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Flashcards in Lec 18 Membranes Deck (28):

Membrane Functions:

1) barrier between cell and environment
-for eukaryotes separate the cell into different compartments
2)contain selective pumps to regulate transport of ions and biological molecules
3)Generates and maintains proton (pH) gradient
4)site of enzyme and reaction pathways
5)recognizes extracellular signals and communicates with cell exterior


What are the 6 membrane features?

1)Bilayer structure
2)Non-covalent assemblies
3)Asymmetry of the layers
4)Selectively permeable
6)Self-annealing ability


Bilayer structure

-made of two lipid layers
-hydrophobic tails interior
-polar head groups exposed to the hydrophilic exterior


Non-covalent assemblies

membrane is assembled using non-covalent polar interactions specifically hydrophobic interactions and van der waals interactions


Asymmetry of the layers

specific proteins are exposed to one surface or another to carry out their specific functions


selectively permeable

-the membrane provides a impenetrable barrier to ionic and polar molecules
-therefore specific membrane proteins transport these molecules across the membrane into or out of the cell



-lipids and proteins of the membranes can move within the membrane and the function of the protein is dependent on this movement
-the degree of fluidity is determined by the proportion of saturated, unsaturated fatty acid tails, and cholesterol found in the membrane


Self-annealing ability

if the membrane is disrupted they can spontaneously reseal immediately.
-the resealing is essential for cell viability
-also crucial during membrane fusion for process such end and exocytosis



contain both polar and hydrophobic regions


What drives the formation of lipid monolayers and bilayers?

-the association of hydrophobic regions to maximize their interaction which releases water


What factors are used to determine the stability of the lipid layer formation

-van der waals interactions between closely packed acyl tails
-hydrophobic effect is the dominant force


Thermodynamically what happens when the lipid layer is formed?

entropy is increased


What are two examples of vesicles?




contain a hydrophobic interior


What molecules form the micelles

the molecules that forms the micelles have short and single acyl chains on a polar head because the interior of the vesicle does not have space to accommodate longer or a larger number of acyl chains


What is the lipid concentration what which micelles are formed called?

critical micelle concentration (CMC)


What is the importance of the fluid nature of the membrane bilayer? why?

it is critical for the life of the cell because:
-acts as a 2 dimensional solvent for membrane proteins (tansporters and signaling molecules)
-analogous to aqueous cytosol acting as a 3 dimensional solvent for soluble proteins


lipid raft

-thicker than other parts of the brain
-function: is to accommodate and concentrate certain types of proteins to enable them to function together. (Ex: proteins converting extracellular signals to intracellular signals--transmembrane signaling)


What are the three types of protein associated with membranes?

1. Integral membrane (transmembrane) proteins
2.Lipid anchored membrane proteins
3. Peripheral membrane protein


integral membrane (transmembrane) proteins

-proteins that completely span a bilayer thus have surfaces exposed to both the interior and exterior of the cells
-have hydrophobic bundles of a helices or Beta sheets which make favorable hydrophobic interactions with the interior of the lipid--this is what causes the protein to be inserted into the membrane


lipid anchored membrane proteins

-proteins with a hydrophobic lipid tail
-inserts into the membrane to hold the protein in place
-hydrophobic interactions important


Peripheral membrane protein

-proteins associated with the membrane through contacts with the polar head groups through charge-charge interactions and H-bonding


What two structural motifs do integral membrane proteins display?

-Membrane spanning alpha-helical bundles
-beta barrels


what are the three types of anchors used for lipid-anchored proteins?

-protein joined to a fatty acid for a tail
-protein joined to a prenyl group for a tail
-protein joined to a phosphotidylinositol through a glycan for a tail


Lateral movement within a membrane

"Fluid Mosaic Model"
-membrane is a dynamic structure in which both lipids and proteins can move rapidly within the membranes bilayers
-this occurs because there is low cost in energy in lateral movement


Transverse movement within a membrane

-movement from one side of the membrane to another
-polar head group would have to travel through two layers of hydrophobic tails
-great energy barrier for this process


Membrane Fluidity

-controlled by fatty acid composition and cholesterol content
-fluidity is dependent on temperature
rigid at low temp
fluid at high temp


What does the phase-transition temp depend on?

-fatty acid chain length (shorter chains have lower Tm)
-fatty acid saturation level (fewer double bonds have higher Tm)
-How much cholesterol is present