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69 Cards in this Set
- Front
- Back
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What is the cell membrane IMPERMEABLE to?
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1. Hydrophilic ions
2. Molecules >4 Angst. radii 3. Molecules w/ C-chains 5+ |
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So what items are incapable of passive transport across the membrane?
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1. Glucose
2. Amino acids 3. Vitamins 4. Multi-valent anions/cations |
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What needs to take place in order for Impermeable items to be able to help keep the cell functional?
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Maintainence of LARGE TRANSMEMBRANE CONCENTRATION DIFFERENCES.
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What are the 4 important features of Membrane-bound Protein Carriers?
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1. Intrinsic (integral) proteins
2. Multispan lipids in memb. 3. Cylindrical geometry 4. Central pore |
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How do membrane carriers NOT act?
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AS FERRYBOATS - shuttling 1 molecule at a time; THAT WOULD BE ENERGETICALLY IMPOSSIBLE
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How do membrane carriers transport items across the membrane?
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By specifically binding item at intrinsic sites INSIDE the channel in sequential steps.
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What are the kinetics of carrier-mediated membrane transport very similar to?
(2 concepts) |
1. ENZYME-SUBSTRATE REACTIONS
2. Drug-Receptor interactions |
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At what substrate concentration is carrier-mediated transport -MORE RAPID?
-Slower? |
-More rapid: LOW concentration
-Slow: high concentration |
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Why does low [substrate] allow transport to be faster?
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B/c it allows transport to be even faster than predicted by Fick's law of simple diffusion!
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Why is transport slower at higher concentration of substrate?
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Because the # of carrier proteins becomes saturated and limits the rate of diffusion.
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What is a very important feature of Carrier-mediated transport?
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Specificity - eg RBCs only will allow d-glucose to be taken up, not l-glucose
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What are 2 ways to regulate carrier transport proteins?
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1. Regulate the Maximum Number of them in the membrane.
2. Regulate the number of binding sites available on them. |
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What is a good example of carrier-mediated transport?
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Glucose - uptake in RBCs is specific for D isomer, not L.
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What are 2 types of mechanisms that can inhibit transporters?
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1. Competitive inhibition
2. Non-competitive inhibition |
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Define competitive inhibition
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competition for the same site on the membrane transporter by 2 separate solutes
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Give an example of competitive inhibition for the RBC
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All different d-hexoses absorbed from the intestine compete for SLGT-1 for uptake.
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What is Noncompetitive inhibition?
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Stearic inhibition - when a substance attaches to the transporter at a site other than the carrier site
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How does stearic inhibition hinder transport?
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By reducing the carrier site's affinity for transported substrate.
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What is Ouabain?
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a cardiac glycoside that is a noncompetitive inhibitor of Na/K ATPase
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What is the mechanism of Ouabain?
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It binds an Extracellular site that prevents K+ from binding its carrier site.
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What are the 3 types of Carrier-mediated transport?
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1. Facilitated diffusion
2. Primary active transport 3. Secondary active transport |
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What is the definition of Facilitated diffusion?
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Carrier-mediated transport of a non-charged molecule DOWN its chemical gradient; or an ION down an electrochemical gradient by means of a specific membrane-bound integral protein.
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2 Important notes about facilitated diffusion:
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-Faster than Fick's law predicts
-Exhibits a Tmax (Vmax) |
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What is the major function of facilitated diffusion?
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To maintain a a rate of transport of essential molecules across membranes that is sufficient to sustain function.
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What is the ENERGY source for facilitated transport?
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Concentration gradient - chemical potential energy
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What is NOT the energy source for facilitated transport?
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Metabolic energy - ATP
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What are 3 types of proteins that allow passive transport?
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1. Pores
2. Channels 3. Carriers |
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What is the difference between pores and channels?
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Pores are always open
Channels have gates |
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How are carriers different from pores and channels?
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They allow facilitated diffusion, and have 2 lids that are never open at the same time.
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What are the steps in facilitated diffusion?
#1 |
1. Outside-open carrier spontaneously binds solute to specific site in pore; causes outside gate to close.
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What are the steps in facilitated diffusion?
#2 |
Inner gate opens and solute can move into cell
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What are the steps in facilitated diffusion?
#3 |
Inner gate re-closes, then outer gate reopens.
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Again, what is the energy source for this transport?
