Passive Transport Flashcards
Define aquaporins
integral membrane proteins that contain channels that only allow the passage of water molecules through them
T or F: aquaporins let water and ions through their channels
FALSE
Aquaporins are specific to water
What kind of cells would have many aquaporins?
Ones that have to secrete or move a lot of fluid
such as
cells lining the ducts of exocrine glands (ex. sweat glands)
kidney cells
Describe the structure of aquaporins
The channel is made of alpha helices and has a narrow pore that is lined with POLAR amino acids allowing water to travel through SINGLE FILE
How does water move through an aquaporin?
the narrow pore in a channel is lined with polar amino acids
the water molecules form temporary hydrogen bonds with the polar amino acids to help move them along
What can happen as a chain of water molecules moves through an aquaporin?
Proton hopping
A free H+ at the bottom of the chain can bind to an H2O to form H3O+
An H+ from H3O+ will leave that H3O+ and bind with a different H2O further along the chain to form another H3O+
The proton will continue to hop through the H2Os in a transmembrane protein until it reaches the other side of the membrane
Why is proton hopping an issue?
It disrupts the chemical gradient
How is proton hopping prevented?
Two key polar residues hydrogen bond with a water molecule at the centre of the water chain, preventing it from being able to bind with the H+ and make H3O+
this stops the forward movement of the H+ proton and only H2O molecules will move to the other side of the membrane
What is an ion?
A charged molecule
aka an electrolyte
What two gradients does the movement of ions depend on?
the chemical gradient (concentration)
the electrical gradient (charge)
How do we refer to the differences in ion concentrations?
as an electrochemical gradient
it includes both chemical/concentration and electrical gradients
Describe membrane potential
a difference in charge created by the maintenance of electrical gradients across a membrane
How can the difference in charge needed for membrane potential arise?
from both passive and active movement of ions
What is a major contributor to the difference in charge that creates membrane potential?
the sodium-potassium pump that constantly moves 3 Na+ out of the cell for every 2 K+ into the cell
creating a net negative charge across the membrane (more positive outside than inside)
AND the facilitated diffusion of K+
What is the net charge across the membrane? why?
negative!!
because the Na+/K+ pump moves 3 Na+ out for every 2 K+ it moves in –> more positive outside the cell than inside
Describe the resting membrane potential?
When there is no net flux of ions
What is the resting membrane potential? Though how can it vary?
Usually around -70 mV
But can vary between -20 mV and -120 mV depending on cell type and organism
What is the driving force for ion movement?
electrochemical gradient
What conditions will allow ion movement inside the cell the best?
when the electrochemical gradient is aligned with a negative internal membrane potential
outside will be high concentration of +
inside will be low concentration of -
so the electrochemical gradient will pull the + from outside to the inside
What conditions will allow ion movement inside the cell the best?
when the electrochemical gradient is aligned with a negative internal membrane potential (positive outside, negative inside)
outside will be high concentration of +
inside will be low concentration of +
so the electrochemical gradient will pull the + from outside to the inside because of the - inside
T or F: the electrochemical gradient will not move ions into the cell without a membrane potential
FALSE
EG will still move ions without potential
How will the electrochemical gradient work if the membrane potential is positive inside?
if membrane is negative outside
positive inside
and there’s high concentration of + ions outside
low concentration of + ions inside
ions will move against their electrochemical gradient toward the + inside
T or F: transport proteins move solutes (such as ions) at a different rate than simple diffusion
true
Why is the speed of facilitated diffusion initially high and then plateaus at a maximum speed?
as the concentration of solute increases, facilitated diffusion will increase rapidly and plateau because the protein channels fill up/become saturated so adding more solute will not make a difference
T or F: as concentration increases, rate of simple diffusion will increase
true because it is only based on the concentration gradient
Describe depolarization
Positive membrane potential deviation from the resting membrane potential
Describe hyperpolarization
Negative membrane potential deviation from the resting membrane potential
Why do charged ions require facilitation by ion channels to cross the cell membrane?
because the core of the cell membrane is hydrophobic
Describe ion channels
transmembrane (integral) proteins that contain a polar aqueous pore
How do ions move through ion channels? Is this movement passive or active?
WITH their electrochemical gradient
PASSIVE
Rapidly
What do ions usually have to do in order to pass through the selectivity filter of the pore in the ion channel?
Dissociate from any water molecules
Describe a selectivity filter
a tight point in the pore of an ion channel that is VERY selective to which solutes can enter and how
it relates to the thickness or narrowness of channel
T or F: ions can move across membranes without facilitation
FALSE, they require facilitation because ions are charged solutes and the membrane has a hydrophobic core
How many ion types do selectivity filters of ion channels usually allow through?
one
What could happen if a single amino acid change occurred in the pore of an ion channel?
loss of ion selectivity
potentially cell death
What are the 2 types of ion channels?
- leak channels
2. gate channels
Describe leak channels
ion channels in which ions can move freely along their gradients at a specific rate
these are always open
Describe gate channels
ion channels that are closed to ion movement until a conformational charge opens the pore
What are the 4 types of gate channels?
