Lecture 17/18 - Transport Through Membranes

Question 1

What are the 3 categories of transporters?

L17/18 S6

Answer 1

Pumps:
-active transport

Carriers:
-traverse membranes without needing extra energy

Channels:
-passive transport

Question 2

What are the types of pumps?

L17/18 S7

Answer 2

P-type:
-phosphorylates self

ABC:
-ATP binding cassette

Question 3

Describe the structure and function of P-type pumps.
What are examples of this type?

L17//18 S8-9

Answer 3

Structure:

• transmembrane domain
• actuator (A) domain; links transmembrane and cytosolic domains
• nucleotide binding (N) domain; binds ATP
• phosphorylation (P) domain; domains that is phosphorylated

Function:

• hydrolysis of ATP from N domain phosphorylates P domain
• eversion to E(2) occurs
• phosphate is released
• eversion back to E(1)
• ATP rebinds

Examples:

• SERCA: moves Ca++ out of ER
• Na+/K+ pump: moves Na+ out and K+ in
• K+/H+ pump: moves K+ out of stomach and H+ in

Question 4

Describe the structure and function of ABC pumps.
What are examples of this type?

L17//18 S13-14

Answer 4

Structure:
-monomer consist of 2 transmembrane domains and 2 ATP-binding cassettes
—or—
-homodimers consisting of a transmembrane domain and an ATP-binding cassette

Function:

• molecule binds and increases ATP affinity
• 2 ATP bind and cause eversion
• molecule is released
• ATPs are hydrolyzed and eversion back to beginning

Question 5

What is the function of co-transporters?

L17/18 S18

Answer 5

All secondary transporters are symporters that use concentration gradients created but primary active transport to transport something else against its gradient.

Question 6

What factors affect passive diffusion rates?

L17/18 S20

Answer 6

• magnitude of concentration gradient
• size of molecule
• surface area:volume ratio
• temperature
• density of solvent
• solubility of solute
• distance to destination

Question 7

What are ions channels and what are two important features about most ion channels?

L17/18 S21

Answer 7

Allows ions across the membrane down their concentration gradient.

Important features:

• selectivity filter: allows only certain ions through the channel (e.g. use of size/charge to limit)
• gate: either voltage gated, ligand gated, or mechanically gated

Question 8

What are gap junctions?

L17/18

Answer 8

Channel connecting two cells.
Channel formed by connexons of both cells.

Pore is always open and does not have a selectivity filter.

Question 9

What are aquaporins?

L18/19 S27

Answer 9

Allows water in but not charged ions.

Selective by requiring cohesion of water with other water molecules, adhesion of water molecules to one part of the pore, and a hydrophobic surface in the pore that allows polar but not charged molecules.

Uses Asn residues to allow proper orientation of water in channel.