Chp 10 cell bio

• a bilayer of phospholipids ~ 3 nm thick provides the basic architecture of membranes
• membrane proteins give each cellular membrane its unique set of functions
• individual phospholipids can move laterally and spin within the plane of the membrane, giving the membrane a fluidlike consistency similar to that of olive oil
• Ø Fluidity & flexibility also enables the dynamic property of membrane budding and fusion

  • When viruses r released from an infected cell
  • When the internal cellular membrane of the Golgi complex bud into vesicles in the cytosol
• Phospholipids are :

  Ø the principal building blocks of biomembrane

  Ø Amphipathic molecules - consist of 2 segment w very diff properties ;

  • A fatty acid-based (fatty acyl) hydrocarbon "tail" that is hydrophobic and partitions away from water
  • A polar "head group" that is strongly hydrophilic, or water lover, and tends to interact with water molecules
• When a suspension of phospholipids is mechanically dispersed in a aqueous solution, the phospholipids aggregate into one of the 3 forms

  • Spherical Micelles : has hydrophobic interior composed entirely of fatty acyl chains
  • Spherical liposomes : phospholipid bilayer surrounding an aqueous center
  • Sheetlike phospholipid bilayers : 2 moL thick
• Each phospholipid layer in this lamellar structure is called a leaflet
• Phospholipid bilayers can be generated in the laboratory
• A preparation of biological membranes is treated with an organic solvent, which selectively solubilizes the phospholipids and cholesterol. Proteins & carbohydrates remain in an insoluble residue
• A planar bilayer can form over a small hole in a partition separating 2 aqueous phases -> used to study the physical properties of bilayers such as their permeability to solutes
• 2 surfaces of cellular membrane :

  • Cytosolic face
  • Exoplasmic face
• Faces of cellular membranes are conserved during membrane budding and fusion
• Natural membranes from diff cell types exhibit a variety of shapes which complement a cell's function

  • The smooth, flexible surface of the erythrocyte plasma membrane allows the cell to squeeze through narrow blood capillaries
  • Some cells have a long, slender extension of the plasma membrane called cilium or flagellum which beats in a whiplike manner
• A typical biomembrane contain 3 classes of amphipathic lipids

  Phosphoglycerides ; Sphingolipids; Sterols
• • Phosphoglycerides :

  Consist of a hydrophobic tail composed of 2 fatty acid-based (acyl) chains esterified to the 2 hydroxyl groups in glycerol phosphates and a hydrophilic (polar) head group attached to the phosphate group
• • Sphingolipids :

  All of theses compounds r derived from sphingosine, an amino alcohol w a long hydrocarbon chain, contain a long-fatty acid attached in amide linkage to the sphingosine amino group. Like phosphoglycerides, some sphingolipids have a phosphate-based polar head group
• Sterols : the basic structure of sterols is a four-ring isoprenoid-based hydrocarbon
• while all phosphoglycerides are phospholipids only certain sphingolipids are and no sterols are
• Steroids : - Steroids are lipids characterized by a carbon skeleton consisting of 4 fused rings

  • Cholesterols, a type of steroid, is a component in animal cell membrane and precursor from which other steroids are synthesized
  • A high level of cholesterol in the blood may contribute to cardiovascular disease.
• n the 2D plane of a bilayer, thermal motion permits lipid moL to rotate freely around their long axes and to diffuse laterally withing each leaflet.

  Bc such movements r lateral or rotational, the fatty acyl chains remain in the hydrophobic interior of the bilayer
• Heat disorders the nonpolar tails & induce a transition from a gel to a fluid. As the chains become disordered, the bilayer also decreases in thickness
• A pure sphingomyelin (SM) bilayer is thicker than one formed from a phosphoglyceride such as phosphatidylcholine (PC)
• Cholesterol has a lipid-ordering effect on phosphoglyceride bilayers that increases their thickness but does not affect the thickness of the more ordered SM bilayer
• A phospholipids such as PC have a cylindrical shape and form essentially flat monolayers whereas those w smaller head groups such as phosphatidylethanolamine (PE) have a conical shape
• bilayer enriched w PC in the exoplasmic leaflet & PE w cytosolic face as in many plasma membrane would have a natural curvature
• The preferential location of lipids on one face of the bilayer is necessary for a variety of membrane-based functions
• The plasma membranes of cells contain combinations of glycosphingolipids and protein receptors organized in glycolipoprotein microdomains termed lipid rafts
• Lipids droplet formation begins w the accumulation of cholesterol ester & triglycerides within the hydrophobic core of the lipid bilayer
• The resulting delamination of the 2 lipid monolayers causes a "lens" to form, the further growth of which creates a spherical droplet that is then released by scission at the neck
• The newly formed droplet is surrounded by a lipid monolayer, derived from a cytosolic leaflet of the ER membrane
• Integral membrane proteins also called transmembrane proteins
• Lipid-anchored membrane proteins are bound covalently to one pr more lipid molecules
• Peripheral membranes proteins do not directly contact the hydrophobic core of the phospholipid bilayer. Instead they r bound to the membrane either indirectly by interactions w integral or lipid-anchored membrane proteins or directly by interactions w lipid head groups
• Proteins containing membranes panning alpha-helical domains are stably embedded in membranes because of : - Hydrophobic & VdW interactions of the hydrophobic side chains in the domain w specific lipids

  • Ionic interactions w the polar head groups of the phospholipids
• Van der Walls interactions (also known as London Dispersion forces) are weak attractions that occur between molecules in close proximity to each other
• Seven membrane-spanning alpha helices :

  • Bacteriorhodopsin, a photoreceptor in certain bacteria
  • G protein-coupled receptors (GPCR)

  (proteins)
• 6 membrane-spanning alpha helixes :

  • aquaporins (water/glycerol channels) a tetramer of 4 identical subunits, each subunit has 6 membrane-spanning alpha helixes
  • Many alpha-helical transmembrane segments do contain polar and/or charged residues. Their amino acid side chains can be used to guide the assembly & stabilization of multimeric membrane proteins.

  (proteins)
• T-cell receptor (TCR) for antigen.

  • Composed of 4 separate dimers : a pair directly responsible for ag recognition and accessory subunits collectively referred to as the CD3 complex. These accessories include the (grec letters)

  (protein)
• Only one membrane-spanning alpha helix: glycophorin A (on RBC) (protein)
• All porins are trimeric transmembrane proteins. Each subunit is barrel shaped w strands forming the wall and a transmembrane pore in the center