2093910126

2093910126 - Further Membrane Functions BSCI230-Lecture-7...

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BSCI230-Lecture-7 Biomembranes II Chp. 4 • Selectively Permeable Barrier • Transporting Substances • Electrical Properties • Energy Transduction Further Membrane Functions Water moves by osmosis across a membrane more readily than ions In = Out Solute Conc. In higher than Out Solute Conc. In lower than Out Solute Conc. Plant cells mostly hypertonic compared to their environment Animal cells in an isotonic world –water movement into cell maintains turgor pressure
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Diffusion Osmosis Plasma membrane is a selective barrier that allows variable solute passage: Simple (passive) diffusion - through bilayer or via channels. Energy input not required. (Oxygen, carbon dioxide, solutes with large partition coefficients). From random thermal movement. Facilitated diffusion (passive) – where normal diffusion rate of substrate is increased through interaction with specific membrane protein (ion channels, glucose transporter). Conformational change, but not a "pump" Active transport – an energy requiring process in which a substance binds to a specific transmembrane protein, changing its conformation for movement against an electrochemical gradient. Energy driven "pump". Passive Nonmediated Solute Movement Passive Active Nonmediated Transport mediated Passive diffusion: Diffusion = a spontaneous process where a substance moves from an area of higher concentration to one of lower concentration Gases simply diffuse through the lipid bilayer Simple ions passing via a channel (aqueous protein) Diffusion of water through a channel
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Selective Membrane Solute Movement Passive Active Nonmediated Transport mediated Ion diffusion requires special protein lined channels-often very specific Usually the channels are gated & controlled by a. Voltage b. Chemical ligands (e.g. selective ion conduction Na + vs. K + ) Ion channel action is very fast, somewhat slower is facilitated diffusion KcsA bacterial K + channel, opened at low pH: conformational change results Example: Bacterial K + ion channel (KcsA) and its function Selection for K + over Na + ions: Unique short pore helix (non-polar pentapeptide) lined with carboxyl groups that face into pore with a 0.3 nm diameter – a favorable distance for interacting with carbonyl groups C=O, but not for smaller Na + Rapid flow of ions is possible because of low energy barrier
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How KcsA (bacterial) channel may open and close: helix bending Gly residue at hinge
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This note was uploaded on 10/22/2008 for the course BSCI 207 taught by Professor Higgins during the Spring '08 term at Maryland.

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2093910126 - Further Membrane Functions BSCI230-Lecture-7...

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