Since the phenomena of life are without exception the results of protoplasmic activity, it is obvious that we must look to the cytoplasm and inside cells for the primary attributes of living matter. The properties which are absolutely diagnostic of living matter are its chemical composition, metabolism including the power of waste and repair, growth.
Chemical Composition of Life
1. Chemical Composition It is impossible to make an analysis of living matter because the disturbance of its molecular organization by chemical reagents kills it. Therefore our knowledge of its chemical composition has of necessity been derived from a study of dead protoplasm. However, since in the trans formation from the living to the non-living state there is clearly no loss of weight, it follows that the complete material basis of life is still present for examination. In other words, the death of protoplasm is a result of disorganization.
Chemical analysis of protoplasm shows that it invariably comprises the elements carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus; and usually also chlorine, potassium, sodium, magnesium, calcium, and iron. Occasionally a num ber of other elements are found normally in the protoplasm of certain parts of various species of animals and plants.
The average composition of the human body is about as follows: Oxygen 65.00% Carbon 18.00 Hydrogen 10.00 Nitrogen 3.00 Calcium 2.00 Phosphorus 1.00 Potassium 0.35 Sulfur 0.25 Sodium 0.15 Chlorine 0.15 Magnesium 0.05 Iron 0.004 Iodine traces Fluorine traces Silicon.
At first glance there is nothing very striking about this list of elements. They are all common ones with which the chemist is familiar in the non-living world. But it is the combination of the elements which is significant, and this results from the capacity of carbon, hydrogen, and oxygen, or carbon and hydrogen together, to form the numerous complex compounds which in turn supply the basis for inti mate associations with other elements. As a matter of fact, the bulk of protoplasm is composed of carbon, oxygen, hydrogen, and nitrogen associated with each other in an apparently infinite series of relationships, in which the carbon seems to play the leading role. Some of these com pounds are relatively simple, such as water (H 2 O) which is quantitatively the most important constituent of all protoplasm, but the majority consist of elaborate atomic arrangements and not a few represent molecular complexes of hundreds and even thousands of atoms.
The compounds of carbon which are characteristic of protoplasm fall into three chief groups: proteins, carbohydrates, and fats.
Proteins invariably consist of the elements carbon, oxygen, hydrogen, nitrogen, and sulfur, and frequently phosphorus and iron. Examples are albumin of the white of egg, casein of milk, gluten of cereals, and myosin of lean meat. The nitrogen particularly distinguishes proteins from the other compounds of the living complex and, as we shall see later when considering the chemical processes in animals and plants, is largely responsible for their commanding position as "the chemical nucleus or pivot around which revolve a multitude of reactions characteristic of biological phenomena." Study of the relationship of nitrogen to the other chemical elements of proteins long since revealed the fact that the protein molecule is a huge complex of linked amino acids, but at the present time it is becoming increasingly patent that the amino acids are, as it were, the nitrogenous units with which organisms deal physiologically, rather than the proteins themselves. An animal, for example, with various proteins available in its food, chemically disrupts these into their amino acid constituents, and then takes an amino acid here and another there and synthesizes the specific proteins it demands. And further, if individual amino acids are supplied, the animal employs them. So it seems highly probable that the specific structure of an or ganism depends upon the chemical specificity of its proteins.
Amino acids an amino acid being an organic acid in which one hydrogen atom is replaced by the amino group, NH 2.
Although the presence of proteins and the power of forming them is the chief diagnostic chemical characteristic of living matter, at the present stage of our knowledge it is impossible to define proteins satisfactorily on the basis of chemical or physiological properties. The most we can say is that the biochemist describes proteins as "huge molecules, complex in structure, labile in character, and therefore prone to chemical change " and the latter characteristic undoubtedly is closely associated with the perennial plasticity and responsiveness of the protoplasmic system itself.
Carbohydrates consist of various combinations of carbon, hydrogen, and oxygen, the latter elements invariably being present in the proportion found in water (H 2 0).
Though more simple in chemical structure than proteins, they range in complexity from the simgle sugars, or monosaccharids, such as glucose and fructose, to polysaccharids such as dextrins, starches, and cellulose.
