You probably know that breathing is necessary to the body through the air we breathe oxygen came, necessary for life, and when you exhale the body releases out carbon dioxide.
Breathes all living things - animals, birds, and plants.
And why living organisms are so necessary oxygen that without it life is impossible? And where in the cell is taken as carbon dioxide, from which the body needs constantly to be free?
The fact that every cell of a living organism is a small, but very active biochemical production. And you know that no production is impossible without energy. All processes that occur in cells and tissues that occur with the consumption of large amounts of energy.
Where did she come from?
With the food we eat - from carbohydrates, fats and proteins. In cells, these substances are oxidized. Most often, the complex chain of transformations of substances leads to the formation of a universal energy source - glucose. As a result of oxidation of glucose energy is released. That's oxidation as times and needs oxygen. The energy released in these reactions, the cell stores energy in the form of a special high-energy molecules - they, like batteries, or batteries lose energy as needed. And the end product of the oxidation of nutrients are water and carbon dioxide, which is removed from the body: cells it enters the bloodstream, which transports carbon dioxide to the lungs, where it is expelled during exhalation. For one hour through the lungs of the person produces from 5 to 18 liters of carbon dioxide and up to 50 grams of water.
By the way...
High-energy molecules that are "fuel" for biochemical processes, called ATP - ATP acid. In humans, the life expectancy of one molecule of ATP is less than 1 minute. The human body synthesizes about 40 kg of ATP per day, but all of it almost immediately spent, and the supply of ATP in the body is almost not generated. For the normal functioning it is necessary constantly to synthesize new molecules of ATP. That is why without oxygen a living organism can survive only a few minutes.
And are there any organisms that do not need oxygen?
Yes, there are. Such organisms are called anaerobic (Greek "EN" - no, "Aer" air), in contrast to the aerobic - those organisms that cannot live without oxygen. To anaerobes are organisms that live in environments devoid of oxygen, many bacteria, some fungi, algae and some animals, for example, virtually all helminths (i.e. worms - parasitic worms). Part of anaerobes can "switch" from an anaerobic type of respiration in aerobic - they are called facultative anaerobes; and some do not tolerate oxygen, dies from his presence - this obligate anaerobes (e.g., the causative agent of tetanus - tetanus Bacillus).
With the processes of anaerobic respiration familiar to each of us! Thus, the fermentation of the dough or kvass is an example of anaerobic process carried out by the yeast: they oxidize glucose to ethanol (alcohol); the process of souring of milk is a result of lactic acid bacteria, which carry out lactic acid fermentation - convert the milk sugar lactose into lactic acid.
Why do we need oxygen breathing, if there is oxygen-free?
Then that aerobic oxidation times more efficient than anaerobic. Compare: in the process of anaerobic breakdown of one glucose molecule produces only 2 ATP molecules, as a result of aerobic decomposition of a molecule of glucose produces 38 molecules of ATP! For complex organisms with high speed and intensity of metabolic processes anaerobic respiration is simply not enough to sustain life - so electronic toy, which requires 3-4 batteries, just will not turn on if it insert only one battery.
And in the cells of the human body may anoxic respiration?
Of course! The first stage of decomposition of glucose molecules, called glycolysis, is held without the presence of oxygen. Glycolysis is a process common to virtually all living organisms. In the process of glycolysis produces pyruvic acid (pyruvate). It goes on with further transformations leading to ATP synthesis as when oxygen and anoxic respiration.
Thus, in muscle ATP reserves are very small - they last only 1-2 seconds of muscular work. If the muscle is necessary short-term, but active, first it is mobilized anaerobic respiration is faster it is activated and provides energy for approximately 90 seconds of active muscles. If the muscle is actively working for more than two minutes, connects aerobic respiration: it is the production of ATP occurs slowly, but the energy it gives enough to support physical activity for a long time (several hours).