The breath and life

We already spoke about the fact that the animals receive their energy for the manifold activities mainly due to destruction in its cells with oxygen environment, i.e. due to the slow combustion of a number of organic substances. Therefore, the organs of respiration, supplying oxygen to the body, are of great significance. Without food, you can live for several weeks without water for several days, without air, it is impossible to live even a few minutes. Not by chance the termination of life symbolized by the cessation of breathing, and in our speech is sufficiently expressive expression "last breath".
Atmospheric air contains mainly nitrogen (79,03 %) and oxygen (20,94 %). Oxygen is one of the most active chemical elements, easily entering into relationship with the substances of the earth's crust. So, he constantly absorbed rocks. Therefore, the presence in the atmosphere more than 1/5 of free oxygen remained a mystery. We now know that the oxygen is released into the air by green plants. When they did not exist, in the atmosphere there was no oxygen. Only with the development of the easiest plants and the appearance of oxygen began to arise from the ancestors of animals - the simplest single-celled creatures - more intensive oxidation processes. Hence, more prominent place occupied the exchange of energy, coupled with an exchange of substances in the narrow sense (the dissolution and creation of proteins of the body) of the second party to the single process of metabolism and energy metabolism. Some of the simplest organisms survived without oxygen metabolism, but all any of perfect single-celled animals, not to mention multicellular, breathing became necessary party of life, the more necessary even than obtaining nutrients.
Thus, animals doubly dependent on plants, which, as we have said, not only provide them with organic substances, but also provide them with pure oxygen. However, consider animals only parasitizing at the expense of plants would be wrong. The carbon dioxide, which is the main food plants and contained in the air 0,03 %, stands out animals as one of unnecessary end products of metabolism. Consequently, the animal and plant worlds are connected dialectical unity, due to the history of their development. Plants create complex organic compounds from carbon dioxide, water and elementary nitrogen compounds. Animals decompose these organic substances to the same source products. So is circulation of substances in nature, and all the energy for the life of living beings is derived ultimately from assimilated by plants energy of sunlight.