Fats are composed of the same elements as the carbo hydrates, but in quite different arrangements. The proportion of oxygen is always less, and therefore they are more oxidizable and richer in potential energy. Fats represent a synthesis of an acid (fatty acid) and glycerine. Examples are butter and all oils of plant or animal origin.
Thus proteins, carbohydrates, and fats represent large classes of substances which are distinctly characteristic of living matter, not being found in nature except as the result of protoplasmic activity; although biochemists now can artificially synthesize certain fats and carbohydrates as well as the amino acid constituents of some proteins. Proteins undoubtedly play the most important part in the organization of protoplasm, while the carbohydrates and fats contribute largely to the supply of available energy. However, it is impossible to draw a hard and fast distinction in regard to their respective contributions because, for example, as we shall see later, carbohydrates farm the foundation upon which proteins are synthesized by green plants.
Proteins, carbohydrates, and fats are frequently referred to as the foodstuffs, but it will be recognized that while, in a way, they constitute the chief groups, all the constituents of protoplasm must be available. Accordingly, inorganic salts, water, and free oxygen are really foodstuffs. Further more, recent investigation has disclosed another class of organic substances which are absolutely necessary for the constructive phases of protoplasmic activity. These are termed Vitamins and must be classified as accessory food substances, although as yet little is known in regard to their chemical structure or mode of action. And then, finally, on the border line of food substances may be mentioned a great group of organic catalyzers, called Enzymes, which play a major role in metabolism. But, when all is said, our knowledge of the chemical complexities of protoplasm affords no adequate conception of how they are related to the phenomena of life. This is beyond present-day biology.
Metabolism
We have emphasized that living matter is continually changing, and this fundamental fact is reflected in nearly all attempts to define life. Aristotle described life as "the assemblage of operations of nutrition, growth, and destruction"; de Blainville, as a "twofold internal movement of composition and decomposition"; and Spencer, as "the continuous adjustment of internal relations to external relations."
This interaction consists of chemical and physical pro cesses in which combustion or oxidation plays the chief role.
Lavoisier et al. in 1780 showed that animal heat results from a slow burning of the materials of the body, involving the consumption of oxygen and the liberation of carbon dioxide; and further, that for a given consumption of oxygen and liberation of carbon dioxide, about the same amount of heat is produced by an animal as by a burning candle. This was an important discovery, because it went far toward establishing the fact that at least certain characteristic vital phenomena are amenable to the laws which hold in the non living world.
But the processes involved in life are not so simple as perhaps might be imagined from the results just mentioned.
Heat represents but one of the many energy transformations within the organism. Indeed the living organism, like a steam engine, is a machine for transforming energy trans forming the potential energy stored in chemical complexes of its own substance into the various vital processes of living into work performed. In these processes many complex substances rich in potential energy, which have entered as food and have in whole or part added to the protoplasmic complex, are reduced to simpler and simpler conditions and finally, with their energy content nearly or entirely exhausted, are eliminated as Excretions. This continual waste must, if life is to persist, be counterbalanced by a proportionate intake of food in order to renew the supply of energy and afford the materials which, after preliminary changes, are made into an integral part of the living organ ism. Thus in living the animal or plant is partially consuming and rebuilding itself continually. This dual process is Metabolism. When constructive metabolism, Anabolism, keeps pace with destructive metabolism, Catabolism, the individual remains essentially unchanged and this is the normal condition of adult life. During youth the anabolic I phases are in the ascendency and growth occurs, while old I age is characterized by a predominance of catabolic processes.
Growth
The results of metabolism force themselves upon our attention chiefly as growth, or permanent increase in the size of the individual. As a rule growth in plants continues more or less rapidly throughout life, while in animals it is confined mainly to the early part of the individual's existence, or youth. Indeed, at birth a child is about a billion times larger than the egg from which it has developed!
