Membranes Flashcards
Size of cell membranes?
5-10 nm thick
Functions of cell membranes? (8)
- compartmentalization
- scaffold for biochemical activities
- provides selectively permeable barrier
- transporting solutes
- responding to external signals
- intercellular interactions
- intracellular interactions
- energy transduction
Composition of cell membranes (3 components)
lipids (bimolecular layer)
proteins
carbs–attached to lipids or proteins
Membrane Lipids are all…
ALL AMPHIPATHIC
Phospholipids
organize so that fatty acid tails face one and other and polar heads face aqueous l outside. In between the fatty acid chains is cholesterol–more = more rigid
If water wants to get through a membrane there must be a…
channel
Movements of lipid bilayer (3 types and associated times)
- flex 10^-9 sec
- lateral shift 10^-6 sec
- transverse diffusion (flip flop) 10^5 sec
need flipases to move from one leaflet to the other
Features of lipid bilayers… (3)
fluid
seal spontaneously
asymmetric because lipids do not flip flop
Membrane carbohydrates exist as…
glycoproteins or glycolipids which are asymmetric and short oligosaccharides
always face away from the cytosol
important in cell-cell interactions and sorting proteins into compartments
Types of linkage between carbs and proteins/lipids
N-linked (amino–attached to nitrogen) more common
O-linked (attached to O by carboxyl) less common
Membrane Proteins (3 types)
~50% by weight
integral
peripheral
lipid anchored
Integral membrane proteins
penetrate right through lipid bilayer
asymmetric
amphipathic–hydrophobic part anchors it into the lipid bilayer really well
Peripheral membrane proteins
weakly attached by electrostatic interactions
dynamic–leave and come back
sit on edge
asymmetrical
Lipid anchored membrane proteins
Are on either side of membrane but don’t go through
Defined by being covalently attached to phospholipid (phosphotidylinositol)
lipid–carb–protein
GPI is attached to a phospholipid only on the outside of cell BUT if attached to myristate or farnesyl then can be either inside or outside and not called GPI
Fluid mosaic model
defines characteristics of cell membrane
fluid–dynamic, there is movement
quasifluid–movement is somewhat restricted
mosaic–asymmetrical, little bit patchy because no flip flop
Example of how phospholipids are asymmetrically distributed
Polar head defines the phospholipid
PC almost exclusively on outside
PE almost exclusively on inside
Technique to study membrane protein mobility
FRAP
Five types of protein movement
- Random (not much restriction)
- No movement ever–completely stuck
FRAP, colours never come back - Defined direction of movement
because being carried along cytoskeleton by motor proteins - No movement (immobilized)
because swarmed by other transmembrane proteins corralled around it - Random movement that is restricted
has to stay within constrains (likely due to cytoskeleton)
How did they prove that proteins are sometimes anchored by the extracellular matrix?
Removed cytoplasmic portion of protein
- Found that they sometimes stayed stuck and other times were free to move
eg: acetycholine receptor at neuromuscular junction
Lipid Rafts
Microdomains within the the cellular membrane that posses decreased fluidity due to the presence of cholesterol, glycolipids, and phospholipids containing longer saturated fatty acids
Lipid rafts 3 features
- tightly packed (high [saturated fat]
- high [sphingolipids]
sphingolipid=no glycerol backbone, long fatty acid tail - A lot of cholesterol (rigidity)
Lipids rafts have a lot of…
GPI anchored proteins and signaling proteins
Experiment showing that diffusion is limited for other lipds
lipid labelled with gold particle get tracings, find that moves randomly but in one area– in cell membrane. If done in an artificial membrane movement not restricted at all
Why are red blood cells good for studying the plasma membrane?
Because they have no nucleus and not a lot of other organelles
Found spectrin which gives a membrane its shape (holds it–biconcave in this case)
