3. Biological Membranes I Flashcards
Functions of cell membrane
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
Structure of cell membrane
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
The cell mebrane and its structure
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
Lipids of the cell membrane
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
Contains different phospholipids
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
Cholesterol + Cell membrane rigidity
belongs to the sterols
very abundant in cell membrane
cholesterol : phospholipid ratio is usually about 1:1
provides rigidity to the membrane: more cholesterol»_space;> more rigid
Variable lipid composition of cell membranes
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
Lipid Bilayers - Spontaneous Formation
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
Formation of sealed lipid compartments
Hydrophylic (polar) lipid heads exposed to polar water
Hydrophobic (non-polar) fatty acid tails not exposed to polar water
Lipid Bilayers - a tool to study function
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)
The cell membrane - 2D fluid
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
Factors affecting cell membrane fluidity
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
Saturated versus unsaturated lipids
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
Factors affecting cell membrane fluidity
(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
The cell membrane and lipid rafts
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