3. Biological Membranes I

Question 1

Functions of cell membrane

Answer 1

 Protects the cell and its contents from surroundings/other cells
 Provides cellular integrity
 Provides selectivity barrier
 Provides cellular compartments
 Allows cell movement
 Allows interaction with other cells
 Provides fluidity/flexibility to the cell

Question 2

Structure of cell membrane

Answer 2

 Composed of both lipids (50%) and proteins (50%)
 Phospholipid bilayer: non-polar basic structure of biological membranes
 Held together by non-covalent bonds
 Dynamic and fluid structure
 Contains membrane proteins (structural proteins, transporters, sensors and receptors

Question 3

The cell mebrane and its structure

Answer 3

Freeze-Fracture Electron Microscopy

 physically breaking apart (fracturing) a frozen biological sample
 structural detail of the fracture plane is visualized by vacuum-deposition of platinum-carbon
 Generation of replica for examination by transmission electron microscopy

Question 4

Lipids of the cell membrane

Answer 4

 phospholipids: most abundant lipids in cell membrane
 mainly phosphoglycerides (glycerol backbone+phosphate)
 14-24 hydrocarbons attached to glycerol backbone via ester bonds
 lipids are amphiphilic

Question 5

Contains different phospholipids

Answer 5

Phosphoglycerides:

Charge (0) = phosphotidyl - ethanolamine
Charge (- 1) = phosphatidyl - serine
Charge (0) = phosphatidyl - choline

Charge (0) = sphingomyelin
= sphingosine ceramide

All have polar groups attached to them held togetehr by hydrogen bonds

Question 6

Cholesterol + Cell membrane rigidity

Answer 6

 belongs to the sterols
 very abundant in cell membrane
 cholesterol : phospholipid ratio is usually about 1:1
 provides rigidity to the membrane: more cholesterol&raquo_space;> more rigid

Question 7

Variable lipid composition of cell membranes

Answer 7

Eukaryotic cell membranes:
 Lipid composition highly variable depending on organism/organelle
 highly complex composition: 500-1000 different molecules

Bacterial cell membranes:
 No cholesterol, mainly phospholipids
 Structural reinforcement providing rigidity: cell wall

Question 8

Lipid Bilayers - Spontaneous Formation

Answer 8

 Lipid bilayers form spontaneously due to
amphiphilic nature of lipids
 Hydrophylic (polar) lipid heads exposed to water
 Hydrophobic (non-polar) fatty acid tails group together
 Lipid bilayer behaves like two-dimensional fluid

Question 9

Formation of sealed lipid compartments

Answer 9

 Hydrophylic (polar) lipid heads exposed to polar water

 Hydrophobic (non-polar) fatty acid tails not exposed to polar water

Question 10

Lipid Bilayers - a tool to study function

Answer 10

Black membranes (planar lipid bilayer)

Liposomes: lipid bilayer vesicles (1-25 nm)

 Lipid bilayer can include specific membrane proteins
 to study function of transporters (channels)

Question 11

The cell membrane - 2D fluid

Answer 11

Monolayer leaflet is driven by lateral diffusion by flexion + rotation

 Mainly lateral diffusion of lipids within monolayer/leaflet

Lateral exchange of neighboring molecules: ~ 10 (7) times per sec

Lipid molecule diffuses length of bacterial cell (~ 2μm) in ~ 1 sec

 Extremely rarely: flip-flop = exchange between monolayers
 Flip-flop only possible when catalyzed by phospholipid translocator proteins

Therefore two-dimensional and NOT 3D fluid

 Fatty acid chains exhibit rotational motion along axis

Experimental evidence for rotation/lateral diffusion of lipids:
 Electron Spin Resonance: paramagnetic signal -
nitroxyl group attached to lipid
 Fluorescent dye or gold particle attached to lipid
 Fluorescence Recovery After Photobleaching (FRAP)

Techniques also used also for measuring movement of proteins

Question 12

Factors affecting cell membrane fluidity

Answer 12

Homeoviscous adaptation:

 Organisms adapt membrane composition to environmental conditions
 Conditions: Temperature range, mechanical forces, pressure, etc

Membrane Fluidity affected by:

(1) Lipid Composition of Membranes

 Saturation level of lipids: more unsaturated lipids, membrane will be thinner (more flexible), more fluid
 Length of hydrocarbons: shorter hydrocarbons, less interaction, membrane will be thinner, more fluid

=> unsaturated hydrocarbon chains with cis-double bonds => saturated hydrocarbon chains

