Internal Organization of the Cell_1

Membrane Structure
1. Plasma membrane
  1. Encloses cell, defines its boundaries and maintains the essencial differences between cytosol and extracellular environment
  2. Eucaryotic cells, membranes, endoplasmic reticulum, golgi apparatus, mitochondria, membrane-enclosed
    1. 4 phospholipids
      1. phosphatidylcholine
      2. phosphatidylethanolamine
      3. phosphatidylserine
        Translocation in apoptotic cell by two mechanism
        The phospholipid translocator is inactivated
        Scramblase that transfer phospholipids in both directions is activared
      4. sphingomyelin
2. Ion gradients
  1. Across membrane, activities of membrane proteins, synthesize ATP, movement of selected solute,produce electrical signals
3. thin film of lipid and protein molecules held by noncovalent interactions
4. Lipid Bilayer
  1. Fluid structure, impermeable barrier to water-solube molecules
    1. Lipid Molecules
      1. Amphipathics that have hydrophilic end and a hydrophobic end
      2. Phospholipids
        Have a polar head group and two hydrophobic hydrocarbon tails
        phospholipases
        activated by extracellular signals to cleave phospholipid
    2. depends on composition and temperature
      1. Cholesterol
        enhance the permeability-barrier
      2. glycolipids
  2. Liposomes
    1. spherical vesicles
  3. Black Membrane
    1. Planar billayers
  4. Phospholipid translocators
    1. Membrane-bound enzymes, catalyze the rapid flip-flop of phospholipids
Membrane properties
1. Glycolipids In the surface of the membrane
  1. The glycolipids are lipid molecules with the most extreme asymmetry
    1. These molecules are found in the noncytosolic monolayer of the lipid bilayer
      1. The glycolipids tend to self-associate,
        Through hydrogen bonds between their sugars
        Are in the cell surface where they have important roles
  2. Some glycolipids provide entry points for certain bacterial toxins
2. Membrane proteins
  1. Membrane proteins perform most of the specific functions of membranes
    1. Most trans-membrane proteins are thought to extend across the bilayer as a single a helix,
      1. Many proteins are attached to the membrane only by noncovalent interactions with other membrane proteins
    2. Membrane Proteins Can Be Associated with the Lipid Bilayer in Various Ways
      1. Many extend through the lipid bilayer of the membrane, with part of their mass on either side
        These transmembrane proteins are amphipathic
        Having regions that are hydrophobic and regions that are hydrophilic
        Their hydrophilic regions are exposed to water on either side of the membrane.
        Their hydrophobic regions pass through the membrane and interact with the hydrophobic
        Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it.
        Proteins bound to the plasma membrane can be readily distinguished by the use of an enzyme called phosphatidylinositolspecific
      2. Many of the proteins of this type can be released from the membrane by relatively gentle extraction procedures
        Such as exposure to solutions of very high or low ionic strength
        Of extreme pH, which interfere with protein– protein interactions but leave the lipid bilayer intact
  2. In Most Transmembrane Proteins the Polypeptide Chain Crosses the Lipid Bilayer in an a-Helical Conformation
    1. A transmembrane protein always has a unique orientation in the membrane.
    2. The hydrogen bonding between peptide bonds is maximized if the polypeptide chain forms a regular a helix as it crosses the bilayer
      1. In single-pass transmembrane proteins, the polypeptide crosses only once
        Whereas in multipass transmembrane proteins, the polypeptide chain crosses multiple times
        Because transmembrane proteins are notoriously difficult to crystallize, relatively few have been studied in their entirety by x-ray crystallography.
  3. Some b Barrels Form Large Transmembrane Channels
    1. Multipass transmembrane proteins are comparatively rigid and tend to crystallize readily.
      1. The number of b strands varies widely, from as few as 8 strands to as many as 22
      2. The b barrel proteins are abundant
        in the outer membrane of mitochondria, chloroplasts
        Many Membrane Proteins Are Glycosylated
        The great majority of transmembrane proteins in animal cells are glycosylated.
        Membrane Proteins Can Be Solubilized and Purified in Detergents
        The transmembrane proteins can be solubilized only by agents
        That disrupt hydrophobic associations and destroy the lipid bilayer.
        As in glycolipids, the sugar residues are added in the lumen of the ER and Golgi apparatus
        In many bacterias
Red Blood Cell
1. Most protein molecules associated with the human red blood cell membrane are peripheral membrane proteins bound to the cytosolic side of the lipid bilayer.
  1. Spectrin
    1. Long, thin, flexible rod about100nm in length. that constitutes 25 % of the membrane-associated protein mass.
  2. Glycophorin
    1. Small, single-pass transmembrane glycoprotein with most of its mass on the external surface of the membrane.. a Helix
  3. Ankyrin
    1. The binding of spectrin depends on a large intracellular attachment protein, which attaches both to spectrin and to the cytosolic domain of the transmembrane protein band 3..
  4. Band 3 protein (anion transporter)
2. The main function of red blood cells is to carry O2 from the lungs to the tissues and to help in carrying CO2 from the tissues to the lungs.
  1. Multipass membrane protein, traversing the membrane in a highly folded conformation. The polypeptide chain is thought to extend across the bilayer 12 times
Cell surface
1. Decorated by carbohydrates, which coat the surface of all eucaryotic cells
2. Membrane Proteins
  1. Membrane proteins do not tumble (flip flop) across the lipid bilayer, but they do rotate about an axis perpendicular to the plane of the bilayer
  2. Bacteriorhodopsin
    1. These proteins function as signal transducers rather than as transporters: each responds to an extracellular signal by activating another protein inside the cell, which generates chemical signals in the cytosol,
Cells can confine proteins and lipids to specific domains within a membrane

	Created by Cesar Corona Flores over 8 years ago

	Copied by LCM over 8 years ago

	Copied by Cesar Corona Flores over 8 years ago

	Copied by MAT ROD over 8 years ago

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Internal Organization of the Cell_1

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