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Cell metabolism is a general term covering all of the chemical reactions that convert energy or use energy inside a cell. The cells of humans and many other multi-cellular organisms use the process of aerobic respiration to convert food into energy. Plants and some microorganisms perform the process of photosynthesis. Other organisms use anaerobic respiration or fermentation, types of cell metabolism which do not require oxygen.
Two types of chemical reactions occur in cell metabolism, catabolic reactions and anabolic reactions. Catabolic reactions produce energy for the cell to use, while anabolic reactions require energy to create molecules that are necessary for the cell to continue functioning. Cells store energy in the form of adenosine triphosphate (ATP), which is created by catabolic reactions and used by anabolic reactions.
Aerobic respiration breaks down organic sources of carbon, such as carbohydrates, proteins, and fats. First, the process of glycolysis breaks down a molecule of glucose — a sugar with six carbon atoms — into two molecules of pyruvate, two molecules of reduced nicotinamide adenine dinucleotide (NADH), and two molecules of ATP. The Kreb's cycle, also called the citric acid cycle (CAC) or the tricarboxylic acid (TCA) cycle, further breaks down the pyruvate created during glycolysis into carbon dioxide and water, creating two more molecules of ATP in the process. A mechanism called an electron transport chain transfers hydrogen atoms from NADH to oxygen. This transfer releases energy, which is used to create 34 more molecules of ATP.
Glycolysis and the Kreb's cycle take place in the same way in anaerobic respiration as they do in aerobic respiration. In the electron transport chain, however, inorganic molecules — molecules that do not contain carbon — are used as the electron acceptor in place of oxygen. The type of inorganic molecule used depends on the organism. For example, some organisms use sulphur-containing compounds, and some use nitrogen-containing compounds. Anaerobic respiration produces a total of 36 molecules of ATP, as opposed to 38 for aerobic respiration.
Fermentation is another form of anaerobic metabolism, but unlike anaerobic respiration, it does not involve an electron transport chain or the Kreb's cycle. Glycolysis breaks down organic molecules to create energy. Since glycolysis is the only reaction that takes place in fermentation, it produces only two molecules of ATP per molecule of glucose.
Plants and some microorganisms, all of which are classified as photoautotrophs, gain their energy from a form of cell metabolism called photosynthesis, rather than respiration. Photoautotrophs get energy from light and convert it to chemical energy in the form of ATP. The cells then use the ATP to convert carbon dioxide to glucose and other nutrients that the organism needs.