Structure Of Biomembranes Flashcards
What is the Fluid Mosaic Model of the membrane?
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
What basic features do biomembranes have in common?
A phospholipid bilayer
Proteins embedded in the bilayer
Lateral mobility of lipids and proteins
Flexibility that permits shape changes
What are amphipathic molecules and why are they important for membranes?
Molecules that exhibit both hydrophilic and hydrophobic regions. These properties cause the membrane to organize into a bilyaer
How does cross-sectional area of the polar and nonpolar regions affect membrane conformation?
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.
Describe the general structure of a glycerol phospholipid
Polar head - amino alcohol — phosphate — glycerol
Non polar tail - fatty acid chain
What is the most abundant type of lipids in the cell membrane?
Phospholipids
Describe the structure of sphingolipids and give an example.
Sphingolipids are phospholipds that contain sphingosine (an amino alcohol with a long hydrocarbon chain)
E.g. sphingmyelin
What are glycolipids and where are they located?
Lipids that contain a carbohydrate moiety
Only occur in the outer leaflet
What is the structure and function of cholesterol?
Cholesterol is an amphipathic sterol that is located within the bilayer of the membrane
It maintains membrane fluidity
Describe the asymmetric distribution of phospholipids in the lipid bilayer.
Inner leaflet contains more phosphotidylethanolamine, phosphotidylserine, and phosphotidylinositol
Outer leaflet - phosphotidylcholine, sphingomyelin, glycolipids
List the steps to create phospholipid bilayers in vitro
1) place the membranes in organic solvent
2) all the solvent to evaporate
3) mechanically disperse the remaining residue into an aqueous solution
Why are lipids able to diffuse laterally within the bilayer?
They are held together by noncovalent forces rather than covalent bonds.
What is the effect of temperature on membrane fluidity?
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.
Describe the effect of cholesterol on membrane fluidity
Cholesterol stabilizes membrane fluidity by both keeping lipids apart and pulling them towards the molecult
Describe the effect of fatty acyl chain saturation on membrane fluidity
Saturation allows close packing of the hydrocarbon tails and decreases fluidity
Unsaturation increases the space between tails and increases fluidity
Describe the effect of hydrocarbon tail length on membrane fluidity
As tail length increases, more interactions occur between the tails and this will decrease fluidity.
What is a lipid raft?
Localized regions of the membrane enriched with cholesterol, sphingolipids, and phospholipids with saturated fatty acyl chains
What is glycophosphoatidylinositol? (GPI)
GPI is a glycolipid that binds a protein and attaches it to the plasma membrane
Where are phospholipids synthesized
The cytosolic surface of the ER
What is the movement of phosphoatidylserine to the external surface associated with?
Apoptosis and cell senescence (aging cells)
What are the differences in the two ways phospholipids move in the membrane?
Lateral diffusion - rapid, unfacilitated
Flip-flop - transfer between monolayers, slow unless facilitated by lipid translocator proteins
What are flippases?
Family ATP-dependent lipid translocators that move a phospholipid from the outer layer to the inner layer
Maintain asymmetric distribution of phospholipids
What are floppases
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
What are scramblases?
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
What are integral membrane proteins?
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
What are peripheral proteins?
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)
What is a common structure of transmembrane proteins?
Membrane-spanning a-helices
Composed of hydrophobic residues
May make a single or multiple passes
What is a porin, what is their structure, and where are they found?
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
How are proteins linked to the membrane through acylation?
A single covalent linkage between a fatty acid acyl chain (palmitate or myristate) and an N-terminal glycine group in the protein.
Cytoplasmic leaflet
How are proteins linked to the plasma membrane via prenylation?
A thioester bond between the thiol group of a C-terminal cystein residue and a hydrocarbon anchor (Farnesyl or geranylgeranyl)
What is gycosylphosphotidylinsoitol used for?
Anchoring proteins to the exoplasmic face via a link between GPI and the C-terminus of the protein.
Name four things that restrict the rate of lateral diffusion of proteins
- 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
What are two ways lateral movement of proteins has been observed.
- Mouse-human cell combination experiment
- Protein bleach experiment
Where are glycoproteins and glycolipids located and name two functions.
Outer leaflet of the membrane.
Formation of the glycocalyx and cell-to-cell recognition
