Transport Across Cell Membranes Flashcards
Is the following statement true: “Some molecules (e.g. sugars, ions) will never diffuse over the lipid bilayer”
No. All molecules will eventually get across, but for some this may take way too long
- In general smaller and more hydrophobic or non-polar molecules will pass more rapidly
Name molecules that pass through quickly and molecules that not so much.
Pass:
- Small uncharged molecules -> dissolve readily in lipid layers e.g. CO2, O2
- Uncharged polar molecules (uneven electric charge)
- not us much but still can -> smaller molecules (e.g. H2O) more easily than bigger (e.g. glycerol)
Fail:
- Charged molecules
- their charge and electrical attraction to water inhibits their movement e.g. K+, Na+
What are the most important inorganic ions? How is it with their inner x outer concentration and balance?
Ions: Na+, K+, Ca2+, Cl-, H+
- Most abundant outside = Na+, most abundant inside = K+
- To prevent cell being torn apart by the strong electrical charges there has to balance on both sides
- For Na+ it is Cl-
- For K+ it is variety of negatively charged molecules e.g. proteins, nucleic acids
How do we call the state at which cell is unstimulated? What is happening with ions - what if the flow changes?
Resting membrane potential - in animal cells anywhere between -20 to -200mV (inside more negative)
- the exchange of cations and anions is exactly balanced -> if there is a disruption a small membrane potential is generated and may propagate further
How do the transmembrane proteins for transport proteins look like?
Their polypeptide chains transverse the lipid bilayer multiple times (multipass) -> crossing back and forth allows them to form pathways/pores which are selective to small hydrophilic molecules while having hydrophobic exterior
What two main classes do we differentiate with membrane transport proteins?
Differ in terms of how they discriminate molecules
1) Transporters = transmit only molecules that can bind to their binding site
2) Channels = discriminate based on size and charge -> if it is small enough and has a specific charge it will pass through (selective since ions differ in their sizes greatly)
What are 2 mechanisms by which molecules are moved through the transporter proteins?
1) Passive transport = molecules move with their concentration gradient (even if they move in both directions across membrane - one direction is leading), no energy is needed
- for charged molecules also includes electrical gradient => electrochemical gradient
2) Active transport = molecules move against its concentration gradient
- specialized proteins = pumps
- requires energy which could be taken from e.g. ATP hydrolysis, sunlight
How do K+ and Na+ differ in terms of their electrochemical gradients?
Na+ is driven inside by both concentration and charge => rapid influx if given chance
K+ is driven outside by concentration, but inside by charge => slower eflux
How does water pass through?
- Water molecules are small and uncharged -> can go directly via lipid bilayer although ut takes some time
- specialized channel proteins may help = aquaporins
- Moves based on osmosis (on the concentration of solutes) -> generally there tends to be more solute (polar and charged molecules) present inside the cell => water will move inside
Sometimes osmotic pressure can cause swelling of cells -> how do cells deal with it?
They could have gel-like cytoplasm (animal), cell wall (plants) - even use it to keep themselves rigit (turgor pressure), contractile vacuole (protozoan)
What kind of transporters can you find e.g. on membrane, mitochondria?
Membrane - transporters for nutrients e.g. amino acids, sugars, nucleotides
Mitochondria - importing pyruvate to be used to generate ATP, exporting ATP
Give an important example of a passive transporter -> how does it work?
Glucose transporter
- polypeptide chain crossing through the membrane multiple times - may create multiple different conformations
- since glucose = uncharged, all that drives the transport is its concentration gradient
- NOTE: very selective, binds only D-glucose and NOT L-glucose
- High glucose content outside the cell e.g. after eating
-> glucose binds to external binding sites of the transporter (one variation of conformation)
-> spontaneously switches -> brings glucose to cytosol - Low glucose level outside e.g. when hungry
-> liver cells want to compensate -> start breaking down glucogen into glucose
-> binds to internal binding sites -> switch -> release to the outside
What are the 3 ways of active transport?
1) ATP-driven pumps = hydrolyze ATP to drive the opposing transport
2) Coupled pumps = link transport of one solute in certain direction to a transport of another solute in the opposite direction
3) Light-driven pumps = using energy derived from the sunlight
NOTE: In a cell tend to be coupled together e.g. ATP-driven Na+ pump exports Na+ out against its electrochemical gradient while Coupled Na+ pump can put it back in while providing energy for transport of other substances
What is likely the most important pump in our body? Elaborate.
