Lecture 17-18

Question 1

Describe in detail the way that 5 ion channels are involved in neuromuscular transmission

Answer 1

1. Voltage gated Ca2+ channels are opened via depolarization
2. Acetylcholine (which was exocytosed) opens ligand gated Na+ channels
3. local depolarization opens adjacent Voltage gated Na+ channels
4. Voltage gated Ca2+ channels open as depolarization spreads
5. Coupled Ca2+ release channels embedded in the SR open

Question 2

Describe P-Type ATPase pumps

Answer 2

P-type ATPase pumps are a type of pumps that phosphorylate themselves, by taking a phosphate from ATP, in order to move a molecule across a membrane

Question 3

Describe an ABC transporter

Answer 3

ABC transporters (ATP-binding Cassette) consume 2 molecules of ATP per “round” of molecule movement bc of their identical domains (of a transmembrane and cytosolic domain)

(does NOT phosphorylate itself)

Question 4

Describe a Secondary Transporter

Answer 4

Secondary transporters use a concentration gradient that is previously established via “primary” active transporters

This allows secondary transporters to move “high value stowaway” molecules against a concentration gradient, across a membrane

This method of transport is still considered to be a form of active transport

Question 5

Describe an Ion channel and include the 2 “important pieces” these include

Answer 5

Ion channels use a selectivity filter to allow specific ions into the transmembrane protein and are then stimulated to open their “gate” via 1. voltage 2. a ligand or 3. stress (mechanical)

1. Selectivity filter
2. Gate

Question 6

Describe a Gap Junction in terms of the subunits it is made up of.

Answer 6

Gap junctions have no specificity filter and allow cytoplasm sharing between neighboring cells via “connexons” that form pores between cells

Question 7

Describe an Aquaporin and the 2 forces that allow it to exhibit selectivity for water

Answer 7

Aquaporins allow water through but NOT ions

A unique property of water is it’s ability to conduct cohesion (water to water) interactions and adhesion (water and another molecule) interactions.

Cohesion and Adhesion interactions between water, itself, and the hydrophobic walls of the aquaporin are what allows water to traverse the aquaporin in a selective manner.

Question 8

State and describe the 2 “states” that you can find P-type ATPases in. Describe the relationship between these 2 states

Answer 8

E1 state: open to the inside of the cell

E2 state: open to the outside of the cell

P-type pumps switch between these 2 states when they phosphorylate themselves
“take in something from the inside, phosphorylate, and release it to the outside”
(sort of how a canal lock system works with boats)

Question 9

Name the type of transmembrane protein that can exist in a monomer and homodimer. Explain these 2 different structures.

Answer 9

ABC transporters can be either monomers or homodimers

Monomers: each transporter is a single unit with 2 transmembrane domains and 2 cytosolic domains

Homodimers: Each transporter is composed of 2 “twin” subunits
(each subunit has 1 transmembrane domain and one cytosolic domain)

(both monomers and homodimer transporters have 4 total domains)

Question 10

Name the 2 primary active transporters we have discussed that will eventually set up a gradient for secondary active transport processes. Describe which of these moves ions and which of these moves small molecules

Answer 10

P-type pumps: move ions

ABC transporters: move small molecules

(both use ATP)

Question 11

True or False:

Primary and secondary active transport both use ATP to move a molecule, thats why they are both forms of active transport. explain.

Answer 11

FALSE

Primary active transport uses ATP but Secondary active transport does not use ATP

However, they are both considered to be “active” forms of transport because they are moving a molecule across a membrane AGAINST it’s own concentration gradient.
(NOT bc of ATP use)

Question 12

Compare passive transport and active transport

Answer 12

Passive transport:
No energy needed
Solute travels down a concentration gradient

Active transport:
Coupled to ATP hydrolysis
Solute travels against a concentration gradient

Question 13

Compare Antiporters, Symporters, and Uniporters in terms of the mechanisms by which they move molecules across membranes

Answer 13

Antiporters: move molecule A and B in opposite directions (Anti = opposite)

Symporters: move molecule A and B in the SAME direction (Adding/removing 2)

Uniporters: moves ONLY 1 molecule at a time

Question 14

Compare Pumps, Carriers, and Channels

Answer 14

Pumps: perform primary active transport (against a conc. gradient)

Carriers: move molecules across the membrane without needing (extra) energy (stowaway)

Channels: are used in passive transport (travels down the concentration gradient ; “stuck in traffic” ex)

Question 15

State the 3 (4 in a way) domains that are found in P-type ATPases and describe them

Answer 15

1. Transmembrane domain: spans the lipid bilayer
2. A/Actuator domain: links the cytosolic domains to the transmembrane domain
3. N/Nucleotide binding domain: binds to ATP
4. P/Phosphorylation domain: accepts the Phosphate group from the ATP molecule (place where it phosphorylates itself)

Question 16

For SERCA, Describe the steps that occur during it’s movement of Ca2+ ions out of the cytoplasm and into the membrane lumen. (6 steps)

Answer 16

1. It begins in the E1 state (unphosphorylated) with Ca2+ ions bound
2. ATP binds, which allows the A domain (hinge) to swing and trap the Ca2+ ions
3. ATP hydrolysis occurs and the SERCA protein phosphorylates it’s P domain
4. Eversion to the E2 state allows for the Ca2+ ions to be released into the membrane lumen
5. Pi (the phosphate that was bond to the P domain of SERCA) is released
6. Eversion back to the E1 state occurs due to the release of Pi (dephosphorylation of the P domain of SERCA)

Question 17

For Na/K pumps, describe the steps that occur during it’s movement of Na+ ions into the membrane lumen and K+ ions into the cytoplasm of the cell.

Answer 17

1. it begins in the E1 state (unphosphoyrlated), Na+ ions are bound
2. ATP binds, and the conformational change that traps the Na+ ions occurs
3. ATP hydrolysis occurs, and the p domain is phosphorylated
4. Eversion to E2 occurs, which releases Na+ ions and allows K+ ions to bind to the extracellular portion of the Na/K pump
5. Pi is released, which traps K+ ions
6. ATP binds again, causing eversion back to E1 and the release of K+
   (different than SERCA here)

Question 18

Explain what Digitalis and Oubain do to Na/K pumps. Describe what happens if you give Digitalis and Oubain to a human

Answer 18

Digitalis and Oubain both lock the Na/K pump in it’s E2 conformation

This keeps Na+ inside of the cell unless it can be removed by a different transmembrane protein that exchanges it with Ca2+. This adds extra Ca2+ to the cell and causes the heart to contract.

Question 19

By definition, all secondary transporters are ______.

Answer 19

symporters

Question 20

For simple diffusion rate, state the 7 factors that affect the diffusion rate

Answer 20

1. Magnitude of the concentration gradient
2. Size of the molecule
3. Surface Area (volume ratio / shape ; microvilli create a lot of this)
4. Temperature
5. Density of the solvent
6. Solubility of the solute
7. Distance to the destination

Question 21

When it comes to an ion that is moving through an ion channel’s selectivity filter, what force is driving it through the filter and causing its movement across the membrane?

Answer 21

Movement of the ion results from electrostatic repulsion

This movement only properly occurs if the ion is the correct size/charge for that specific ion channel (this adds selectivity)