Membrane Transport

Question 1

what can cross the lipid bilayer without the aid of a channel or carrier?

Answer 1

small, uncharged, hydrophobic molecules moving down their concentration gradients

Question 2

When is membrane voltage not a factor for membrane transport?

Answer 2

during transport of uncharged molecules like sugars and glycerol

Question 3

passive v. active transport

Answer 3

down the gradient = passive transport (simple and facilitated diffusion)
up the gradient = active and secondary active transport

Question 4

the ACh-gated ion channel

Answer 4

a multisubunit complex present in the plasma membranes of muscle cells. binding of ACh to the alpha subunits of the channel changes its conformation, allowing sodium to flow into the cell, along its concentration gradient (passive transport). The muscle cell depolarizes and contracts.

Question 5

Can channels engage in active transport?

Answer 5

NO! Channels can only facilitate the diffusion of molecules down their electrochemical gradient and therefore do not engage in active transport.

Question 6

uniport carriers

Answer 6

move a single molecule at a time

Question 7

symport carriers

Answer 7

simultaneously transport two different molecules in the same direction

Question 8

antiport carriers

Answer 8

transport two different molecules in opposite directions (aka exchangers)

Question 9

GLUT protein families

Answer 9

the GLUT protein family consists of a group of glucose uniporters (GLUT 1 to 4) that transport glucose accross a variety of tissues. They are all passively transport glucose though the plasma membrane. They have different binding affinity for glucose (low and high kms) The varied binding affinities and their tissue specific expression enable a hierarchy of glucose within the body: uptake by the brain occurs at all concentrations (especially at low concentrations) whereas uptake by adipose tissue and muscle occurs at [higher]. GLUT proteins alternate between two conformations: they first bind glucose on one side of the membrane, then undergo a conformatonal change which releases the glucose on the other side of the membrane.

Question 10

GLUT1

Answer 10

(odds have more affinity) Tissue distribution: Brain, erythrocytes, placenta, fetal tissue.
Important features: high affinity transporter. This efficient binding allows relatively constant uptake of glucose. Since the concentration of glucose is much higher in the plasma than in the erythrocyte, the GLUT1 transporter facilitates import of glucose into the erythrocyte.

Question 11

GLUT2

Answer 11

Tissue distribution: Liver, Kidney, intestine, pancreatic beta-cells
Important Features: low-affinity transporter. GLUT2 facilitates glucose export by the liver and also acts as a glucose sensor for pancreatic beta-cells.
This is the transporter that brings in glucose into the basolateral side of the intestinal epithelial cells

Question 12

GLUT3

Answer 12

(odds have more affinity)
Tissue distribution: BRAIN
Important Features: high-affinity transporter. Very efficient binding allows preferential uptake in hypoglycemia

Question 13

GLUT4

Answer 13

Tissue distribution: Muscle, adipose tissue
Important Features: medium affinity transporter (insulin regulated). Insulin-signaling recruits GLUT4 transporters from intracellular stores to increase uptake

Question 14

paracellular transport

Answer 14

one way nutrients (such as glucose) in the intestinal lumen can access the capillary beds which lie under the intestinal epithelial. In this way the solute basses BETWEEN adjacent cells, through tight junctions that have loosened in response to a meal. (This is passive)

Question 15

Transcellular Transport

Answer 15

one way nutrients (such as glucose) in the intestinal lumen can access the capillary beds which lie under the intestinal epithelial. The solute is imported into the cell at the apical surface and expelled from the cell at the basolateral surface. (this is primarily how glucose transport takes in the intestines takes place.)

Question 16

secondary active transport

Answer 16

maintainence of an ion gradient to provide energy for transporting a molecule

Question 17

The Na+ driven glucose transporter

Answer 17

uses the energy of the Na+ gradient (established by the Na+/K+ ATPase on the basolateral end) to import glucose into the cell from the intestinal lumen. The transporter coordinately binds and transports one glucose molecule and two Na+ ions into the cell. (ATP is NOT used by the gluocose transporter)

Question 18

is the Na+/K+ ATPase on the apical or basolateral side of the intestinal epithelial cells?

Answer 18

basolateral side! the Na+/Glucose co-transporter is on the apical side.

Question 19

mechanism of the Na+/K+ ATPase Pump

Answer 19

(2PumpKin and Na+/K+. . . in the name it says which is pumped out first; the three Na+s out first)
(phosphorylation/dephosphorylation marks the release of both ions [Na+ first])
-begins with a conformation which leaves it open to the INSIDE of the cell, which binds to 3 Na+ molecules.
-the pump is phosphorylated (using ATP) which causes it to change conformation, which causes the pump to expel the Na+ ions to the outside of the cell membrane.
-during the second half of the cycle, the pump is open to the outside of the cell, and two K+ ions bind to the pump.
-dephosphorylation of the pump triggers a reversion to the pump’s original conformation, which leaves it open to the inside of the cell, expelling the two K+ ions into the cell, and back to the initial state.

Question 20

Physiological function of the Na+/K+ ATPase

Answer 20

pumps out 3 molecules of Na+ for every 2 K+ it imports.

1) Establishes a (negative) resting membrane potential
2) establishes osmotic balance by playing an important role in regulating cell volume (by controlling water influx or efflux)
3) establish an Na+ electrochemical gradient for secondary active transport.

Question 21

The Na+ driven glucose transport

Answer 21

on the apical surface you find only the Na+-driven glucose symport (secondary active transport again glucose gradient)
on the basolateral surface you find 2 carriers
1)Na+/K+ATPase (primary active transport against the Na+ gradient [establishes Na+ gradient for secondary active transport])
2) GLUT2 (uniport, passive transport, glucose going down its gradient)
NOTE: Na+ at the basolateral side leaks back through tight junctions to the apical side, resulting in high levels of extracellular Na+ at the apical surface (paracellular transport)