Biological Membranes

• Every living cell is surrounded by a thin, flexible cell membrane called the cell surface membrane; similar structure in all cells, its partially permeable but the level of this varies cell to cell dependent on function
• Membranes regulate entry/exit of all materials: allow small polar (charged) molecules (water and carbon dioxide) and small non-polar molecules (oxygen and fatty acids) to pass in and out
• Stop large molecules (enzymes, DNA, RNA) leaving the cell, enzymes may leave the cell when in a lysosome (exocytosis) as they have there own membrane; never has to be a rip cytoplasm could spill from
• Lets ions and some larger polar molecules to be transported in and out of the cell (glucose and amino acids)
• The fluid mosaic is a ‘model’ of the membrane structure: phospholipids form a thin, flexible sheet, while the proteins “float” in it, like icebergs - carbohydrates extend out from the proteins; it has receptor molecules (proteins, glycoproteins and glycolipids) on its surface
• Membranes are visible in the TEM (magnifications of x100,000) as two dark lines separated by a clear space - the difference across them is about 7nm
• The Fluid Mosaic Model of Cell Membrane

  A) Phospholipid bilayer

  B) Transport Protein

  C) Phospholipid

  D) Proteins

  E) Carbohydrate Chains

  
• It is called the fluid mosaic structure because all the components can move around (fluid), and the many different components fit together like a mosaic

  A) Phospholipids

  B) Cholesterol

  C) Cytoskeleton

  D) Proteins

  E) Glycocalyx

  F) Communication

  G) transport

  H) actions

  I) Peripheral Proteins

  J) Integral Proteins

  
• Cholesterol: Steroid in animal membranes, has polar (bind to phosphate heads) and non-polar regions (bind to fatty acids tails); key for stabilising, strengthening, fluid permeability (less chance of leak) - but weaker chemical bonds so not rigid