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62 Cards in this Set
- Front
- Back
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What's the function of the plasma membrane?
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Defines cell boundaries, interacts selectively and dynamically with the cell's environment.
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What's a more recent theory in the fluid mosaic model, beyond just "proteins floating in a sea of membrane"?
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The proteins are anchored to each other and to structures inside the cell by cytoskeletal filaments, which may modulate membrane protein function.
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Name six ways molecules get across cell membranes.
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Simple diffusion, diffusion through ion channels, carrier-mediated transport, active transport, endocytosis, exocytosis.
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How long does it take a molecule to diffuse one micron?
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Half a millisecond.
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How long does it take a molecule to diffuse one cm?
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14 hours.
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What is the driving force behind diffusion?
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Concentration gradients; movement occurs through random molecular motion.
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What is Fick's Law?
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States the rate of diffusion is proportional to the concentration gradient and the diffusion coefficient of a particular solute. If applicable, permeability coefficient applies as well.
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What besides concentration gradients determines diffusion in charged molecules?
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Electric field effects. In ions, you usually talk about the electrochemical gradient, not just the concentration gradient.
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What kind of things can diffuse across the PM?
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Small, uncharged molecules. Hydrophobic molecules.
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Name some molecules that can diffuse across a PM. Why?
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O2, N2, benzene (hydrophobic). H2O, urea, CO2, glycerol (small, uncharged).
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In weak electrolytes, what determines membrane permeation?
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pH. The pH that makes the electrolyte non-ionized will promote permeation.
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What effect could abnormal pH have on a drug to cause mis-dosing?
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Can increase or decrease permeability. If pH is low outside a cell, a certain drug HA would remain undissociated and would readily cross PM, where it could dissociate inside a cell and build up as A- and H+.
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What is osmolarity?
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Number of moles of solute particles per liter of solution.
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What is osmolality?
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Number of moles of solute per kg of solution.
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What's the relationship between osmolarity and osmolality in a dilute solution?
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They are about equal.
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Define osmotic pressure.
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Amount of pressure required to resist the osmotic flow of water.
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What's the van't Hoff relation?
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Osmotic Pressure is proportional to RT(ic)
R=Gas constant T=Kelvins i=# particles formed by dissociation of a solute molecule c=molar/molal concentration of solute. |
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In the van't Hoff relation, what does sigma correct for?
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It's the reflection constant, and corrects for particles bouncing off the membrane and not penetrating.
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In the van't Hoff relation, what does psi correct for?
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It's the osmotic coefficient, and corrects for solute particles interacting with each other in solution and preventing each other from penetrating.
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At zero degrees C, a solution containing one osmole per liter exerts an osmotic pressure of:
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22.4 atmospheres.
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What do permeant vs. impermeant solutes do to the steady state cell volume?
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Permeant solutes have no effect. Impermeant solute determine the osmotic pressure difference.
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What is an ion channel?
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A water-filled protein pore that allows ions to cross the PM.
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What are some functions ion channels are involved in?
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Nerve and muscle excitation, hormone secretion, maintenance of PM resting potential, epithelial transport, sensory transduction, second-messenger mediated processes.
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How do we classify ion channels?
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By ion selectivity and gating mechanism.
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What determines the selectivity of an ion channel?
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Selectivity filters, such as chemical groups within the pore. Ex: carbonyl groups allow cations to come through.
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What drives the flow of ions through an ion channel?
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Electrochemical gradients.
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What's faster? Carriers or ion channels? What's the usefulness of this?
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Ion channels. This means they are used in transmitting nerve impulses.
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Local anesthetics target ____.
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Ion channels.
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____ and ____ result from ion channel defects.
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Cystic fibrosis, heart arrythmias.
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Snake toxins attack ____.
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Ion channels.
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What are the classes of ion channels?
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Ligand gated, voltage gated, stretch gated (mechanosensitive), temperature-gated, and light gated.
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What's an example of a ligand-gated ion channel?
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They open when one or more activating molecules bind to site on the intracellular or extracellular surface of the protein. An example is channels opened by neurotransmitters. Some ligand-gated channels also have some voltage gating.
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Where are V-gated ion channels found?
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In the nerve impulse.
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How do V-gated ion channels work?
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They contain charges or dipoles within the membrane electric field. These dipoles control a gate or conformational change in response to changes in membrane voltage.
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Where would you find stretch-gated ion channels?
