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In the introductory chapter we learned that science concerns itself with matter, and that the science of biology is concerned with the study of this matter when it is in a living state. In order to understand this definition we must first get a conception of what matter really is.
If you take a piece of ice in your hand , you are aware that it is cold, and that it has weight and a certain form. We call it a solid. A few minutes’ exposure to the warmth of your hand will change this solid into a liquid. If the water thus formed be heated over a flame until it boils, it may be changed again, this time into a gas which passes off into the air and be comes invisible. The ice has successively changed from a solid to a liquid and then to a gas. In each state we could measure it and weigh it. In each form it occupies space. It must be considered matter, whether in the form of a solid, a liquid, or a gas.
I. Inorganic Matter
The sciences which treat chiefly of the properties and forces of inorganic (dead) matter, and of the rela tions of the parts of the substances composing it, are known as the sciences of physics and chemistry.
All the building materials of this universe, both living and lifeless, are classified by chemists as either chemical elements or chemical compounds. A chemical element is so simple in its structure that it cannot be broken or decomposed into a simpler substance. Examples of such substances are oxy gen, making up about one fifth of the atmosphere; nitrogen, com posing nearly all the remainder of pure air; carbon, an element that enters into the composition of all organic living things or those that once possessed life; and over sixty others of more or less importance to us in the study of biology.
Oxygen may be easily prepared in the school room or at home in the following manner.1 Heat half a teaspoonful of black oxide of manganese with a little more than its bulk of chlorate of potash in a, test tube over a bunsen flame or a spirit lamp. Vapors will be seen to arise as the mixture becomes heated. After a moment insert a glowing match into the mouth of the test tube; it bursts into a bright flame. In what form does oxygen pass off from the two chemicals in the test tube? How could you determine the presence of oxygen in a substance?
The physical proper ties of oxygen are those which we determine with our senses. Oxygen, when carefully prepared, is found to be a colorless, odorless, and tasteless gas. It is known to form nearly one half of the earth’s surface, to form eight ninths of all water and over three fourths of the weight of the plants and animals inhabiting this world of ours. It has the very important chemical property of causing things placed in it to burn. If, for example, a piece of picture wire is heated red-hot, and then placed in a jar of oxygen, the metal will burn with a bright flame.
Light carefully a small piece of magnesium wire and then place it in a tost tube in which you have previously made oxygen. Notice the very brilliant flame. A light-colored ash remains. This is magnesium oxide. In the above experiment the oxygen in the test tube unites with the magnesium so rapidly as to form a flame. This process is known as a combustion. The chemical union of oxygen with any other substance is called oxidation.
Can you distinguish between combustion and oxidation? Oxidation takes place wherever oxygen is present. These facts, as we shall see later, have a far-reaching significance in the understanding of some of the most important problems of biology.
The simple process of striking a sulphur match gives us another illustration of this process of oxidation. The head of the match is formed of a composition of phosphorus, sulphur, and some other materials. Phosphorus is a chemical element distinguished by its extreme inflammability. It unites with oxygen at a comparatively low temperature. Sulphur is another chemical element that combines somewhat easily with oxygen but at a much higher temperature. The rest of the match head is made up of red lead, niter or some other substance that will release oxy gen, and some glue or gum to bind the materials together. The heat caused by the friction of the match head against the striking surface is enough to cause the phosphorus to ignite; this in turn ignites the sulphur and finally the wood of the match, composed largely of the element carbon, is lighted and oxidized. If we could take out the different chemical elements of which the match is formed and oxidize them separately we should find that the amount of heat needed to start the oxidation of the substances would vary greatly. The element phosphorus, for example, is kept under water in a glass jar because of the extreme readiness with which it unites with oxygen.
Oxidation may take place with very little heat present, although heat is always a result of oxidation. Place an iron nail in a bottle of water, and cork and seal the bottle. Place another nail in a saucer in which is kept a little water. Note the for mation of rust on the nail in the saucer and the absence of rust on the nail in the bottle. Rust is iron oxide and is formed by the union of iron and oxygen. This kind of oxidation is said to be a slow oxidation. Slow oxidations are constantly taking place in nature and result in the process of decay and breaking down of complex materials into simpler materials.
