The Chemistry of Life

THE CHEMISTRY OF LIFE Characteristics of Living Organisms.—If organic evolution has taken place, it must be living matter that has evolved. Let us, therefore, examine what living organisms are composed of, what properties they have in common, how they can be distinguished from non-living matter. An analysis shows that organisms are entirely composed of the ordinary elementary chemical substances found in their inorganic surroundings, but that these elements are built up into highly complex compounds of peculiar molecular structure, and occurring only in living organisms or in their prod ucts. These compounds may be grouped in order of complexity into three classes : carbohydrates, fats and proteins. The molecule of a carbohydrate (such as sugar, starch, cellulose, etc.) is formed entirely of the elements carbon, hydrogen and oxygen; likewise the molecule of fat which is, however, more complex in structure. Proteins are the most important of these substances, since their molecules contain nitrogen and sulphur in addition to carbon, hydrogen and oxygen. To these essential elements are added small quantities of phosphorus, iron, potassium, sodium, calcium, mag nesium and chlorine. The molecules of a protein are, therefore, very large and may contain many hundreds of atoms in combina tion, some of which form subordinate groups within them. With every increase in size and complexity, these organic molecules acquire new and often very important properties. For just as the chemical combination of atoms of oxygen and hydrogen into mole cules produces a substance, water, with new physico-chemical properties not possessed by either of its constituents, so with every advance in molecular structure will new properties emerge. Many of these new properties displayed by large and complex molecules are of the highest importance in the carrying out of the functions of life.

Living organisms feed, breathe, grow and reproduce. The most characteristic property of living matter is its capacity to undergo perpetual change, and this fundamental process going on in all living things we call metabolism. It involves a perpetual trans formation of material and energy. Speaking generally, animals are continually taking in food and oxygen and giving off waste products which are got rid of by excretion. The food consists chiefly of carbohydrate fats and proteins, high compounds in which the energy used in building them up is stored as potential energy which may be freed and transformed into motion or kinetic energy when these substances are again broken down into simpler compounds. This breaking down is brought about by their oxida tion or burning by means of the oxygen taken in in respiration. All material entering the organism as food and oxygen eventually leaves as waste products (carbon dioxide, water and urea), except in so far as some is retained for repair and growth. All the energy brought in is balanced by work done in the performance of the animals' various activities, and by heat given off. During the whole process of metabolism, no matter or energy is either pro duced or destroyed, but merely transformed. One of the most fundamental generalizations of modern biology is that the prin ciples of conservation of matter and the conservation of energy hold good in living organisms as they do in the inorganic world.

What has been said above of metabolism in animals applies also to plants. But whereas in animals the powers of synthesis are restricted to the building up of their substance from organic materials, most plants can form carbohydrates, fats and even proteins from the simplest inorganic compounds. Thus green plants, with the help of chlorophyll, can build up starch from water and atmospheric oxygen by a synthetic process in which the energy of sunlight is absorbed, and can synthesize protein by the addition of simple salts of nitrogen derived from the soil. Many of the lowest plants (Bacteria) can build up proteins without chlorophyll from inorganic compounds, and some can use the free nitrogen of the air. Thus carnivorous animals depend on vegetable feeders, these in turn on plants for nutriment, and ultimately all living matter is derived from the non-living matter of the environment.

Now in the living organism the food does not all pass through merely as fuel; the machinery itself is involved in the process of change. The food serves to build up the complex living substance, part of which is perpetually being broken down again into non living matter. The highest compounds are unstable, and the com plete process of metabolism includes a double process of building up and breaking down. Energy is stored in the first and freed in the second. The organism has at its disposal the excess of energy set free by the oxidation of the food material over that required for the working and upkeep of the machinery. There is thus in living substance a mixture of compounds, some leading up to the highest complexity and others leading down to the waste products, and this mixture of substances undergoing these physico–chemical changes constitutes the living matter known as protoplasm, well named by T. H. Huxley, the physical basis of life. Protoplasm, the essential living substance present in all living organisms and the seat of all their activities, is a viscid colourless substance of granular appearance under the microscope, being apparently formed of minute globules suspended in a more fluid medium. Dead protoplasm can be analysed into a variety of proteins asso ciated with mineral salts. Suspended in the protoplasm may be found granules of food material about to be used up, various products of its own activities, and waste material about to be rejected.

