Biological Membranes

• Biological Membranes

  • Every living cell is surrounded by a thin, flexible cell membrane
  • Cell surface membrane
  • Similar structure in all cells
  • Partially permeable, level of this varies cell to cell depending on function
  • Regulate entry/exit:
  • Allow
  • Small polar/charged (H_2O and CO_2)
  • Small non-polar (O_2 and fatty acids)
  • Stop
  • Large molecules (enzymes, DNA, RNA)
  • Enzymes may leave cell when in lysosome (exocytosis)
  • Have own membrane
  • Never a rip cytoplasm could spill from
  • Lets ions and some larger polar molecules be transported in and out (glucose and amino acids)
• Fluid Mosaic Model

  • Model of membrane structure
  • Phospholipids form thin, flexible sheet
  • Proteins “float” in it, like icebergs
  • Carbohydrates extend out from proteins
  • Membranes are visible in TEM (magnifications x100,000) as two dark lines separated by clear space
  • Difference across them is ~7nm
  • Cholesterol:
  • Steroids in animal membranes
  • Polar (bind to phosphate heads) and non-polar regions (bind to fatty acids tails)
  • Key for stability, strength, fluid permeability (less leaky)
  • Weaker chemical bonds so not rigid