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202 Cards in this Set
- Front
- Back
lipid bilayer established as
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the universal basis of membrane structure
|
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most abundant lipid in cell membranes
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phospholipids
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phospholipids
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molecules in which the hydrophilic head is linked to the rest of the lipid through a phosphate group
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most common type of phospholipid in cell membranes
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phosphatidylcholine
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phosphatidylcholine
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has the small molecule choline attached to a phosphate as its hydrophilic head and two long hydrocarbon chains as its hydrophobic tails
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amphipathic
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hydrophilic and hydrophobic properties
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decrease in entropy makes a structure energetically ___
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unfavorable
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flexibility
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the ability of the membrane to bend
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fluidity
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movement of lipid molecules within the plane of the bilayer
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types of synthetic bilayers
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liposomes
flat phospholipid bilayers |
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liposomes
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form if pure phospholipids are added to water
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flat phospholipid bilayers
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can be formed across a hole in a partition between two aqueous compartments
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flip flop
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phospholipid molecules tumble from one monolayer to the other-- very rare
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temperature affects lipid movement
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temperature decrease = drop in thermal energy = decrease rate of lipid bilayer (less fluid)
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how fluid a lipid bilayer is at a given temperature depends on
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its phospholipid composition, and on the nature of the hydrocarbon tails (closer=less fluid)
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shorter chain length
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reduces tendency of the hydrocarbon tails to interact with one another and therefore increases fluidity of bilayer
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lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are ____ fluid than those with lower portions
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more
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types of movement in membrane
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lateral diffusion
flexion rotation flip flop (rare) |
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in animal cells, membrane fluidity is modulated by the inclusion of
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the sterol cholesterol
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because cholesterol molecules are short and rigid, they
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fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails
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cholesterol tends to
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stiffen the bilayer, making it more rigid and less permeable
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transfer of phospholipids to outer part of membrane catalyzed by
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enzymes called flippases
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cell membranes are generally
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asymmetrical (different sets of phospholipids and glycolipids on each monolayer)
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lipid asymmetry is preserved during
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membrane transport
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nearly all new membrane synthesis in eukaryotic cells occurs in the membrane of
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the endoplasmic reticulum
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new membrane assembled at ER, then
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bits of the bilayer pinch off from the ER to form small vesicles, which then fuse with another membrane.
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cytosolic face
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always adjacent to the cytosol
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noncytosolic face
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exposed to either the cell exterior or the interior space of an organelle
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glycolipids are located mainly in ______ and are found only in ____
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the plasma membrane; found only in the noncytosolic half of the bilayer
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most membrane functions are carried out by
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membrane proteins
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transmembrane proteins
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-have hydrophobic regions in interior of bilayer
-hydrophillic regions exposed to aqueous environment on either side of membrane (pg 373 diagrams) |
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monolayer associated membrane proteins
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located entirely in the cytosol, associated with the inner leaflet of the lipid bilayer by an amphipathic alpha-helix exposed on the surface of the protein
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lipid-linked membrane proteins
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lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups
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protein attached membrane proteins
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bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins
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integral membrane proteins
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directly attached to a lipid bilayer; can be removed only by disrupting the bilayer with detergents
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peripheral membrane proteins
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can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact
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a polypeptide chain usually crosses the bilayer as
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an alpha helix
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why alpha helix?
