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202 Cards in this Set

  • Front
  • Back
lipid bilayer established as
the universal basis of membrane structure
most abundant lipid in cell membranes
phospholipids
phospholipids
molecules in which the hydrophilic head is linked to the rest of the lipid through a phosphate group
most common type of phospholipid in cell membranes
phosphatidylcholine
phosphatidylcholine
has the small molecule choline attached to a phosphate as its hydrophilic head and two long hydrocarbon chains as its hydrophobic tails
amphipathic
hydrophilic and hydrophobic properties
decrease in entropy makes a structure energetically ___
unfavorable
flexibility
the ability of the membrane to bend
fluidity
movement of lipid molecules within the plane of the bilayer
types of synthetic bilayers
liposomes
flat phospholipid bilayers
liposomes
form if pure phospholipids are added to water
flat phospholipid bilayers
can be formed across a hole in a partition between two aqueous compartments
flip flop
phospholipid molecules tumble from one monolayer to the other-- very rare
temperature affects lipid movement
temperature decrease = drop in thermal energy = decrease rate of lipid bilayer (less fluid)
how fluid a lipid bilayer is at a given temperature depends on
its phospholipid composition, and on the nature of the hydrocarbon tails (closer=less fluid)
shorter chain length
reduces tendency of the hydrocarbon tails to interact with one another and therefore increases fluidity of bilayer
lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are ____ fluid than those with lower portions
more
types of movement in membrane
lateral diffusion
flexion
rotation
flip flop (rare)
in animal cells, membrane fluidity is modulated by the inclusion of
the sterol cholesterol
because cholesterol molecules are short and rigid, they
fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails
cholesterol tends to
stiffen the bilayer, making it more rigid and less permeable
transfer of phospholipids to outer part of membrane catalyzed by
enzymes called flippases
cell membranes are generally
asymmetrical (different sets of phospholipids and glycolipids on each monolayer)
lipid asymmetry is preserved during
membrane transport
nearly all new membrane synthesis in eukaryotic cells occurs in the membrane of
the endoplasmic reticulum
new membrane assembled at ER, then
bits of the bilayer pinch off from the ER to form small vesicles, which then fuse with another membrane.
cytosolic face
always adjacent to the cytosol
noncytosolic face
exposed to either the cell exterior or the interior space of an organelle
glycolipids are located mainly in ______ and are found only in ____
the plasma membrane; found only in the noncytosolic half of the bilayer
most membrane functions are carried out by
membrane proteins
transmembrane proteins
-have hydrophobic regions in interior of bilayer
-hydrophillic regions exposed to aqueous environment on either side of membrane (pg 373 diagrams)
monolayer associated membrane proteins
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
lipid-linked membrane proteins
lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups
protein attached membrane proteins
bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins
integral membrane proteins
directly attached to a lipid bilayer; can be removed only by disrupting the bilayer with detergents
peripheral membrane proteins
can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact
a polypeptide chain usually crosses the bilayer as
an alpha helix
why alpha helix?
-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
how do transmembrane proteins allow water soluble molecules to pass
-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
membrane proteins can be solubilized in _____ and _____
detergents; purified
detergents
small, amphipathic, lipidlike molecules that have both a hydrophilic and hydrophobic region; used to destroy lipid bilayer by disrupting hydrophobic associations
detergents+membranes
-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
the plasma membrane is reinforced by
the cell cortex; framework of proteins attached to the membrane via transmembrane proteins
membrane domains
functionally specialized regions created by confining plasma membrane proteins to localized areas within the bilayer
plasma membrane proteins can be linked to
-fixed structures outside the cell
-or to relatively immobile structures inside the cell
cells can create barriers that
restrict particular membrane components to one membrane domain
carbohydrate layer
sugar coating on noncytosolic side of the membrane
three major classes of membrane lipid molecules
phospholipids, sterols, glycolipids
membrane transport proteins
span the lipid bilayer, providing private passageways across the membrane for select substances
transporter
has moving parts; can shift small molecules from one side of the membrane to the other by changing its shape
channels
form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion
ion channels
channels that let through inorganic ions only
___ is the most plentiful cation outside the cell
Na +
___ is the most plentiful cation inside the cell
K+
the high concentration of sodium outside the cell is balanced chiefly by
extracellular Cl-
rate at which a molecule diffuses across a membrane depends on
the size of a molecule and its solubility properties (in lipid)
small nonpolar molecules: diffusion
rapidly diffuse across lipid bilayers (CO2, O2)
Uncharged Polar Molecules: diffusion
diffuse rapidly if small enough (water, ethanol); slowly if larger (glucose, amino acids, nucleosides)
ions and charged molecules: diffusion
membranes are highly impermeable to these, no matter how small
why can't charged molecules diffuse across a membrane easily?
