L9. Membrane structure & function

Question 1

how does the plasma membrane act?

Answer 1

• as a selective barrier
• as a means of information relay

Question 2

plasma membrane - selective barrier

Answer 2

• separates cell form its surroundings
• enables cells to differ from its environment

Question 3

plasma membrane - information relay

Answer 3

• receiving information using receptor proteins
• import and export of small molecules using transport proteins
• capacity for movement and expansion

Question 4

explain membrane composition

Answer 4

• all membranes are composed of lipids and proteins
• the lipids are arranged in two closely apposed sheets forming a lipid bilayer
• this bilayer serves as a permeability barrier to most water-soluble molecules

Question 5

membrane composition - lipids

Answer 5

• each lipid has a hydrophilic head and a hydrophobic tail
• thus are known as amphipathic

Question 6

membrane composition - phospholipids

Answer 6

lipids that have a phosphate-containing hydrophilic head linked to a pair of hydrophobic tails

Question 7

phospholipids - hydrophobicity and hydrophilicity

Answer 7

• the hydrophobic tails are insoluble in water bc they are uncharged and nonpolar
• the hydrophilic heads dissolve readily in water bc they are charged and polar

Question 8

explain the spontaneous closing of the bilayer

Answer 8

because of the phospholipids hydrophobic and hydrophilic properties it is energetically favorable for the molecules to create the bilayer

Question 9

spontaneous closing of phospholipid bilayer - what are liposomes

Answer 9

they are pure phospholipids that forms closed spherical vesicles when added to water

Question 10

what does the fluidity of the phospholipid bilayer depend on

Answer 10

• hydrocarbon tail lengths
• saturation

Question 11

fluidity of phospholipid bilayer - hydrocarbon tail length

Answer 11

• longer tails interact more and makes the membrane less fluid
• shorter tails interact less and makes the membrane more fluid

Question 12

fluidity of phospholipid bilayer - saturation

Answer 12

• unsaturated tail = has kinks (double bonds present) = more fluid
• saturated tail = has no kinks (no double bonds present) = less fluid

Question 13

phospholipid fluidity - bacteria and yeast

Answer 13

• they are able to modify the conditions with temperature
• higher temp = longer tails and fewer double bonds = less fluid

Question 14

what do scramblases do

Answer 14

they are enzymes that randomly transfer phospholipid molecules from one monolayer to the next

Question 15

scramblases - where are they located and when are they used

Answer 15

• located in ER membrane
• used when phospholipid synthesis adds new phospholipids to the cytolytic side of the bilayer

Question 16

what do flippases do?

Answer 16

they are enzymes that catalyzes the transfer of specific phospholipids to the cytolytic monolayer

Question 17

flippases - where are they located and when are they used

Answer 17

• located in Golgi membrane
• used when membranes leave the ER and are incorporated into the Golgi

Question 18

flippases - how is it important for apoptosis (cell death)

Answer 18

• When a cell is undergoing programmed cell death, flippases stops working
• PS (Phosphatidylserine) goes up the bilayer instead of staying at the bottom (inner leaflet)
• When PS (Phosphatidylserine) is detected the cell is cleaned up by macrophages

Question 19

what is the role of cholestrol

Answer 19

• stiffens the bilayer by filling the spaces made by unsaturated kinks
• can impact fluidity based on temperature
• facilitates lipid-raft formation

Question 20

cholesterol - how is fluidity impacted by temperature

Answer 20

• at high temp: reduces fluidity and permeability
• at low temp: reduces crystallization and maintains fluidity

Question 21

cholesterol - what is the lipid-draft formation

Answer 21

• they are pockets where there is more cholesterol packed and has phospholipids with longer tails
• they have a longer region of hydrophobicity so proteins can have longer transmembrane domains

Question 22

explain alpha-helical transmembrane proteins

Answer 22

• hydrophobic side chains of alpha-helix contact the hydrophobic hydrocarbon chains of the phospholipid molecules
• hydrophilic parts of the alpha-helix form hydrogen bonds with one another on the inside of the helix

Question 23

explain how membrane proteins can be extracted

Answer 23

• with detergents
• when mixing membranes and detergent together, hydrophobic ends of the detergent interacts with hydrophobic regions of transmembrane proteins
• this disrupts the lipid bilayer and separates most proteins from the phospholipids

Question 23

explain beta-barrel transmembrane proteins

Answer 23

• they roll up into a cylinder
• hydrophilic side chains will face the inside of the barrel
• hydrophobic portion will contact the core of the bilayer

Question 24

what is the cell cortex

Answer 24

- it is a fibrous protein meshwork that stabilizes animal cells - it is rich in actin (used for structure) and myosin (used for movement)

Question 25

how can the plasma membrane's mobility be restricted

Answer 25

- lateral mobility restriction - tight junctions

Question 26

restricting plasma membrane mobility - lateral mobility restriction

Answer 26

- proteins can be tethered to: 1. the cell cortex inside the cell 2. to extracellular matrix molecules 3. to proteins on the surface of another cell

Question 27

restricting plasma membrane mobility - tight junctions

Answer 27

- a barrier formed along the line where a cell is sealed to adjacent cells - creates a seal between the plasma membranes and the proteins cannot diffuse past the junction

Question 28

what is the glycocalyx

Answer 28

- a sugar coating on the outside of the cell - the coat is connected to the transmembrane proteins - its purpose is to help protect the cell surface from mechanical damage