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-atoms forming the backbone are driven to form H bonds with one another. H bonding is maximized if the polypeptide chain forms a regular alpha helix
-hydrophobic regions on outside of helix; hydrophilic H bonds on inside of helix |
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how do transmembrane proteins allow water soluble molecules to pass
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-alpha helixes line up side by side to form a cylinder shape with a hydrophobic outer region and hydrophilic inner region
-Beta sheets form a beta-barrel |
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membrane proteins can be solubilized in _____ and _____
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detergents; purified
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detergents
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small, amphipathic, lipidlike molecules that have both a hydrophilic and hydrophobic region; used to destroy lipid bilayer by disrupting hydrophobic associations
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detergents+membranes
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-hydrophobic ends of detergent molecules bind to the membrane-spanning hydrophobic region of the transmembrane proteins, as well as to the hydrophobic tails of the phospholipid molecules
-separating the proteins from most of the phospholipids |
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the plasma membrane is reinforced by
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the cell cortex; framework of proteins attached to the membrane via transmembrane proteins
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membrane domains
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functionally specialized regions created by confining plasma membrane proteins to localized areas within the bilayer
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plasma membrane proteins can be linked to
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-fixed structures outside the cell
-or to relatively immobile structures inside the cell |
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cells can create barriers that
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restrict particular membrane components to one membrane domain
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carbohydrate layer
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sugar coating on noncytosolic side of the membrane
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three major classes of membrane lipid molecules
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phospholipids, sterols, glycolipids
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membrane transport proteins
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span the lipid bilayer, providing private passageways across the membrane for select substances
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transporter
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has moving parts; can shift small molecules from one side of the membrane to the other by changing its shape
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channels
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form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion
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ion channels
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channels that let through inorganic ions only
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___ is the most plentiful cation outside the cell
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Na +
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___ is the most plentiful cation inside the cell
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K+
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the high concentration of sodium outside the cell is balanced chiefly by
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extracellular Cl-
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rate at which a molecule diffuses across a membrane depends on
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the size of a molecule and its solubility properties (in lipid)
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small nonpolar molecules: diffusion
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rapidly diffuse across lipid bilayers (CO2, O2)
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Uncharged Polar Molecules: diffusion
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diffuse rapidly if small enough (water, ethanol); slowly if larger (glucose, amino acids, nucleosides)
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ions and charged molecules: diffusion
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membranes are highly impermeable to these, no matter how small
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why can't charged molecules diffuse across a membrane easily?
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their charge and strong electrical attraction to water molecules inhibit them from entering the hydrocarbon phase of the bilayer
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channels discriminate mainly on the basis of
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size and electric charge: if a channel is open, an ion or a molecule that is small enough and carries the appropriate charge can slip through
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transporters allow passage only to
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those molecules or ions that fit into a binding site on the protein
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membrane transport proteins are responsible for
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the selective transfer of water-soluble molecules (solutes) across a membrane
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in many cases, the direction of transport depends on
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the relative concentrations of the solute
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passive transport
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high concentration --> low concentration
-facilitated diffusion -no energy |
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Active transport
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-against concentration gradient
-needs energy -carried out only by special types of transporters that can harness some energy source to the transport process |
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pumps
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transporters that drive the transport of solutes against their concentration gradient
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selective transport can lead to
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the differential distribution of solutes inside and outside the cell (and also between cytosol and organelles)
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trade off between channel proteins and carrier proteins (transporters)
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specificity vs. speed
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channel proteins: ____ transport ONLY
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passive
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may be passive or active
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carriers (transporters)
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membrane transporters ____ the rate of diffusion
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increase
|
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why does the rate of facilitated diffusion plateau
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proteins are all saturated (all in use)
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each cellular membrane contains
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a set of different transporters appropriate to that particular membrane
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a conformational change in a transporter could
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mediate the passive transport of a solute such as glucose
|
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for an uncharged molecule, the direction of transport is dependent solely on
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concentration
|
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membrane potential
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cell membranes have a voltage across them, a difference in the electrical potential on each side of the membrane
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membrane potential (E) exerts
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a force on charged molecules
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the net force driving a charged solute across the membrane is a composite of two forces
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-one due to the concentration gradient
-and the other due to the voltage across the membrane |
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net driving force
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electrochemical gradient
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cells carry out active transport in 3 main ways
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coupled transporters; ATP driven pumps; Light driven pumps
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coupled transporters
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couple the uphill transport of one solute across the membrane to the downhill transport of another
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ATP-driven pumps
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couple uphill transport to the hydrolysis of ATP
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Light-driven pumps
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(found mainly in bacterial cells) couple uphill transport to an input of energy from light
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when Na+ flows back in
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it flows through coupled transporters, providing an energy source that drives the active movement of many other substances into the cell against their electrochemical gradients
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pumps always use
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ATP
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the ATP driven Na+ pump is not only a transporter but also
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an enzyme (ATPase)
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The energy from ATP hydrolysis is used to
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drive Na+ (3) out and K+ (2) in, both against their electrochemical gradients. These activities establish membrane potential
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under normal conditions, the interior of most cells is at a
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negative electric potential
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Oubain inhibits the pump by
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preventing K+ binding
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Na+-K+ pump uses about __% of total cellular ATP
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30
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The Na+-K+ pump helps maintain
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the osmotic balance of animal cells
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osmosis
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movement of water from a region of low solute concentration to a region of high solute concentration
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aquaporins
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specialized water channels in cell membranes that facilitate flow of osmosis
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osmotic pressure
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driving force for water movement; equivalent to a difference in water pressure
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how do animal cells maintain osmotic balance?
