Membrane Transport1 Not responsible for: Nernst Equation, other than to know what it’s used for. Chapter 12 Membrane Transport Questions in this chapter you should be able to answer: Chapter 12: 2-14, 16, 17, 19, 20, 21A, B, 22, 23

Membrane Transport2 To what are membranes permeable? Where are membrane transport proteins found? Aquaporin transporters can facilitate H 2 O transport in some cells

Membrane Transport3 How do we describe the properties of membrane transport proteins? 1. Symmetry of transport 2. Mechanism of transport Transport symmetry

Membrane Transport4 3. Energy requirements What defines ‘active’ transport? What potential sources of energy for active transport? Concentration gradients ATP Light Electrical attraction

Membrane Transport5 What is “membrane potential” What ions are commonly involved? -- Na+, K+, fixed ions Outside Inside low [ K+] High [K+] High [ Na+] Low [Na+] Read text of Figure pg 399 Movement of K+ through the ‘K Leak Channel’ establishes the cell’s ‘resting potential’ -- concentration gradient -- electrical attraction to -200 mA inside outside

Membrane Transport6 What are examples of transport coupled to electrochemical gradients? Na + gradients H + gradients called 2 O active transport What is an electrochemical gradient? concentration gradient + membrane potential ECG can drive or impede transport

Membrane Transport7 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 Question 12-2, pg 395 Na/K pump

Membrane Transport8 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

Tick and Sick-49 At sensory cells, opening of ion transporters creates nerve impulse Stimulus opens Na + channelse.g., stereocilia of inner ear Na + rushes into cell Changes membrane potential

Tick and Sick-410 Nerve impulses travel from sensory neurons  motor neurons

Membrane Transport11 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 Transport12 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 Transport13 An action potential can be studied experimentally? “Voltage clamping” can be used to manipulate membrane potential -- & study effects Stimulating electrode: -- alters membrane potential Measuring electrode: -- measures current (ion) flow Voltage-gated ion gates respond Threshold potential Refractory period Measure current Set (‘clamp’) membrane potential Schematic of Voltage Clamp Device axon Stimulating voltage Membrane Potential (mV)

Tick and Sick-414 Nerve impulse ion flow During propagation of Action Potential, waves of Na + and K + ions move back and forth across membrane Due to opening and closing of ion channels -- why does A.P. move??

Membrane Transport15 How can the properties of different ion channels be studied? Patch Clamping Can measure current through a single channel Question 12-4, p 405

Tick and Sick-4 16 K+ then flows out of cell  restores Resting Potential Na+ flows into cell  Action Potential Action Potential triggers opening and closing of “voltage-gated Na + channels” “voltage-gated K + channels”

Membrane Transport17 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 closed open closed State of K+ Channels State of Na+ Channels The “wave”

Tick and Sick-418 Flow of Na+ ions opens Voltage-gated Na + channels voltage-gated Na + channels voltage-gated K + channels Propagate impulse Restore Resting potential Action Potential Action Potential

Tick and Sick-4 19 What happens when nerve impulse reaches the end of axon? Neurons pass signals at a synapse

Membrane Transport20 What happens when an action potential reaches the synaptic complex? Presynaptic membranePost-synaptic membrane Voltage-gated Ca++ channels Ligand-gated Na+ channels Exocytosis of vesicles Voltage gated Na+ channels Synaptic Signaling

Tick and Sick-421 “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-422 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 23 Cell process both excitatory and inhibitory signals -- may signal or not

Membrane Transport24 Adapted form question 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?