biological membranes

• Plasma membranes


  a barrier between the cell and its environment, controlling which substances enter and leave the cell.
• partially permeable 


  only let certain molecules through
• The membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm
• membranes can form vesicles to transport substances between different areas of the cell
• Membranes within cells can be the site of chemical reactions.

  The membranes of some organelles are folded, increasing their surface area and making chemical reactions more efficient.
• The structure of all membranes is basically the same. They're all composed of lipids (mainly a type called phospholipids), proteins and carbohydrates (usually attached to proteins or lipids).
• • In 1972, the fluid mosaic model was suggested to describe the arrangement of molecules in the membrane
  • In the model, phospholipid molecules form a continuous, double layer (called a bilayer).
  • This bilayer is 'fluid' because the phospholipids are constantly moving.
  • Protein molecules are scattered through the bilayer, like tiles in a mosaic.
  • Some proteins have a carbohydrate attached - these are called glycoproteins.
  • Some lipids also have a carbohydrate attached - these are called glycolipids.
  • Cholesterol molecules are also present within the bilayer.
• fluid mosaic model

  A) glycoprotein

  B) glycolipid

  C) phospholipids

  D) cholesterol

  E) protein

  F) channel protein

  
• phospholipid bilayer 


  permeable to fat soluble molecules and small uncharged molecule so impermeable to ions. allows the membrane to be flexible so the cell can change shape
• cholesterol 


  reduces the mobility of phospholipids and gives stability to the membrane
• intrinsic proteins 


  span the entire membrane and can act as carrier proteins. carrier proteins transport molecules or ions from one side of the membrane to the other either by facilitated diffusion or active transport
• channel proteins 


  span the entire membrane and have a channel to allow hydrophilic molecules to diffuse across by facilitated diffusion
• extrinsic proteins 


  can act as enzymes in catalysing reactions at a membrane
• glycolipids and glycoproteins 


  allows similar cells to come together to form tissues. enables white blood cells to recognise foreign cells. receptors for hormones