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In auditory and touch sensation, and in regulation of cell volume, shape, and motility. The mechanism isn't well known, but they open or close in response to stretch in the membrane.
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What are TRP channels and where are they located?
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Thermo-regulated. Usually found in sensory cells, including nocioceptors. They are also frequently ligand-gated. Some are activated by capsaicin, and some by menthol.
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Why are light gated channels useful?
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They're not found in mammals, but they're used to study the mammalian nervous system. It can be GE into a nerve cell and then excited to study a neuron.
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T/F: Ion channel gating is usually controlled by only one type of stimulus.
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False. There are usually several at play, but we name them based on the most predominant one.
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In terms of ion selectivity, the major classes of ion channels are:
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K, Na, Ca, Non-selective cation channels, Cl, Gap junction channels.
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T/F: Transport by ion channels involves active transport.
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False. ATP isn't expended directly. However, ATP is used indirectly in maintaining concentrations, etc.
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Do ions just "freely diffuse" through ion channels?
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Not exactly. They interact with other molecules, with chemical groups at the mouths, and with chemical groups along the pore walls. One such site is the selectivity filter, which determines which kinds of ions are able to pass through the pore. It can select by size, charge, electron density, and by an ion's affinity for water (dehydration is required for passage through some ion channels).
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How does the speed of carrier-mediated transport compare to that of ion channels?
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It's relatively slow since only a few molecules are transported with each change in conformation.
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This kind of cell transport exhibits specificity, saturation, kinetics, and competition between transported substrates.
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Carrier-mediated transport.
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What are the three major classes of carriers?
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Facilitated diffusion, coupled carrier, and active transport.
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Which way does facilitated diffusion go on a gradient?
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It always goes down the gradient.
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What's the advantage of facilitated diffusion compared to simple diffusion?
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A carrier molecule speeds the movement of its specific substrate.
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What's an important example of facilitated diffusion?
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Glucose moves this way into all animal cells.
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In coupled carrier transport, can a molecule move against its electrochemical gradient?
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Yes. In fact, one of the two molecules usually moves against its gradient.
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How does Na/Glucose symport work?
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Sodium increases the affinity of the carrier for glucose. Once glucose binds, the movement of Na down its gradient powers the movement of glucose up its gradient.
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How can facilitated diffusion be blocked?
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By heavy metals, but not by metabolic poisons.
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How can coupled carrier transport be blocked?
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Both metabolic poisons and ouabain. These poisons work by reducing Na/K pump activity and reducing the Na gradient into the cell.
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How does cellular uptake of AAs happen?
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By Na-driven coupled carrier transport.
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What is secondary active transport?
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Used to describe carriers that don't directly use ATP but rely on gradients that are controlled by pumps.
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What's the most famous exchange carrier?
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The Na/Ca exchanger, which transports Na into the cell along its gradient iot drive Ca out of the cell against its gradient. This is how low intracellular Ca is maintained-- a condition essential for regulation of intracellular enzymes, secretory processes, and muscle contractile systems.
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The maintenance of these is critical for maintenance of cell resting potential, cell volume, intracellular enzyme function, coupled transport processes, nerve and muscle impulse conduction, and epithelial transport of NaCl and water.
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Constant Na and K gradients--high intracellular K and low intracellular Na.
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A large fraction of the body's resting energy is used to drive this one active transporter:
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The Na/K pump.
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What are some examples of pumps in the body?
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Na/K pump, Ca pumps, proton pumps, K/H exchange pumps, and Ca/H exchange pumps.
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What is furosemide and how does it work?
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It's a diuretic and disables the Na/K/Cl cotransporter. Water flows back into the kidney (reabsorption), returning to the body, along with these transported ions. When the diuretic stops the carrier, less water is taken back up and more goes out as urine.
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What are ouabain and digitalis and how do they work?
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They are drugs called cardiac glycosides. They stop the Na/K pump by binding to the K binding site on the extracellular side of the pump. Inhibition of the pump causes membrane depolarization, which opens v-gated Ca channels. This Ca stimulates more forceful heart pumping.
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Where would you find "leaky epithelia?"
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Where maintaining large osmotic gradients isn't critical. You see them in the renal proximal tubule, small intestine, and the choroid plexus. They transport isosmotic fluid.
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Where do you see really tight epithelia?
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Where osmotic gradients (renal collecting tubule) or ionic gradients (stomach) are critical.
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How does the concentration of Ca inside the cell compare to outside?
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It's low outside the cell, and even lower inside the cell.
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