Question 13

Saturated versus unsaturated lipids

Answer 13

saturated => 10 Carbons with Oxygen double bond on 1st carbon and OH group

unsaturated => 10 Carbons with Oxygen double bond on 1st carbon and OH group in addition to Carbon = Carbon double bond on 7th carbon

Question 14

Factors affecting cell membrane fluidity

Answer 14

(2) Temperature

 Phase transition temperature of membranes
 The temperature at which membranes change from liquid to rigid crystalline (gelling) - freezing point

This is linked to lipid composition:

 more unsaturated lipids: membrane is thinner (more flexible), more fluid, more difficult to freeze
 Length of hydrocarbons: shorter hydrocarbons, less interaction, more fluid, more difficult to freeze

Question 15

The cell membrane and lipid rafts

Answer 15

Original hypothesis: (fluorescent dye added)
-> lipids randomly and equally distributed (van der Waals forces
=> Liposomes made of phosphatidylcholine and sphingomyelin (1:1)

New:
-> specific lipids found in rafts/microdomains
=> Liposomes made of phosphatidyl choline, sphingomyelin, and added cholesterol (1:1:1)

 Are specialized regions of the cell membrane
 Involved in organization (concentration) of specific membrane proteins
 Contain elevated levels of cholesterol and glycosylated sphingolipids
 Glycosylated sphingolipids make lipid rafts more rigid + tightly packed
 Lipid rafts are dynamic structures
 Are Involved in cellular signaling and other important processes

Question 16

Lipid rafts and lipid droplet formation

Answer 16

 Lipid droplets bound by phospholipid monolayer
 Storage of lipids
-> Lipids are used as building blocks for membrane
-> synthesis or as energy source (beta-oxidation)

Question 17

Cell membranes are assymetric

Answer 17

e.g. membrane of human red blood cell

Extracellular space:
Mainly phosphatidylcholine and sphingomyelin (choline
headgroups)

Cytosol:
Mainly phosphatidylethanolamine and phosphatidylserine
(-1 charge) minor amounts of phosphatidylinositol

 Each lipid monolayer (leaflet) can show a difference in composition
 Significant difference in charge between monolayers: negative inside
 Asymmetry is functionally important for direction of nutrient transport, cellular signaling etc.

Question 18

Cell membrane assymetry - phosphatidylserine

Answer 18

Role of Phosphatidylserine in Apoptosis (Programmed Cell Death)
 Phosphatidylserine normally located on cytosolic side of membrane

In apoptotic cells
 Phospholipid translocator inactivated Transports lipids from cytosolic to non-cytosolic monolayer
 Scramblase activated Transfers phospholipids non-specifically in both directions
 Phosphatidylserine appears on extracellular surface of cell membrane
 Phosphatidylserine recognized by macrophages
 phagocytosis: digestion of non-functional cells

Viable Cell:
CD31 -> repulsion => phagocyte

Apoptotic Cell:
CD31 -> eat me signals -> engulfment receptors => phagocyte

Question 19

cell membrane asymmetry - Phosphatidylinositol

Answer 19

Phosphatidylinositol important for cellular signaling

 minor (not abundant) phospholipid
 has multiple negative charges
 only found in inner monolayer/leaflet of lipid bilayer

Question 20

Phosphatidylinositol - Cellular Signaling

Answer 20

extracellular signal (activated recptor protein) -> activated PI 3 kinase => Cytosol (phosphorylated inositol phospholipid -> docked intracellular signalling protein) -> relay signal

 phosphorylated by lipid kinases (PI 3-kinase)
 Recruits specific proteins to membrane

extracellular signal (activated recptor protein) -> phospholipase C -> signalling fragments -> relay signal

phospholipase C cleaves phosphatidylinositol
Activates protein kinase C

Question 21

Cell mebrane asymmetry - Glycolipids

Answer 21

Neutral glycolipid

A) galactocerebroside

Charged glycolipids

B) GM 1 ganglioside
C)sialic acid (NANA)

 Sugar-containing lipids
 Asymmetric distribution exclusively found in non-cytosolic monolayer
 they self-associate
 found mainly in lipid rafts
 Up to 5% in outer membrane monolayer/leaflet of animal cells
 Sugars are added in Golgi-apparatus

Different Functions:

 Cell recognition processes
 Cell-cell adhesion

Epithelial cells:

 Protection of membrane against low pH and degrading enzymes

Charged gangliosides:

Electrical properties/field