14Basic Mechanisms of Signal Transduction

In order to survive, all organisms need to constantly adapt to changes in the external environment. Unicellular organisms mainly detect physical and chemical changes, such as temperature and nutrients for cellular adaptation. In multicellular organisms, however, not only the direct responses of individual cells, but also integrated response by the entire organism and cell-to-cell communication for maintaining internal homeostasis are very important in order to adapt to external changes. All biologic processes in a multicellular organism -from growth, daily physiological functions, to all kinds of actions- are the result of communication between cells within the organism.

Such exchange of information (i.e., signal transduction) can be categorized into intercellular communication through signal molecules and intracellular communication, which is the process that takes place within a cell from the reception of the signal to the final response of the cell. Basically, intracellular signal transduction is mediated by protein phosphorylation, G proteins, second messengers and protein degradation. Hormones, which act on various physiological processes in an organism, are an example of signal molecules that play a role in endocrine signal transduction. Another group of signal molecules is the proliferative factors (also often called as growth factors), which promote cell proliferation. It is gradually being discovered that growth factors have various other functions besides their roles in cell proliferation.

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