What is a plasma membrane and how does it work? 9/22

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 vs. Trans-artificial– Kinks No Kinks– Lower Tm 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• Prokaryotes-• Plants-• Yeast-

• 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. SlowPhospholipid 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!

• 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!