life? After more than a century of investigation the question still cannot be answered with certainty.

Only some time ago the best answer would have been twenty. Since then four more elements have been shown to be essential for life, for example, for the growth of animals, such as fluorine, silicon, tin, and vanadium.

Nickel is thought by the scientists soon to be added to the list.

In many cases the exact role played by these elements would remain unknown or unclear. Both chemists and biologists have long been surprised by the way the evolution has selected certain elements as the building-blocks of living organisms.

Thus the composition of the earth and its atmosphere obviously sets a limit on what elements are available.

The solar system, like the universe, seems to be 99 per cent hydrogen and helium. In the earth's crust helium appears to be essentially non-existent, except in a few rare deposits, hydrogen atoms constituting only 22 per cent of the total.

Eight elements provide more than 98 per cent of the atoms in the earth's crust, namely oxygen 47 per cent, silicon 28 per cent, aluminium 7.9 per cent, iron 4.5 per cent, calcium 3.5 per cent, sodium 2.5 per cent, potassium 2.5 per cent, magnesium 2.2 per cent.

Of these eight elements only five are among the eleven that account for more than 99.9 per cent of the atoms in the human body.

Two elements, hydrogen and oxygen, account for 88.5 per cent of the atoms in the human body, hydrogen supplying 63 per cent of the total and oxygen 25.5 per cent.

Carbon accounts for another 9.5 per cent and nitrogen 1.4 per cent. The remaining 20 elements now thought to be essential for life account for less than 7 per cent of the body's atoms.

Silicon is known to be 146 times more plentiful than carbon in the earth's crust. Silicon like carbon has the capacity to gain four electrons and form four covalent bonds. Carbon was selected over silicon as the central building-block. The difference that led to the preference for carbon compounds over silicon compounds can be explained: 1) by the unusual stability of carbon dioxide, and 2) by almost unique ability of carbon to form long chains and stable rings with five or six members.

The versatility of the carbon atom is responsible for the millions of organic compounds found on the earth.

If some generalization were made about the role of various elements it would be interesting to note that six elements carbon, nitrogen, hydrogen, oxygen, phosphorus and sulphur make up the molecular building-blocks of living matter: amino acids, sugars, fatty acids, purines, pyrimidines and nucleotides.

These molecules not only have independent biochemical roles but also are the constituents of the following large molecules: proteins, glycogen, starch, lipids, and nucleic acids. This is the first essential group.

The electrochemical properties of living matter depend on elements or combination of elements that either gain or lose electrons when they are dissolved in water, thus forming ions.

The principal positively charged ions are provided by four metals: sodium, potassium, calcium and magnesium. The principal anions (ions with negative charge) are provided by the chloride ion, sulphur, and phosphorus. These seven ions maintain the electrical neutrality of a body fluid and cells. This is considered to be the second essential group. The third group of essential elements consists of the trace elements. The fact that they are required in extremely minute quantities in no way diminishes their great importance.

Notes on the text:

building-block – зд. основа

to set a limit – установить предел

to be responsible for – обусловливать, быть причиной

living matter – живая материя

in no way – никоим образом

Задания к тексту:

Просмотрите текст, найдите в нем абзацы, в которых говорится об основных элементах, из которых состоит наше тело, и догадайтесь по контексту о значении слова account for.

Выпишите названия всех химических элементов, встречающихся в тексте; вспомните, как они звучат по-английски, проверив правильность произнесения по транскрипции в словаре.

Найдите в словаре значения следующих глаголов и выучите их наизусть:

to occur, to add, to remain, to surprise, to select, to constitute, to gain, to lead, to explain, to note, to form, to charge, to provide, to require, to diminish.

 

Найдите в тексте следующие слова и словосочетания, дайте их русские эквиваленты и запомните их:

naturally occurring elements, to answer with certainty, in many cases, exact, obviously, available, the solar system, the universe, except, the earth’s crust, covalent bonds, long chains, stable rings, versatility, amino acid, fatty acids, protein, starch, trace elements, in extremely minute quantities.

Найдите пары синонимов и запомните их:

lately, to account for, recently, essential, to provide, to explain, to supply, amount, important, quantity.

 

Найдите пары антонимов и запомните их:

existent, clear, disappear, negative, unlike, charge, appear, like, stable, to gain, artificial, unclear, unstable, to lose, discharge, positive, non-existent, natural.

 

Ответьте на вопросы:

Is it possible to answer the question about the number of naturally occurring elements with certainty?

What have the chemists and biologists long been surprised by?

What does the solar system consist of?

Is there much helium in the earth crust?

What eight elements can provide more than 98 per cent of the atoms in the earth's crust?

What two elements account for 88.5 per cent of the atoms in the human body?

What element is more plentiful, silicon or carbon?

What elements make up the molecular building-blocks of living matter?

What do the electrochemical properties of living matter depend on?

What ions maintain the electrical neutrality of a body?

 

Прочтите внимательно текст и найдите в нем предложения с Complex Subject, переведите их на русский язык.

