The ability of a cell to respond to its environment is essential for its survival. Short-term responses to environmental stimuli, which can occur rapidly and are generally reversible, most often result from modification of existing proteins. Long-term responses are usually the result of changes in gene transcription. Extracellular signals that induce long-term responses to influence many aspects of cellular function; division, differentiation and even communication with other cells. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Alterations in these signaling pathways cause many human diseases including cancer, diabetes, and immune defects. To illustrate the variety of mechanisms used to activate key transcription factors, the eight classes of cell surface receptors and the intracellular signaling pathways they activate are important. Ligand binding to many receptors causes activation by causing two or more receptor molecules to form a complex on the cell surface. Most signaling pathways involve one or more protein kinases. Many extracellular signaling molecules are synthesized and released by signaling cells within the organism. In all cases, the signaling molecules produce a specific response only in target cells that have receptors for the signaling molecules. Many types of chemicals are used as signals: small molecules, peptides, soluble proteins, and many proteins presented on the surface of a cell or found in the extracellular matrix. Most receptors that bind a signaling molecule are a group of closely related molecules. Most signaling molecules, however, are too large and too hydrophilic to penetrate the plasma membrane. These bind to cell surface receptors which are integral proteins on the plasma membrane. Cell surface receptors generally consist of three distinct segments: a segment on the extracellular surface, a segment that spans the plasma membrane, and a segment facing the cytosol. Signaling molecules act as a ligand that binds to a structurally complementary site on the extracellular surface or membrane-expandable domains of the receptors. Ligand binding induces a conformational change in the receptor that is transmitted through the membrane-spanning domain to the cytosolic domain resulting in binding and subsequent activation of other proteins in the cytosol or attached to the plasma membrane. The overall process of converting extracellular signals into intracellular responses, as well as the individual steps of these processes, is termed signal transduction. The molecules involved in the process are called signal transducers. Most ligands are responsible for cell-to-cell signaling (including neurotransmitters, peptide hormones, and growth factors) that bind to receptors on the surface of target cells. An important aspect in understanding cell-cell signaling is understanding the mechanisms by which cell surface receptors transmit signals initiated by ligand binding. Cell surface receptors, including receptors for peptide hormones and growth factors, act by regulating the activity of intracellular proteins. . These proteins then transmit signals from the receptor to a number of additional intracellular targets, often including relay factors. The binding of the ligand to a receptor on the cell surface.