We know that living organisms are characterized by certain structural and functional features which set them apart from the nonliving world. One of these features is the phenomenon of metabolism. In the broadest sense, metabolism refers to the sum total of all the chemical charges or reactions which take place in a living organism.
However, the term is often used in a more restricted sense to include only those chemical reactions which occur within individual cells. According to this definition, then, metabolism is entirely a cellular phenomenon, and the term can be applied to a multicellular organism only in the sense that it exhibits multiple cellular organisms only in the sense that it exhibits multiple cellular characteristics.
This viewpoint would not take into consideration any aspects of organismic metabolism which might consist of emergent properties.
The metabolic link between photosynthesis and finished living matter is a large variety of synthesis reactions. These produce the chemicals that a cell does not obtain directly as prefabricated environmental nutrients or as secretions from other cells.
Such missing ingredients include most of the critically necessary compounds for cellular survival: nucleic acids, structural and enzymatic proteins, polysaccharides, fats, and numerous other groups of complex organic substances. In most cases such synthesis reactions are endergonic and ATP-requiring.
A cyclical interrelation is therefore in evidence. On the one hand, breakdown of organic compounds leads to a net build up of ATP through respiration. On the other, breakdown of ATP leads to a net buildup of organic compounds through chemicals synthesis.
Essentially, metabolism consists of three phases: nutrition, synthesis, and respiration. Nutrition is that aspect of metabolism by means of which the raw materials for synthesis and respiration are supplied. Synthesis is that aspect of metabolism during which protoplasm or protoplasmic components are constructed from the raw materials supplied in nutrition. Generally speaking the chemical reactions which are characteristic of this phase of metabolism are “energy- consuming”, that is, they exhibit a+AF.
Respiration is that aspect of metabolism in which the raw materials supplied in nutrition are degraded, or broken down, and chemical energy residual in such materials is made available to the cell or organism. In general, the chemical reactions which are characteristic of this phase of metabolism are “energy-yielding”, that is, they exhibit a-AF.
Before entering into the body of our discussion, let us make one additional point. It might seem that we are indulging in circular reasoning by stating that metabolism is a characteristic of living organisms which sets them apart from the nonliving world, and then proceeding to define metabolism as the.sum total of the chemical activity which occurs within a living organism.
Obviously, chemical activity can take place outside a living organism. It would appear that we are in the rather awkward position of partially defining life in terms of metabolism, and in turn, defining metabolism on the basis of its occurrence within a living system.
The processes of nutrition, synthesis, and respiration not only must take place within a living organism but they must occur in a certain relationship to one another. In other words, these three phases are regulated by one another, and in this sense, they are in balance.
Herein lays the difference in the chemical activity we call metabolism and chemical activity which occurs in test tubes. Metabolism is precisely controlled and ordered, and is thus inherently self-perpetuating.
This, within itself, is an excellent example of steady-state control. Continued metabolism is a result of control, and at the same time, the control mechanisms are dependent upon continued metabolism.
Perhaps an analogy will serve to clarify the matter. Let us consider a mechanical apparatus, say, a gasoline engine. The operation of the engine is analogous to metabolism, the fuel supply is analogous to nutrition, the combustion of this fuel is analogous to respiration and maintenance or repair of the engine is analogous to synthesis.
Obviously, each of these processes must be precisely geared to the others if the engine is to run for very long. In other words, controls must be built into the engine whereby each aspect of its operation “considers” the other aspects.
Like all analogies, this one is valid only to a point, but in this case we are very interested in the point at which it breaks down. An engine is not inherently capable of repair. It cannot take a broken piston and use fuel to make a new one because it lacks the organization and control for doing so, and this is the essential difference between nonliving and living systems.
In the living system, metabolism runs the machinery of the organism, and continued metabolism is made possible by the running machinery. In the final analysis, it is really this property that we call lice, and we might even define life as controlled metabolism.