Introduction to cell membranes

Question 1

What does the appearance of biological membranes differ depending on?

Answer 1

Location and function

Question 2

What is the axon surrounded by and how is it formed?

Answer 2

-surrounded by myelin sheath
-formed from plasma membrane of a Schwann cell

Question 2

What do plasma membranes do?

Answer 2

-provides cell boundary
-prevents movement of materials into + out of cell

Question 3

What do organelle membranes do?

Answer 3

Divide cytoplasm into compartments

Question 4

What are the fundamental properties of membranes?

Answer 4

-acts as a barrier
-flexible, self-repairing, continuous
-selectively permeable

Question 4

What is the advantage of compartments?

Answer 4

Each organelle can function optimally

Question 4

Describe the composition of membranes

Answer 4

Composition of all membranes are basically similar:
-lipids
-proteins
-carbohydrates

Question 5

What are linked to proteins or lipids in the membrane and how?

Answer 5

-oligosaccharide chains made up of sugar residues linked covalently in branched chains
-covalently linked

Question 6

What can phospholipids do in the bilayer?

Answer 6

-can rotate or exchange in the lateral plane of the membrane
-normally move from 1 leaflet to the other (flip-flop)

Question 7

What determines the fluidity of the lipid bilayer?

Answer 7

-number of double bonds in fatty acid chains of phospholipids
-number of carbon atoms in fatty acid chains of phospholipids
-more double bonds + shorter acyl chains = less tightly packed molecules, so more fluid

Question 8

What does it mean to say phospholipids are amphipathic?

Answer 8

Contain a polar head group + non-polar hydrophobic tail

Question 9

What isomers are possible for phospholipids to have and which is most common?

Answer 9

-cis + trans isomers
-cis isomer usually found in nature as it increases packing density

Question 10

What are the charges of the major membrane phospholipids?

Answer 10

Balanced charges:
-phosphatidyl-ethanolamine
-phosphatidyl-choline
-sphingo-myelin
Negatively charged:
-phosphatidyl serine

Question 11

What is the structure of cholesterol?

Answer 11

-small polar head group
-rigid steroid ring structure
-long hydrocarbon tail

Question 12

What is the function of cholesterol and how does it do this?

Answer 12

-makes membranes less permeable
-packs between phospholipids, making membrane less deformable at surface
-at high conc. it stops membrane becoming crystalline

Question 13

Describe the behaviour of hydrophobic + hydrophilic molecules in aqueous solutions

Answer 13

-polar molecules can be completely dissolved in water
-non polar molecules can’t be - forms crystalline structure

Question 14

Describe the behaviour of lipids in aqueous solutions

Answer 14

-lipids form micelles or bilayer
-hydrophobic tails protected from aqueous environment by facing inward
-hydrophilic heads face outwards

Question 15

How do peripheral membrane proteins fit into the membrane?

Answer 15

-associate with integral membrane proteins or directly bind lipids
-covalently bound lipids are inserted into membrane

Question 15

What are the 2 types of membrane proteins?

Answer 15

-integral membrane proteins
-peripheral membrane proteins

Question 15

How do integral membrane proteins fit into the membrane and how can they be extracted?

Answer 15

-directly inserted into membrane by a hydrophobic domain
-can be extracted using detergents + high pH

Question 16

What modification do small G proteins have + what does this allow them to do?

Answer 16

-have a fatty acid modification
-allows them to cycle on + off membranes
-active on membrane + inactive in cytosol

Question 17

What can membrane lipids form and why?

Answer 17

-can form clusters or microdomains
-not homogenous

Question 18

What can cholesterol and sphingolipids form?

Answer 18

-microdomains called rafts
-membrane is slightly thicker in the microdomain

Question 19

What do tight junctions do?

Answer 19

Prevent movement between apical + basolateral membranes