Transmembrane Transport Flashcards
what is facilitated diffusion done via
channel or carrier proteins
what is active transprot done via
pumps
Concentration gradients move from what
high to low
What determines how well a substance can diffuse through a lipid bilayer
hydrophobicity of substance
size
steepness in concentration gradient
lipid/protein composition in the bilayer
Charge difference between the 2 compartments
what can diffuse through the membrane
small gasses (O2, CO2, N2) nonpolar molecules some small uncharged polar molecules
what does hydrophobicity have to do with rate of diffusion
more hydrophobic = faster diffusion
What cant diffuse through the bilayer
ions
large uncharged polar molecules (sugar)
Charged polar molecules (amino acids, proteins, ATP)
four main classes of ATP powered transport proteins
P-class pumps v-class pumps F-class pumps ABC
What are active transport pumps
transmembrane proteins that have ATP binding site in cytoplasmic domain
Why do we need active transport
cells want to bring in nutrients from environment even if cytoplasm has high concentration of that nutrient
How to active pumps function
to strictly control the pH and ionic composition of the cytosol
What ionic gradients exist between the cytosol and extracellular fluid
pH
Na+
K+
Ca2+
What do P class pumps transport
ions only
What is special about the P class pumps
it is the only pump to get phosphorylated
what are the subunits of P pumps
alpha catalytic subnuit- binds to ATP and transports ions
beta- regulatory unit
what gets phosphorylated in the p class pumps
the alpha sub unit
Where is Ca2+ stored
sarcoplasmic reticulum
What is the sarcoplasmic reticulum
smooth ER of muscle cells
How does Ca effect muscles
release of Ca into cytosol triggers muscle contraction
in order for muscle to relax Ca must be pumped back in
Examples of P class pumps
Sarcoplasmic reticulum Ca2+ pump
Na+/K+ pump
How do sarcoplasmic reticulum Ca2+ pumps work
- cytoplasmic portion of alpha subunit binds to two Ca ions and a molecule of ATP
- ATP is hydrolyzed into ADP +P
- this phosphorylation causes a conformational change in the pump
- Ca ions come off and go into lumen
- the phosphate is removed and the protein goes back to its original conformation
What do the Na+/K+ pumps help
keep high K+/ low Na+ in cytosol and high Na+/low K+ in the extracellular
How do Na+/K+ pumps work
- 3Na+ ions and ATP bind to the alpha subunit
- ATP hydrolyzed and P is transferred as before
- Conformational changes allow Na+ to be released outside
- 2 K+ ions bind to the other sites in the alpha subunit
- phosphate removed, protein reverted back to original conformation
- bound K+ ions are released into cytosol
END RESULT: 3 Na+ out and 2K+ in
what do V class proton pumps pump
protons only
primary function to pump protons into lysosomes, endosomes and plant vacuoles
Why is a pump needed to pump protons into a lysosome
it is very acidic in a lysosome so protons are pumped in against a concentration gradient
what are the two v class pump domains
V1 hydrophilic cytosolic domain
V0 hydrophobic transmembrane domain
How V class pumps work
ATP is bound and hydrolyzed by the V1 subunit
provides power to pump H+ through the V0 domain
Nothing is phosphorylated
why must a Cl- must be pumped when a H+ is transported into lysosome
to keep the charge balance neutral
What is the structure of ABC pumps
2 transmembrane domains (T) and 2 cytosolic domains (A) that function to bind to ATP
comes together where two T units meet also where it opens
What do ABC pumps pump
various dif substances
sugars, amino acids, toxins not just ion
where are ABC pumps found
all over bacterial and eukaryotic plasma membranes
what are the ABC pumps in bacteria called
permeases
type of mammal ABC pumps
MDR1 (multidrug resistance transport protein)
pumps various toxins and drugs from cytosol to outside
CFTR = chloride pump that when mutated leads to cystic fibrosis, Cl required to keep mucus thin and moving through lungs
what is a hydrophilic passageway
channel proteins create them and water ions and small organic molecules move across
what makes up a hydrophilic passageway
single polypeptide folded
complex dif polypeptides
what is the difference between gated and non gated channels
gated open and close in response to stimuli of changes in voltage while non gated are unaffected by these factors
What channel proteins does water diffuse through
aquaporins
Is there ever a difference in water concentration
no but there can be a difference in amount of free water molecules
what are free water molecules
not bound to anything
capable of diffusing as they are not associated to an ion or something else
high solute concentrations will have what levels of water
low
Water diffuses in what way
area of low solute to area of high solute
Hypotonic
environment containing less solute
Hypertonic
environment contain more solute
Isotonic
when solute concentration is equal in the 2 environments
What would happen if osmosis went unchecked
water would constantly rush into the cell as there is an enormous amount of solute in the cytoplasm
cause osmotic lysis
How is osmotic lysis prevented in plant fungal cells
cell walls allow for cell swelling
the swelling causes turgor pressure which prevents more water from coming in
How is osmotic lysis prevented in animal cells
they do not contain a cell wall so they constantly ship out ions into the extracellular fluid in an attempt to make fluid isotonic with respect to cytoplasm
How is osmotic lysis prevented in protozoans
contractile vacuole that is constantly collecting extra water from cytoplasm and shipping it out