Na+/K+ ATPase
- uses energy from ATP via hydrolysis (removal of the phosphate group)
- Sends Na+ out, brings K+ in
- delicate, fast process of multiple steps -> may one step go wrong the whole process is stopped e.g. toxin ouabain can inhibit the binding of extracellular K+
- If ions are unavailable it stops to not waste ATP
- Na+ re-enters the cell passively due to its strong electrochemical gradient
What other pump functions similarly as Na+ pumps? In what way yes, in what not?
Ca+ pumps
- ATP-driven mechanism that exports Ca+ outside the cells to keep the inner concentration at low (the lower the background concentration the more sensitive to Ca+ level changes the cell is)
- Ca+ used as an intracellular signaling agent -> can bind to variaty of proteins -> change metabolic/chemical activity of the cell
- BUT returns to normal conformation without the need to bind another ion
What kinds of coupled pumps do we have?
Coupled pump = active transporters that utilize movement of 2 molecules (organic or inorganic) to drive itself
- Symports = moves both solutes in the same direction
- Antiports = moves solutes in opposing directions
NOTE: if only one molecule is moved (e.g. glucose) = uniport
How does symport benefit epithelial cells in lining of the gut? Are there other transporters present?
Epithelial cells make use of glucose-Na+ symport i.e. transport of Na+ which “grabs” glucose with it
- molecules are cooperative -> the movement occurs only if both are binded
- helps with the need to extract glucose even if the concentration is lower outside
- at the apical domain (facing the lumen of the gut) -> BUT wouldn’t be able to release glucose
At the basal site, the cells have uniform passive glucose transport (based on concentration) -> this can release the glucose from the epithelial cells to the neighboring tissues
What about an example of antiport?
E.g. Na+-H+ exchanger -> pumps Na+ in, releases H+
- used to regulate the pH level in the cell, preventing cytosol from becoming too acidic
Plants, fungi, and bacteria don’t have Na+ pumps - so what do they use? What mechanism? Where else can we find it?
They make use of H+ pumps - functions as symport of H+ and e.g. sugars, amino acinds into the cell
- H+ also has pretty high gradient (driving force)
- in some bacteria it is light-driven (e.g. bacteriarhodopsin), while in other ATP-driven
- could be also found in membranes of organelles (e.g. lysosomes, vacuole) -> actively transports H+ into the organelle keeping the cytosol more neutral while the organelle insode more acidic
How could we transport small water-soluable molecules across membrane? What characterizes them + example.
The best way would to create a hydrophillic pore within the hydrophobic membrane environment => Channels
- Function on the basis of passive transport BUT are highly selective
- e.g. aquaporins will let through only uncharged water molecules (not even the smallest ions could cross)
What differentiates ion channels from a hole in a membrane?
Contrary to the hole in the membrane:
1) Ion channels are highly selective -> they allow only a specific ion based on its size, shape and charged amino acids of the channel
- each ion is surrounded by a small shell of water molecules -> dehydratation when entering the channel -> ion is forced into contact with the channel walls -> only those who fit will cross
2) Ion channels open only after being triggered
- they are “gated” = specific stimulus determines whether the channel is open or closed
What differentiates ion channels from a transporter?
Channels don’t have to undergo conformation changes for each ion -> allows them to pass through milions of molecules
BUT they cannot use this “crossing” as a source of energy
What is happening with a resting cell (in terms of ions, channels?) How do we call it and can calculate it?
- Mostly permeable to K+
- Na+/K+ ATPase pump is importing K+ inside the cell (to restore its normal position)
- K+ leak channels randomly open and close to let K+ flow down its electrochemical gradient
-> if open -> K+ will have a tendency to rush out (higher concentration of K+ is inside BUT inside is more negatively charged - not as strong as for Na+) -> if K+ would keep on passing at one point it would reach an equilibrium = state at which the driving forces are exactly equal and K+ net flux = 0
=> resting membrane potential, may range between -20 to -200 mV (can be calculated by Nernst equation)
What technique could be used to measure electrical changes in a membrane and its effect on channels? What for instance was demonstrated in this way?
Patch-clamp technique - glass tube (microelectrode) is applied to a membrane -> small suction -> isolate a channel -> put it under varying conditions e.g. changing concentration of ions, adding a specific ligand
It was shown that channels open and close randomly sometimes (likely the thermal movement pushes them into one or the other conformation) -> if we apply a stimulus the random behavior becomes biased e.g. spending more time in the open conformation
