Midterm 2

• The plasma membrane is the outer boundary of the cell that separates the cellular contents from the outside world. You need to use a light microscope to see them.
• Membranes were found to be mostly composed of lipids because of their dissolving power corresponding to that of oil.
• By measuring the surface area of red blood cells and the area covered when they're spread over water, a 1:2 ratio was found, thus membranes are lipid bilayers.
• The most energetically favourable orientation for polar head groups is facing the aqueous compartments outside of the bilayer
• Bilayers are stabilized by van der Waals interactions in the fatty acyl chains (weak but there are many)
• Ionic and hydrogen bonds between the polar head groups with each other and with water in plasma membranes
• Proteins are present in the form of individual protein molecules and protein complexes that penetrate a lipid bilayer and extend out into the surrounding aqueous environment
• Due to lipid bilayer fluidity, membranes are dynamic structures in which the components are mobile and capable of coming together for temporary interactions
• Compartmentalization: defines boundaries of a cell and organelles
• Scaffolding for biochemical activities: provides a framework that organizes enzymes for effective interactions
• Selective permeability barrier: allows regulated exchange of substances
• Solute transport: membrane proteins facilitate movement of substances
• Response to external stimuli: receptors transduce signals in response to specific ligands
• Cell-cell communication: mediates recognition and interactions
• Energy transduction: membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy
• Plasma Membrane function:

  • Compartmentalization
  • Scaffolding for biochemical activities
  • Selective permeability barrier
  • Solute transport
  • Response to external stimuli
  • Cell-cell communication
  • Energy transduction
• Membranes are lipid-protein assemblies held together by noncovalent bonds
• Lipid bilayer: the structural backbone and barrier to prevent random movements of materials into and out of the cell
• Membrane proteins: carry out the more specialized functions
• Specialized/differentiated cell types have unique properties which result from the expression of distinct groups of membrane proteins
• The lipid-to-protein ratio is linked to function and varies, depending on the:

  • Type of cellular membrane
  • Type of organism
  • Type of cell
• Mammalian membranes are primarily composed of 3 types of lipids:

  • Phosphoglycerides
  • Sphingolipids
  • Cholesterol
• Phosphoglyceride structure:

  • At a minimum, contain 2 fatty acids, a glycerol, and a phosphate
  • 2 hydroxyl groups of glycerol are linked to fatty acids
  • The third hydroxyl group of glycerol is linked to a hydrophobic phosphate group
  • Most have a small hydrophilic head group linked to phosphate: choline, ethanolamine, serine, or inositol
• Phosphoglycerides are amphipathic. They have fatty acid chains at one end (hydrophobic) and a polar head group at the other end (hydrophilic).
• Phosphoglycerides usually contain one unsaturated and one saturated fatty acid chain
• Sphingolipids are derivates from ceramide, which is hydrophobic
• Sphingolipid Structure:

  • Derivatives from ceramide (sphingosine + a fatty acid)
  • Additional groups can be added to the terminal alcohol
  • If carbohydrates are added, it's called a glycolipid
  • Mutiple sugars can be added (ganglioside)
• Glycolipids are found exclusively on the exoplasmic face of the plasma membrane
• Glycolipids = glycosphingolipids (in this course)
• The sugar groups are added to the sphingolipids in the lumen of the Golgi
• Cholesterol:

  • Amphipathic because of the hydroxyl group and nonpolar hydrocarbon tail
  • The main sterol in animal cell membranes
• Cholesterol is need to stabilize and maintain membranes (important for fluidity)
• A cell's membranes have distinct lipid compositions, differing in lipid types, head groups, and species of fatty acid chains
• Lipid composition can determine the physical state of the membrane and influence membrane protein activity
• Membrane lipids can be precursors for highly active chemical messengers that regulate cellular function
• The hydrophobic nature of the hydrocarbon chains means they cannot be exposed to the aqueous environment --> membranes are always continuous unbroken structures (they form entensive inter-connected networks within the cell)
• Lipid bilayers are flexible and deformable. Their shape can change, as occurs during locomotion or cell division
• Lipid bilayers can self-assemble spontaneously
• In vitro, phospholipids can assemble spontaneously to form fluid-filled spherical vesicles called liposomes. These structures are good for research because they are a simpler environment than that of a natural membrane
• Distinct leaflets of the same lipid bilayer also have different lipid composition. Lipid bilayers can be thought of as two independent monolayers with differnt physical and chemical properties