Week 8 Textbook Flashcards
what are membrane transport proteins
transmembrane proteins - path for the movement of select substances across a cell membrane
- import/export nutrients, sugars, AAs, remove toxins
what direction does simple diffusion occur
from high conc to low con
what is facilitated transport
membranes that have specialized membrane transport proteins
smaller and hydrophobic/non polar molecules diffuse more rapidly across the bilayer
true
explain which kinds of molecules can diffuse through the membrane easily
small, nonpolar = dissolve readily in lipid bilayers and diffuse rapidly
uncharged polar molecules = uneven distribution of charge - will only diffuse if small enough (water, ethanol)
larger uncharged polar molecules = glucose, hardly cross
ions = impermeable, no matter how small, their strong electrical attraction to water molecules inhibit the entry
how do the transmembrane proteins allow hydrophilic molecules to pass thru the hydrophobic part of the bilayer
the transmembrane segments cluster together and allow the hydrophilic AA side chains to face the inside and interact with the hydrophilic molecules following in
what are the 2 main classes of membrane transport proteins
- transporters
- transfers only ions that fit into specific binding sites on the protein - need specificity (specific binding site that gives transporters their selectivity) - channels
- discriminate mainly on size and electric charge, when the channel is open, ions of good size and charge can pass thru
T/F channels are slower than transporters since they need to select which molecules are charged and of size
false
the transports are slower
- they undergo conformational charges as they open and close on either side and have a specific binding site
where can you usually find lots of Na+ and K+
Na+ outside the cell
K+ inside the cell
the charges inside and outside must be balanced
the high conc of Na+ outside the cell is balanced by Cl- charges outside the cell
the high conc of K+ inside the cell is balanced by lots of anions or other nucleic acids
what is a membrane potential
when theres an electrical imbalance of charges across the membrane = potential
what occurs when the the cell is in resting membrane potential
the voltage difference across the cell membrane
the charges are balanced - holding steady
animal cell resting potential = -20 to -200mV
why is the resting potential represented in negative mV
bc the interior of the cell is more negatively charged than the exterior
what is passive transport
when the substances flow downhill from high to low concentration
there is no other driving force
passive transport across through channel and transport membrane proteins
moves along its concentration gradient
what is active transport
moving a solute against its conc gradient
driving from low to high
- needs ATP
- has transmembrane proteins (pumps) that use the hydrolysis of ATP
what is the electrochemical gradient
the net driving force which determines the direction in which a charged solute will flow across the membrane via PASSIVE transport (channel, transporter)
one force = conc gradient
another force = membrane potential
Na+ (high outside the cell) tends to go into the cell
K+ movement is small, even when the K+ is open bc the electrochemical gradient across the plasma membrane of resting cells are small
which type of membrane transport proteins can perform both active and passive transport?
transporters
T/F there are many transport proteins for each type of selectivity
true
each cell membrane contains a characteristic set of different transporters appropriate to that particular membrane
- transporters that import nutrients, sugar, AAs, nucleotides, lysosomes, H+
T/F glucose is negatively charged
false
neutral uncharged molecule
what passive transport movement works with glucose
glucose transporter, reversible but highly selective to D-glucose
- has lots of conformations, one of the conformations exposed binding sites for glucose on the exterior of the cell
glucose = neutral - electrochemical gradient is zero
after eating a meal, lots of glucose, binds to external site on transporter, conformation changes, glucose is carried inside the cell where the conc of glucose is low, conformation switches again and is able to transport another glucose
what are the three types of transmembrane pumps
gradient driven pumps
- moves one solute across a membrane depending on the concentration (high to low)
ATP-driven pumps
- uses energy released from the hydrolysis of ATP to drive uphill transport (low to high)
light-driven pumps
- found mainly in bacterial cells which uses energy derived from sunlight to drive uphill transport
explain how the influx of Na+ through gradient driven pumps is linked with active transport pumps
the influx of Na+ going towards the concentration graident
provides the energy for active transport of many other substances into the cell against their gradient
Na+ pump has a central role in the active transport
Na+-K+ Pump Process
transporter in most animal cells that actively pumps Na+ out of the cell and K+ in the cell
using energy derived from ATP hydrolysis
- if any step is prevented from occurring the entire cycle stops
- ouabain inhibits the Na+ pump by preventing the binding of extracellular K+
since this pump is used for coupling reactions, it only operates when it needs to to waste hydrolysis of ATP
Process:
3 Na+ binds from inside the cell to be transported out
phosphorylated and the conformation changes which allows Na+ to be ejected
K+ binds to the open outside conformation, this triggers the dephosphorylation of the pump, which returns it to its original conformation - K+ is ejected
T/F Na+ pump provides the cell with a steep concentration gradient and = energy
true
Na+ outside the cell = large volume of energy
explain the movement of Ca2+ and the Ca2+ pump
less available than Na+
Ca2+ like Na+ is kept at low concentration in the cytosol compared to its concentration in the extracellular fluid
Ca2+ binds tightly to a variety of proteins in the cell
has a large concentration difference using the ATP driven Ca2+ pumps
