Flip flop and Scrambling

Q: What two characteristics of lipid bi-layers can make a membrane more fluid
A: Short FA chains and cis double bonds

Q: What is the phospholipid/cholesterol ratio in eukaryotic membranes
A: 1:1

Q: Cholesterol makes a lipid bilayer more rigid (less fluid),
at the same time it prevents the hydrocarbon chains from doing what
A: Crystallizing

Q: Describe the asymmetrical distribution of phosholipdis in a bilayer
A: PS, PI, PE face the cytoplasm; PC and sphingomyelin face the outer surface

Q: Flipases, Flopases and scrambleases are what type of transporters
A: Flip and Flop ATP dependent; scramble is ATP independent

Q: How can you remember which direction flipase and flopase work
A: flIp goes In: flOp goes Out


In steady state the choline containing phospholipids,
sphingomyelin (SM) and phosphatidylcholine (PC) are mainly found on the
outside of the bilayer while the aminophospholipids are mainly (phosphatidylethanolamine,
PE), or exclusively (phosphatidylserine, PS), found in the inner monolayer.

This organization is maintained by an active set of transport systems
that "flip" and "flop" phospholipids across the membrane.

The flipase actively transports aminophospholipids from the outer to the
inner monolayer,

while the scramblase, when activated, moves all phospholipids
in both directions, thereby scrambling the phospholipid distribution.

PS exposed on the surface forms a docking site for hemostatic factors
such as the prothrombinase complex (factor Xa, Va and II). In addition,
PS is recognized by macrophages and interacts with proteins such as annexin

V.