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constantly pumping out unwanted solutes (and ions)
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plant cells are prevented from swelling by
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their tough cell walls
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Ca2+ is kept at ___ concentration in the cytosol compared with extracellular fluid
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low
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Coupled Transporters
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downhill movement of the first solute down its gradient provides the energy to drive the uphill transport of the second.
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symport
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both solutes moved in same direction
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antiport
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solutes moved in different directions
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uniport
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carries only one type of solute across the membrane (not a coupled transporter)
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secondary active transport
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ATP not directly coupled; coupled transporters
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The Na+ gradient generated by the Na+-K+ pump is used in animal cells as
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an energy source to drive transport of many other solutes by coupled transport
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The uphill transport of glucose can be driven by
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the downhill transport of Na+
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two types of glucose transporters in epithelial cells
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-actively transported into the cells by Na+ driven glucose symports at apical surface; released from the cell down its concentration gradient by passive glucose uniports at the basal and lateral surfaces
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Ion channels
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-ion-selective, gated
-passive transport |
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ion selectivity depends on
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the diameter and shape of the ion channel and on the distribution of the charged amino acids that line it
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ion transport flow is controlled by
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closed and open conformations of the ion channel
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voltage gated channel probability of being open controlled by
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the membrane potential
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ligand gated channel probability of being open controlled by
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the binding of some molecule (ligand) to the channel
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stress-gated channel opening controlled by
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a mechanical force applied to the channel (auditory hair cells in the ear)
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membrane potential
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electrical chemical difference across plasma membrane
|
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resting membrane potential
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the membrane potential in such steady-state conditions, so that no further difference in charge accumulates across the membrane. (-60mV)
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the membrane potential is determined by
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both the state of the ion channels in the membrane and the ion concentrations in the cytosol and extracellular medium
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The fundamental task of a neuron
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is to receive, conduct, and transmit signals
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action potential
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an explosion of electrical activity in the plasma membrane that is propagated rapidly along the membrane of the axon and sustained by automatic renewal all along the way
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a signal is communicated as
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change in membrane potential
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A stimulus causes
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a localized membrane depolarization
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A localized membrane depolarization large enough to pass a critical threshold will activate
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voltage-gated Na+ channels
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an action potential in a neuron is typically triggered by
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a sudden local depolarization of the plasma membrane; that is, a shift in the membrane potential to a less negative value (towards 0)
|
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A stimulus that causes a sufficiently large depolarization to pass a certain threshold value promptly causes
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voltage-gated Na+ channels to open temporarily at that site, allowing a small amount of Na+ to enter the cell DOWN ITS ELECTROCHEMICAL GRADIENT.
|
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The influx of positive charge
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depolarizes the membrane further (membrane potential made even less negative); causing opening of more voltage gated Na+ channels; causing even further depolarization
|
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action potential: membrane potential shifts
|
from about -60mV to about +40 mV
|
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voltage gated Na+ channel inactivated state
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membrane is still depolarized, so channel is open, but becomes inactivated (more stable form) to prevent further influx of Na+ ions
|
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The membrane is further helped to return to its resting value by
|
the opening of VOLTAGE GATED (not leak) K+ channels; stay open as long as membrane remains depolarized
|
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How is the membrane re-polarized?
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Voltage gated Na+ channels are inactivated (Na+ cant come in)
-Voltage-gated K+ channels are activated (K+ goes out) --K+ leak channels continue to function |
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An action potential can spread long distances by
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depolarizing neighboring regions of the membrane
|
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neurons in vertebrates have
|
a myelin sheath
|
|
myelin sheath allows
|
much faster propagation of an action potential
|
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Multiple Sclerosis
|
Autoimmune disorder resulting in the gradual destruction of the myelin sheath
|
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when an action potential reaches the nerve terminals the signal is
|
transmitted to the target cells at synapses
|
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for the message to be transmitted from one neuron to another...
