cell communication

	gap junctions between animal cells, plasmodesmata between plant cells, cell-cell recognition (animal cells)
	two cells in an animal may communicate by interaction between molecules protruding from their surfaces
	a secreted molecule that influences cells in the vicinity
	local signaling: paracrine signaling, synaptic signaling, long-distance signaling: hormonal signaling
	a secreting cell acts on nearby target cells by discharging molecules of a local regulator into the extracellular fluid
	a nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell
	specialized endocrine cells secrete hormones into body fluids, often the blood,
	in multicellular organisms, one of many types of secreted chemicals that are formed in specialized cells, travel in body fluids, and act on specific target cells in other parts of the body to change their functioning
	reception, transduction, response
	the target cell detects a signaling molecule coming from outside the cell, a chemical signal is "detected" when the signaling molecules binds to a receptor protein located at the cell's surface or inside the cell
	the binding of the signaling molecule changes the receptor protein in some way, initiating this process, the stage converts the signal to a form that can bring about a cellular response, sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules... a signal transduction pathway (the molecule in the pathway are often called relay molecules)
	the transduced signal finally triggers a specific cellular response, the cell-signaling process helps ensure that crucial activities like these occur in the right cells, at the right time, and in proper coordination with the other cells of the organsim
	a molecule that binds specifically to another molecule, usually a larger one, bind reversibly to signal receptors (usually in the plasma membrane, some times intracellular)
	when a ligand binds to it, the receptor transmits information from the extracellular environment to the inside of the cell by changing shape or aggregating
	receptor found in either the cytoplasm or nucleus of target cells,a chemical messenger passes through the target cell's plasma membrane to reach such a receptor
	a signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activating a G protein
	(GTP or guanosine tripohosphate)a GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G protein-coupled receptor, to other signal transduction proteins inside the cell
	loosely attached to the cytoplasmic side of the membrane, the G protein functions as a molecular switch that is either on or off, depending on which of the two guanine nucleotides is attached, GDP or GTP, when GDP is bound to the G protein, the G protein is inactive, the receptor and G protein work together with another protein, usually an enzyme, when the appropriate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape, its cytoplasmic side then binds to an inactive G protein, causing a GTP to displace the GDP
	the activated G protein dissociates from the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme
	the changes in the enzyme and G protein are only temporary, because the G protein also functions as a GTPase enzyme... in other words, it then hydrolyzes its bound GTP to GDP, now inactive again, the G protein leaves the enzyme, which returns to its original state. the G protein is now available for reuse, the GTPase function of the G protein allows the pathway to shut down rapidly when the signaling molecule is no longer present
	a receptor protein in the plasma membrane, the cytoplasmic part (intracellular) of which can catalyze the transfer of a phosphate group form ATP to a tyrosine on another protein, receptor tyrosine kinases often respond to the binding of a signaling molecule by dimerizing and then phosphorylating a tyrosine on the cytoplasmic portion of the other receptor in the dimer, the phosphorylated tyrosines on the receptors then activate other signal transduction proteins within the cell
	before the signaling molecules binds, the receptors exist as individual polypeptides, each has an extracellular ligand-binding site, an a helix spanning the membrane, and an intracellular tail containing multiple tyrosines
	the binding of a signaling molecule causes two receptor polypeptides to associated closely with each other, forming a dimer (dimerization)
	dimerization activates the tyrosine kinase region of each polypeptide; each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine on the tail of the other polypeptide
	now that the receptor protein is fully activated, it is recognized by specific relay proteins inside the cell, each such protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound protein, each activated protein triggers a transduction pathway, leading to a cellular response
	a protein pore in cellular membranes that opens or closes in response to a signaling chemical (its ligand), allowing or blocking the flow of specific ions
	the ligand-gated ion channel receptor's gate remains closed until a ligand binds to the receptor, when the ligand binds to the receptor and the gate opens, specific ions can flow through the channel and rapidly change the concentration of that particular ion inside the cell, this change may directly affect the activity of the cell in some way
	when the ligand dissociates from this receptor, the gate closes and ions no longer enter the cell
	the binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in the chain of molecular interactions that leads to a particular response within the cell, the signal-activated receptor activates another molecule, which activates yet another molecule (relay molecules) and so on, until the protein that produces the final cellular response is activated
	an enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein
	an enzyme that removes phosphate groups from (dephosphorylates) proteins, often functioning to reverse the effect of a protein kinase
	a relay molecule activates protein kinase 1, active protein kinase 1 transfers a phosphate from ATP to an inactive molecule of protein kinase 2, thus activating this second kinase, active protein kinase 2 then catalyzes the phosphorylation (and activation) of protein kinase 3, finally, active protein kinase 3 phosphorylates a protein that brings about the cell's response to the signal
	enzymes called protein phosphatases (PP) catalyzes the removal of the phosphate groups from the proteins, making them inactive and available for reuse
	a small, nonprotein, water-soluble molecule or ion, such as a calcium ion (Ca 2+) or cyclic AMP, that relays a signal to a cell's interior in response to a signaling molecule bound by a signal receptor protein
	(cAMP) cyclic adenosine monophosphate, a rung-shaped molecule made from ATP (by Adenylyl Cyclase)that is a common intracellular signaling molecule (second messenger) in eukaryotic cells
	an enzyme that converts ATP to cyclic AMP in response to a signal
	the first messenger activates a G protein-coupled receptor, which activates a specific G protein, in turn, the G protein activates adenylyl cyclase, which catalyzes the conversion of ATP to cAMP, the cAMP then acts as a second messenger and activates another protein, usually protein kinase A, leading to cellular responses
	(IP3) produced by the cleavage of a certain phospholipid in the plasma membrane, a second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger, a rise in cytoplasmic Ca 2+ concentration (releases CA2+ from the cell's ER)
	(DAG) a second messenger produced by the cleavage of a certain kind of phospholipid in the plasma membrane
	a sinaling molecule binds to a receptor, leading to activation of phospholipase C, phospholipase C cleaves a plasma membrane phospholipid called PIP2 into DAG and IP3, DAG functions as a second messenger in other pathways, IP3 quickly diffuses through the cytosol and binds to an IP3-gated calcium channel in the ER membrane, causing it to open, calcium ions flow out of the ER (down their concentration gradient), raising the CA 2+ level in the cytosol, the calcium ions activate the next protein in one or more signaling pathways
	the initial signaling molecule, a local regulator called a growth factor, triggers a phosphorylation cascade, once phosphorylated, the last kinase in the sequence enters the nucleus and there activates a gene-regulating protein, a transcription factor, this protein stimulates a specific gene so that an mRNA is synthesized, which then directs the synthesis of a particular protein in the cytoplasm
	the particular proteins a cell possesses determine what signaling molecules it responds to and the nature of the response