Lecture 7: Membrane Transport

Question 1

Name four classes of molecules

Answer 1

1- small hydrophobic (O2, CO2, N2)
2- small uncharged polar (H2O, glycerol, ethanol)
4- large uncharged polar (amino acids, glucose, nucleotides)
4- charged (H+, Na+, K+, Ca2+, Mg2+, Cl-, HCO3-)

Question 2

Define a synthetic bilayer

Answer 2

protein-free artificial, impermeable to large uncharged polar molecules and charge ions.

Question 3

Cell membranes are permeable to

Answer 3

Large uncharged polar molecules and charged ions. These water-soluble molecules are transferred through transport proteins.

Question 4

Transport proteins are

Answer 4

substrate specific and responsible for maintaining cells specific chemical composition.

Question 5

Every membrane has its own characteristic set of transport proteins and why(T/F)

Answer 5

True, and therefor organelles perform unique functions due to their unique chemical composition.

Question 6

The concentrations of cations in and out of cell

no electrical forces

Answer 6

• low [Na+] in
• high [K+] in
• [Na+]in + [K+]in=[Na+]out + [K+]out

Question 7

The concentrations of anions in and out of cell

no electrical forces

Answer 7

• high [protein-] in
• low [Cl-] in
• low [HCO3-] in

Question 8

The quantity of positive charge ___________ must be ______ by an almost exactly equal quantity of negative charge.

Answer 8

• inside the cell

- balanced

Question 9

Name three mechanisms by which small molecules cross the bilayer

Answer 9

• passive diffusion
• facilitated diffusion
• active transport

Question 10

Define passive diffusion

Answer 10

• hydrophobic and small polar uncharged molecules dissolve in lipid bilayer
• membrane protein not required
• no energy required
• hydrophobic molecules diffuse down their concentration gradient

Question 11

The direction of transport of molecules in passive and facilitated diffusion is determined

Answer 11

only by the relative concentrations of the molecule in and out of cell, at equilibrium there is no transport.

Question 12

Difference between passive and facilitated diffusion

Answer 12

In facilitated diffusion membrane transport proteins are involved, used to transport large polar molecules and charged ions without interacting with hydrophobic interior of the membrane.

Question 13

Define active transport

Answer 13

• membrane transport proteins required
• energy driven
• molecules move against their concentration gradient through the hydrolysis of ATP

Question 14

Name three classes of transport proteins

Answer 14

1) ATP-powered pump
2) channel proteins (ion channels)
3) carrier proteins (transporters)

Question 15

Relative rate of transport of transport proteins in order from most to least molecules transferred

Answer 15

Ion channels (10^7-10^8) > transporters (10^2-10^4) > ATP- powered pump (10^0-10^3)

Question 16

Why does ATP-powered pump have a lower rate of transport than ion channels?

Answer 16

• ATP-powered pumps transport a) one molecule at a time, b) limited reserve of ATP, c) require hydrolysis of ATP.
• whereas, ion channels transport a) many ions at once, b) movement is thermodynamically favoured, c) doesn’t require external use of energy.

Question 17

Define ion channels

Answer 17

• transport many specific ions at once down their concentration gradient.
• hydrophilic pore.
• exist in open or closed conformation as polypeptide chain relocates.

Question 18

Key difference between ion channels are carrier proteins (transporters)

Answer 18

-the binding of a molecule changes the conformation of the carrier protein.

Question 19

Name three types of transporters

Answer 19

1) uniporter
2) symporter
3) antiporter

Question 20

Describe a uniporter

Answer 20

• transport single type of molecule down its concentration gradient.
• moves glucose or amino acids across plasma membrane into mammalian cell.

Question 21

Describe a symporter

Answer 21

• couple movement of an ion or polar molecule against its concentration gradient and different molecule down its concentration gradient.
• both ions or polar molecules move in the same direction.
• The couple movement of energetically favoured reaction drives the other energetically unfavoured movement.

Question 22

Describe a antiporter

Answer 22

• couple movement of an ion or polar molecule against its concentration gradient and different molecule down its concentration gradient.
• move in opposite direction.
• The couple movement of energetically favoured reaction drives the other energetically unfavoured movement.

Question 23

Symporters and antiporters are called

Answer 23

cotransporters

Question 24

What role do Na+/K+ ATPase play in mammalian cells

Answer 24

• maintain intracellular K+ and Na+ concentrations.

- responsible for the couple movement of Na+ and K+ in and out of cell.

Question 25

Steps in the mechanism of Na+/ K+ ATPase

Answer 25

step 1: binding of 3 Na+ ions (E1)

• E1 conformation, 3 high affinity Na+ binding sites, 2 low affinity K+ binding sites on cytosolic face.
• low conc of Na+ inside cell, high affinity of Na+ binding sites.
• high conc of K+ inside cell, low affinity of K+ binding sites.

step 2: binding of ATP, phosphorylation of aspartate (E1~P)

• ATP binds to cytosolic surface.
• ATP hydrolyzed to ADP, and the liberated phosphate is transferred to aspartate forming high energy acyl phosphate bond.

step 3: conformation change, transport of 3 Na+ ions (E2~P)

• 3 Na+ ions move outward through protein.
• generates 2 high affinity K+ binding sites on extracellular face.

step 4: dissociation of 3 Na+ ions, binding of 2 K+ ions

• 3Na+ ions dissociate into extracellular medium from low affinity Na+ binding site.
• 2 K+ ions bind to high affinity sites despite low conc of K+ in extracellular medium.

step 5: hydrolysis of aspartate phosphate (E2~P –> E1)

• back to E1 conformation.
• during conformation transition, 2 K+ move inward through protein.

step 6: dissociation of 2 K+ ions
- 2 K+ ions dissociate into cytosol from low affinity K+ binding sites on cytosolic surface, despite high conc of K+ in cytosol.

Question 26

Define two properties of ion channels

Answer 26

1) ion selectivity,because of narrow pores.
2) gating, because ion channels are not permanently opened. open and closed state are regulated by gates that open in a response to specific stimuli.

Question 27

Define ion selectivity of Na+ channels

Answer 27

permit the passage of Na+ bound to a single water molecule, but interferes with passage of K+.

Question 28

Name and define three type of gated ion channels

Answer 28

1) voltage-gated: electric potential
2) ligand-gated: binding of chemical ligand in or out of cell
3) stress-activated: mechanical force

Question 29

Medically important ligand-gated ion channel

Answer 29

Cl- in epithelial cells (CFTR)
CFTR= Cystic Fibrosis Transmembrane conductance Regulator - is unusual in that it requires both ATP hydrolysis and phosphorylation in order to open.

Question 30

Cystic fibrosis caused by and is

Answer 30

by: recessive mutation in the CFTR
is:
- most common lethal inherited disease of Caucasians.
- production of thick mucus by cells in respiratory and gastrointestinal tracts.
- respiratory disease.
- diagnosed if presence of excessive salt in sweat.
- is fatal, along with lung disease with 95% mortality rate.