Найдите в тексте условное предложение. Определите тип условного предложения и переведите его на русский язык.

Выберете правильный ответ на вопрос: “What elements make up the molecular building-blocks of living matter?”

Sulphur and phosphorus make up the molecular building-blocks of living matter.

Carbon and nitrogen make up the molecular building-blocks of living matter.

Six elements, namely carbon, nitrogen, hydrogen, oxygen, phosphorus and sulphur, make up the molecular building-blocks of living matter.

TEXT 5. FATS

Fats occur extensively in nature. Like the carbohydrates and proteins, they are constituents of all plant and animal organisms; they are one of the main components of our food.

It is customary to distinguish fats of animal and plant origin. In animals fatty substances are deposited mainly on the internal organs and in the subcutaneous tissue (hypodermic tissue); considerable amounts of fats are contained in the milk of mammals. In plants they accumulate mostly in the seed and the fruit. In sunflower seeds, for example, the fat content reaches 45%; in linseed oil, 47%.

Animal fats are, as a rule, solids. Vegetable fats are mostly liquid and are called oils. There are, however, some liquid fats of animal origin (such as cod-liver oil), just as there are some solid fats of plant origin (such as coconut oil).

All the fats are lighter than water. They are also insoluble in water. This property is familiar to everyone who has tried to wash greasy dishes with cold water or to wash greasy hands without soap. Fats dissolve well in many organic solvents: dichloroethane, petrol, etc. The fact that they dissolve in organic substances is utilized in removing grease stains from garments and in extracting the oil from plant seeds.

The structure of fats was established by the work of the French chemists Chevreul and Berthelot. By heating fats with water (in the presence of an alkali), Chevreul in the early 19th century found that they add water and undergo decomposition into glycerol and carboxylic acids: stearic, oleic, and others. Berthelot (1854) brought about the reverse reaction. He heated a mixture of glycerol with acids and in this way prepared substances analogous to fats. From these facts it is not difficult to draw a conclusion about the structure of fats.

Evidently, Chevreul carried out the reaction of the hydrolysis of an ester, whereas Berthelot effected the reaction of esterification, i. e., the synthesis of an ester.

Fats are esters of the trihydroxy alcohol glycerol and carboxylic acids.

Fats are mostly formed by the higher saturated and unsaturated carboxylic acids, mainly by stearic acid C₁₇H₃₆COOH, palmitic acid C₁₅H₃₁COOH, and oleic acid C₁₇H₃₃COOH. The lower acids play a lesser part in forming fats. Fats do, however, contain butyric acid C₃H₇COOH (a constituent of butter), caproic acid C₅H₁₁COOH,and others.

We know that the saturated higher acids are solids, while the unsaturated are liquids. The fats formed primarily by saturated acids also turn out to be solid (beef and mutton tallow). With a rise in the content of unsaturated acids, the melting point of fats declines; they become softer (lard, butter). The liquid fats are formed primarily by unsaturated acids (linseed oil, sunflower oil, and other oils).

The chemical properties of fats are determined by the fact that they belong to the class of esters. Their most characteristic reaction is therefore hydrolysis. It was by effecting this reaction that Chevreul was able to throw light on the question of what fats are.

The reaction of the hydrolysis of fats, as of other esters, is reversible.

In order that this reaction should proceed from left to right, hydrolysis is conducted by heating the fat with water in the presence of an alkali.

Fats experience hydrolysis if they are stored for a long time in the air. When fats become rancid, it is because of this very phenomenon, which takes place under the influence of enzymes contained in the fat.

When the fats in food are assimilated by the organism, they also undergo hydrolysis. Some of the industrial uses of fats are likewise based on the reaction of hydrolysis.

Fats are an important part of our food. Their oxidation in the organism generates much more heat than does the oxidation of equal amounts of proteins or carbohydrates.

Since they are insoluble in water, fats cannot be absorbed by the organism directly from the digestive tract. Under the influence of an enzyme in the pancreatic and intestinal juices, they are first broken down in the small intestines into glycerol and acids. These products of hydrolysis are absorbed by the walls of the intestines, where they are reconverted to fats, but fats characteristic of that particular organism. The synthesized fat is conveyed by the lymphatic system into the blood, which in turn carries it to the fatty tissues. From there fats reach other organs and tissues of the organism. In the process of continuous metabolism in the cells of the organism the fats are again hydrolyzed and then gradually oxidized, ultimately to carbon dioxide and water. These exothermic reactions provide the organism with the energy it requires. The expenditure of fats in the fatty tissues is made up by the process of nutrition.

Notes on the text:

subcutaneous tissue – подкожная ткань

mammals – млекопитающие

linseed oil – льняное масло

garment – одежда

rancid – прогорклый

primarily – главным образом

digestive tract – пищеварительный тракт

pancreatic juice – желудочный сок

intestinal – кишечный

small intestine – тонкая кишка

nutrient – питательное вещество

 

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