their charge and strong electrical attraction to water molecules inhibit them from entering the hydrocarbon phase of the bilayer
channels discriminate mainly on the basis of
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
transporters allow passage only to
those molecules or ions that fit into a binding site on the protein
membrane transport proteins are responsible for
the selective transfer of water-soluble molecules (solutes) across a membrane
in many cases, the direction of transport depends on
the relative concentrations of the solute
passive transport
high concentration --> low concentration
-facilitated diffusion
-no energy
Active transport
-against concentration gradient
-needs energy
-carried out only by special types of transporters that can harness some energy source to the transport process
pumps
transporters that drive the transport of solutes against their concentration gradient
selective transport can lead to
the differential distribution of solutes inside and outside the cell (and also between cytosol and organelles)
trade off between channel proteins and carrier proteins (transporters)
specificity vs. speed
channel proteins: ____ transport ONLY
passive
may be passive or active
carriers (transporters)
membrane transporters ____ the rate of diffusion
increase
why does the rate of facilitated diffusion plateau
proteins are all saturated (all in use)
each cellular membrane contains
a set of different transporters appropriate to that particular membrane
a conformational change in a transporter could
mediate the passive transport of a solute such as glucose
for an uncharged molecule, the direction of transport is dependent solely on
concentration
membrane potential
cell membranes have a voltage across them, a difference in the electrical potential on each side of the membrane
membrane potential (E) exerts
a force on charged molecules
the net force driving a charged solute across the membrane is a composite of two forces
-one due to the concentration gradient
-and the other due to the voltage across the membrane
net driving force
electrochemical gradient
cells carry out active transport in 3 main ways
coupled transporters; ATP driven pumps; Light driven pumps
coupled transporters
couple the uphill transport of one solute across the membrane to the downhill transport of another
ATP-driven pumps
couple uphill transport to the hydrolysis of ATP
Light-driven pumps
(found mainly in bacterial cells) couple uphill transport to an input of energy from light
when Na+ flows back in
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
pumps always use
ATP
the ATP driven Na+ pump is not only a transporter but also
an enzyme (ATPase)
The energy from ATP hydrolysis is used to
drive Na+ (3) out and K+ (2) in, both against their electrochemical gradients. These activities establish membrane potential
under normal conditions, the interior of most cells is at a
negative electric potential
Oubain inhibits the pump by
preventing K+ binding
Na+-K+ pump uses about __% of total cellular ATP
30
The Na+-K+ pump helps maintain
the osmotic balance of animal cells
osmosis
movement of water from a region of low solute concentration to a region of high solute concentration
aquaporins
specialized water channels in cell membranes that facilitate flow of osmosis
osmotic pressure
driving force for water movement; equivalent to a difference in water pressure
how do animal cells maintain osmotic balance?
constantly pumping out unwanted solutes (and ions)
plant cells are prevented from swelling by
their tough cell walls
Ca2+ is kept at ___ concentration in the cytosol compared with extracellular fluid
low
Coupled Transporters
downhill movement of the first solute down its gradient provides the energy to drive the uphill transport of the second.
symport
both solutes moved in same direction
antiport
solutes moved in different directions
uniport
carries only one type of solute across the membrane (not a coupled transporter)
secondary active transport
ATP not directly coupled; coupled transporters
The Na+ gradient generated by the Na+-K+ pump is used in animal cells as
an energy source to drive transport of many other solutes by coupled transport
The uphill transport of glucose can be driven by
the downhill transport of Na+
two types of glucose transporters in epithelial cells
-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
Ion channels
-ion-selective, gated
-passive transport
ion selectivity depends on
the diameter and shape of the ion channel and on the distribution of the charged amino acids that line it
ion transport flow is controlled by
closed and open conformations of the ion channel
voltage gated channel probability of being open controlled by
the membrane potential
ligand gated channel probability of being open controlled by
the binding of some molecule (ligand) to the channel
stress-gated channel opening controlled by
a mechanical force applied to the channel (auditory hair cells in the ear)
membrane potential
electrical chemical difference across plasma membrane
resting membrane potential
the membrane potential in such steady-state conditions, so that no further difference in charge accumulates across the membrane. (-60mV)
the membrane potential is determined by
both the state of the ion channels in the membrane and the ion concentrations in the cytosol and extracellular medium
The fundamental task of a neuron
is to receive, conduct, and transmit signals
action potential
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
a signal is communicated as
change in membrane potential
A stimulus causes
a localized membrane depolarization
A localized membrane depolarization large enough to pass a critical threshold will activate
voltage-gated Na+ channels
an action potential in a neuron is typically triggered by
a sudden local depolarization of the plasma membrane; that is, a shift in the membrane potential to a less negative value (towards 0)
A stimulus that causes a sufficiently large depolarization to pass a certain threshold value promptly causes
voltage-gated Na+ channels to open temporarily at that site, allowing a small amount of Na+ to enter the cell DOWN ITS ELECTROCHEMICAL GRADIENT.
The influx of positive charge
depolarizes the membrane further (membrane potential made even less negative); causing opening of more voltage gated Na+ channels; causing even further depolarization
action potential: membrane potential shifts
from about -60mV to about +40 mV
voltage gated Na+ channel inactivated state
membrane is still depolarized, so channel is open, but becomes inactivated (more stable form) to prevent further influx of Na+ ions
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
How is the membrane re-polarized?
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
An action potential can spread long distances by
depolarizing neighboring regions of the membrane
neurons in vertebrates have
a myelin sheath
myelin sheath allows
much faster propagation of an action potential
Multiple Sclerosis
Autoimmune disorder resulting in the gradual destruction of the myelin sheath
when an action potential reaches the nerve terminals the signal is
transmitted to the target cells at synapses
for the message to be transmitted from one neuron to another...
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.
neurotransmitters are stored
in synaptic vesicles
the action potential activates _______ in the nerve terminal
VOLTAGE GATED Ca2+ channels in the nerve terminal
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
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)
The binding of neurotransmitter to its receptors causes
a change in the membrane potential of the target cell, which can trigger the cell to fire an action potential
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.
excitatory neurotransmitters
initiate an action potential (acetylcholine, glutamate (usually Na+ channels))
Inhibitory neurotransmitters
prevent action potential (GABA, glycine (usually Cl- channels; block depolarization))
GABA and glycine: ligand-gated Cl- channels
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
Tranquilizers (Valium, Halcion, temazepam)
bind to GABA Cl- channels, make them easier to open; cells become more sensitive to inhibition by GABA
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
Neurons can have thousands of synapses allowing
wide-spread transmission of information
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)
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!!!!!
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
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
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