BMB 251 1st Edition Lecture 32 Outline of Last Lecture I. Review Lecture for Exam Foura. Modular eukaryotic gene activators/repressorsb. DNA in the major and minor grooveOutline of Current Lecture II. Channel proteinsIII. Na+/K+ portsa. Antiportb. Symportc. UniportIV. Membrane PotentialV. AquaporinsVI. NeuronVII. Action potentialCurrent Lecture- Channel proteins form hydrophilic pores across membranes; most have narrow, highly selective pores that open and close rapidly ion channelso Function of ion channels: allow Na+, K+, Ca2+ and Cl- to diffuse rapidly down their electrochemical gradients across lipid bilayer- Na-port is more commonly used than K-port. If you want to move something up its concentration gradient, you must use a source of energy (for example, you could use an Na-gradient)- The reaction which pairs Na+ moving down its concentration gradient and glucose moving up its concentration gradient is spontaneous- Clicker Question 1: Transporter shows one Na+ ion moving up its concentration gradient, one K+ ion moving up its concentration gradient in the opposite direction across membrane, and ATP being phosphorylated. What type of transporter is it?o Active antiport- Clicker Question 2: Transporter shows both glucose and Na+ ion moving up their concentration gradients (out of the cell) in the same direction. What type of transporter is it?o An active symport- Clicker Question 3: Transporter shows a single glucose molecule moving down its concentration gradient. What type of transporter is it?o Passive uniport- Two properties distinguish ion channels from single aqueous pore:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Ion selectivity: permits some inorganic ions passage ions of certain size and charge can pass through narrowest part of channel (aka selectivity filter) which limits rate of passage Selectivity filter allows ions to transiently contact the hydrophilic side chains lining the channel pore As ion concentration increases, the flux of ion through channel increases proportionally, but level off at maximum rate (becomes saturated)o Channels are not continuously open, they are gated Main types of stimuli to open a gate: change in voltage, mechanical stress or binding of a ligand- Clicker Question 4: Why is flux though an ion channel so much faster than through a typical transporter protein?o Diffusion through the channel protein does not depend on its under going a conformational changeo The pore allows almost unhindered diffusion of its iono More than one ion can occupy the pore at any give timeo All of the above are trueo None of the above are true- Membrane potential arises when there is a difference in electrical charge on the two sides of a membraneo Slight excess of positive and negative charge on one side and slight deficit of that charge on the other o **Passive ion diffusion contribute the most to potential, followed by electrogenic pumps- Resting membrane potential: equilibrium condition in which there is no net flow of ions across the plasma membrane o Calculated by using Nernst equation- Na+-K+ pump: pumps three Na+ molecules out of cell for every two K+ ions pumped in closing it creates an initial decrease in membrane potential- K+ channel is made from four identical transmembrane subunits which come together to form a pore through the membrane; anions on inside of cell make the channel cation-selectiveo Each subunit has two alpha helices, one pore helix connecting them and a selectivity loop that creates the selectivity filter (cone shaped when put all together)o **K+ ion loses all water molecules to enter the filter and then bind to carbonyl-oxygens lining the filter in the exact same way that it would make the bonds with its hydration shell; Na+ can’t enter filter because it’s hydration shell is too small to bind with carbonyl-oxygens- Water channels (aquaporins): allow water to move across membrane (highly condensed in epithelial cells in kidney)o Allow rapid passage of water while blocking ions much the opposite of ion channels- Neuron (nerve cell): receive, conduct and transmit signals o Contains a cell body with a nucleus, axon (conducts signals away form cell body) and dendrites (protrude from cell body to receive signals from axons of other neurons).o Axon divides its far end into many branches, sending many signals simultaneously- Action potential (nerve impulse): traveling wave of electrical excitation that rapidly carries a message without attenuation across a neuron direct consequence of voltage-gated cation channelso Triggered by depolarization of plasma membrane (shift of membrane to less negative value inside) o Opens voltage-gated Na+ channels which cause greater depolarizationo Na+ channels close so K+ channels become activated, which help bring activated plasma membranes more rapidly back to its initial negative
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