Energy is necessary to create new molecules and, in the long run, to build new cells. The organs and tissues also need it for their work. All the energy used by an organism is supplied by the oxidation of proteins, fats and carbohydrates or, in other words, by the combustion of these substances.

Oxidation cannot take place without oxygen. It is the task of the respiratory organs to deliver oxygen. In man this function is performed by the lungs. However, one should not restrict the term ‘respiration’ to those rhythmic movements of the chest which result in air being drawn into and forced out of the lungs. This is not respiration proper, but merely transportation of the oxygen necessary for it.

Respiration consists essentially of oxidation processes, which are only slightly reminiscent of combustion and can by no means be identified with it. During ordinary combustion oxygen combines directly with the substance being oxidized. But during the biological oxidation of proteins, fats or carbohydrates hydrogen is extracted from them. This hydrogen then, in its turn, reduces oxygen and forms water. Remember this scheme of tissue respiration because we shall return to it later.

Oxidation is a most important means of obtaining energy. This is why astronomers studying the planets of the solar system are anxious to know whether they have oxygen and water since life may be expected where they are present. It is quite understandable why the good news of the world’s first soft landing by the Soviet interplanetary station Venus 4 on the planet Venus was overshadowed by the report that its atmosphere contains hardly any free oxygen and very Httk water, while the temperature is as high as 300°C.

But one should not be too pessimistic about this. Even if there are no traces of life on Venus, that planet is still not without hope. It may be possible to populate its upper atmosphere, where it is not so hot, with primitive unicellular plants which would consume carbon dioxide and produce oxygen. The very dense atmosphere of Venus will make it possible for tiny unicellular living organisms to float in it without dropping onto the planet’s surface. Such organisms would ultimately change the gas composition of the atmosphere on Venus.

This is quite a feasible task for green plants. The atmosphere of the Earth, in the form we know it now, was created by living organisms. Each year the plants on the Earth consume 650 thousand million tons of carbon dioxide and produce 350 thousand million tons of oxygen. There was a time when the Earth’s atmosphere also contained much less oxygen and much more carbon dioxide than it does now. It is only a question of time. Several hundred million years will probably be sufficient for radical changes to occur in the atmosphere of Venus. There are grounds for supposing that by that time the temperature on the planet will have dropped considerably (wasn’t it once hot on the Earth too?). When this happens, Earthmen will be quite at home there.