Explain how an original signal molecule can produce a cellular response when it may not even enter the target cell?

Answer 1

There are a few ways that this happens. One way is through synaptic signalling. This signalling occurs in the nervous system. An electric signal along a nerve cell triggers the secretion of a chemical signal in the form of neurotransmitter molecules. These diffuse across the synapse, these neurotransmitters stimulate the target cell. Another type of signalling is paracrine signalling. The secreting cell acts on nearby target cells by discharging molecules of a local regulator like a growth factor into the extracellular fluid. Both animals and plants use hormones for long distance signalling. With this cell communication, specialized endocrine cells secrete hormones into body fluids, often the blood. Hormomes may reach virtually all body cells. What happens when a cell encounters a signal? The signal must be recognized by a specific receptor molecule, and the information it carries must be changed into another form, transduced before the cell can respond. So generally the cells generally communicate via chemical messengers targeted for cells. Addition of general pathways:A signal molecule such as a peptide hormone produce a cellular response by binding to receptor proteins on the cell membrane. These molecules may be involved in endocrine (systemic-global), paracrine (tissue-local), autocrine (self-local), or nervous (restricted to synaptic junctions) signaling. The receptor proteins for these signal molecules have very high specificity for a particular kind of molecules and respond by either directly opening a gated channel, creating a secondary messenger molecule, or directly phosphorylating downstream molecules to initiate a signal cascade that result in a subtle or gross alteration in the cell's state of operation through the activation/deactivation of enzymes and the activation/deactivation of transcription factors controlling gene expression. Recurring archetypes of these receptors include ligand gated channels (example: acetylcholine receptors in neuromuscular junctions. Binding of acetylcholine causes the opening of ion channels that propagate the action potential), G-protein coupled receptors (example: adrenergic receptors. Binding of adrenaline initiates G-protein activation and results in the production of secondary messenger molecules cAMP, which activate downstream target molecules that effect changes), Receptor tyrosine kinases (example: insulin receptor. Binding of insulin leads to the recruitment of downstream proteins and their activation through phosphorylation on tyrosine residues).