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Chemical gradient - potential energy
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What determines the direction of flux in facilitated diffusion?
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Direction of chemical gradient
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What will happen to net flux if the chemical gradient equalizes on both sides of membrane?
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Flux will cease.
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What is the classic example of facilitated diffusion?
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Glut2 and Glut4 - on RBCs, liver , and fat cells.
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What increases Glut2 in RBCs and myocytes?
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Insulin - but not in liver cells.
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What is Primary Active Transport?
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ATP-driven transport of charged or uncharged molecules up their chemical or electrical gradients.
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3 agents that inhibit primary active transport:
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1. Cyanide
2. Dinitrophenol 3. Azide |
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Prime example of Primary active transport:
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Na/K ATPase
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What is the initial state of Na/K ATPase?
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E1 - atp binds the alpha subunit and the pump is open to the intracellular space.
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What binds Na/K ATPase when it is in the E1 state? Where on Na/K ATPase?
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3 Na+ ions from inside the cell - to lumen sites on the alpha subunit.
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What happens after Na binds the alpha subunit of Na/K ATPase?
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ATP is hydrolyzed to ADP and the Pi phosphorylates the alpha subunit.
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What is the result of phosphorylating the a-subunit of Na/K ATPase?
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A conformation change to the E2-P state.
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What does the E2P change cause?
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Opening of the extracellular side of the channel - Na+ exits and E2P then switches to low-energy state.
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What happens to Na/K ATPase in the Low-Energy E2P state?
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2 K+ ions bind luminal sites in the membrane.
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What happens to Na/K ATPase after K+ binds its lumen?
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A conformation change back to the E1 state - hence K+ ions release
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What is the stoichiometry of Na/K ATPase?
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-3 Na+ out
-2 K+ in Net of one Pos charge extruded |
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What term describes Na/K ATPase net pumping of one charge?
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Electrogenic
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What does electrogenic mean?
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That Na/K ATPase contributes to the magnitude of the cell Em
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What are the Cardiac Glycosides?
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-Oubain
-Digoxin -Digitalis |
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What is the mechanism of the cardiac glycosides?
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Non-competitive inhibition - Binding of the empty E2P and changing its conformation to block K+ before it has the chance to bind.
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Which cardiac glycosides are plant-derived steroids?
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-Oubain
-Digoxin |
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Give another example of primary active transport that is NOT Na/K ATPase:
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Ca2+ ATPases
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What are two primary active transporters of calcium?
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1. Ca2+ ATPase in the SR membrane of all myocytes
2. Ca2+ ATPase in the plasma membrane of all cells. |
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What is the function of the Ca2+ ATPase that is in the SR membrane of myocytes?
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To sequester 2 Ca2+ ions (and 2 H+) from the myocyte cytoplasm into the SR per one ATP.
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What is the function of the Ca2+ ATPase that is in all cell membranes?
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To extrude one Ca2+ (and take up 1 H+) per ATP
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What is one other example of an important primary active transport protein?
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The H+/K+ pump
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Where is the H+/K+ pump found?
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In Parietal cells of
-Gastric glands -Kidney tubule cells -Intestinal cells |
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What is accomplished by the H+/K+ pump?
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Puts 2 H+ into gland lumen and takes up 2 K+ per ATP molecule.
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What is the functional importance of the H+/K+ pump?
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Gastric digestion and K+ and Acid/base balance in the kidney.
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So what are the three important types of Primary Active Transport?
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Na/K ATPase
Ca ATPase (both types) H+/K+ ATPase |
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What is the 3rd type of Carrier-mediated transport?
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Secondary active transport
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Define 2ndry active transport:
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Same thing as primary but a different energy source.
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What is the energy source for 2ndry active transport?
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Chemical potential energy released when a couple molecule (Na) moved passively down its own electrochemical gradient.
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So by definition what is secondary active transport?
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Coupled transport - either:
-Co-transport (symport) or -Counter-transport (antiport) |
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Define Secondary Active Co-Transport:
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Mvmt of 1 solute down its electrochem gradient coupled to mvmt of another solute AGAINST its own gradient, but both in the SAME DIRECTION.
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What is an important 2ndry Active Counter-transport protein at the end of muscle contraction?
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Na-Ca exchange
-3 Na+ go into cell -1 Ca++ goes out |
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In general, what do counter-transport proteins exchange?
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-Cations for cations
-Anions for anions |
