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Download 01.22.10 Lecture 5: Membrane transport
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01.22.10 Lecture 5: Membrane transport Ion concentrations within the cell are different from those outside Few molecules cross the membrane by passive diffusion Each cell membrane transports specific molecules Solutes cross membranes by passive or active transport • Passive transport is driven by concentration gradients & electrical forces • Active transport is requires energy An 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 potassium There are 3 main classes of membrane transport proteins Passive transport by glucose carrier protein (GLUT2) protein randomly switches between two states • Carrier Glucose moves down it’s concentration gradient • Active 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 absorbtion Example: the Na+-K+ pump • Uses ATP hydrolysis to pump sodium out, potassium in • Helps to maintain a negative electric potential inside the cell Example: the Na+-K+ pump Sodium gradients do work: glucose transport • • Glucose-Na+ symport protein Electrochemical Na+ gradient drives import of glucose Two types of glucose carriers enable epithelial cells to transport glucose in the gut Ion 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 close Gated ion channels respond to different kinds of stimuli The membrane potential is produced by the distribution of ions on either side of the bilayer K+ leak channels establish the membrane potential across the plasma membrane The action potential provides rapid, longdistance 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 distance Action potentials are propagated along an axon Voltage-gated Na+ channels mediate action potentials • Exist in 3 states: closed opened, and inactivated Action potentials are propagated along an axon Conversion of an electrical signal to chemical signal Conversion of biochemical signal back into electrical