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the electrical signal is converted into a chemical signal, in the form of a neurotransmitter
|
|
synaptic cleft
|
20 nm gap between target cells and nerve terminal
|
|
action potential cannot cross
|
a synapse
|
|
how is an electrical signal converted into a chemical signal?
|
When an action potential reaches a nerve terminal, it opens voltage-gated Ca2+ channels in the plasma membrane, allowing Ca2+ to flow into the terminal. The increased Ca2+ in the nerve terminal stimulates the synaptic vesicles to fuse with the plasma membrane, releasing their neurotransmitter into the synaptic cleft.
|
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neurotransmitters are stored
|
in synaptic vesicles
|
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the action potential activates _______ in the nerve terminal
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VOLTAGE GATED Ca2+ channels in the nerve terminal
|
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Ca2+ influx activates
|
a docking protein in the vesicle membrane, which triggers the fusion of synaptic vesicles with the plasma membrane to release the stored neurotransmitters into the synaptic cleft
|
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The released neurotransmitter...
|
rapidly diffuses across the synaptic cleft and binds to neurotransmitter receptors (ligand gated ion channels!!!!) concentrated in the postsynaptic membrane of the target cell (post synaptic cell)
|
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The binding of neurotransmitter to its receptors causes
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a change in the membrane potential of the target cell, which can trigger the cell to fire an action potential
|
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A chemical signal is converted into an electrical signal by
|
transmitter-gated ion channels at a synapse; neurotransmitter binds to receptor, opens channel, letting ions flow in, altering membrane potential.
|
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excitatory neurotransmitters
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initiate an action potential (acetylcholine, glutamate (usually Na+ channels))
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Inhibitory neurotransmitters
|
prevent action potential (GABA, glycine (usually Cl- channels; block depolarization))
|
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GABA and glycine: ligand-gated Cl- channels
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whne the neurotransmitter binds, the channels open; when Na+ channels are open and Na+ flows in, Cl- will flow in, neutralizing the effect of the Na+ influx, making the target cell membrane much harder to depolarize
|
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Tranquilizers (Valium, Halcion, temazepam)
|
bind to GABA Cl- channels, make them easier to open; cells become more sensitive to inhibition by GABA
|
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Antidepressant (Prozac)
|
blocks re-uptake of serotonin (excitatory neurotransmitter); target cells remain activate long after the presynaptic signal has faded. Unknown why this relieves depression
|
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Neurons can have thousands of synapses allowing
|
wide-spread transmission of information
|
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patch clamp recording
|
provides a direct and surprising picture of how individual ion channels behave
|
|
most important fuel molecules
|
sugars
|
|
energy stored as "high-energy" chemical bonds
|
covalent bonds that release large amounts of energy when hydrolyzed (ATP and NADPH)
|
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reactions that dominate energy production in most animal cells
|
breakdown of glucose
|
|
living cells use ___ to carry out the oxidation of sugars in a tightly controlled series of reactions
|
enzymes
|
|
animal cells make ATP in two ways
|
-enzyme-catalyzed reactions are directly coupled to ADP+Pi--> ATP
-in mitochondria: uses energy from activated carrier molecules to drive ATP production |
|
catabolism
|
breakdown process that uses enzymes to degrade complex molecules into simpler ones
|
|
3 stages of breaking down food molecules
|
1. digestion: large macromolecules to simple subunits
2. glycolysis: glucose to 2 pyruvate to acetyl CoA + production of limited ATP and NADH 3. Citric Acid Cycle: acetyl CoA to H2O + CO2 + production of a lot of ATP in mitochondrion |
|
stage 1 mostly occurs
|
outside of cell
|
|
stage 2 occurs
|
in cytosol except for conversion of pyruvate to acetyl CoA (mitochondria)
|
|
stage 3 occurs
|
in mitochondria
|
|
glycolysis is the conversion of
|
glucose (6C) to two molecules of pyruvate (3C each)
|
|
glycolysis __ steps
|
10 enzymatic
|
|
All steps of glycolysis occur in
|
the cytosol!!!!!