what is the main difference in the Ca2+ pumps
they return to their original conformation without a requirement for binding and transporting a second ion
Na+ and Ca+ pumps have similar AA sequences and structures which show that they have common evolutionary origin
Explain the significance of gradient-driven pumps
A gradient of any solute across a membrane, like the electrochemical Na+ gradient generated by the Na+ pump, can be used to drive the active transport of a second molecule. The downhill movement of the first solute down its gradient provides the energy to power the uphill transport of the second solute
explain the symport/uniports/antiports gradient pumps
symport
the pump moves a pair of solutes in the same direction across the membrane
uniport
gradient driven transporters that only move a single solute across the membrane at a time
the passive diffusion of a solute down its concentration gradient - they arent pumps (passive glucose transporters)
antiport
moves 2 solutes from either sides of the membrane to the opposite sides
explain the glucose-Na+ symport pump in terms of epithelial cells
symports of Na+ are impact driving force for transporting glucose in the gut
they take up glucose from the gut lumen even when the conc of glucose is higher in the cells than in the gut
bc the electrochemical gradient for Na+ is so steep when Na+ moves into the cell down its gradient, glucose binds to Na+ and are both driven
- if one solute is missing, the other will not bind - needs both solutes to bind into the protein for transport to occur
- the importance of this is ot make sure that the coupling Na+ doesn’t leak into the cell without doing useful work
what is the occluded occupied state of the glucose-Na+ symport
when the protein is closed from both sides with BOTH solutes inside
it can also be in this state when BOTH solutes are not in the spots (empty)
what would happen if the epithelial cells only had glucose-Na+ symport
they would always take up glucose and never release it for the other cells to use
- the epithelial cells have 2 symports on either ends of the cell
in the basal and lateral domains they have glucose uniports which passively release glucose down its conc gradient for other tissues to use
in an epithelial cell, how are the two glucose transported segregated?
tight junctions around the apex of the cell
= barrier preventing mixing of membrane components
what is an example of an antiport used in the plasma membrane of many animal cells
Na+–H+ exchanger (antiport)
uses the downhill influx of Na+ to pump H+ out of the cell
- used to control pH in the cytosol to prevent the cell interior to become too acidic
what do plants, fungi and bacteria use if they dont have Na+ gradients
rely on H+ to import solutes into the cell
H+ pumps (creates the acid pH in the medium)
the import of other solutes are mediated by the H+ symports (electrochemical gradient in the same way Na+ is used in animals)
- uses energu of ATP hydrolysis to pump H+ out of the cell
- in lysosomes/vacuoles of some plant, animal. fungal cells they have different H+ pumps in the membrane - which actively transport H+ out of the cytosol into the organelle
what are bacteriorhodosin pumps
located in the plasma membrane of some bacteria
active export of h+
light as energy source
explain channel proteins
transmembrane pores that allow the passive movement of small water soluble molecules
porins - that form pores in the outer membrane of mitochondria and some bacteria
what are ion channels
- they are not always open, needed specific stimulus to trigger open
- doesn’t need to undergo conformational changes for an ion to pass thru when open
- they show ion selectivity, allowing ion selectivity - depends on the size and shape of the ion channel and the distribution of the charged AA that line it
- since every ion is surrounded by water molecules, the channels have selectivity filter which allows the narrowest part of the ion channel to force ions into contact with the interior of the channel wall - removing the water surrounding - since it select for the ion, it then moves down and then re binds with lots of water molecules on the other side of the membrane
Explain K+ leak channels
allow K+ to move freely across the membrane
when K+ moves out of the cell its down its conc gradient
the loss of positive inside the cell creates a voltage difference (membrane potential)
no further movement of K+, in equilibrium conditions K+ is kept inside the cell which is strong enough to counteract the tendency of K+ to move down its concentration gradient L+ = zero
____ where the flow of positive and negative ions are precisely balanced so no further differences in charge accumulate across the membranr
resting membrane potential
what is the nernst equation
expresses the equilibrium quantitatively and makes it possible to calculate the theoretical resting membrane potential
Voltage = 62 Log (Co/Ci)
what are the two components that drive an ion across a membrane
- due to the electrial membrane potential
- due to the concentration of the ion gradient
what is a mechanically gated channel
an ion channel that allows the passage of select ions across a membrane in response to a physical perturbation
- opened by a physical force
what is a ligand-gated channel
the opening of the protein is based on the binding of a molecule which is either intracellular or extracellular ligand
what is a voltage gated channel
it is the opening of the membrane protein which is controlled by the membrane potential
how can a cell restrict movement of its membrane proteins
cells can create barriers that restrict particular membrane components to one membrane domain (functionally and structurally specialized region of the membrane which is in the presence of specific proteins)
explain tight junctions
junctions in certain areas of the cell that allow junctional proteins to form a continuous belt around the cell where the cell contacts its neighbors - this creates a seal between adjacent plasma membranes
- membrane proteins are unable to diffuse past the junction
- thus restricting them to particular domains
