11: Membrane transport of small molecules and the electrical properties of membranes

Question 1

What two main factors determine the rate of diffusion across a membrane?

Answer 1

Size
Relative hydrophobicity

Small, nonpolar (hydrophobic) = fast

Question 2

What are the two main classes of membrane transport proteins? Differences?

Answer 2

Transporters:
Conformational change when bound to a specific solute. Alternate exposure of solute-binding site at each side of the membrane.
Passive or active.

Channels:
More weak interaction.
Form continuous pores that extend across the membrane.
Faster transport, passive down concentration (electrochemical) gradient.

Question 3

What are the three conformations of a transporter protein?

Answer 3

Open
Closed
Inactive

(outward-open, occluded, inward-open)

Question 4

What are the three main ways of active transport?

Answer 4

Coupled transporters:
One molecule uphill, one downhill

ATP-driven pumps:
Uphill transport coupled with ATP hydrolysis

Light- or redox-driven pumps:
Uphill transport coupled with energy from light or a redox reaction

Question 5

Briefly describe the structure of a transporter

Answer 5

10 or more transmembrane α helices.
Solute- and ion binding sites in the middle of the membrane.
Pseudosymmetric: built from inverted repeats, the structure of the two halves are similar but inverted
=> alternate access to binding sites

Question 6

Which type of antiport is generally used to maintain pH in cells? What is the normal intracellular pH?

Answer 6

Na+ antiport/exchangers

Intracellular pH~7.2

Question 7

What are the basic differences between the three classes of ATP-driven pumps (transport ATPases)?

Answer 7

P-type:
Phosphorylate themselves
Ion pumps, maintain gradients of ex. Na+, K+, H+, Ca2+…

ABC transporters:
Largest family
ATP-binding cassette, 2 domains
Pump small molecules
Might function as transporters or gated channels

V-type:
Turbin-like, constructed from multiple subunits.
Transfer H+ into organelles.
Structurally similar to F-type (ATP synthase, work in reverse to drive ATP synthesis)

Question 8

What is the role of the P-type Ca2+ pump and how does it work?

Answer 8

Pump Ca2+ actively out of a cell to maintain a steep concentration gradient.

Ca2+ pump is in the membrane of the sarcoplasmic reticulum. Action potential depolarizes a muscle cell membrane => influx of Ca2+ through Ca2+ release channels.
Ca2+ pump moves Ca2+ from the cytosol back into the SR.

Have 2 centrally binding sites for Ca2+. At non-phosphorylated state: only accessible from the cytosolic side of the SR memb.
Binding => phosphorylation and conformation change. Binding of new ATP opens to SR lumen side => Ca2+ out.
Replaced by H+ and H2O

Question 9

What is the sarcoplasmic reticulum?

Answer 9

Network of specialized smooth ER that is important in transmitting the electrical impulse as well as in the storage of calcium ions.
Forms a network of tubular sacs in the muscle cell cytoplasm.

Question 10

What type of transport ATPase is the Na+-K+ pump?

Answer 10

P-type:
Phosphorylate themselves
Ion pump

Question 11

What is the general structure of an ABC transporter?

Answer 11

2 hydrophobic domains, each containing 6 transmembrane α helices.
Can be one or more polypeptides.
Have 2 cytosolic ATPase domains.

Question 12

How does an ABC-transporter function?

Answer 12

Specific for a molecule or molecule class.
Variety of substrates: aa, oligo/polysaccharides, peptides, proteins, inorganic ions…

Some function as transporters, others as gated channels.

Eukaryotic: mostly export from the cytosol into the
extracellular space or ER (via TAP).

Prokaryotic: transport molecules in both directions
across membranes.

Important clinically

Question 13

What is special about the P-type ATPases?

Answer 13

Phosphorylate and dephosphorylate themselves during each pumping cycle.

E.g. the Ca2+ pump and the Na+-K+ pump

Question 14

Which ions do channels primarily transport?

