Lecture 14: Transport Through Membranes II

Question 1

Describe the basic overview of ion channels

Answer 1

• Integral, polytopic membrane proteins
• Over 400 genes in human genome encode channels.
• Mediate facilitated passive transport
• Transport ions 1000x faster than pumps.
• Highly selective and specific for ions (Na, K, Ca)
• Respond to physical and chemical changes in their environment and undergo precisely timed conformational changes
• Opening and closing of channels shape the membrane potential

Question 2

List the three types of ion channels

Answer 2

1. Voltage gated: respond to change in membrane potential.
2. Chemically gated: respond to ligands such as neurotransmitters.
3. Mechanically Gated: respond to mechanical stimuli, detect vibration, pressure, stretch, touch, sounds, tastes, smell, heat, volume, and vision. Found in sensory neurons

Question 3

Describe the acetylcholine receptor

Answer 3

• Ligand is the neurotransmitter acetylcholine
• Present on postsynaptic side
• Binding of acetylcholine to its receptor opens the channel
• Channel equally permeable to Na+ and K+
• Pentamer of 4 kinds of membrane spanning subunits ( α 2 βγ delta) arranged in the form of a ring that creates a pore through the membrane

Question 4

Describe neurotransmission

Answer 4

• Most important manifestation of membrane transport - the basis for communication in the nervous system
• Nerve impulse is an electrical signal produced by the flow of current across the plasma membrane
• The inside of the neuron has high potassium and low sodium
• Generated by sodium/potassium ATPase.

Question 5

Give the nernst equation

Answer 5

Veq = (-2.303) (RT/zF) log10 ([Xin]/[Xout])

Where Veq = membrane potential
R = Gas Constant
F = Faraday's Constant
Z = charge on ion
X= ion

Question 6

Describe the action potential

Answer 6

• Action potential arises from large transient changes in the permeability of the axonal membrane to ions
• The conductance for sodium changes first due to openings of the sodium channels
• Depolarization decreases permeability to sodium, which begins to flow in via the sodium channel due to high concentration outside
• Depolarization then opens the potassium channels and potassium moves out.

Question 7

List the 9 steps undergone during an action potential

Answer 7

1. Resting membrane (around -60 mV)
2. Depolarizing stimulus
3. Sodium Channels open
4. Rapid sodium entry causes further depolarization
5. Sodium channels close, Potassium Channels open
6. Potassium moves out
7. Membrane Hyperpolarization
8. Potassium Channels close
9. Membrane returns to resting potential

Question 8

Give the 7 steps of a synaptic transmission action potential

Answer 8

1. Action potential reaches presynaptic terminal
2. Depolarization opens up the VGCC
3. Calcium enters cell
4. Causes fusion of synaptic vesicles
5. Exocytosis release of NT in synaptic cleft
6. NT bind to its receptor on post-synaptic side
7. Opens up a ligand-gated channel

Question 9

Describe Gap Junctions

Answer 9

• Special form of ion channels
• Called “cell to cell” channels.
• Passages between contiguous cells.
• Important role in cell to cell communication.
• Membrane proteins line up next to each other to form a continuous channel

Question 10

Describe the structural features of gap junctions

Answer 10

• Packed in hexagonal array
• Lumen (around 20Å)
• Distance between gap junctions of 2 adjacent cells is about 35 Å

Question 11

Describe the properties of gap junctions

Answer 11

• Small hydrophilic molecules and ions can pass through these channels
• Less than 1 kDa molecules e.g., sugars, nucleotides, amino acids can pass through
• Proteins, polysaccharides, and nucleic acids, can Not pass through

Question 12

Describe the structure of gap junctions

Answer 12

• Meant to allow cells to have direct communication with one another
• Each channel is made up of 12 molecules of a transmembrane protein called conexins.
• Six conexin molecules are hexagonally arrayed to form a half channel called a connexon
• Two connexons join together end to end in the intercellular space to form a functional channel. (Each connexon is a “hemichannel”)

Question 13

Describe the physiological relevance of gap junctions

Answer 13

• Very important in cardiac tissue - the signal to contract is passed via gap junctions
• Induction of labor, all muscle cells contract in synchrony
• Important in lens and bone tissue to distribute nourishment
• Sealed by high amounts of calcium and hydrogen (to prevent normal cells from injured or dying cells)
• Prevents the bystander effect…whatever that is.

Question 14

What are the major differences between ion channels and gap junctions?

Answer 14

Gap Junctions

• Traverse 2 Membranes
• Connect from cytoplasm to cytoplasm
• Synthesized by two different cells
• Remain open for seconds to minutes

Question 15

Describe water-channels/aquaporins

Answer 15

• Increase rate of water flow across membranes
• Found in red blood cells, kidneys, and the cornea
• Is a 24 kDa protein
• Have 6 transmembrane alpha-helical domains
• May wanna understand how the aquaporin totally works coughREADBOOKcough