Metabolism occurs even without life, Engels notes. In other words, a chemical process which consistently reproduces itself. We might also think about the carbon cycle, which involves the processes of geology, also. Engels uses the example of sulphuric acid produced by burning sulphur.
“In this process sulphur dioxide, SO2, is produced, and when steam and nitric acid are added, the sulphur dioxide absorbs hydrogen and oxygen and is converted into sulphuric acid, H2SO4. The nitric acid gives off oxygen and is reduced to nitric oxide; this nitric oxide immediately absorbs new oxygen from the air and is transformed into the higher oxides of nitrogen, but only to transfer this oxygen immediately to sulphur dioxide and to go through the same process again; so that theoretically an infinitely small quantity of nitric acid should suffice to change an unlimited quantity of sulphur dioxide, oxygen and water into sulphuric acid.” (p 101)
He also gives the example of liquids passing through organic or inorganic membranes, as with Traube's artificial cells. So, metabolism, of itself, cannot be definitive. He offers as an alternative,
“Life is the mode of existence of proteins, and this mode of existence essentially consists in the constant self-renewal of the chemical constituents of these bodies.” (p 102)
He notes that, wherever we find life, we find proteins. These proteins form building blocks of life, but organisms also also absorb proteins, in order to build and repair existing cells.
“But what are these universal phenomena of life which are equally present among all living organisms? Above all the fact that a protein absorbs other appropriate substances from its environment and assimilates them, while other, older parts of the body are decomposed and are excreted.” (p 102-3)
Non-organic matter also decomposes. A rock into sand, a metal into rust, an so on. But, this does not apply to organic life. Organic life, first, grows for example, a seed into a plant, a child into an adult. It is not a simple wear and tear as with the erosion of a rock, or even the passing of time itself that causes organic life to deteriorate and die. We know much more about this process, now, than Engels could know at that time. He writes,
“From which it follows that if chemistry ever succeeds in producing protein artificially, this protein must show the phenomena of life, however weak these may be. It is certainly open to question whether chemistry will at the same time also discover the right food for this protein.” (p 103)
In fact that has happened. Understanding of amino acids, of DNA and RNA enables such artificial life to be created in the laboratory, and grown in the appropriate medium. Similarly, using existing cells, its now possible to grow entire organs in the laboratory. But, we also know, as a result of the discover of DNA, that the cause of ageing, and ultimately death, is errors in the replication of this DNA in cells, as hey replace themselves, and that thee is a limit to the number of times a cell can replicate itself, determined by telomeres. However, science appears to be on the verge of being able to deal with these problems, so that ageing can be prevented, or even reversed. Understanding the role of epigenetics, is also significant in this respect.
“From metabolism - the essential function of protein – by means of nutrition and excretion, and from its peculiar plasticity there are derived all the other most elementary processes of life: capacity for excitation, which is already included in the interaction between the protein and its food; contractibility, which is shown, already at a very low stage, in the consumption of food; the possibility of growth, which in the lowest stage includes propagation by division; internal movement, without which neither the consumption nor the assimilation of food is possible.” (p 103-4)
Engels notes that this definition of life is also, inevitably, deficient, because limited. Fans of science-fiction will know that such organic life is based on carbon, whereas it has been suggested that, elsewhere in the universe there may be silicon based life forms. As the development of I shows, a self-aware computer could have all the required conditions of life, and would be able to engage in a process of metabolism and reproduction. It would not have the limitations and fragility of existing, carbon based, organic life-forms.
“But for ordinary usage such definitions are very convenient and in places cannot well be dispensed with; nor can they do any harm, provided their inevitable deficiencies are not forgotten.” (p 104)
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