Answer 14

Na+
K+
Ca2+
Cl-

Question 15

What is special about aquaporins relative to ion channels?

Answer 15

Permeable to water, but not to ions.

Question 16

What structural components of an aquaporin are important for its function?

Answer 16

Continuously open.

Narrow pore - one water molecule at a time, too narrow for hydrated ions.
Carbonyl oxygens on one side.
Hydrophobic aa on the other side.

2 asparagines in the middle bind to the O atom of the central H2O => bipolarity. Cannot participate in an H+ relay => pore impermeable to H+

Question 17

What is a selectivity filter and where is it found?

Answer 17

The narrowest part of an ion channel.
Limits the rate of passage for ions.
Allows contact between the ions and the channel wall so that only ions of appropriate size and charge can pass.

Question 18

What are the different types of ligands that bind ligand-gated ion channels?

Answer 18

Extracellular mediator e.g., neurotransmitter
Intracellular mediator e.g., ion
Nucleotide

Question 19

What main types of stimuli can cause an ion channel to open?

Answer 19

• voltage across a membrane
• mechanical stress
• binding of a ligand

Question 20

What characterize ion channels / how do we distinguish them from each other?

Answer 20

Ion it conducts
Mechanism by which it is gated
Abundance and localization in the cell/specific cells

Question 21

What is the role of K+ leak channels?

Answer 21

Open in the plasma membranes of all cells, even in unstimulated/resting.
Makes the plasma membrane much more permeable to K+ than to other ions => maintains the membrane potential.

Question 22

What structural components of the voltage-gated K+ channel are important for its function?

Answer 22

4 identical transmembrane subunits that together form a pore through the membrane.

Each subunit contributes 2 transmembrane α helices, tilted outward in the membrane and together form a cone. Wide end faces outside of the cell (K+ exit)

Polypeptide connecting transmembrane helices form a short α helix (pore helix) and a crucial loop extending into the wide section of the cone, forming the selectivity filter. Contains carbonyl O.
Pore helices guide K+ into the selectivity filter

-C=O O’s are too far away from each other for Na+ to pass

Gating: movement of the helices in the membrane to obstruct/open path for ions.

Question 23

Which ion channel is important for generating and propagating action potentials in nerve cells?

Answer 23

Voltage-gated cation channels.

Na+ channels in nerve and skeletal muscle cells.

Question 24

What happens during the depolarization reaction in a membrane?
Key words: depolarization, Ca2+, Na+, neurotransmitter

Answer 24

Depolarization of the membrane at the axon terminal => voltage-gated Ca2+ channels in presynaptic memb. open, Ca2+ influx.

Release of neurotransmitters from synaptic vesicles into synapse. Bind to transmitter/ligand-gated receptors at neighbor cell => opening of cation channels.

When sufficient depolarization: Opening of voltage-gated Na+-channels => Na+ influx (diffusion)
=> more depolarization
=> more Na+ channels open (self-amplification, positive feedback)

After reaching +50 mV:
- Inactivation of Na+ channels
- Opening of voltage-gated K+ channels, K+ efflux
=> restoring of natural memb. potential (-70 mV, neuron, to -40 mV)

Question 25

What structural components of the voltage-gated Na+ channel are important for its function?

Answer 25

Chapter 11 - p. 622
Central channel (4 ppt)
Every domain has:
Voltage sensor
Selectivity filter

Between 3rd and 4th domains: inactivation gate

Question 26

What is the “refractory period”?

Answer 26

Time necessary for a sufficient number of Na+ channels to recover from inactivation to support a new action potential.

Question 27

What type of channel are channelrhodopsins and what is the associated photopigment?

Answer 27

Photosensitive ion (cation) channels.

Has a covalently bound retinal group that absorbs light and undergoes an isomerization reaction, which triggers a conformational change in the protein, opening an ion channel in the plasma membrane.

Question 28

How can light-responsive ion channels be used in science?

Answer 28

Use genetic engineering techniques to express it in target cells e.g., specific neurons in the brain.
Implant an optic cable near the relevant brain region.
Flash of light activates/inactivates the neurons to fire action potentials depending on the type of channel.