- voltage gated
- pressure/mechanically gated
- ligand gated
- pH gated
What function do leak channels serve for the membrane?
Long with Na+/K+ pump, they help establish and maintain the resting membrane potential
How do leak channels help establish resting membrane potential?
as Na+/K+ pump actively moves 2 K+ into the cell and increases the K+ concentration inside
leak channel allows K+ to diffuse passively out of cell (from high to low K+)
each K+ that leaves, leaves an unbalanced - charge inside the cell = helps create the RMP
the negative RMP prevents further efflux of K+
How does creating a negative RMP prevent further efflux of K+ from a cell?
the influx of 2 K+ ions by the Na+/K+ pump establishes a high concentration of K+ inside the cell
the presence of leak channels for K+ will allow K+ to diffuse out because of the high concentration
but keeping the inside negative, as K+ ions diffuse out, will prevent too many K+ ions from leaving because attractive forces will keep the + ions inside the - environment
When will the net efflux of K+ by leak channels stop? why?
when the membrane potential reaches -70 mV (RMP)
the electrical force that drives K+ back into the cell is balanced by the chemical force moving K+ out of the cell and the electrochemical gradient = 0
Describe the basic structure of ion channels
a bundle of at least 4 transmembrane amphipathic alpha helices
each helix:
outer hydrophobic surface to be in the core of the membrane
inner hydrophilic surface to move charged solutes (ions)
Give an example of a leak ion channel
Bacterial K+ channel
tetramer of 4 identical subunits
each subunit has 2 transmembrane helices
each has a short ‘pore helix’ in outer leaflet
central transmembrane helices are kinked to form a cone with narrow end facing inside
What is the selectivity filter of an ion channel lined with?
carbonyl groups (C=O)
How does the selectivity filter select for K+ but not Na+ even though they are very similar in size?
the filter strips away any water from the K+ and
the channel is just wide enough for K+ to be able to properly bind to the oxygen in the carbonyl group
Na+ would not be able to orientate itself in a way to bind properly with the O in the carbonyl
Describe voltage gating ion channels
Ion channels that are gated/closed at the pore and will open or close based on the voltage
What causes a voltage gated ion channel to open or close?
a change in the membrane potential
Deviation from the resting membrane potential
What do multicellular organisms use voltage gated ion channels for?
To create nerve impulses
Regulate secretion
Regulate muscle contraction
Rapidly change cell volume
How many different domains do voltage gated ion channels have?
at least 2
Define domains
Regions of a tertiary protein that have distinct functions and usually are separated by a segment of unstructured protein
Define subunit (just to remind yourself)
The polypeptides (other protein structures) that make up a quaternary protein
What are the two domains in a voltage gated ion channel?
Pore domain
Voltage sensor domain
Describe the pore domain of a voltage gated ion channel
Allows ions to move through in the direction of their electrochemical gradient
Describe the voltage sensor domain of a voltage gated ion channel
The domain that reacts to the voltage differences in the membrane and changes conformation
How many subunits do most voltage gated ion channels have?
4
Each with 6 transmembrane alpha helices
Describe the structure of voltage gated ion channels
4 subunits
each subunit has 6 alpha helices
What do helices S1-S4 of a voltage gated ion channel subunit do?
contribute to the voltage sensing domain
What do helices S5-S6 of a voltage gated ion channel subunit do?
Contribute to the pore domain
What do all the helices of the subunits of a voltage gated ion channel contribute to?
the selectivity filter
What does each subunit of a voltage gated ion channel provide to the middle of the channel? What does this form?
Oxygen groups
forms a 3D ring
Which helices in each subunit of a voltage gated ion channel line the pore? and what is their function?
Helices S6 line the pore
their conformation determines whether the channel is open or closed
gate is formed by the inside cytoplasmic ends of the helices
Explain how the gate opens in voltage gate ion channels
S4 helix has positively charged side chains within the helix
membrane potential depolarizes (gets more +) on the inside
repulsion of positive charge
causes S4 helices to move outwards
indirectly causes movement in S6 helices that form the gate
Explain how the gate closes in voltage gate ion channels
Hyperpolarization of the membrane potential (more - inside?) triggers
attraction of positive charge
causes S4 helix to move inwards
indirectly causes S6 helices to close gate
What is the inactivation region? Where is it located?
One cytoplasmic domain in a voltage gated ion channel usually has this
globular piece of the protein automatically moves to block the pore after it has been open for a period fo time
T or F: inactivation will occur even if the membrane is depolarized
True, it just depends on how much time the pore has been open
What triggers the opening of a voltage gated ion channel?
depolarization
causes the membrane potential to become more positive and the positive charges repulse to push the S4 helices outwards, pulling the S6 gate helices with them
What triggers the closing of a voltage gated ion channel?
Hyperpolarization
causes the membrane potential to become more negative and attracts positive charges, pulling the S4 helices inwards and pulling the S6 gate helices with them to close the gate
What is a ligand?
