Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 2401.21.11Lecture 5: Membrane transportIon concentrations within the cell are different from those outsideFew molecules cross the membrane by passive diffusionEach cell membrane transports specific moleculesSolutes cross membranes by passive or active transport•Passive transport is driven by concentration gradients & electrical forces•Active transport is requires energyAn electrochemical gradient is driven by 2 forces•Concentration gradient - ions move across a membrane from high to low concentrations•Voltage across the membrane•High for sodium, low for potassiumThere are 3 main classes of membrane transport proteinsPassive transport by glucose carrier protein (GLUT2)• Carrier protein randomly switches between two states• Glucose moves down it’s concentration gradientActive transport is mainly driven in 3 ways•1. Coupled transporters couple uphill transport of one solute to the downhill transport of another•2. ATP-driven pumps use hydrolysis of ATP to uphill transport•3. Light driven pumps couple transport to light absorbtionExample: the Na+-K+ pump•Uses ATP hydrolysis to pump sodium out, potassium in•Helps to maintain a negative electric potential inside the cellExample: the Na+-K+ pumpSodium gradients do work: glucose transport•Glucose-Na+ symport protein •Electrochemical Na+ gradient drives import of glucoseTwo types of glucose carriers enable epithelial cells to transport glucose in the gutIon channels are selective pores in the membrane •Ion channels have ion selectivity - they only allow passage of specific molecules•Ion channels are not open continuously, conformational changes open and closeGated ion channels respond to different kinds of stimuliThe membrane potential is produced by the distribution of ions on either side of the bilayerK+ leak channels establish the membrane potential across the plasma membraneThe action potential provides rapid, long-distance communication•Action potential (nerve impulse): a wave of electrical activity propagated along the length of a neuron•Very fast (~100 m/sec), dose not weaken over distanceAction potentials are propagated along an axonVoltage-gated Na+ channels mediate action potentials•Exist in 3 states: closed opened, and inactivatedAction potentials are propagated along an axonConversion of an electrical signal to chemical signalConversion of biochemical signal back into
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