Frequency of light flashes determines the frequency of action potentials.

Question 29

Which cell types form the myelin sheath of neurons?

Answer 29

Nonneuronal supporting cells, glial cells:

• Schwann cells in the peripheral nerves
• Oligodendrocytes in the CNS

Question 30

What are the advantages of the nodes of Ranvier in the myelin sheath?

Answer 30

Nodes of Ranvier:
Regularly spaces without myelin where almost all the Na+ channels of the axon are located.
Action potential “jumps” from node to node => faster, more effective.
Energy conservation as the excitations is confined to small regions of the membrane.

Question 31

How are the secreted neurotransmitters removed?

Example: acetylcholine

Answer 31

Destroyed by enzymes in the synaptic cleft.
Acetylcholine is hydrolyzed and degraded by acetylcholinesterase.

Taken up by the presynaptic nerve terminal or by surrounding glial cells.
Reuptake mediated by Na+ dependent NT symporters.

Rapid removal => temporal and spatial precision of signaling at a synapse.

Question 32

What is the main function of a transmitter-gated ion channel?

Answer 32

Rapidly conversion of extracellular chemical signals into electrical signals at chemical synapses.

Open transiently in response to the binding of NT molecules => brief permeability change of membrane.

Size of action potential depends on the number of NTs released at the synapse and how long it persists there.

Relatively insensitive to memb. potential, cannot produce a self-amplifying excitation.

Question 33

What are the main differences between excitatory and inhibitory chemical synapses?

Answer 33

Both are generally immediate, simple, and brief.

Excitatory:

• Excitatory NTs (acetylcholine, glutamate) open cation channels (often Na+, or Ca2+)
• Depolarization of the postsynaptic membrane, action potential

Inhibitory:

• Inhibitory NTs (GABA, glycine) open Cl- or K+ channels
• Suppresses firing, hyperpolarization
• Harder to fire action potential

Sum of excitatory and inhibitory signals
decide strength of the signal

Question 34

Name some features of the acetylcholine receeptor

Answer 34

5 transmembrane ppt, where 2 are identical and bind acetylcholine.
Ring of hydrophobic aa blocks ion flow in the closed state.
Clusters of negatively charged aa at both ends help to attract cations and repel anions.
Na+ and K+ (some Ca2+) enter by diffusion => little selectivity

Opened transiently after binding of 2 acetylcholine molecules and conformational change.
Acetylcholinesterase hydrolyzes acetylcholine and returns the receptor to its original state.

If acetylcholine is present for a prolonged time (>20 milliseconds): desensitivization.

Question 35

Give a brief description of the different types of ion channels at the axon hillock at a neuron

Answer 35

Voltage-gated Na+
1 selective for Ca2+
3 selective for K+: delayed, rapidly activating, and Ca2+-activated K+ channels

Delayed K+ channels:
Repolarize the membrane after each action potential by K+ efflux. Close after.
Slower kinetics, only open when the Na+ channels are inactivated.

Rapidly activating K+ channels:
Ensure a firing rate that is proportional to the strength of the depolarization stimulus.

Ca2+-activated K+ channels:
A neuron that is stimulated continuously for a prolonged period (more Ca2+ in) becomes gradually less responsive to the constant stimulus; adaptation. K+ leakage out of the cell => harder to depolarize

Question 36

What is long-term potentiation (LTP) and when does it happen?

Answer 36

Occurs in a specific subset of neurons in the hippocampus and is responsible for molecular processes forming the prerequisite for learning.

Occurs when a presynaptic cell fires simultaneously with the post-synaptic membrane already being depolarized.

Glutamate - main excitatory NT in the mammalian CNS.
AMPA and NMDA receptors mediate the process => Ca2+ influx, depolarization.

AMPA receptors mediate normal signaling via membrane depolarization, NMDA regulate
the abundance of AMPA receptors dependent on glutamate presence and membrane depolarization