Plasma Membrane (PM) bilayer

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Plasma Membrane (PM) bilayer

• Function characteristics
• Composition
• Spontaneous closure
• Asymmetry
• Fluidity

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What features make lipid bilayer an advantageous
container?

• Permeability -gt nutrients, waste, membrane
  potential
• Deformability -gtmovement, division
• Fluidity -gtreactions, subdomain assembly
• Asymmetry -gtspecialization of each face

5nm
What accounts for these features?
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Properties arise from composition

• Fundamental structure and function of all cell
  membranes depends on lipids (phospholipids,
  steroid derivatives)
• Specific function of each membrane depends on the
  membrane proteins that are present in that
  specific membrane
• Membrane lipids and proteins may be glycosylated

Lipid/protein ratios weight 5050 501
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Phosphoglycerides (phospholipid) most abundant
type think glycerol, 2 acyl chains,
phosphate, polar head group Sphingolipids
may be a phospholipid e.g. sphingomyelin may
be a glycolipid key is sphingosine (no
glycerol) Cholesterol polar OH steroid back
bone acyl chain
3 main types
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5.3 Phospholipid structure
Figure 5-27a
Figure 5-28
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Sphingolipids (including glycolipids)
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One type is phospholipidand sphingolipid
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Cholesterol, polar steroid with acyl chain
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5.3 Due to the amphipathic nature of
phospholipids, these molecules spontaneously
assemble to form closed bilayers
Figure 5-30
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Experimental formation Of pure phospholipid bilaye
rs
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5.3 Each closed compartment has two faces
The two faces of a membrane are asymmetric in
terms of lipid and protein composition
Proteins do not flip flop! Lipids may with aid of
flipase
Figure 5-31
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Phospholipases demonstrate asymmetry
Outside cellPLC releases choline, (SM) Inside
cell PLC releases inositol, ethanolamine, serine
gt exo PC, SM cytoPS, PE, PI
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5.3 Lipids and integral proteins demonstrate
(rotational ) lateral mobility in biomembranes
The Fluid Mosaic Model
Figure 5-35

• Mobility (diffusion) of a given membrane
• components depends on
• the size of the molecule
• its interactions with other molecules
• temperature
• lipid composition (tails, cholesterol)
• Mobility can be measured by FRAP

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5.3 Fluorescence recovery after photobleaching
(FRAP)
Figure 5-36
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• Fluidity depends on temperature and composition
• Temp phase transition
• Composition acyl chain length saturation,
  cholesterol

• Short or kinksgtfewer van der Waals interactions
• Cholesterol has opposing effects is tightly
  regulated
• Polar head group restricts phospholipid head
  group movement-gt decreases fluidity
• Planar steroid separates phospholipid acyl
  tails-gtincreases fluidity

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5.3 Functions of the plasma membrane

• Regulate transport of nutrients into the cell
• Regulate transport of waste out of the cell
• Maintain proper chemical conditions in the cell
• Provide a site for chemical reactions not likely
  to occur in an aqueous environment
• Detect signals in the extracellular environment
• Interact with other cells or the extracellular
  matrix
• (in multicellular organisms)