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The sum of chemical reactions which transpire within cells. The cell performs chemical, osmotic, mechanical, and electrical work, for which it needs energy. Plant cells obtain energy from sunlight; using light energy, they convert simple compounds such as carbon dioxide and various nitrogen, phosphate, and sulfur compounds into more complex materials. The energy in light is thus “stored” as chemical substances, mostly carbohydrates, within plant cells. Animal cells cannot use sunlight directly, and they obtain their energy by breaking down the stored chemical compounds of plant cells. Bacterial cells obtain their energy in various ways, but again mostly by the degradation of some of the simple compounds in their environment. See Bacterial physiology and metabolism, Photosynthesis, Plant metabolism
Cells have definite structures, and even the chemical constituents of these structures are being constantly renewed. This continuing turnover has been called the dynamic state of cellular constituents. For example, an animal cell takes in carbohydrate molecules, breaks down some of them to obtain the energy which is necessary to replace the chemical molecules that are being turned over, while another fraction of these molecules is integrated into the substance of the cell or its extracellular coverings. The cell is constantly striving to maintain an organized structure in the face of an environment which is continuously striving to degrade that structure into a random mixture of chemical molecules.
All the large molecules of the cell have specific functions: Carbohydrates, fats, and proteins constitute the structures of the cells; these, particularly the former two, are also used for food, or energy, depots; the nucleic acids are the structures involved in the continuity of cell types from generation to generation. All these large molecules are really variegated polymers of smaller molecules. These smaller molecules interact with one another in chemical reactions which are catalyzed at cellular temperature by enzymes. All these reactions are very specific each enzyme only reacting with its own specified substrate or substrates. At present, about a thousand chemical reactions are known which occur within cells; thus, about a thousand specific enzymes are known. By studying how enzymes operate and what substrates they attack, the biochemist has learned in general, and in many cases in specific, how fats, carbohydrates, proteins, and nucleic acids are synthesized and degraded in cells. Mainly through the use of radioactive tracer atoms, the pathways of many chemical compounds within the cell have been realized. For example, it is known what part of the carbohydrate molecule is used for energy production, what part is used in fat storage, and what parts end up in proteins and nucleic acids. See Carbohydrate metabolism, Enzyme, Lipid metabolism, Nucleic acid, Protein metabolism
Via the vast array of enzymatic reactions which go on inside cells, the substances which a cell brings in are completely changed, becoming transformed into cell substance. This changeover needs energy for accomplishment. This energy comes from a chemical compound called adenosine triphosphate (ATP); it is synthesized enzymatically by the cell in a number of reactions in which various compounds coming from foodstuffs are oxidized, and the energy gained as a result is stored in ATP. Subsequently, all cellular reactions which require synthesis of cell-specific substances use this ATP as a source of energy. See Adenosine triphosphate (ATP)
Remarkably, even with these constant replacements going on, the cell never loses its own distinctive structure and function. The reason is that the ordering of the cell resides in a code of nucleic acids, which directs the syntheses of specific enzymes designed to do specific tasks; when these enzymes are degraded and have to be resynthesized, they are made again in exactly the same way as before. In this way continuity is ensured. See Deoxyribonucleic acid (DNA), Genetics, Ribonucleic acid (RNA)
Although a great deal is known of the metabolism of a large variety of compounds (their degradation, syntheses, and interactions), little is known of how these multitudinous reactions are regulated within the cell to effect growth, particular size, and division into daughter cells having the same structure and functioning characteristics as those of the parent cells. It is known that enzymatic reaction activities within cells are strictly governed so that in quite a few cases knowledge has been gained of how a cell shuts off the synthesis of a compound of which it has enough, or speeds up the syntheses of those in short supply. This is done by an enzyme so constructed that the compound which it synthesizes, say, can interact with the enzyme to inhibit its further activity.
Almost the sole justification for cell metabolism is the functioning of a vehicle whose major task is to reproduce as precise a replica of itself as possible. The efficiency of this metabolism has been maximized with this goal in view. See Cell (biology)