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Cell Communication
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Enduring Understanding:
Cells communicate with one another
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Essential Question:
How do cells communicate?
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Cells communicate by signal conduction pathways
signal on cell's surface is converted to specific cell response
evolved first in prokaryotes and single celled eukaryotes
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Yeast (Saccharomyces cerevisiae) use cell signaling to identify mates
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Intercellular communication
cell junctions signal molecules dissolved in cytosol that passes directlyplasmodesmatagap junctions
membrane bound cell surface molecules (animals)important for embryonic development & immune response
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messenger molecules secreted by cells
local regulators if travel short distanceex. growth factor stimulate cell to grow & multiply
paracrine signaling when many cells respond due to one single cell in the vicinity
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synaptic signaling(nervous tissue of animals)
electrical signal in nerve cell triggers secretion of chemical signal (neurotransmitter), diffuse across synaptic cleft to target cell
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Hormones used for long distance signalingcells release hormones, travel through circulation to target cells in organism
ex. pituitary gland releases Follicle stimulating hormone (FSH) into blood stream, goes to ovary, ovary starts maturing egg for ovulation
in plantsgrowth regulators travel via other cells or diffusion in air as gas (ethylene fruit ripening)
nervous system can also be considered long distance signaling
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Local and Long distance signaling
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Sutherland's Research Nobel prize 1971
investigated how epinephrine stimulates the breakdown of storage polysaccharide glycogen in liver and skeletal cells
epinephrine activates a cytosolic enzyme, glycogen phosphorylase in intact cells
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results: found epinephrine did not interact directly with enz. for glycogen breakdown intermediate steps must take placeplasma membrane involved in epinephrine signal
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three stages in cell communication:
1. Reception target cell detects signal molecule from outside cellsignal molecule binds to receptor protein
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2. Transductionbinding of signal molecule changes receptor proteinconverts signalEx. binding of epinephrine to receptor activates glycogen phosphorylasecan be single or multiple steps (signal conduction pathway)
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3. Response transduced signal triggers a specific cell response
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Receptionsignal molecule acts as a ligand (molecule that binds to another molecule)causes receptor protein to change shapeactivates the receptor allows interaction with other cell molecules some can aggregate
intracellular receptor proteins in cytoplasm or nucleusmust pass through plasma membrane to get to receptor (hydrophobic)
Ex. steroid hormones, thyroid hormones, nitric oxide
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Testosteronepasses through membranebinds to receptor proteinhormone/receptor complex enters nucleus & binds to specific genesstimulates transcription of gene to mRNAmRNA transcribed to protein
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Water soluble signal moleculesbind to receptor proteins in plasma membrane
Three types of membrane receptors:
1. Gproteinlinked receptors
2. receptor tyrosine kinases
3. ion channel receptors
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Gproteinlinked receptor
a plasma membrane receptor that works with help of a G protein
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have role in embryonic development & sensory reception (vision & smell)
system evolved early
involved in diseasescholerawhooping cough (pertussis)botulismtoxins interfere with Gprotein function
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a. Gprotein works as an on or off switch depending if GDP or GTP attached to it
GDP attached = inactive
work together with enzyme
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b. when signal molecule bindsto receptor, receptor is activated changes shape
cytoplasmic side binds an inactive G protein
causes GTP to displace GDP
activates G protein
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b.activated G protein dissociates from the receptor and diffuses along the membrane
binds to enzyme activates it
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c. G protein then functions asGTPase enzyme, hydrolyzes bound GTP to GDP
becomes inactive again can reuse it
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Tyrosinekinase receptor
can trigger more than 1 cell pathway at once (10 or more)kinase = enzyme that catalyzes transfer of phosphate groups ( phosphate from ATP to tyrosine)
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tyrosine kinase are individual polypeptideshave extracellular binding sitehave an alpha helix through membraneintracelluar tail of tyrosines
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signal molecule binds two polypeptides come together= dimer
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dimerization activates tyrosinekinase regioneach tyrosine kinase adds a phosphate from ATP to tyrosine
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activated protein is recognized by relay proteinseach relay protein binds to a specific phosphorylated tyrosinecauses cell response
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ligand gated ion channel receptors
acts as a gate when receptor changes shapecan cause gate to open or close
Ex. sodium ions, calcium ions
important in nervous system in synapses
some gated ion channels are regulated by electrical signals instead of ligands
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gate is closed until ligand (signal molecule) binds
ligand binds, gate opensspecific ions can go through channelchanges ion concentration in cell
ligand dissociatesgate closes
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Transduction
multistep pathwayallows amplification of the signalsmall # of signal molecules produce a large cellular responsebetter for coordination and regulation
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Signal transduction pathway = chain of molecular interactions that leads to a particular cellular response
domino effect one signalactivated receptor activates another protein, which activates another protein and so on until final cell response
use relay molecules (proteins)original signal does not enter cellcauses conformational changes in a protein
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Protein phosphorylation and Dephosphorylation
regulates protein activityprotein kinase an enzyme that transfers phosphate
groups (is a cytoplasmic protein kinase)act on different proteinsphosphorylate serine or threonine, not tyrosine
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Phosphorylation Cascade
each step involves a conformational change
changes protein from inactive to active form
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2% of genes code for protein kinases
abnormal activity of a kinase can cause abnormal cell growth and cancer
phosphorylation cascade also has protein phosphatases remove phosphate groups from proteins = dephosphorylation
*turns off signal transduction pathway
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components other than proteins involved in signaling pathways
1. second messengers small, water soluble molecules or ions
(signal molecule = 1st messenger)move by diffusion in cellfound in both Gprotein and receptor tyrosine
kinase pathways
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Ex. Cyclic AMP = cylic adenosine monophosphate
cAMP
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Adenylyl cyclase in membrane converts ATP to cAMP in response to epinephrine
makes many cAMPs in cytosol
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increase in cAMP concentration activates serine/threonine kinase called protein kinase A
then phosphorylates other proteins
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ex. Vibrio cholerae causes cholera
produces toxin that modifies G protein involved in water and salt secretionunable to hydrolyze GTP to GDP stuck in active formcontinuous stimulation of adenylyl cyclase to make cAMPcauses intestines to secrete high amounts of water/salts= diarrhea
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Viagra works due to signaling pathways (cyclic GMP as a signal molecule)
originally made to relax smooth muscle for increase blood flow to heart muscleallows greater blood flownow used for erectile dysfunction
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2. Calcium ions and inositol trisphosphate (IP )3
calcium = second messengerincreases in cytosol and causes responses like
muscle contractionin plants causes greening in response to light
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light areas = low Calcium
dark areas = high calcium
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IP = inositol trisphosphate DAG = diaclyglycerol3
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Response
can occur in cytoplasm or nucleus
can regulate enzymes
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can regulate synthesis of enzymes or other proteins
can turn genes on or off
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signaling pathways with multiple steps:1. amplify the signal2. specificity of response
can have same hormone create two different responses depending on the cell
ex. epinephrinein liver cells helps breakdown glycogenin heart cells causes increased contraction
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Specificity of cell signaling
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Scaffolding proteins
large relay proteins where other several relay proteins can attach
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importance:enhances speed and signal transfer between cells
Termination of the signal:signal molecule binding to receptor is reversibledecrease in signal molecule concentrationsignal molecule not attached inactivates receptor
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