Cell Bio - Ch.12 Notes
Cell Bio - Ch.12 Notes BIOL 3510
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This 4 page Class Notes was uploaded by Michelle Notetaker on Friday October 7, 2016. The Class Notes belongs to BIOL 3510 at University of North Texas taught by Dr. Amanda Joy Wright in Fall 2016. Since its upload, it has received 3 views. For similar materials see Cell Biology in Biology at University of North Texas.
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Date Created: 10/07/16
Cell Bio - Ch.12 Notes: Membrane Transport 2 Types of Transport Proteins: ● Transporters - shift small organic/inorganic ions from one side of the membrane to the other by changing shape. - Discriminate based on shape. ● Channels - form tiny hydrophilic pores across the membrane for substances to pass via diffusion. - Discriminate based on size & electrical charge. ● Ion channels - only permit passage of inorganic ions. Lipid Bilayer: ● Permeable to: - Nonpolar molecules - Small, uncharged polar molecules ● Impermeable to: - Ions - Large, uncharged polar molecules Ion Concentrations within & outside of Cell (mM): ● Cations - Na+ (5-15 inside, 145 outside) - K+ (140 inside, 5 outside) ● Anions - Cl- (5-15 inside, 110 outside) - HCO3-, proteins, nucleic acids, etc. (lots inside, little outside) - Differences in ion concentrations create a m embrane potential - voltage difference across the membrane. Passive vs Active Transport: - Hydrophilic molecules must cross the membrane by facilitated transport using transport proteins. ● Passive transport - no expenditure of energy by the transport protein. - Spontaneous movement from high solute concentration to low solute concentration following the concentration gradient. ● Active transport - energy expenditure required in the form of ATP, transmembrane ion gradient, or sunlight. - Movement against concentration gradient. - Carried out by transporters called “pumps”. ● Electrochemical gradient - net force driving a charged solute across a cell membrane. - Composite of 2 forces: 1) concentration gradient 2) membrane potential. Transporters: - Move most small water-soluble, organic, and some inorganic molecules across the cell membrane. - Each membrane has specific set of transporters for that type of membrane. ● Passive transporters: - Glucose transporter - uncharged, so movement depends solely on concentration gradient. ● Active transporters: 1) ATP-driven pump (Na+/K+ pump) - Na+ binding promotes phosphorylation of pump & conformational change, K+binding promotes dephosphorylation; active import of Na+ and export of K+. (Ca²+ pump) - Ca²+ binding promotes phosphorylation of aspartic acid & conformational change; active export of Ca²+. 2) Coupled pump - transport a molecule and cotransport an ion. - Symport: molecule and ion move in same direction. (glucose-Na+ symport) - requires active and passive transport; active import of glucose. - Antiport: molecule and ion move in opposite directions. 3) Light-driven pump (Bacteriorhodopsin) - Active export of H+. Ion Channels - Use passive transport, but are not open pores. ● Selective - some channels allow passage of specific ions. ● Gated - open/close in response to stimulus. ● Membrane potential - accumulation of charge across a membrane. - Potential = 0 if equal charges are on both sides of membrane. - Potential is negative in resting animals; K+ tends to leak out down concentration gradient. - K+ leak channels randomly flicker open and closed, letting K+ in and out, so membrane is much more permeable to K+ than other ions. - Patch Clamp Recording - detects ion flow across small area of membrane. ● Types of Ion-gated channels: 1) Voltages-gated - opening/closing controlled by membrane potential. (ex. Neuron action) action potential triggered by depolarization of the membrane (influx of Na+ ions). - 3 conformations: 1)Closed - before action potential. 2) pen - during action potential (depolarization). 3)Inactivated - as repolarization is occurring (efflux of K+ ions). 2) Ligand-gated - opening/closing controlled by binding of some molecule (ligand) to the channel. (ex. Neuron action) 3) Mechanically-gated - opening/closing controlled by some mechanical force applied to the channel. (ex. Ion channels in inner ea detection of sound vibrations cause stereocilia to move and ion channels to open. ● Neuron signaling: - Neurons combine, interpret, and record signals. - Electrical signals convert to chemical signals (neurotransmitters) at synapses, vice versa. - Action potentials open Ca2+ channels prompting release of neurotransmitters into the synapse. - Neurotransmitter binding opens transmitter-gated ion channels (ligand-gated) initiating membrane depolarization. - Excitatory neurotransmitters: influx of Na+ depolarizes membrane and increases action potential occurrence. (acetylcholine and glutamate) - Inhibitory neurotransmitters: influx of Cl- polarizes membrane and decreases action potential occurrence. (GABA and glycine)
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