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