membranes

Question 1

briefly describe some of the functions of a cell membrane

Answer 1

• flexibility accomodates cell growth and movement
• provides compartmentalization
• fusion (endo and exocytosis), slitting (mitosis)
• selective permeability determines what goes in/out of cell
  -makes it easier for processes like signaling to occue

Question 2

what is meant by “sidedness”?

Answer 2

• the inner and outer leaflets have different lipid composistions

Question 3

is the lipid makeup of the membrane static?

Answer 3

• no, can change based on cell compartment and organism

Question 4

list three functions of proteins in membranes

Answer 4

• receptors, enzymes, transport, adhesion

Question 5

name 2 types of integral membrane proteins (i.e. how they are connected to the membrane

Answer 5

1. polytopic- spans cell membrane multiple times
2. monotopic - anchroed to only one side of the membrane
3. bitopic - spans the lipid bilayer only once

Question 6

what is a peripheral membrane protein

Answer 6

• they associate noncovalently with membrane components
• associated to proteins or lipid heads

Question 7

how do you interpret a hydropathy plot and how can it predict transmembrane segments

Answer 7

• calculates the average hydropathy index for a window of a certain size
• the hydrophobic regions are typically transmembrane segments

Question 8

why can beta barrels extend through a membrane while a beta sheet cannot?

Answer 8

• the beta barrel can hide the hydrophilic regions on the inside while the hydrophobic regions face outward towards the hydrophobic lipids
• sheets cannot hide hydrophilic regions like this

Question 9

what are lipid rafts and what components are they enriched in?

Answer 9

• less fluid microdomains of the plasma membrane
  -enriched in cholesterol and sphingolipids with a long saturated fatty acid chain

Question 10

what is the difference between lateral diffusion and transverse diffusion of a membrane lipid? which requires energy and why?

Answer 10

• lateral diffusion - molecules move sideways, changing places with neighboring lipids
• transverse diffusion - lipid moves from one side of the bilayer to the other
• necessary to maintain sidedness
• ## transverse requires ATP to move one lipid, but not to move two at the same time

Question 11

what is one possible function of lipid rafts?

Answer 11

• localize molecules involved in a cartain pathway near each other

Question 12

what is Fluorescence Recovery After Photobleaching (FRAP)

Answer 12

• attachment of probes to head groups of lipids
• bleach small area with a laser beam to make it nonfluorescent
• see how long it takes for the labeled lipid molecules to diffuse into the bleached region to get diffusion rate

Question 13

do all proteins have the ability to diffuse laterally? why or why not?

Answer 13

• no, many are attached to the cytoskeleton, which limits their movement

Question 14

passive diffusion of polar molecules through a membrane has a high activation barrier, why?

Answer 14

• the middle of the bilayer is very nonpolar so it takes a lot of energy to get a polar molecule through it
• lots of energy to remove solute shell and then readd it

Question 15

what are the 2 broad classes of proteins that help molecules cross membranes and what is a major difference between them?

Answer 15

• channels and transporters
• channels are basically just open highways - cannot be saturated
• transporters can move molecules against their electrochemical gradient, transporters cannot
• transporters can be active or passive channels are all passive

Question 16

Define membrane potential

Answer 16

• voltage difference across a biological membrane
• typically negative on the inside

Question 17

what is an electrochemical gradient

Answer 17

• defined by the combined effect of the concentration and electrical gradient across the membrane
• drives the movement of ions

Question 18

what type of proteins are necessary to move molecules against their electrochemical gradient

Answer 18

• active transport proteins (transporters)
• primary expends energy, usually ATP
• secondary couples uphill transport with a downhill transport