Chapter 12 Flashcards
protein-free artificial lipid bilayer (liposome)
impermeable to most water-soluble molecules
cell membranes
contain transport proteins, each of which transports a specific type of molecule
EX: selective transport includes active pumping of specific molecules into or out of the cell
combined action of transport proteins can…
allow solutes to build up inside a membrane enclosed compartment (ex: cytosol or and organelle)
interior of the lipid bilayer is…
hydrophobic
creates a barrier that hydrophilic molecules can’t pass
why doesn’t simple diffusion work for hydrophilic molecules?
it is way too slow!
how do we accelerate passage of hydrophilic molecules through lipid bilayers?
facilitated transport
can all molecules diffuse across a lipid bilayer?
YES, but some rates are super slow
what type of molecules diffuse more quickly across the membrane?
smaller, more hydrophobic (aka more nonpolar) molecules
fewer favorable interactions with water (less polar) = diffuses more quickly
solutes
substances dissolved in water that are polar
need transporters to cross cell membranes
lipid bilayer permeability
small nonpolar molecules
O2, CO2
dissolve readily, rapidly diffuse
important for cellular respiration
lipid bilayer permeability
uncharged polar molecules
molecules w/ uneven distribution of electric charge
diffuse readily if small enough
H20, ethanol cross quickly
glycerol crosses very slowly
glucose is large and hardly crosses
lipid bilayer permeability
charged molecules
highly impermeable!
includes inorganic ions
why?: strong electrical attraction to water inhibits entry into inner hydrocarbon phase of the bilayer
most plentiful ion outside the cell?
Na+
most plentiful ion inside the cell?
K+
high conc Na+ outside cell is balanced by?
Cl- inside cell
high conc K+ inside cell is balanced by?
negatively charged ions including nucleic acids, proteins, and cell metabolites
membrane potential
electrical imbalances in plasma membrane
generate a voltage difference across the membrane
resting membrane potential
cell is unstimulated, exchange of cations and anions across membrane is balanced
resting potential is negative bc inside of cell is more neg than outside
most membrane transport proteins have…
polypeptide chains that cross lipid bilayer many times (multipass transmembrane proteins)
how do we use multipass transmembrane proteins?
to allow small, hydrophilic molecules to cross the membrane without contacting hydrophobic interior of lipid bilayer
how do transporter and channels differ?
in the way they discriminate between solutes
channels discriminate on
size and electric charge
open channel = any ion or molecule small enough and with the right charge can pass through
transporter
transfers only molecules or ions that fit into specific binding sites on the protein
bind solutes with great specificity (just like enzymes binding substrates)
transporter mechanism
undergoes conformational changes to transfer small solutes across lipid bilayer
channel mechanism
when open, forms a pore across the bilayer through which specific inorganic ions or polar organic molecs can diffuse
which is faster channel or transporter?
channels!
passive transport
no energy required
solute moves down concentration gradient spontaneously
this occurs to keep equilibrium
all channels and many transporters facilitate passive transport
active transport
drives flow against conc gradient by coupling it to a process to provides input of energy
uses pumps to harness an energy source
energy sources for active transport
ATP hydrolysis, transmembrane ion gradient, sunlight
what influences passive transport of charged solutes
BOTH conc gradient AND membrane potential
what influences passive transport of uncharged solutes
conc gradient ONLY
cytosolic side of membrane charge?
negative
extracellular membrane charge
positive
positively charged solutes are typically brought
into the cell since the interior is more negative
electrochemical gradient
net force of conc gradient and membrane potential
determines direction of solutes for passive transport
steep electrochemical gradient
when voltage and conc gradients run in same direction
EX: Na+
small electrochemical gradient
when voltage and conc gradient have opp effects
EX: K+
little movement even when channels are open
osmosis
passive movement of water down its conc gradient
which direction does water typically move?
into the cell
bc solute is usually more concentrated outside the cell
aquaporins
specialized channel proteins that speed up flow of water
hourglass shaped
what happens if osmosis occurs without constraint?
the cell swells
how do animal cells resist osmotic swelling?
gel-like cytoplasm
transporter
move small, water-soluble, organic molecules (and some inorganic ions) across cell membranes
highly selective
glucose charge
uncharged
chemical part of electrochemical gradient is 0
ONLY conc gradient drives glucose
glucose transporter
in plasma membrane
switches conformation to expose glucose binding sites to exterior, interior, whatever is needed
highly selective
transmembrane pumps
actively transport a solute against its electrochemical gradient
why do we need active transport?
1) maintain appropriate intracellular ionic composition of cells
2) import solutes that are at a lower concentration outside the cell than inside
3 active transport methods
1) ATP-driven pumps
2) coupled pumps
3) light-driven pumps
ATP-driven pumps
hydrolyze ATP to drive uphill transport
coupled pumps
link uphill transport of one solute across a membrane to the downhill transport of another
light-driven pumps
mainly in bacteria
use energy from sun to drive uphill transport
symport
pump that moves both solutes in same direction across the membrane
antiport
pump that moves both solutes in opposite direction across the membrane
uniport
A transporter that ferries only one type of solute across the membrane (and is therefore not a coupled
transporter)
electrochemical Na+ gradient drives
coupled pumps in plasma membrane
Na+ flows in
drives other solutes into animal cells
EX: epithelial cells in the gut
why do we need the glucose-Na+ symport?
if we only had the passive glucose uniport, they would release glucose into the gut after fasting AS freely AS they take it up from the guy after a feast
NO regulation!
why do we need glucose-Na+ symport
so we can take up glucose from gut lumen even when concentration of glucose is higher in cell’s cytosol than in its gut lumen
how does glucose-Na+ symport work?
electrochemical gradient for Na+ is steep, so when Na_ moves into the cell (down its gradient), glucose is dragged into the cell
binding of Na+ and glucose is…
cooperative: binding of one enhances binding of the other
if one solute is missing, the other fails to find
both Na+ and glucose must be present for coupled transport to occur
What would happen if gut epithelial cells only had glucose-Na+ symport?
they could never release glucose for use by other cells of the body
SO, we need two types of symporters
2 types of glucose transporters
at opposite ends of cell
1) in apical domain of plasma membrane (faces gut lumen)
2) In basal and lateral domains of plasma membrane
allow gut epithelial cells to transfer glucose across epithelial lining of gut
glucose transporters in apical domain
face gut lumen
take up glucose actively, create high glucose conc in cytosol
basal and lateral glucose transporters
passive glucose uniports
release glucose down its conc gradient for use by other tissues
how are 2 types of glucose transporters kept in place
segregated in their proper domains of plasma membrane by a diffusion barrier formed by tight junction around apex of the cell
why do we need to keep glucose transporters apart
to prevent mixing of membrane components between the two domains
