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Signal Transduction Mechanisms:? II Messengers and Receptorshttp://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-14/1401.jpgCell communication occurs through chemical signals and cellular receptors by either the 1) direct contact of molecules on two cell’s surfaces or the 2) release of a chemical signal recognized by another cell (near or far). Hormones are carried by the circulatory systems to many sites. Growth factors are released to act on nearby tissues. Ligands are signals that bind cell surface receptors (as observed with insulin (a ligand) and the insulin receptor) or that can pass into the cell and bind an internal receptor (such as the steroid hormones). Signal TransductionSignal transduction is defined as the ability of a cell to change behaviour in response to a receptor-ligand interaction. The ligand is the primary messenger. As the result of binding the receptor, other molecules or second messengers are produced within the target cell. Second messengers relay the signal from one location to another (such as from plasma membrane to nucleus). Often a cascade of changes occur within the cell which results in a change in the cell’s function or identity. The signal transduction pathway can act to amplify the cellular response to an external signal. Messenger molecules may be amino acids, peptides, proteins, fatty acids, lipids, nucleosides or nucleotides. Hydrophilic messengers bind to cell membrane receptors. Hydrophobic messengers bind to intracellular receptors which regulate expression of specific genes. A ligand binds its receptor through a number of specific weak non-covalent bonds by fitting into a specific binding site or pocket. In situations where even low concentrations of a ligand will result in binding of most of the cognate receptors, the receptor affinity is considered to be high. Low receptor affinity occurs when a high concentration of the ligand is required for most receptors to be occupied. The dissociation constant (Kd,) is the conce