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Transcript
Chapter 12 Membrane Transport Not responsible for: Nernst Equation, other than to know what it’s used for. Questions in this chapter you should be able to answer: Chapter 12: 2-14, 16, 17, 19, 20, 21A, B, 22, 23 Membrane Transport 1 To what are membranes permeable? Where are membrane transport proteins found? Aquaporin transporters can facilitate H2O transport in some cells Membrane Transport 2 How do we describe the properties of membrane transport proteins? 1. Symmetry of transport 2. Mechanism of transport Transport symmetry Membrane Transport 3 3. Energy requirements What defines ‘active’ transport? What potential sources of energy for active transport? Concentration gradients ATP Light Electrical attraction Membrane Transport 4 What is “membrane potential” Outside low [ K+] High [ Na+] Inside High [K+] Low [Na+] What ions are commonly involved? -- Na+, K+, fixed ions Read text of Figure 12-21 pg 399 Movement of K+ through the ‘K Leak Channel’ establishes the cell’s ‘resting potential’ outside -- concentration gradient -- electrical attraction -- -20 to -200 mA inside Membrane Transport 5 ECG can drive or impede transport What is an electrochemical gradient? concentration gradient + membrane potential What are examples of transport coupled to electrochemical gradients? Na+ gradients H+ gradients called 2O active transport Membrane Transport 6 What are the properties and functions of the Na-pump (Na/K pump) -- 3 Na+ out for 2 K+ in What are its… Symmetry? Mechanism? Energy requirement? What are its functions? maintenance of tonicity maintenance of the Na+ gradient Na/K pump Question 12-2, pg 395 Membrane Transport 7 How can cells alter membrane potential and respond to its changes? -- ‘gated’ ion channels Nerve impulse; muscle contraction -- Voltage-gated and ligand-gated Sense of hearing; Plant movements -- Mechanically-gate channels Dirurnal cycling; Phototaxis -- Light-gated channels Membrane Transport 8 At sensory cells, opening of ion transporters creates nerve impulse Stimulus opens Na+ channels e.g., stereocilia of inner ear Na+ rushes into cell Changes membrane potential Tick and Sick-4 9 Nerve impulses travel from sensory neurons motor neurons Tick and Sick-4 10 What is a nerve impulse?? A nerve impulse is an altered membrane potential = “action potential” Wave-like movement Electrodes can show movement of the action potential Membrane Transport 11 The cellular foundation of “neurobiology” -- the giant neuron of the squid Used to study factors that affect action potential -- e.g., effect of [Na+] Membrane Transport 12 An action potential can be studied experimentally? Schematic of Voltage Clamp Device “Voltage clamping” can be used to manipulate membrane potential -- & study effects Measure current Voltage-gated ion gates respond Threshold potential Refractory period Stimulating voltage Measuring electrode: -- measures current (ion) flow axon Membrane Potential (mV) Stimulating electrode: -- alters membrane potential Set (‘clamp’) membrane potential Membrane Transport 13 During propagation of Action Potential, waves of Na+ and K+ ions move back and forth across membrane Nerve impulse ion flow Due to opening and closing of ion channels -- why does A.P. move?? Tick and Sick-4 14 How can the properties of different ion channels be studied? Patch Clamping Can measure current through a single channel Question 12-4, p 405 Membrane Transport 15 Action Potential triggers opening and closing of “voltage-gated Na+ channels” “voltage-gated K+ channels” Na+ flows into cell Action Potential K+ then flows out of cell restores Resting Potential Tick and Sick-4 16 Why do the Na+ and K+ channels open and close at different times? -- Membrane potential alters their state voltage-gated Na+ channels -- 3 states voltage-gated K+ channels -- 2 states State of Na+ Channels State of K+ Channels closed open closed The “wave” Membrane Transport 17 Flow of Na+ ions opens Voltage-gated Na+ channels Action Potential voltage-gated Na+ channels Propagate impulse voltage-gated K+ channels Restore Resting potential Tick and Sick-4 18 What happens when nerve impulse reaches the end of axon? Neurons pass signals at a synapse Tick and Sick-4 19 What happens when an action potential reaches the synaptic complex? Presynaptic membrane Post-synaptic membrane Voltage-gated Ca++ channels Exocytosis of vesicles Ligand-gated Na+ channels Voltage gated Na+ channels Synaptic Signaling Membrane Transport 20 “Neuro transmitters” are the chemical signal passed from cell-to-cell Common examples -- Dopamine -- Serotonin -- Acetylcholine Some are inhibitory -- GABA Bind to receptors -- Ligand-gated Na+ Channels Tick and Sick-4 21 Excitatory Neurotransmitters open Na+ channels -- Na+ flows through open channels -- alters membrane potential -- create action potential in next cell Synaptic Signaling Tick and Sick-4 22 Cell process both excitatory and inhibitory signals -- may signal or not Tick and Sick-4 23 Adapted form question 19-19. The inside of endosomes is acidic, which is achieved by the presence of a H+ pump in the endosome membrane. The endosome membrane also contains a transporter which pumps Cl- into the endosome. If a mutation eliminates the Cl- pump, acidification of the endosome is impaired. A, Draw a diagram of the endosome showing the two transporters and the movement of ions. B. Why does a mutation to the Cl- pump impair endosome acidification? C. Could a Ca++ pump substitute for the Cl- pump? Why or why not? Membrane Transport 24