Cellular respiration is the process by which most of the cells generate energy from complex chemical molecules. In simplest terms, the complex chemical molecules (such as glucose, etc.) are broken down in a series of reaction. At each step, these molecules (also known as donors) donate electrons, which are accepted by an electron acceptor (which is oxygen, in the case of respiration). In the end, after complete oxidation of the chemical molecule, carbon dioxide and water are generated along with energy in the form of adenosine triphosphate (ATP) molecules. The ATP molecules can be converted to energy, as and when required by the cells. The aerobic process of cellular respiration converts as much as half of the energy stored in the chemical substrate (food) into energy usable by the organism. In the absence of oxygen, a process known as fermentation or anaerobic respiration is used by the cells to generate energy.
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