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Lecture 5: Membrane Transport and Electrical Properties

Lecture 5: Membrane Transport and Electrical Properties. Ion concentrations inside and outside the cells are often different. The relative permeability of a synthetic lipid bilayer. Two main classes of membrane transport proteins. (aka carriers, permases, or transporters). Conformation

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Lecture 5: Membrane Transport and Electrical Properties

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  1. Lecture 5: Membrane Transport and Electrical Properties

  2. Ion concentrations inside and outside the cells are often different

  3. The relative permeability of a synthetic lipid bilayer

  4. Two main classes of membrane transport proteins (aka carriers, permases, or transporters) Conformation change Both: Specificity Multi-TM Hydrophilic solutes cross membrane via hydrophilic protein pathway Aqueous Pores (faster)

  5. Passive transport, active transport, electrochemical gradient

  6. Ionophores are tools to increase permeability of membranes to specfic ions Made by microorganisms Mobile ion carriers Channel formers A23187

  7. Reversible conformational change in a carrier protein

  8. Like an enzyme reaction--it saturates

  9. Three ways of driving active transport

  10. Active transport can be driven by ion gradients Primary active transport: ATP-driven Secondary active transport: ion-driven Na is the usual co-transported ion

  11. Binding of Na and glucose is cooperative Binding of Na and glucose is cooperative

  12. In bacteria, yeast and membrane organells proton gradient is more predominant Lactose permease

  13. Transcellular transport Asymmetric distribution of carrier proteins

  14. Na-K pump Both Na and K are transported to higher [ ]

  15. Reversible! P-type transport ATPases

  16. F-type ATPases=ATP synthases Work in reverse of transport ATPases Chapter 14

  17. MDR ABC transporters ATP binding= dimerization

  18. ABC transporters: Amino acids, sugars, ion, polysaccharides, peptides, proteins, flipping of lipids MDR gene: hydrophobic drugs, chemotherapy Malaria: chloroquine Yeast mating pheromone Peptides from degration into ER Cystic fibrosis:regulator of Cl- channel

  19. Channels form hydrophilic pores, selective We will not talk about gap junctions here Porins stay in outer membrane of bacteria, mitochondria and Chloroplasts Channels in the plasma membrane are narrow and selective And can open and close--ion channels, only “downhill”

  20. Channels are “gated” “Phosphorylation”, “desensitized”, “inactived”

  21. A typical vertebrate neuron

  22. Voltage-gated Na+ channels Voltage-gated K+ channels

  23. The “ball-and-chain” model of voltage-gated K+ channel inactivation Driven by state of lowest energy 20 aa

  24. Axon myelination Schwann cells More mature Just beginning To myelinate axon

  25. Patch-clamp recording of current flow through Individual channels All or nothing Conductance Duration Rate

  26. synapses

  27. Acetylcholine receptor is the first ion channel to be purified, cloned, Reconstituted, recorded single channel,3-D structure Neuromuscular junction is one of the best studied synapse

  28. Schematic structure of AchR Ligand-gated ion channels Cations: Na+, K+, Ca2+

  29. Five sets of channels are involved in neuromuscular transmission (from a nerve impulse to muscle contraction)

  30. Summary Ions and larger polar molecules cannot cross the lipid bilayer; Two types of transport proteins: carriers and channels; Passive and active transport; Three types of active transport; Mechanisms of cotransport; Na-K pump and ABC transporters; Channels: voltage-gated and ligand gated; Selectivity of K channels; Action potential, voltage-gated Na channel, voltage-gated K channel, myelination; 10. Single-channel recording; 11. Neuromuscular junction as an example of synapse, functions of ion channels.

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