Module 9 - 3

Question 1

What is simple diffusion?

Answer 1

Non-polar gases and hydrophobic molecules cross the membrane down a concentration gradient.

Question 2

What is facilitated diffusion?

Answer 2

Membrane transporters lower the activation energy barrier of crossing the bilayer.

Question 3

What is the difference between channels and carriers in facilitated diffusion?

Answer 3

Channels are membrane pores that transport molecules down a concentration gradient, while carriers are membrane proteins that undergo substrate-induced conformational change.

Question 4

What is the advantage of channels over carriers in facilitated diffusion?

Answer 4

Channels tend to be faster and do not saturate.

Question 5

What is the advantage of carriers over channels in facilitated diffusion?

Answer 5

Carriers are highly specific for the molecule they carry and can saturate.

Question 6

What is the facilitated diffusion through a carrier glucose permease of erythrocytes?

Answer 6

Facilitated diffusion of glucose at 50,000X faster than simple diffusion, specific for D-glucose, and follows a pattern resembling M-M kinetics.

Question 7

What is the Kt of the carrier glucose permease of erythrocytes?

Answer 7

Kt is about 1/3 the concentration of blood glucose so the transporter is nearly saturated and operates near Vmax.

Question 8

What is the conductance rate of channels in facilitated diffusion?

Answer 8

High conductance rates because they bind the substrate very weakly.

Question 9

What is the conductance rate of carriers in facilitated diffusion?

Answer 9

Slower because they bind the substrate quite strongly.

Question 10

What is the structure of carriers in facilitated diffusion?

Answer 10

Carriers tend to be beta barrels.

Question 11

What is the activation energy for removing the hydration shell from a polar solute and transferring it into the non-polar environment in the core of the bilayer?

Answer 11

Very high.

Question 12

What do membrane transporters do in facilitated diffusion?

Answer 12

They lower the activation energy for crossing the membrane by replacing the hydration shell with interaction with polar groups along the transfer path in the protein interior.

Question 13

What determines the direction and rate of movement of molecules in simple diffusion?

Answer 13

Their concentrations on either side of the membrane.

Question 14

What is the disadvantage of facilitated diffusion?

Answer 14

It can only result in the net movement down a concentration gradient.

Question 15

What is the advantage of facilitated diffusion over simple diffusion?

Answer 15

Facilitated diffusion can transport molecules at a faster rate.

Question 16

What is the disadvantage of channels in facilitated diffusion?

Answer 16

They tend to bind the substrate very weakly.

Question 17

What is the disadvantage of carriers in facilitated diffusion?

Answer 17

They tend to bind the substrate quite strongly.

Question 18

What are lipid-anchored membrane proteins?

Answer 18

Proteins covalently attached to lipids to anchor them to the membrane.

Question 19

Are lipid modifications reversible?

Answer 19

Yes, sometimes allowing for regulation of cellular location.

Question 20

Where are GPI anchored proteins located?

Answer 20

Always at the outer face of the membrane.

Question 21

Where are proteins with single chain hydrocarbons located?

Answer 21

Always at the inner face of the membrane.

Question 22

What amino acids are involved in lipid anchoring?

Answer 22

Cystine or serine.

Question 23

What are integral membrane proteins?

Answer 23

Proteins that span the membrane.

Question 24

What is the positioning of integral membrane proteins?

Answer 24

Specific and directional.

Question 25

What are the three varieties of integral membrane proteins?

Answer 25

Single pass a-helical, a-helical bundles, and b-barrels.

Question 26

What are the side chains within the transmembrane region?

Answer 26

Non-polar

Question 27

What are the polar groups in each peptide bond?

Answer 27

Carbonyl and amide groups

Question 28

Why are polar unpaired carbonyl and amide groups in the bilayer core energetically unfavorable?

Answer 28

Because they are hydrophilic in nature and the bilayer core is hydrophobic

Question 29

What has to be done to the carbonyl and amide groups of the protein backbone within the bilayer?

Answer 29

They have to be hydrogen-bonded

Question 30

What is the distribution of charged residues in integral membrane proteins?

Answer 30

Located mostly within the intra and extracellular portions of the protein

Question 31

What type of residues dominate inside the hydrophobic slab of the bilayer in integral membrane proteins?

Answer 31

Residues with non-polar side chains dominate inside the hydrophobic slab of the bilayer.

Question 32

Where do tryptophan and tyrosine cluster in integral membrane proteins?

Answer 32

Tryptophan and tyrosine cluster at the interface between the hydrocarbon chain and polar head group region.

Question 33

What are membrane spanning regions?

Answer 33

Regions predicted from amino acid sequence.

Question 34

What is the criterion for predicting membrane spanning regions?

Answer 34

Stretches of ~20 hydrophobic residues.

Question 35

What is a hydropathy index?

Answer 35

Predicts transmembrane regions based on hydrophobicity.