One of the most important effects of oxidation lies in the fact thai, when anything is oxidized, heat is produced. This heat maybe of the greatest use. Coal, when, oxidized, gives off heat; this heat boils the water in the tubes of a boiler; steam is generated, wheels of an engine turn, and work is performed. The energy released by the burning of coal may be transformed into any kind of work power. Energy is the ability to perform work.
Another chemical element of much importance to us is carbon. This element makes up an important part of all things that now have or at any time had life. Such matter we call organic. Carbon is found making up part of the bodies of plants and animals, of coal, and in a nearly pure state in the diamond. The presence of carbon can often be detected by the fact that the substance containing it turns black upon being heated in a flame.
Heat separately on a tin plate some leaves, sticks of wood, gravel, sand, and rich black earth. Place them over a hot flame for some minutes. Which of the above materials contains carbon? If some substance. that contains carbon, as a piece of wood, is burned in a far with a tight-fitting cover, the flame will be seen to go out after a short time. This will occur before all the wood is consumed. Another splinter of wood, placed in a jar with the cover off, will burn slowly but completely. A third piece of wood burned in the air will be quickly and completely consumed. If now a little limewater is poured into the jar which was closed, and the contents shaken up, the limewater will be found to turn a milky color. This milky appearance is due to the formation within the jar of a material known as calcium carbonate. This is thrown down in the liquid as a result of the union of carbon with lime. Evidently some of the carbon from the wood has passed in the form of a gas into the limewater and there united with the calcium in the lime. Remembering what we know about oxidation, we see that the carbon of the wood has passed off and united with oxygen of the air in the jar. Thus, by the uniting of the two chemical elements, a chemical compound has beenformed. The presence of carbon dioxide is known by the fact that it puts out a flame and that it turns limewater milky. This compound is known to chemists as carbon dioxide.
There is another gaseous substance that will not support combus tion; this is the element nitrogen. Its presence in the atmosphere is shown by the following experiment:
Invert a bell jar in a large, deep dish of water, having previously placed within the jar on the surface of the water a piece of phosphorus sup ported on a flat bit of wood or cork. Leave the experiment for at least two days undisturbed (or, the phosphorus may be lighted and then the jar left for a few hours untouched). After that time the water will be found to have risen considerably in the jar.s If you make a mark on the cover to show the amount of nitrogen present in the air. to show where the water stood, you may measure the space occupied by the water in the jar. This space will be found to be almost exactly one fifth of the cubic contents of the jar. It was occupied by the oxygen of the air, this having been used up by the oxidation of the phosphorus. The remaining space at the completion of the experiment is occupied by the nitrogen, which makes the remaining four fifths of the atmosphere. The physical properties of nitrogen are its lack of color, taste and odor. Its chief chemical characteristics are its inability to support combustion and its slight affinity for other substances.
Mineral Matter in Living Things
We saw in the experiment for the detection of carbon by burning, that the sand or gravel contained no carbon. If a piece of wood is burned in a very hot fire, the carbon in it will all be consumed, and eventually nothing will be left except a grayish ash. This ash is well seen after a wood fire in the fireplace, or after a bonfire of dry leaves. This ash consists entirely of mineral matter which the plant has taken up from the soil, dissolved in water, and which has been stored in the wood or leaves. If we were able by careful analysis to reduce a plant and an animal to the chemical compounds of which they were formed, we should discover that both contained mineral or inorganic material. We have just seen examples of this in plants. Mineral matter is found in bone, in the shells covering mollusks, and in many of the other parts of the bodies of animals.
Water in Living Things
Water forms an important part of the sub stance of plants and animals. This can easily be proved by weighing a number of green leaves, placing them in a hot oven for a few moments, and then reweighing. How much weight of a given quantity of leaves is made up of water? Make the same experiment with some soft-bodied animal, as an oyster removed from the shell. Some jellyfish are composed of almost 99 per cent water. The human body contains 60 per cent water.
Some gases are found in a free state in the bodies of plants or animals. Oxygen is of course present wherever oxidation is taking place. Some plants and animals form nitrogen. Many other examples might be given.
