The Plasma Membrane

Membrane Functions Form specialized compartments by selective permeability Unique environment Creation of concentration gradients pH and charge (electrical, ionic) differences Asymmetric protein distribution Cell-Cell recognition Site for receptor molecule biding for cell signaling Receptor binds ligand (such as a hormone) Induces intracellular reactions Controls and regulates reaction sequences Product of one enzyme is the substrate for the next enzyme Can "line up" the enzymes in the proper sequence

Membrane Structure According to the Davson-Danielli 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 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

Membrane Lipids

• Phospholips 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 sphingolipds - 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

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 fliuid
    • 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 - Asymetry 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