7.2    Metabolism 

Metabolismis a set of processes performed by the living beings that allow them tointerchange matter and energy with their environment. The living beings use themetabolic processes to grow and organize themselves internally. This is one ofthe visible manifestations of life. Unlike the biotic (living) systems, some abiotic (inert, without life) systems growexternally; for example, a copperchloridecrystalgrows toward outside adding material inits periphery or forming nuclei of growth around itself. The living beings growinternally although they do not reproduce, that is, even if they do not producedescendants. Therefore, the metabolism is a set of processes used by the livingbeings for maintaining their structure and molecular organization in a quasi-stable state.

The metabolism consists of two basic types of interdependent phases, the phase inwhich the energy is freed and the phase in which energy is captured. The phasethat consists of the disintegration of complex organiccompound to release energy is known as catabolism, whereas the phase that consists of thearrangement of organic compounds from simpler compounds to store energy iscalled anabolism.

An example of catabolism is that a cell intakes a molecule of glucose from thesurroundings and disintegrates it to release energy (glycolysis). Immediately after the energy has beenreleased, it is captured and stored by a specialized molecule, the adenosine triphosphate or ATP. When the energy that was storedby the ATP is used in the synthesis of more complex compounds (e.g. bondingseveral monosaccharides to build molecules of disaccharides or polysaccharides) the process is anabolism.

•Generally, the materials obtained duringthe catabolic phase will be used in the anabolic phase. Many molecules are usedto form cellular structures, others are used to transmit messages orinformation (signal transduction) and others are used like energy sourcesto perform other cellular functions.

The carbohydrates constitute the main source of energy for all living beings.Lipids are the second source of energy, which could be used when the primarysource is exhausted or independently of the exhaustion of the primary source ofenergy. Proteins would be the ultimate organic compounds to which the cellswould resort like an energy source because most proteins function to regulateprocesses and/or to form structures. When an organism is compelled to make useof its proteins, it could be suffering from undernourishment.

Catabolism means disintegration, whereasanabolism means reorganization. The catabolism implies release of energy,whereas anabolism implies capture of energy. Catabolism implies disorganization of matter, whereas anabolism implies a more complex reorganization of matter.

glycolysis [glai'kɔləsis] n. 糖酵解

adenosine triphosphate 腺苷三磷酸

monosaccharide [mɔnə'sækəraid] n. 单糖

disaccharide [dai'sækəraid] n. 二糖

polysaccharide [pɔli'sækə.raid] n. 多糖

cellular structure 细胞结构

signal transduction 信号传导

cellular function 细胞功能