03_Fluidity and Membrane Proteins Flashcards
What are the three fluidity phases of biological membranes? List from least to most fluid.
solid gel< liquid-ordered “raft” < liquid-disordered
What is a liposome? Why does it form?
A liposome is a spherical bilayer with a water-filled inner compartment, two leaflets, and a hydrophilic outer surface. It is formed spontaneously by pure phospholipid bilayers to avoid free ends (where the the hydrophobic core would be otherwise exposed to an aqueous environment).
Cholesterol packs well with ________ and __________.
types of lipids; where applicable indicate saturation
sphingolipids
saturated phospholipids
What group in cholesterol faces the membrane surface?
small hydrophilic hydroxyl head group
What is the implication of the flat & rigid rings of cholesterol in the lipid bilayer?
They interfere with movement of phospholipid fatty acid tails
What are lipid rafts?
The membrane is composed of a fluid ocean of lipids with more rigid regions (microdomains – enriched with sterols and sphingolipids).
Some proteins associate with these microdomains; therefore, lipid rafts are thought to be locations for proteins to exit/enter.
Define fluidity and viscosity.
Fluidity: measure of the ease of flow
Viscosity: measure of the resistance of flow
How does temperature affect membrane fluidity?
Explain the basis.
Bilayers are more fluid at higher temperatures due to “melting” of lipid-lipid interactions.
What is the transition temperature? What does it depend on?
Tm is the temperature at which phase transition occurs.
It depends on the lipids that are present in a particular bilayer.
A membrane is (less/more) ordered at a temperature below the Tm.
more
How does the length of fatty acid acyl side chains affect the fluidity of a bilayer?
Longer chains –> less fluid bilayer
Shorter chains –> more fluid bilayer
How is the viscosity-temperature graph affected by longer acyl chains?
graph shifts to the right
at the same temperature, viscosity is greater
How does saturation affect membrane fluidity? Explain the basis.
Saturated bonds decrease fluidity
Unsaturated bonds increase fluidity.
Saturated fatty acid chains are organized more regularly. Unsaturated bonds introduce kinks into acyl chains (cis double bonds), and more room is needed to accommodate the kink. Therefore, side chains are less packed, the bilayer is more fluid.
How is Tm affected by acyl chain saturation?
Tm increases with higher degrees of acyl chain saturation.
How is the graph of viscosity-temperature affected by more unsaturation in acyl chains?
graph shifts to the left
at same temperature, viscosity is smaller
How does the present of cholesterol affect membrane fluidity?
Cholesterol creates intermediate fluidity (gets rid of sharp transition temperatures).
How does the presence of cholesterol affect the durability and permeability of a membrane?
Increases durability
Decreases permeability
What are membrane lipids and proteins with attached carbohydrates (sugars) called?
Glycolipids
Glycoproteins
What type of membrane proteins cross the entire bilayer?
Integral membrane proteins
What is needed to remove integral membrane proteins?
Necessary to disrupt the bilayer (with detergent)
What type of membrane proteins are Beta barrel proteins in bacteria?
Integral membrane proteins
What are peripheral membrane proteins?
Proteins that are not inserted in the bilayer; instead, they are found on either side of the membrane. They interact with the membrane, or with integral membrane proteins.
What are some methods for removing peripheral membrane proteins?
high [salt], low/high pH
without disrupting lipid bilayer
How are lipid-anchored proteins inserted in the bilayer?
They have a hydrophobic group (usually an acyl chain) that allows the protein to insert into the cytoplasmic (inner) leaflet.
What type of modification is the addition of the acyl chain to lipid-anchored proteins?
post-translational
What type of proteins insert specifically into the external leaflet of the bilayer?
GPI-anchored proteins
GPI = Glycosylphosphatidylinositol
Describe the components of a GPI-anchored protein, and how they are attached.
A protein portion is covalently attached through glycosylation to the phosphate head group of a phosphatidylinositol
The sugar groups on glycoproteins lie mostly on the (inner/outer) side of the plasma membrane.
Outer
What is LPS?
Lipid A + polysaccharide
found in outer lipid bilayer of bacteria
Are there any sterols in bacteria?
no.
