In biology,<unk>  refers to any process by which a cell converts one kind of signal or<unk> imulus into another, most often involving ordered sequences of biochemical reactions inside the cell, that are carried out by enzymes and linked through second messengers resulting in what is thought of as a "second messenger pathway". Such processes are usually rapid, lasting on the order of milliseconds in the case of ion flux, to minutes for the activation of protein and lipid mediated kinase cascades.  In many<unk>  processes, the number of proteins and other molecules participating in these events increases as the process eminates from the initial<unk> imulus, resulting in a "signal cascade" and often results in a relatively small<unk> imulus eliciting a large response.
In bacteria and other single-cell organisms, the variety of<unk>  processes of which the cell is capable influences how many ways it can react and respond to its environment. In multicellular organisms, a multitude of different<unk>  processes are required for coordinating the behavior of individual cells to support the function of the organism as a whole.  As may be expected, the more complex the organism, the more complex the repertoire of<unk>  processes the organism must possess. Thus, sensing of both the external and internal environment at the cellular level, relies on<unk> .  Many disease processes such as diabetes, heart disease, autoimmunity and cancer arise from defects in<unk>  pathways, further highlighting the critical importance of<unk>  to biology as well as medicine.
Signal transduction
Cell signaling
Gene expression
Drug metabolism
Energy transformation
Phosphorylation cascade
Carbohydrate metabolism
Excretion