|
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glycolysis: ATP production
|
2 go in, produces 4; net=2
|
|
also yielded in glycolysis
|
NAD+ --> NADH (yields two NADH per glucose)
|
|
substrate level phosphorylation
|
production of ATP by direct transfer of a high-energy phosphate group to ADP
|
|
there is no ___ phosphate with substrate level phosphorylation
|
inorganic
|
|
oxidative phosphorylation
|
proton electrochemical gradient (in mitochondria) drives ATP synthesis
|
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most ATP is produced by
|
oxidative phosphorylation
|
|
oxidative phosphorylation depends on
|
electron transport within the mitochondrial membrane and the transport of ions across it
|
|
electron transfers of electron transport chain release energy that is used to..
|
pump protons across the membrane and thus generate an electrochemical proton gradient
|
|
An ion gradient across a membrane is
|
a form of stored energy that can be harnessed to do useful work when the ions are allowed to flow back across the membrane down their gradient
|
|
when H+ flows back in through ATP synthase,
|
ATP synthase is like a turbine, permitting the proton gradient to drive the production of ATP
|
|
chemiosmotic coupling
|
linkage of electron transport, proton pumping, and ATP synthesis
|
|
chemiosmotic mechanisms allow cells to
|
harness the energy of electron transfers in much the same way that the energy stored in a battery can be harnessed to do useful work
|
|
what happens when membranes are rendered permeable to proteins?
|
H+ ions pumped across the membrane flow back into the mitochondria in a futile cycle; energy released as heat (not ATP) weight loss because fat reserves used more rapidly to feed electron transport chain. Similar process in hibernating animals for heat; brown fat cells (lots of mitochondria)
|
|
__ molecules of ATP for each molecule of glucose oxidized
|
30
|
|
outer membrane of mitochondria contains
|
porins
|
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outer membrane equivalent to
|
cytosol
|
|
inner membrane (passage of molecules)
|
impermeable to the passage of ions and most small molecules, except where a path is provided by membrane transport proteins
|
|
Most of the proteins embedded in the inner mitochondrial membrane are
|
components of the electron transport chains required for oxidative phosphorylation
|
|
mitochondria use ___ as fuel
|
pyruvate and fatty acids
|
|
Electrons from NADH and FADH2 are...
|
transferred to an electron transport chain where released energy is used to pump protons across the mitochondrial inner membrane
|
|
redox reactions
|
oxidation-reduction reactions
|
|
reduced
|
picks up a proton
|
|
oxidized
|
looses a proton
|
|
pairs of compounds with most negative redox potential have
|
the weakest affinity for electrons (strongest tendency to donate electrons)
|
|
NADH strong tendency to
|
donate electrons
|
|
O2 strong tendency to
|
accept electrons
|
|
proton motive force due to
|
voltage; pH (7 outside, 8 inside)
|
|
coupled transport due to proton electrochemical gradient
|
ATP (4-) goes out, while ADP (3-) goes in (coupled)
-dependent only on membrane potential |
|
Active transport due to proton electrochemical gradient
|
pyruvate and Pi goes in with H+ through a symport
|
|
Cellular respiration is
|
the complete oxidation of food molecules, like glucose, to CO2 and H2O
|
|
fate of O2
|
reduced to H2O, NOT released as CO2 (acetyl CoA)
|
|
net yield of ATP is variable
|
-dependent on concentration of reactants and products
-account for energy spent for coupled transport |
|
electrons are transferred from FADH2 to
|
ubiquinone
|
|
___ in mitochondria is analogous with ___ in chloroplasts
|
matrix; stroma
|
|
In chloroplasts, the light capturing systems, the electron transport chains, and ATP synthase are all contained in
|
the thylakoid membrane
|
|
difference between NADH and NADPH
|
NADH used to break down molecules (catabolic)
NADPH used to build up molecules (anabolic) |
|
primary light absorbing pigment
|
chlorophyll
|
|
When chlorophyll molecules absorb protons,
|
an electron is raised to an orbital with higher energy
|
|
work done by electron transport chain
|
generate proton gradient
|
|
energy converted to chemical work by
|
electron transfer
|