What is Life?
I am not going to answer this question. In fact, I doubt if it will ever be possible to give a full answer, because we know what it feels like to be alive, just as we know what redness, or pain, or effort are. So we cannot describe them in terms of anything else. But it is not a foolish question to ask, because we often want to know whether a man is alive or not, and when we are dealing with the microscopic agents of disease, it is clear enough that bacteria are alive, but far from clear whether viruses, such as those which cause measles and smallpox, are so.
So we have to describe life in terms of something else, even if the description is quite incomplete. We might try some such expression as "the influence of spirit on mater." But this would be of little use for several reasons. For one thing, even if you are sure that man, and even dogs, have spirits, it needs a lot of faith to find a spirit in an oyster or a potato. For another thing, such a definition would certainly cover great works of art, or books which clearly show their author's mind, and go on influencing readers long after he is dead. Similarly, it is no good trying to define life in terms of a life force. George Bernard Shaw and C. E. M. Joad think there is a life force in living things. But if this has any meaning, which I doubt, you can only detect the life force in an animal or plant by its effects on matter. So we should have to define life in terms of matter. In ordinary life we recognise living things partly by their shape and texture. But these do not change for some hours after death. In the case of mammals and birds we are sure they are dead if they are cold.
This test would not work on a frog or a snail. We take it that they are dead if they will not move when touched. But in the case of a plant the only obvious test is whether it will grow, and this may take months to find out. All these tests agree in using some kind of motion or change as the criterion of life, for heat is only irregular motion of atoms. They also agree in being physical rather than chemical tests. There is no doubt, I think, that we can learn more about life from a chemical than from a physical approach. This does not mean that life has been fully explained in terms of chemistry. It does mean that it is a pattern of chemical rather than physical events.
What is common to all life is the chemical events. And these are extraordinarily similar in very different organisms. We may say that life is essentially a pattern of chemical happenings, and that in addition there is a building of a characteristic shape in almost all living things, characteristic motion in most animals, and feeling and purpose in some of them. The chemical make-up of different living things is very different. A tree consists largely of wood, which is not very like the constituents of a man,though rather like a stuff called glycogen which is part of most, if not all, of our organs. But the chemical changes which go on in the leaves, bark, and roots of a tree, particularly the roots, are surprisingly like those which go on in human beings. The roots need oxygen just as a man does, and you can see whether a root is alive, just as you can see whether a dog is alive, by measuring the amount of oxygen which it consumes per minute. And the oxygen is used in the same kinds of chemical processes, which may roughly be described as controlled burning of foodstuffs at a low temperature. Under ordinary circumstances oxygen does not combine with sugar unless both are heated. It does so in almost all living things through the agency of what are called enzymes. Most of the oxygen that we use has to first unite with an enzyme consisting mainly of protein, but containing a little iron. Warburg discovered this in yeast in 1924. In 1926 I did some rather rough experiments which showed the same, or very nearly the same, enzyme in green plants, moths, and rats. Since then it has been found in a great variety of living things.
Just the same is true for other processes. A potato makes sugar into starch and your liver makes it into glycogen by substantially the same process. Most of the steps by which sugar is broken down in alcoholic fermentation and muscular contraction are the same. And so on. The end results of these processes are of course very different. A factory may switch over from making bren guns to making sewing machines or bicycles without very great changes. Similarly the chemical processes by which an insect makes its skin and a snail its slime are very similar, though the products differ greatly.
In fact, all life is characterised by a fundamentally similar set of chemical processes arranged in very different patterns. Thus, animals use up foodstuffs, while most plants make them. But in both plants and animals the building up and the breaking down are both going on all the time. The balance is different.
Engels said that life was the mode of existence of proteins (the word which he used is often translated as "albuminous substances"). This is true in so far as all enzymes seem to be proteins. And it is true in so far as the fundamental similarity of all living things is a chemical one. But enzymes and other proteins can be purified and will carry on their characteristic activities in glass bottles. And no biochemist would say they were alive.
In the same way Shakespeare's plays consist of words, whereas words are a very small part of Eisenstein's films. It is important to know this, as it is important to know that life consists of chemical processes.
This pattern has special properties. It begets a similar pattern, as a flame does, but it regulates itself as a flame does not except to a slight extent. And, of course, it has many other peculiarities. So when we have said that life is a pattern of chemical processes, we have said something true and important.
But to suppose that one can describe life fully on these lines is to attempt to reduce it to mechanism, which I believe to be impossible. On the other hand, to say that life does not consist of chemical processes is to my mind as futile and untrue as to say that poetry does not consist of words.
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