II. Organic Matter
The organic or living part of a plant or animal is made up largely of the elements carbon, hydrogen, oxygen, and nitrogen, with a very minute amount of several other elements, which collectively we may call mineral matters. If we were to separate a plant or animal chemically into various organic compounds, we should find it composed of various groups of tissues, the chemical compositions of which are more or less alike. For example, the living part of a plant corresponds chemically witn the living part of an animal. The starch found in grains or roots of plants has nearly the same chemical formula as the anjmal starch found in the liver of man; the oils of a nut or fruit are of composition closely allied to the fat in the body, or in a sheep or cow. These building materials of a plant or animal may be placed in one of the three following groups of organic substances: carbohydrates, materials containing a certain proportion of carbon, hydrogen, and oxygen; organic fats and oils, which contain chiefly hydrogen and carbon; and nitrogenous, or protein substances, which contain nitrogea in addi tion to the above-mentioned elements. The above three kinds of organic materials also form the organic foods of all animals and plants.
What is a food ? We know that if we eat a certain amount of proper foods at regular times, we shall go on doing a certain amount of work, both manual and mental. We know, too, that day by day, if our general health is good, we are adding weight to our body, and that added weight comes as the result of taking food into the body. What is true of a boy or girl is equally true of plants. If food is supplied in proper quantity and proportion, they will live and grow; if the food supply is cut off, or even greatly reduced, they will suffer and may die. From this, the definition which follows is evident. A food is a substance that forms the material for the growth_or repair of the body of a plant or animal or that furnishes energy for it.
Food substances may be classed into a number of groups, each of which may be detected by means of its chemical composition. Such food substances are known as nutrients. Let us now examine a few of the nutrients that we are likely to meet in our daily life, and see how we could test chemically for their presence.
Starch and sugar are common examples of this group of substances.
If the substance to be tested is a solid, break or crush it and add water to it. Pour over it a few drops of iodine solution diluted with water. Notice the color of the iodine, a dark brown; after it has touched the material supposed to contain starch, note any change in color. If starch is present, it will turn dark blue.
There are several forms of sugar commonly used as food; for example, cane sugar, beet sugar, and grape sugar, the latter commonly known as glucose. Glucose, or grape sugar, is manufactured commercially by pouring sulphuric acid over starch. It is used as an adulterant for many kinds of foods, especially in sirups, honey, and candy.
The presence of grape sugar is determined by the following test : Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling’s solution, which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show loss decided red. This change also appears if Fehling’s solution is boiled with cane sugar. A more accurate test is obtained by placing the substance believed to contain grape sugar in a test tube containing Fehling’s solution and allowing the mixture to remain over night in a moderately warm room. If grape sugar is present, a red deposit or precipitate (copper oxide) will be found in the tube the next morning.
2. Organic Fats and Oils
Rub the material believed to contain oil several times on paper and hold the paper to the light. If oil is present, the paper will show a translucent grease spot. Try this with several different nuts and decide which has the most oil. A second test for oil is as follows: Heat the substance to be tested in an oven on a piece of paper. If oil is present, the paper will show a grease spot. Third test: Reduce the substance to small pieces and pour benzine, ether, or other volatile oil over it. Allow the benzine or ether to evaporate; the oil that remains is the extracted oil from the substance tested.
Nitrogenous foods, or proteins, contain the element nitrogen in addition to carbon, hydrogen, and oxygen of the car bohydrates and hydrocarbons. They include some of the most com plex substances known to the chemist, and as we shall see, have a chemical composition very near to that of living matter. Proteins occur in several different forms, but the following tests will cover most cases commonly met. White of egg, lean meat, beans, and peas are examples of substances composed in a large part of protein.
Place in a test tube the substance to be tested; for example, a bit of hard- boiled egg. Pour over it a little strong (80 percent) nitric acid. Note the color that appears —-a lemon yellow. Now wash the egg in water and add a little ammonium hydrate. The color now changes to a deep orange, showing that a protein is present. If the protein is in a liquid state, its presence may be proved by heating; if it coagulates or thickens, as does the white of an egg, when boiled, then proteid in the form of an albumen is present. Another characteristic protein test easily made at home is by burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.
Source: Hunter, Elements of Biology (1907)
Modern Biology, Lesson 02 Assessment