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+
