Signal transduction

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Signal transduction comes from the verb to 'transduce' meaning to 'lead across'. In biology signal transduction is the process by which an extracellular signaling molecule activates a membrane receptor that in turn alters intracellular molecules to create a response.[1] Transmembrane receptors span the cell membrane, with part of the receptor outside and part inside the cell.  The chemical signal binds to the outer portion of the receptor, changing its shape and conveying another signal inside the cell.  Some chemical messengers, such as testosterone, can pass through the cell membrane, and bind directly to receptors in the cytoplasm or nucleus.

Sometimes there is a cascade of signals within the cell.  With each step of the cascade, the signal can be amplified, so a small signal can result in a large response.[2]  Eventually, the signal creates a change in the cell, either in the expression of the DNA in the nucleus or in the activity of enzymes in the cytoplasm.

These processes can take milliseconds (for ion flux), minutes (for protein- and lipid-mediated kinase cascades), hours, or days (for gene expression).

Contents

Signaling molecules

Most signal transduction involves the binding of extracellular signaling molecules (and ligands) to cell-surface receptors. While triggering events inside the cell, such receptors typically face outward from the plasma membrane. Intracellular signaling cascades can also be triggered through cell-substratum interactions. One example is integrins, which bind ligands found within the extracellular matrix. Steroids are another example of extracellular signaling molecules that may cross the plasma membrane due to their lipophilic or hydrophobic nature.[3]  Many, but not all, steroid hormones have receptors within the cytoplasm, and usually act by stimulating the binding of their receptors to the promoter region of steroid-responsive genes.[4]  Within multicellular organisms, numerous small molecules and polypeptides serve to coordinate a cell's individual biological activity within the context of the organism as a whole.  These molecules have been functionally classified as:

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