Download What makes cell membranes work? 9/23

What is a plasma membrane and how does it work?
9/16 (Chapter 7-not on Monday lecture test)
What is the history of what we know about the plasma membrane?
– 7 researchers, years and progressive theories
• What are the lipids in a membrane?
• What is membrane fluidity and what factors determine it?
• How is the lipid melting point affected by fatty acid tail length,
polyunsaturation, trans-fats and sterols?
• What classes of protein are associated with membranes?
• What is a hydropathy analysis?
• Why can’t some proteins move freely in the plasma membrane?
• What is happening at this interface between cytosol and
extracellular material? Check out this YouTube video:
• http://www.youtube.com/watch?v=GW0lqf4Fqpg&feature=related
What is the current
theory of the “Fluid
Mosaic Model”?
Describe the
evolution of ideas,
the names of these
researchers, and
their relative dates.
(7 ideas/dates)
[6 People/dates]
What are the 3 main types of plasma membrane lipid?
Phospholipids, Glycolipids, and Sterols
• PL: Polar Group+Phosphate+Glycerol+2FA
• or (FA+Sphingosine+Pi+PolarGrp)
– Length of FA tail and Tail Saturation important for function!
– Main membrane component!
– Polar groups are quite variable!
• Glycolipid: Sphingosine+FA tail+Carbohydrate
– Mono or Polysaccaride
– Important for cell ID: Antibody/Antigen Rxns
• Sterols: 4 ring dead-end metabolite in humans
– Subtypes: Sterols, Phytosterols and Hopanoids
– Polar groups anchor to outside PM
– Non-polar groups stay inside PM
Phospholipids can have any one of several different polar heads.
Triglycerides are storage lipids and NOT located in a PM.
What things factor into estimates of membrane fluidity?
• Why have a flexible membrane? Can it be TOO fluid?
– Permit conformational changes to membrane proteins!
– Flex without tearing PM!
– Regularity to PM permeability!
• PL density- #/um2
• FA Tail length- long or short?
– Long Tail= + m.p.
Short Tail= -m.p.
• Unsaturated FA: example: omega fish oils
– Cis-natural
– Kinks
– Lower Tm
vs. Trans-artificial
No Kinks
Raise Tm
– Ultimately many factors combine together!
Why do long FA tails make the fluid (melted)
state so tough to maintain?
• Rigid stacking
• + Bilayer width
•
•
•
•
Homeoviscous Adaptation
ProkaryotesPlantsYeast-
• Each membrane PL ratio is unique!
The fluidity of a membrane is increased (M.P. or Tm lowered) when:
The tail length is shorter OR FA- tails are polyunsaturated
Different cells/organelles have unique membrane lipid
compositions to reflect different requirements/environments!
With respect to H2O temp, what FAtail lengths are expected
in Antarctic Icefish (-3 C) or Devils Hole Pupfish (40 C)?
Why do cis-bonds –M.P. and trans-FA modify fluidity?
• Cis-Fats creates spaces in the P.M.!
• Spaces in the P.M. create area for flex!
• FA Content: Animal Fats vs. Vegetable Fats
– Problems and solution for industry?
– Trans fats DO NOT KINK!
– Trans fats in diet promote disease!
• Trans Fats are created to prevent rancidity!
• Trans Fats help keep margarine solid at Room Temp!
Sterols can either increase or decrease membrane fluidity!
• Basic Shape:
– Membranes are up to 50% sterol!
• As temp goes down –OH inhibit tight PL-tail packing!
• As temperature goes UP- rings prevent PL Dispersion!
• CH also decreases PM permeability by filling in
holes/spaces between FA tails!
• Sterols are produced by all eukaryotic cells!
– Especially in the human liver at night!
– WE need not have them in out diet (except Vit D)!
Membrane Flip-Flop: Fast vs. Slow
Phospholipid diffusion across membrane
How do detergents work at the level of a lipid bilayer?
How does membrane fluidity impact human disease?
• Coronary Heart Disease: What are dietary risk factors?
– Cholesterol and Saturated fat
– Trans-fatty acids
– Fish Oils
• Fluidity Balance: flexibility vs. membrane tearing in
arterial wall and endothelial cells lining wall.
• Cell dysfunction and death!
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•
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Neurological Function: How diet affects nerve function?
Cholesterol and permeability to ions like Na+ and K+!
Neuronal flexing and tearing?
Links to hyperactivity and neurological degenerative
diseases?
What are the classes of protein one finds associated
with the plasma membrane?
Remember that not ALL membrane proteins may be free to diffuse
about the membrane, some may be anchored to a particular place.
Typically charges on amino (outside), carboxyl (inside), and
carbohydrate groups make membrane protein flip-flip an extreme rarity!
• How do hydropathy
analysis of a DNA
sequence help us
predict protein
functions?
• Why is this useful?
• Human Genome
Project and our
proteins/diseases!
Are all the proteins associated with the PM floating freely about
the PM? Certainly Not!
Some proteins HAVE to be anchored in the cytosolic side to
certain locations by the proteins actin, ankyrin, and spectrin!