BIO 3324 1st Edition Lecture 3 Outline of Last Lecture I. Active TransportII. Mediated transportIII. exocytosisIV. osmosisV. membrane potentialOutline of Current Lecture II. concurrent effects III. Membrane electrical statesIV. Local signalingV. Signal transductionVI. Control systemsVII. Response loopsCurrent LectureConcurrent effects of K+ and Na+ on membrane resting potential:• Na+ and K+ potentials exist only under experimental conditionsEK+ = -90mVENa+ = +60 mV• In living cells, the effect of both Na+ and K+ must be taken into account• The greater the permeability of the plasma membrane for a given ion, the greater is the tendency for that ion to drive the membrane potential towards the ion’s own equilibrium potential.The membrane of cells is significantly more permeable to K+ than Na+The effect of Na+ & K+ on the resting membrane potential:• The -70 mV potential is insufficient to counterbalance the concentration gradient of K+ (-90 mV), so K+ still leaks out of the cell• The -70 mV potential favors the passive influx of Na+ down its concentration gradient (+60 mV), so Na+ still leaks into the cellThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.• The Na+-K+ pump actively returns the ions back to their respective sides to offset the leaking & maintain the -70 mV potentialMembrane Electrical States:• Changes in the membrane potential result from changes in ion permeability• PolarizationAny state when the membrane potential is other than 0 mV• DepolarizationMembrane becomes less polarized than at resting potential (a reduction in magnitude)• RepolarizationMembrane returns to resting potential after having been depolarized or hyperpolarized• HyperpolarizationMembrane becomes more polarized than at resting potential (an increase in magnitude)Cell-to-Cell communication:• Two types of physiological communicationElectrical – changes in the cell’s membrane potentialChemical – molecules secreted into the ECF • Four basic methods of cell-to-cell communicationGap junctionsContact-dependent signalingLocal signalingLong-distance signalingGap junctions & Contact-dependant signaling:• Gap junctionsCytoplasmic bridge between cellsFormed by membrane spanning proteins called connexins; roughly 20 types of connexinsAllows small molecules including ions, ATP, amino acids and chemical messages to be passed freely between connected cells• Contact-dependant signalingMembrane-bound cell-surface molecules allow adjacent cells to communicate by direct contact.Process is called cell-cell recognition.CAMs (cell adhesion molecules) are proteins that act as receptors in this type of signalingLocal signaling:• Autocrine signaling occurs when a cell releases a factor that act on that same cell• Paracrine signaling occurs when a cell releases factors (local regulators) that affect nearby cells• Signal molecules diffuse through the interstitial fluid and thus are limited by distanceLong-distance signaling:• Chemical message travels through the organism to a distance far away from the signalingcell.Secreted into the blood and distributed throughout the body by the circulatory systemOnly acts on those cells that have a receptor for the chemical message• Responsibility of both the nervous system and the endocrine system• The chemical message is called a hormone.Vary in size and structureNervous system signaling:• Nervous system uses both chemical and electrical signals to communicate.Electrical signals travels along a nerve cell until it reaches the end of the cell where it is converted into a chemical signal.The chemical signal is called a neurocrine.• Neurotransmitter – chemical signal that diffuses across a narrow space (synapse) causinga rapid effect• Neuromodulator – chemical signal that acts more slowly. Acts either in an autocrine or paracrine fashion• Neurohormone – a chemical signal released by a neuron into the bloodstream. (Only difference with hormones is the cell origin that secretes the chemical).Signaling Pathways:• Process where extracellular signals are conveyed to the target cell’s interior for execution• Dependent on the presence of receptor moleculesA cell cannot respond to a chemical signal if there is not an appropriate receptor for that signal• Four common features of all signal pathwaysSignal molecule (ligand) brings the message to the target cellLigand-receptor binding activates the receptorThe receptor activates one or more intracellular signal moleculesThe last signal molecule in the pathway activates protein synthesis or modifies existing proteins to create a responseTwo categories of chemical signals:• Hydrophilic or lipophobicH2O soluble and/or low lipid solubilityMost are peptides and/or proteinsBind to membrane-bound receptors Usually alters the activity of existing proteinsExamples: Insulin, chatecholamines, FSH• LipophilicPoorly soluble in H2OReadily diffuse through the plasma membraneBind to intracellular receptorsDirectly activates genes in the target cell causing the production of new proteinsExamples: Steroids, thyroid hormones (T3, T4)Signal Transduction:• The process by which an extracellular signal molecule activates a membrane receptor that in turn alters an intracellular molecule to create a response• Commonly referred to as a second messenger systemChemical messenger = “first” messenger“first” messenger sends signal through the receptor to activate the “second” messenger• Conversion of a chemical signal into a different formChemical signal into a response• The signal is often amplifiedSingle first messenger is converted into multiple second messenger by an amplifier enzymeBasic signal transduction:Cascade effect – a signal gets amplified at each step, thus for very little hormone a large response can be obtainedAffect multiple processes at once with a single signalReceptor-enzymes:• Two regions: Receptor region on the ECF side & an enzyme region on the ICF sideSome cases the two regions are the same protein (ex: Protein Tyrosine Kinases)Other cases the two regions are different proteins (ex: guanylyl cyclaseG proteins:• Transducer molecule are trimeric proteins with 3 subunits (a, b, & g)Inactive form all 3 subunits are boundActive form a separates from b-g to activate its target• Links the receptors (GPCR) to the amplifier enzymes• Binds guanosine