What are aquaporins made of
4 identical subunits containing 6 membrane spanning alpha helices
what provides specificity in aquaporins
hydrogen binding- hydrophilic R groups extend in middle of channel and perfectly hydrogen bond with water molecules
Non-gated ion channels are
highly selective
Structure of non gated ion channels
must contain 4 subunits arranged around a central pore
each subunit contains a loop that extends into the pore
each loop perfectly takes place of water shell that forms around ion
membrane potential
difference in charge between two sides of the membrane
which side is more negative in plasma membrane of eukaryotic cells
more negative on the inside
Why is the inside more negative then the outside
cytoplasm contains high concentration of large anionic molecules that are unable to exit the cell
K+ is allowed to move down its concentration gradient and diffuse out of the cell
Why is K+ being allowed to move down its concentration gradient make it more negative within the cell
makes it more positive outside of the cell as Na+ and Ca2+ are not allowed to diffuse into the cell easily
What is normal resting membrane potential
-70 mV
What is the patch clamp technique used to measure
ion flow through channels
measure inward or outward flow of ions through a single ion channel complex
How does the patch clamp technique work
utilizes 2 electrodes
- one attached to membrane that should only contain a few ion channels
- one inserted into the cytoplasm
use frogs as they have huge cells
Patch clamp technique with Na+ ions
if measuring how many Na+ ions leave the cell, the intracellular electrode will remove an e-
measures how long these channels stay open and how many ions move through the channel in that time
What is oocyte expression
frog eggs cotnain few or no ion channels so can inject mRNA encoding ion channel into egg, thus ion channel will be made and inserted into plasma membrane
Stimuli that trigger ion channels to open/close
- voltage
- ligands- NT and hormones bind to ion channel causing them to change conformation
- activated g proteins
- light
- many others
Structure of voltage gated ion channels
4 polypeptides or 1 polypeptide with 4 separate domains arranged in cylinder with central pore
voltage sensing alpha helices that contain lots of lysine and arginines (+ charge)
When and what blocks the pore of voltage gated ion channels
after alpha helices return to their original position
channel inactivating protein
Structure of neuron
cell body - nucleus and other organelles
dendrites- receive signals from environment or from other neurons
axon- send electrical impulses to another cell
axon termini - contact these other cells
Steps of impulse generation on a neuron
- small depolarization (Na+ into cell causing a more positive)
- depolarization triggers opening of Na+ voltage gated channels
- Na+ rush into cytoplasm which causes K+ channels to open
- Na+/K+ pumps send Na+ back out and K+ back in
Ligand gated ion channels
channel that open following binding of a specific environmental chemical ligand
What is nicontinic acetylcholine receptor (NAR)
prototypical ligand gated ion channel
involved in transfer of electrical impulse from neuron to muscle cell
receptor and channel in one
How is NAR activated
secretion of acetylcholine triggered from arrival of action potential at end of axon
binds to NAR on muscle cell, triggers Na+ to flow into cell
action potential is initiated
What does a G protein do
regulates opening of some ion channels
How does a G protein work
binding of a ligand to its receptor signals for the recruitment and activation of G protein
G leaves receptor and binds to nearby plasma membrane ion channel
bind to channel and induce conformational change
channel opens and allows for the diffusion of some ion
What is special about G protein binding
it is delayed
What are transporters
integral membrane proteins that grab various ions/nutrients undergo conformational change and transport cargo across membrane
types of transporters
uniporters- bind to single substance
symporters- 2 or more in same direction
antiporters- 2 or more in opposite directions
How a cotransporter works
one moves with gradient thus causing release of energy and this energy is used to transfer second agains the gradient
example of uniporter
GLUT1 with 2 conformational states
example of cotransporters
Na+/ glucose symporter - Na+ moves into cell with gradient and the energy released is used to bring glucose in with it against its gradient
Na+/ amino acid symporter
Na+/Ca+ antiporter
pH controlling antiporter
what would increase rate of glucose update in liver cells
increase glucose in medium
transfect cells with wild type GLUT-1 gene
What does the CO2 transport in blood transport require
Cl-/ HCO3- antiporter
CO2 and O2 in systematic capillaries
High CO2
Low O2
CO2 and O2 in pulmonary capillaries
low CO2 and High O2
What forms of transport require a protein
facilitated, active, cotransport
What forms of transport has solute against gradient
active, cotransport
What forms of transport couple to ATP hydrolysis
Active
What forms of transport is driven by movement of another ion down its gradient
cotransport
Example of molecules transported in simple diffusion
O2, CO2, steroid hormones, many drugs
Example of molecules transported in facilitated
glucose and amino acids (uniporters) ion and water (channels)
Example of molecules transported in active transport
ions, small hydrophilic molecules, lipids
Example of molecules transported in cotransport
glucose, amino acids (symporters) various ions and sucrose (antiporters)
