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Lecture 5

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Plasma Membrane (II) Protein Mobility: Some freely mobile •Others have restricted mobility -tethered to the underlying cytoskeleton -bound to the external ECM •Studied by FRAP: fluorescence recovery after photo- bleaching: Can not identify individual protein units. You don’t know if the individual proteins are moving or they are attached to something else Membrane Lipid Mobility •Phospholipids are small and make up the bilayer –Would expect them to move freely –They don’t, instead seem to move freely within a “compartment” but movement is restricted between compartments –Removing membrane skeletal components removes some barriers to movement Membrane Domains & Cell Polarity •Membranes are rarely homogenous –Distinct variations in protein composition and mobility –Eg. Sperm •Distinct parts, continuous plasma membrane Movement across PM: plasma membrane •The plasma membrane has two main functions –Retain dissolved materials in a cell –Allow necessary materials in •Charged and polar molecules can’t pass through hydrophobic membrane easily •Membranes must be selectively permeable. •Two means of movement across PM: –Passively by diffusion from high to low concentration [can involve protein channel] –Actively by energy-coupled process : energy driven protein pump. Can move against concentration gradeint Solute movement Diffusion = –Spontaneous process –Moves from high concentration to low •When solute is an ion (charged) movement depends on concentration gradient and electrical gradient –If both forces = electrochemical gradient - Voltage/potential difference •Electrochemical gradient can be very strong. -intermembrane space is +ve -matrix is –ve. Therefore H+ wants to move into the matrix because they are positive, A) Simple Diffusion through bilayer: Passive •Substances must go down a gradient and membrane must be permeable to them Solutes: –Non-polar: dissolve in lipids –Small, uncharged (polar) molecules –gases etc. -Not large, polar molecules -Water: •Small polar molecule •Moves more rapidly than solutes •Process called = osmosis •Towards hypertonic, away from hypotonic -Hypotonic solution: water flows inside, cell swells: net gain -Hypertonic solution: water flows outside, cell shrinks: net loss -Isotonic solution: no net loss or gain. B) Simple Diffusion through protein channels: Passive Ions: •Move through ion channels •Movement is bidirectional •Channels are selective to particular ion down its * +, voltage or electrochemical gradient -channels are gated: either an open or closed configuration I. Ligand gated channels –A specific molecule (ligand) binds and opens channel –Ligand is usually NOT the solute that passes through Eg. Ach (acetylcholine)-neurotransmitter II. Voltage-gated channels –Opens depending on differences in ionic charge across membrane - Nerves there are membrane potential - Depolarizes towards 50 and repolarizes towards -100 - Depolarizes: Sodium flows into the cell. Repolarizes: Na Closes (opposite for K. k flows out of the cell) Eg. Voltage-gated K+ and Na+channels Voltage-gated and ligand-gated channels work together to transmit nerve signals Eg. Muscle contraction 1) Arrival of an action potential causes opening of voltage-gated Ca2+ channels 2) Causes release of acetylcholine, which triggers opening of ligand-gated channel in postsynaptic cell 3) Influx of Na+ causes localized membrane depolarization, leading to opening of voltage-gated Na+ channels and generation of action potential 4) Spreading of depolarization reaches T tubules, triggers opening voltage-gated Ca2+ channels, and release of Ca2+ from sarcoplasmic reticulum into cytosol •Causes muscle contraction C) Facilitated Diffusion: Passive •Substance binds to integral protein (transporter) and causes a conformational change which facilitates diffusion. Facilitative transporter (the actual molecule that has to be transported is facilitating diffusion) –Can move solutes equally well in either direction –Depends on concentration gradient –Does NOT use energy •important for entry/exit of larger polar solutes (amino acids and sugars). •at least 5 related transporters (isoforms) in humans •Glucose phosphorylated after entering cytoplasm, lowering intracellular glucose concentration •Increase in blood glucose levels triggers insulin secretion by pancreas •Insulin triggers transfer of vesicles containing GLUT to membrane •Stimulates glucose uptake into various cells D) Active Transport •Moves solutes against concentration gradient –Requires energy (ATP) –Uses protein “pumps”: eg proton (H+) pump in lysosomes •Plasma membrane poten
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