Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell gains useful energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction. The overall reaction is broken into many smaller ones when it occurs in the body, most of which are redox reactions themselves. Although technically, cellular respiration is a combustion reaction, it clearly does not resemble one when it occurs in a living cell. This difference is because it occurs in many separate steps. While the overall reaction is a combustion reaction, no single reaction that comprises it is a combustion reaction.
Cellular respiration allows organisms to use (release) energy stored in the chemical bonds of glucose (C6H12O6). The energy in glucose is used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a process in which the energy in glucose is transferred to ATP.
The carbon atoms of the sugar molecule are released as carbon dioxide (CO2).
The complete breakdown of glucose to carbon dioxide and water requires two major steps: 1) glycolysis and 2) aerobic respiration. Glycolysis produces two ATP. Thirty-four more ATP are produced by aerobic pathways if oxygen is present.
In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP.
NAD+ + 2H --> NADH + H+
FAD + 2H --> FADH2
During glycolysis, glucose (C6) is broken down to two molecules of pyruvate (C3). (Note that compounds that end in "___ate" can be called "___ic acid". For example, lactate is lactic acid and malate is malic acid.)
Glycolysis occurs in the cytoplasm (cytosol) and does not require oxygen.
Probiotics are organisms such as bacteria or yeast that are believed to improve health. They are available in supplements and foods. The idea of taking live bacteria or yeast may seem strange at first. After all, we take antibiotics to fight bacteria. But our bodies naturally teem with such organisms.