Membrane Structure

1
Membrane Structure

• Permeability mechanical characteristics
• Composition
• Spontaneous closure
• Fluidity
• Asymmetry
• Membrane Proteins
• Types
• Transmembrane domain prediction by hydropathy
  ploty

2
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

What accounts for these features
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Properties arise from composition

• Membrane structure and function depends on lipids
• Specific function depends membrane proteins
• Membrane lipids and proteins may be glycosylated

Lipid/protein ratios weight 5050 number 501
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Phosphoglycerides, Sphingolipids and Sterols are
the major lipids in cell membranes
phosphoglycerides
5
Sphingolipids (including glycolipids)
6
Cholesterol
7
Due to the amphipathic nature of phospholipids,
these molecules spontaneously assemble to form
closed bilayers
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Lipids and integral proteins demonstrate
rotational lateral mobility in biomembranes
107 exchanges/sec
The Fluid Mosaic Model

• Mobility (diffusion) of given membrane
• components depends on
• the size of the molecule
• its interactions with other molecules
• temperature
• lipid composition (packing)
• acyl chain length
• acyl chain saturation
• cholesterol concentration

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Fluorescence recovery after photobleaching (FRAP)
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Despite fluidity lipid bilayers can form rafts
GPI-anchored long TMD proteins may partition
into glycolipid rafts where SM increases bilayer
width
111 PC, SM, chol
11 PC, SM
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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
phospholipid translocator (flipase)
Exo PC, SM Cyto PS, PE, PI
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Each cell membrane has a set of specific membrane
proteins that allows the membrane to carry out
its distinctive activities.

• Types are
• Integral
• Transmembrane domain (TMD), single pass
• Multiple TMDs, multi-pass
• Lipid anchored covalently attached hydrocarbon
  chains
• Peripheral associated with membranes through
  interactions with integral proteins

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Peripheral proteins are removed by ? pH or
salt Integral proteins are solubilized only with
detergent
pH or salt
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Anatomy of a Transmembrane Domain (TMD) eg.
Glycophorin

• Approx. 20 hydrophobic amino acids
• Forms alpha helix-gt extends about 4 nm

• Notes
• charges prevent slippage
• Side chains interact w/ acyl chains (hydrophobic
  Van der Waals)
• CO imino NH of helix are H-bonded

Figure 3-33
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TMD prediction by hydropathy plot
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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)