Biological Signal Transduction PDF

Biological signal transduction  In all organisms, cells those are not in physical contact, communicate with one another. Cell–cell communication is one of the most dynamic research areas in biology.  Cell–Cell Signaling in Multicellular Organisms Biologists have classified many types of sig...

Biological signal transduction  In all organisms, cells those are not in physical contact, communicate with one another. Cell–cell communication is one of the most dynamic research areas in biology.  Cell–Cell Signaling in Multicellular Organisms Biologists have classified many types of signaling molecules that keep distant tissues in touch. One type, neurotransmitters, may open or close ion channels in the plasma membrane of distant cells, changing the electrical properties of the membrane. The best-studied means of distant signaling, however, may be via hormones—information- carrying molecules that are secreted by plant and animal cells into bodily fluids and act on distant target cells. Hormones are usually small molecules and include certain peptides, steroids, and even gases. How do cells receive and process signals from distant cells?  Signal Reception Hormones and other types of cell–cell signaling molecules deliver their message by binding to receptor molecules. The key characteristic of this interaction is that it changes the shape, or conformation, of the receptor. Cells in a wide array of tissues may respond to the same signaling molecule, though, if they have the appropriate receptor. Identical receptors in diverse cells and tissues allow long-distance signals to coordinate the activities of cells throughout a multicellular organism. Where does the interaction between a signaling molecule and its receptor occur—inside the target cell or outside?  Signal Reception Most lipid-soluble signaling molecules can diffuse across the hydrophobic region of the membrane and enter the cytosol of their target cells. The receptors for these molecules exist inside the cell. Large or hydrophilic signaling molecules are lipid insoluble, and most cannot cross the plasma membrane. To affect a target cell, they have to be recognized at the cell surface. Their receptors are usually located in the plasma membrane.  Signal Receptors Receptors are dynamic. The number of receptors in a particular cell may decline if hormonal stimulation occurs at high levels over a long time. The ability of a receptor to bind tightly to a signaling molecule may also decline in response to intensive stimulation. As a result, the sensitivity of a cell to a particular hormone may change over time. Receptors can be blocked. The drugs called beta- blockers, for example, bind to certain adrenaline receptors. Processing Lipid-Soluble Signaling Molecules Processing Lipid-insoluble Signaling Molecules Intracellular Signals May Be Amplified and Diversified When a hormone arrives at the cell surface, the message it transmits may be amplified as it changes form. An increased number of intracellular signals makes it possible for hormones to affect different molecules in the cell. Amplification may occur in a variety of ways, depending on the mechanism of signal transduction. In general, the arrival of a single signaling molecule results in a secondary signal that involves many ions or molecules.  Signal transduction and amplification systems G-protein-coupled receptors initiate the production of intracellular “second messengers,” which then amplify the signal. Enzyme-linked receptors amplify the signal by triggering the activation of a series of proteins inside the cell, through the addition of phosphate groups. Signal Transduction via G-Protein-Coupled Receptors Many signal receptors span the plasma membrane and are closely associated with membrane-anchored proteins inside the cell called G proteins. G-proteins got their name because their activity is regulated by the type of guanine nucleotide they are bound to: either guanosine triphosphate (GTP) or guanosine diphosphate (GDP). When GTP binds to a G protein, the addition of the negative charges alters the protein’s shape. Changes in shape produce changes in activity. G proteins are activated when they bind GTP; they are inactivated when a phosphate group, and thus a negative charge, is removed from GTP to form GDP.  Second messengers Second messengers aren’t restricted to a single role—the same second messenger can initiate dramatically different events in the same cell or in different cell types receiving the same signaling molecule. More than one type of second messenger may be involved in triggering a cell’s response to the same extracellular signaling molecule. Signal Transduction via Enzyme-Linked Receptors