The most characteristic properties of living matter are irrita bility, growth and reproduction; they all depend on metabolic processes taking place in protoplasm. Like other physico-chemical processes, metabolism is limited by definite conditions. The essen tial elements, food, oxygen, water, must be present, and it can only take place within a certain range of temperature, not so high as to coagulate or destroy the proteins, or so low as to stop chem ical action. Usually the metabolic processes take place with the help of certain proteins known as ferments or enzymes (q.v.), which hasten and facilitate chemical action. It has recently been proved that for the carrying out of many processes, special corn pounds called vitamins (y.v. ) are necessary.

A living organism may be likened to a whirlpool—new non living matter is always being brought in from the outside, caught up and moulded into protoplasm, and passed out again at the periphery is dead matter having yielded the energy necessary for its activities. All the phenomena of life on which these activities are based are strictly determined and limited by external conditions on the one hand, and the properties of the materials taking part on the other. Protoplasm is not one particular chemical compound, but a mixture of substances; some are materials about to be built up, others the products of their decomposition. This life process forms a chain of interdependent actions, every link of which is essential, and the full attributes of life are displayed only by the whole series of substances and their interactions in the process. No single link by itself can be said to be living; no hard-and-fast line can be drawn between the living and the non-living. There is no special living chemical substance, no special vital element differing from dead matter, and no special vital force can be found at work. Every step in the process is determined by that which preceded it and determines that which follows.

That universal property of living organisms known as irrita bility, or the power of responding to a stimulus, brings them into relation with their environment, and depends on the fact that protoplasm is in a state of unstable equilibrium which can be dis turbed by changes in the surrounding conditions. Neither the amount nor the character of the response, i.e., the resulting change in metabolism, is directly related to that of the stimulus, but both are determined by the structure of the mechanism stimulated and the amount of energy stored in it. Thus, just as the pressure of a button may cause a bell to ring or an engine to start, so a given stimulus may cause a plant to grow or an animal to move. But there is an internal environment as well as an external environ ment, for the various parts of an organism may react on each other. One organ may stimulate another and so regulate its action, as does, for instance, the governor of a steam engine. Thus may arise a series of co-ordinated responses started by a single external stimulus. A special organ for securing this result is seen in the nervous system of animals ; but in all organisms the various parts are closely interdependent, and special substances, hormones (q.v.), may be secreted by one organ to regulate the action of another. This integration of parts is one of the most striking characteristics of living organisms, and becomes more and more perfected in the higher forms.

A living organism, then, from the point of view of the scientific observer, is a self-regulating, self-repairing, physicochemical com plex mechanism. What, from this point of view, we call "life" is the sum of its physicochemical processes, forming a continuous interdependent series without break, and without the interference of any mysterious extraneous force. It is true that we are still far from being able to give a complete scientific explanation of all the processes involved, but their analysis is pushed further every day, and there is no good reason to believe they are not all capable of such explanation.

First Steps in Evolution.

Although we are ignorant of the origin of life, we may suggest what may have been the first steps in its evolution. It used to be thought that simple organisms, such as the Bacteria of putrefaction and fermentation moulds, Infusoria and even small worms, arose spontaneously de novo in liquids con taining dead organic matter (theory of abiogenesis [q.v.]). But Pasteur and others in the last century definitely proved that such creatures arise only from living germs or spores already present in the liquids, and that no organisms of any kind will appear and no putrefaction take place in substances in which the germs have been killed and from which new germs have been rigidly excluded. The important principle of biogenesis (q.v.) was thus established, according to which all living organisms are derived from pre-exist ing living organisms without break. It is one of the chief founda tion stones of the modern doctrine of organic evolution.

But although this principle of continuity applies to all organisms at the present day, which have a long history behind them and have no doubt departed greatly from the initial stages in the evolution of living matter, there must have been a time when protoplasm first appeared. It must be,, supposed that long ago, when conditions became favourable, relatively high compounds of various kinds were formed. Many of these would be quite un stable, breaking down almost as soon as formed ; others might be stable and merely persist. But still others might tend to reform, to assimilate, as fast as they broke down. Once started on this track such a growing compound or mixture would inevitably tend to perpetuate itself, and might combine with or feed on others less complex than itself. These first steps in the elaboration of liv ing matter probably occurred in the sea, for protoplasm contains the same salts as sea-water and in much the same proportions.