Biological Membranes and the Cell Surface

Membrane Structure According to the Davson-Danielli Model (the sandwich model)

• NOTE: this model is not correct but shows the progression of the current model
• Lipid bilayer composed of phospholipids
  • Hydrophobic tails inside
  • Hydrophilic heads outside
  • This forms two separate water-interacting surfaces
• Proteins coat outer surface
  • This forms a protein-lipid sandwich
  • Proteins do not permeate the lipid bilayer
    
• Problems with this model
  • This model assumes that all membranes are identical - this was known to be false
  • The membrane proteins would be exposed to hydrophilic environments on all sides (from the phospholipids and from the water of the cytoplasm). This is not a stable configuration.

Membrane Structure According to the Fluid Mosaic Model of Singer and Nicolson

• The membrane is a fluid mosaic of phospholipids arranged in a lipid bilayer and proteins
• Two main categories of membrane proteins - integral and peripheral
  • Peripheral proteins - bound to the surface of the membrane
  • Integral proteins - permeate the surface of the membrane
• Membrane regions differ in protein configuration and concentration
  • Outside vs. inside - different peripheral proteins
  • Proteins only exposed to one surface
  • Proteins extend completely through - exposed to both surfaces
  • Membrane lipid layer fluid
    • Proteins move laterally along membrane

Comparison of these two models

Membrane Lipids

• Phospholipids most abundant
  • Phosphate may have additional polar groups such as choline, ethanolamine, serine, inositol
  • These increase hydrophilicity
• Cholesterol - a steroid
  • Can comprise up to 50% of animal plasma membrane
  • Hydrophilic OH groups toward surface
  • Smaller than a phospholipid and less amphipathic (having both polar and non-polar regions of the molecule)
• Other molecules include ceramides and sphingolipids - amino alcohols with fatty acid chains
• These lipids distributed asymmetrically

Bilayer Formation

• Membrane components are Amphipathic (having both polar and non-polar regions of the molecule)
• Spontaneously form bilayers
  • Hydrophilic portions face water sides
  • Hydrophobic core
• Never have a free end due to cohesion
  • Spontaneously reseal
  • Fuse
• Liposome - Circular bilayer surrounding water compartment
  • Can form naturally or artificially
  • Can be used to deliver drugs and DNA to cells

Animation - this is an excellent animation describing lipid bilayer formation and function

Membrane Fluidity

• Membrane is Fluid
• Lipids have rapid lateral movement
• Lipids flip-flop extremely slowly
• Lipids asymmetrically distributed in membrane
  • Different lipids in each side of bilayer
• Fluidity depends on lipid composition
  • Saturated fatty acids
    • All C-C bonds are single bonds
    • Straight chain allows maximum interaction of fatty acid tails
    • Make membrane less fliuid
    • Solid at room temperature
    • "Bad Fats" that clog arteries (animal fats)
  • Unsaturated fatty acids
    • Some C=C bond (double bonds)
    • Bent chain keeping tails apart
    • Make membrane more fluid
    • Polyunsaturated fats have multiple double bonds and bends
    • Liquid at room temperature
    • "Good Fats" which do not clog arteries (vegetable fats)
  • Cholesterol
    • Reduces membrane fluidity by reducing phospholipid movement
    • Hinders solidification at low (room) temperatures

How Cells Regulate Membrane Fluidity

• Desaturate fatty acids
• Produce more unsaturated fatty acids
• Change tail length (the longer the tail, the less fluid the membrane)

Membrane Carbohydrates

• Face away from cytoplasm (on outside of cell)
• Attached to protein or lipid
  • Blood antigens - Determine blood type - bound to lipids (glycolipids)
  • Glycoproteins - Protein Receptors
  • Provide specificity for cell-cell or cell-protein interactions

Membrane Proteins

• Peripheral Proteins
  • completely on membrane surface
  • ionic and H-bond interactions with hydrophilic lipid and protein groups
  • can be removed with high salt or alkaline
• Integral Proteins
  • Possess hydrophobic domains which are anchored to hydrophobic lipids
  • alpha helix
  • more complex structure

An Example - Asymmetry of Intestinal Epithelial Cell Membranes

• Apical surface selectively absorbs materials
  • Contains specific transport proteins
• Lateral surface interacts with neighboring cells
  • Contains junction proteins to allow cellular communication
• Basal surface sticks to extracellular matrix and exchanges with blood
  • Contains proteins for anchoring