Topic 3B - Transport Across Membranes Flashcards
How do small hydrophobic molecules cross the membrane?
Diffusion
How do inorganic ions and small organic polar molecules cross the membrane?
Transporters and channels (may have a small amount of diffusion)
How do macromolecules cross the membrane?
Endocytosis and exocytosis
What two factors determine rates of diffusion?
Charge and size
Diffusion across a membrane absolutely cannot happen when a molecule is…
Charged
Define passive transport
Movement of particles in accordance with their concentration gradient
Define active transport
Movement of particles against their concentration gradient
Does diffusion have a maximum rate?
No
Does transporter-mediated diffusion have a maximum?
Yes - transporters can become saturated
What are the 3 kinds of active transport?
- Coupled transporters
- ATP-driven pumps
- Light-driven pumps
Define a symport
Both molecules being transported by a given transporter are moving the same way
Define an antiport
Two molecules are being moved by a transporter in opposite directions
In the lumen of the gut, glucose is packed into the cell by…
Coupled active transport of Na+ into the cell by a symport
What are the 3 kinds of ATP pumps?
- P-type
- ABC transporter
- V-type and F-type pumps
What is the difference between V-type and F-type transporters?
F-type synthesizes ATP and V-type uses ATP
Both are proton pumps, but they pump protons in opposite directions
What do ABC transporters move?
Small molecules like sugars, peptides, and ions
What do P-type pumps move?
Ions
What do V and F-type pumps move?
Protons
What are the 3 extracellular domains of P-type ATPases?
- Nucleotide binding domain
- Phosphorylation domain
- Activator domain
Give an example of a P-type ATPase
the Ca++ pump
In ATP driven pathways, at what point is ADP phosphorylated into ATP?
Never, always switched out
The Na+-K+ pump is a kind of (1)-type pump
P-type
How much of a cell’s energy is devoted to the Na/K+ pump?
1/3rd
Sodium inside the cell is…
Low
Potassium in the cell is in (high/low) concentration
High
What does ABC (of ABC transporters) stand for?
ATP-binding cassette
What is a key difference between eukaryotic and prokaryotic ABC transporters?
Pro: important for import and export
Euk: mostly important for export
How many kinds of ABC transporters are there in mammals?
50-70
What % of the E. coli genome codes for ABC transporters?
5%
Why are ABC transporters implicated in cancer? (2)
- ABC remove drugs from the cancer cell so they are less responsive to treatment
- Cancer cells have a larger number of ABC transporters
Most ABC transporters are (1)-directional
Unidirectional
The largest family of membrane transporter proteins are…
ABC transporters
What are 4 key characteristics of ion channels?
- High selectivity
- Gated pores
- High throughput rates
- Always use passive transport
Is there such a thing as an active channel?
No! All channels use passive transport
What are the 4 kinds of gating used by ion channels?
- Voltage-gated
- Ligand-gated (extracellular ligand)
- Ligand-gated (intracellular ligand)
- Mechanically gated
In addition to voltage gating and ligand gating, this process can affect opening and closing of ion channels
Phosphorylation and dephosphorylation
K+ diffuses via its channel according to its chemical gradient until…
Resting membrane potential is reached
What 2 proteins are especially important in the establishment of the resting membrane potential?
K+ leak channels and the Na/K+ pump
Why is the K+ channel impermeable for Na+? (2)
- Dehydrated Na+ cannot interact favourably with the carbonyl oxygens of the K+ selectivity filter - ENERGY BARRIER
- Na+ is too big to fit through the channel in a hydrated state
A key residue in the selectivity for Na+ ions in the Na channel is…
E177
Why is the Na+ channel wider than the K+ channel?
Na+ is transported with its water shell (partially hydrated), but K+ is too big in a partially hydrated state to pass through
What is transported by aquaporins?
Water
What is not transported by aquaporins?
Ions
How many subunits make a functional aquaporin? (N = …)
N = 4
How do aquaporins keep ions from passing through the channel?
1 side of the channel is hydrophobic. Since the channel is too narrow for hydrated ions to pass, and the energy cost to dehydrate these ions due to the hydrophobic layer is too large, ions do not cross the channel
How do aquaporins keep protons from crossing the channel?
Disrupt the channel in the middle with two asparagine molecules which protons cannot travel across
Why is the Na+ channel one single subunit?
At some point in evolutionary history the 4 subunits would have fused together
What is the function of the lateral portals of the Na+ channel?
make the central cavity accessible from the hydrophobic core of the bilayer, pore blocking drugs (like anaesthetics) can block transmission through these
What transmembrane domain(s) make up the voltage sensor of the Na+ channel?
S1-S4
Where is the inactivation gate of the Na+ channel located?
In the S3-S4 loop
What are the three states of the Na+ channel?
- Open
- Closed
- Inactivated
In the key position of the inactivation gate, substituting F103 with other, larger aa will (increase/decrease) inactivation times
Increase - larger inactivation, more transient current
What prevents backflow of propagation of APs?
Inactivation of Na+ channels
The peak speed of an AP is…
100 m/s
When different Na+ channels are open, how do their conductances differ
They do not, conductance of individual channels is the same
When Na+ channels are depolarized equally and open, describe their rate of current
Rate of current constant
What kinds of channels do excitatory neurotransmitters open?
Cation channels
What kinds of channels do inhibitory neurotransmitters open?
Cl- or K+
Which subunits on ach receptors bind the ligand?
alpha-subunits
Describe the gate of ACh channels
Hydrophobic molecules keep ions from passing
How does Prozac target ACh receptors?
Inhibits Na+ driven symports which clear neurotransmitter in the cleft for prolonged transmission