A general term for a small molecule that binds to a protein
What does ligand binding or dissociating cause?
conformational changes in proteins that can open or close a channel
Describe ligand-gated ion channels
Ion channels open/close when ligands bind or dissociate causing the conformational changes of the transport proteins that open or close the channel
What is an example of ligand bonding?
Acetylcholine-gated sodium channel
Describe the acetylcholine-gated sodium channel
Location: plasma membrane of skeletal muscle cells and postsynaptic membranes of neurons
Binding of acetylcholine triggers the opening of the channel and allows sodium to flow into cell
Describe the acetylcholine-gated sodium channel
has 5 subunits
2 Ach binding sites
resting conformation is closed
Gate is near middle of lipid bilayer - here hydrophobic leucine R groups project into pore to block the channel
How is the acetylcholine-gated sodium channel gated?
Near the middle of the bilayer, hydrophobic leucine groups project into the pore to block the channel
Describe the process of opening the acetylcholine-gated sodium channel
ACh binds to both of the two binding sites = conformation change
moves alpha helices with the leucines out of the way
How come when the acetylcholine-gated sodium channel is open only positive ions can get through?
there are negatively charged side chains at either end of the pore
so negatively charged ions will not be able to enter due to repulsion
How do we study ion channels?
By evaluating the movement of an ion (a current) through a single channel as it opens and closes
Describe a technique for studying ion channels
Patch clamping
Micropipette-electrodes made of polished glass are placed on the outer cell surface and sealed to the membrane by suction and the membrane is pulled away to get only a patch
What is the ideal result of patch clamping?
to get a small number of channels in the membrane patch isolated
What is the benefit of patch clamping?
you can control what is in the solution in the micropipette (EC side of cell) and the surrounding solution (Cytoplasmic side of cell)
ex. you can add Ach or Na+ ligand to surrounding solution
How does studying voltage gated channels differ from ligand gated channels?
Both use patch clamps
but for voltage, we can ‘clamp’ (maintain) the voltage across a membrane at a consistent voltage where the channel is closed
then the adjust voltage so channel opens and monitor current that way
What does it mean if the graph from a voltage patch clamp shows current levels?
ions are moving across the isolated membrane patch
Define transporters
integral membrane proteins that allow for the diffusion of larger, polar solutes (ex. amino acids/sugars)
What kind of molecules can transporters move? Give examples
large, polar
ex. amino acids, sugars
What are the main differences between transporters and ion channels?
Transporters:
- move large, polar solutes
- slower
- quickly become saturated
Channels:
- small molecules (water, ions)
- faster
Describe the basic mechanism of transporters
Ligand binds to protein transporter on side of membrane (very high affinity = very tight bond)
conformational change triggered in transporter protein
new conformation brings ligand to other side of membrane where binding affinity for ligand is low
ligand dissociates
What causes the conformational change in transporter proteins required to move solutes?
ligand binding to the transporter protein on one side of the membrane
Which direction can transporters move solutes?
always down the concentration gradient = passive
What determines the direction of transport in transporter proteins?
the ligand concentration gradient
Give an example of a transporter
the glucose transporter
Describe the structure of the glucose transporter
One peptide chain with 12 transmembrane alpha helices
Briefly describe how the glucose transporter works
glucose binds on one side –>
changes conformation of transporter protein –>
glucose is released on other side of membrane –>
glucose is phosphorylated and cannot leave cell
What happens to glucose as soon as it enters the cytoplasm?
it is phosphorylated
Why is glucose phosphorylated as soon as it enters the cytoplasm?
to keep glucose concentration in the cell low to maintain the facilitated diffusion of glucose into the cell
Why can’t glucose that has entered the cell leave the cell through the glucose transporter?
because it has been phosphorylated and is no longer just glucose
Why is the glucose transporter and the phosphorylation of glucose necessary?
the phosphorylation of glucose keeps the concentration of glucose in the cell low
which ensures the constant diffusion of glucose into the cell
Cells need a constant supply of glucose to function
Define isoforms
structurally similar proteins with the same function that are produced by slightly different genes or different mRNA’s from the same gene
How many isoforms does the glucose transporter have for facilitated diffusion? What are they?
5
GLUT1-GLUT5
Why are there 5 isoforms of the glucose transporter?
They are all slightly different because they are found in different tissues of the body
they need to have different levels of activity and different mechanisms of activation depending on where they are in the body
Give an example of a glucose transporter in the human body
when blood glucose levels are high, insulin is released by the pancreas into the blood
Describe the mechanism of the blood-glucose transporter
high blood-glucose levels in body triggers release of insulin by pancreas
insulin binds to the insulin receptor on a cell’s membrane
GLUT4 transporters are embedded in the cytoplasmic vesicles of most cells waiting for the signal
when insulin binds to receptor, a signal cascade tells vesicles to move to surface and fuse with plasma membrane
GLUT4 transporters bind glucose and move it into cell to lower blood-glucose levels