Growth means that the organism makes over the materials which it receives in the form of food from its environment and fits them into the protoplasmic organization here and there throughout as needed. This method of addition of materials, which is termed growth by INTUSSUSCEPTION, is highly characteristic of life. When growth occurs in the non-living world, it is typically by accretion; as, for example, in crystals where new material of the same kind is superimposed upon the surface. But protoplasm, with materials and energy taken from its environment, constructs more protoplasm and, if the available materials are adequate, the specifically organized living substance tends to increase indefinitely. Thus it is not only the method of growth which is diagnostic of animals and plants, but also the fact that when the individual body has reached a certain physiological balance, or maturity, in which it ceases to increase in size, under normal conditions it expresses the inherent growth power of living matter by setting free certain living units, which go through a cycle of growth phenomena that result in re-productions of the parent individual.
Reproduction
So far as is known, living matter never arises except under the direct influence of preexisting living matter. We have seen that this transformation is continually going on in the constructive phase of metabolism in the animal or plant, and brings about repair and growth of the individual; but it is in reproduction that what may be termed the over; the individual results in the production of a new one. A larger or smaller part of the parent generation is detached and becomes the new generation, so that in ultimate analysis reproduction is division. This is a highly unique characteristic of living things which provides for the continuation of the race / species.
Adaptation
The discussion of metabolism has emphasized the close interrelationship between the living complex and its surroundings, and the dependence of life upon the interplay and interchange between protoplasm and its environment.
As a matter of fact the plant or animal retains its individual ity lives solely by its powers of developing and maintaining exquisite adjustments to its surroundings. This results from the IRRITABILITY of living substance : its inherent capacity of reacting to environmental changes by changes in the equilibrium of its matter and energy. The inciting changes, known as STIMULI, may be chemical, electrical, thermal, photic, or mechanical, but the nature of the response is determined rather by the fundamental character of the protoplasmic system itself than by the nature of the stimulus.
Muscle protoplasm contracts however it is stimulated. The reaction of living matter by virtue of its intrinsic irritability implies not only response to a stimulus but also conduction so that the protoplasmic system as a whole is directly or indirectly influenced. It responds as a coordinated unit an individual. It adapts itself structurally and functionally to the exigencies of its existence. . This power of adaptation, as exhibited in active adjustment between internal and external relations, overshadows every manifestation of life and contributes, more than any other factor, to the " enormous gap that separates even the lowest forms of life from the inorganic world."
Organization
Finally, adaptation implies that living things are not homogeneous, but exhibit reciprocal structural and physiological organization. Accordingly animals and plants are referred to as organisms. Indeed a major part of the present volume is devoted to the organization of organisms.
The characteristics which we have described chemical composition, metabolism including waste and repair, growth by intussusception, reproduction, adaptation, and specific organization individually and collectively are diagnostic of living matter. It is possible, to be sure, to take exception to one or another; e.g., to say that growth by intussusception occurs in non-living things when a salt is dissolved in water; but such formal objections only emphasize the unique conditions which obtain in life.
The reader may be surprised to note that the power of movement has not been mentioned as a characteristic of life, but a moment's thought will make it apparent that visible movement is not confined to living matter. Though this is so, movement is one of the most obvious manifestations of life and depends, of course, in every instance, upon molar changes resulting from tumultuous ultramicroscopic chemical changes of protoplasm itself.
And it is to these changes that, in the last analysis, we must turn for the energy which brings about the visible movements in animals and plants, such as the contraction of the muscles of animals, the streaming movement (amoeboid movement) of the simple animals known as the Amoeba, the rotation and circulation of the protoplasm in certain of the living units of plants and, finally, the lashing of threads of cytoplasm (cilia) which not only enables many a tiny plant and animal to swim, but also aids in numerous ways in certain parts of the bodies of higher organisms. The phenomena of life are quite generally expressed in visible movements, but the latter arc not peculiar to living things.
In our discussion thus far we have endeavored to describe the characteristics of matter in the living state on the basis of the fundamental vehicle of life manifestations protoplasm. We have not attempted formally to define 'life' or ' protoplasm ' because they are so unique that it is impossible to resort to the lexicographer's trick of comparing them with something else; and because the expressions 'cytoplasm' and 'life' are abstractions; one indicating that all individual animals and plants have to a large extent a common organizational foundation, and the other that they exhibit certain characteristic actions and reactions. The living organism is a microcosm which exhibits a permanence and continuity of individuality correlated with specific behavior, and this it transmits to other matter which it makes a part of itself, and to its offspring in reproduction.
For more on the organizational nature of life see:
