Structure Of Biomembranes

Question 1

What is the Fluid Mosaic Model of the membrane?

Answer 1

Cell membranes are fluid structures, composed of a lipid bilayer embedded with proteins, in which individual lipids and proteins are able to move laterally within the plane of the membrane

Question 2

What basic features do biomembranes have in common?

Answer 2

A phospholipid bilayer

Proteins embedded in the bilayer

Lateral mobility of lipids and proteins

Flexibility that permits shape changes

Question 3

What are amphipathic molecules and why are they important for membranes?

Answer 3

Molecules that exhibit both hydrophilic and hydrophobic regions. These properties cause the membrane to organize into a bilyaer

Question 4

How does cross-sectional area of the polar and nonpolar regions affect membrane conformation?

Answer 4

If the CS area of the polar group is larger, micelles will form

If the two CS areas are similar, they will form a hollow vesicle called a liposome.

Question 5

Describe the general structure of a glycerol phospholipid

Answer 5

Polar head - amino alcohol — phosphate — glycerol

Non polar tail - fatty acid chain

Question 6

What is the most abundant type of lipids in the cell membrane?

Answer 6

Phospholipids

Question 7

Describe the structure of sphingolipids and give an example.

Answer 7

Sphingolipids are phospholipds that contain sphingosine (an amino alcohol with a long hydrocarbon chain)

E.g. sphingmyelin

Question 8

What are glycolipids and where are they located?

Answer 8

Lipids that contain a carbohydrate moiety

Only occur in the outer leaflet

Question 9

What is the structure and function of cholesterol?

Answer 9

Cholesterol is an amphipathic sterol that is located within the bilayer of the membrane

It maintains membrane fluidity

Question 10

Describe the asymmetric distribution of phospholipids in the lipid bilayer.

Answer 10

Inner leaflet contains more phosphotidylethanolamine, phosphotidylserine, and phosphotidylinositol

Outer leaflet - phosphotidylcholine, sphingomyelin, glycolipids

Question 11

List the steps to create phospholipid bilayers in vitro

Answer 11

1) place the membranes in organic solvent
2) all the solvent to evaporate
3) mechanically disperse the remaining residue into an aqueous solution

Question 12

Why are lipids able to diffuse laterally within the bilayer?

Answer 12

They are held together by noncovalent forces rather than covalent bonds.

Question 13

What is the effect of temperature on membrane fluidity?

Answer 13

Low temps - decreased fluidity because fatty acyl chains pack close together and don’t move around

High temps - increased fluidity because the lipids have enough energy to overcome the intermolecular foces

*thickness of the bilayer is also decreased.

Question 14

Describe the effect of cholesterol on membrane fluidity

Answer 14

Cholesterol stabilizes membrane fluidity by both keeping lipids apart and pulling them towards the molecult

Question 15

Describe the effect of fatty acyl chain saturation on membrane fluidity

Answer 15

Saturation allows close packing of the hydrocarbon tails and decreases fluidity

Unsaturation increases the space between tails and increases fluidity

Question 16

Describe the effect of hydrocarbon tail length on membrane fluidity

Answer 16

As tail length increases, more interactions occur between the tails and this will decrease fluidity.

Question 17

What is a lipid raft?

Answer 17

Localized regions of the membrane enriched with cholesterol, sphingolipids, and phospholipids with saturated fatty acyl chains

Question 18

What is glycophosphoatidylinositol? (GPI)

Answer 18

GPI is a glycolipid that binds a protein and attaches it to the plasma membrane

Question 19

Where are phospholipids synthesized

Answer 19

The cytosolic surface of the ER

Question 20

What is the movement of phosphoatidylserine to the external surface associated with?

Answer 20

Apoptosis and cell senescence (aging cells)

Question 21

What are the differences in the two ways phospholipids move in the membrane?

Answer 21

Lateral diffusion - rapid, unfacilitated

Flip-flop - transfer between monolayers, slow unless facilitated by lipid translocator proteins

Question 22

What are flippases?

Answer 22

Family ATP-dependent lipid translocators that move a phospholipid from the outer layer to the inner layer

Maintain asymmetric distribution of phospholipids

Question 23

What are floppases

Answer 23

Family of ATP-dependent lipid translocators that move phospholipids and sterols from the inner leaflet to the extracellular leaflet

Belong to ABC cassette family transporters

Question 24

What are scramblases?

Answer 24

Family of transporters that move a phospholipid from one leaflet to the next along its concentration gradient.

Not ATP-dependent

Responds to increase in intracellular calcium levels

Question 25

What are integral membrane proteins?

Answer 25

Proteins inserted into the membrane, can be transmembrane or covalently attached to one leaflet by a lipid anchor

Can only be released using organic solvents

Question 26

What are peripheral proteins?

Answer 26

Proteins that are associated with integral proteins and membrane lipids by non-covalent interactions

Can be removed by high salt or extreme pH solutions (do not disrupt the bilayer)

Question 27

What is a common structure of transmembrane proteins?

Answer 27

Membrane-spanning a-helices

Composed of hydrophobic residues

May make a single or multiple passes

Question 28

What is a porin, what is their structure, and where are they found?

Answer 28

Porins are a class of transmembrane proteins.

Multiple B-sheets are folded into a barrel conformation

Outward hydrophobic side chains

Inward hydrophilic side chains

Present in some bacteria and outer membrane of mitochondria and chloroplast

Question 29

How are proteins linked to the membrane through acylation?

Answer 29

A single covalent linkage between a fatty acid acyl chain (palmitate or myristate) and an N-terminal glycine group in the protein.

Cytoplasmic leaflet

Question 30

How are proteins linked to the plasma membrane via prenylation?

Answer 30

A thioester bond between the thiol group of a C-terminal cystein residue and a hydrocarbon anchor (Farnesyl or geranylgeranyl)

Question 31

What is gycosylphosphotidylinsoitol used for?

Answer 31

Anchoring proteins to the exoplasmic face via a link between GPI and the C-terminus of the protein.

Question 32

Name four things that restrict the rate of lateral diffusion of proteins

Answer 32

• Protein-protein interactions within the membrane
• cytoskeletal interactions with membrane proteins
• interactions of membrane protein with protein on another cell
• interaction of membrane protein with proteins in ECM

Question 33

What are two ways lateral movement of proteins has been observed.

Answer 33

• Mouse-human cell combination experiment

- Protein bleach experiment

Question 34

Where are glycoproteins and glycolipids located and name two functions.

Answer 34

Outer leaflet of the membrane.

Formation of the glycocalyx and cell-to-cell recognition