Biological Membranes


Cell Surface Membranes (plasma membranes)

  • Controlls substances leaving and entering
  • Partially mermeable (can move across plasma membrane by diffusion, osmosis or active transport
  • Allow recognition of other cells eg: cells of immune system
  • Allow cell communication
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Membranes within cells

  • Membrane around organelles divide cell into compartments- barrier for organelle and cytoplasm        

                - makes functions more efficiant (eg: substances for respiration are kept together in                              mitochondria)

  • Can form vesicles to transport substances
  • Controll what enteres and leaves organelle
  • Can be reaction sites - eg: inner membrane of mitochobdria cintains enzymes needed for respiration
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Fluid Mosaic Model

  • Structure of proteins are usually same: composed of lipids (mainly phospholipids), proteins and carbohydrates
  • Fluid mosaic model describes the arrangement of molecules in the membrane 
  • Phospholipids form a continuous double bilayer 
  • Cholesterol molecules present within bilayer
  • Bilayer is fluid; constantly moving
  • Proteins are scattered through bilayer (like mosaic)
  • Some proteins have a polysaccaride chain attatched- glycoproteins 
  • Some lipids have a polysaccaride chain- glycolipids
  • About 7nm thick
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Role of Phospholipids

  • Form a barrier to dissolve substances 
  • Have a hydrohpillic head and hydrophobic tail
  • Molecules automatically arrange themselves to form bilayer (head faces out on either side of membrane)
  • Middle of bilayer is hydrophobic so doesnt let ions through (no water solubles molecules, but fat-soluble substances can pass through)
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Role of Cholesterol

  • Gives membrane stability
  • Cholestrol = type of lipid
  • Present in all membranes (excluding bacterial cell membranes)
  • Cholesterol molecules fit between phospholipids, makes them pack closer together  (makes it less fluid, more rigid)
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Role of Proteins

  • Proteins controll what enter and leave cell
  • Some proteins form channels, these allow small or chaeged particles through
  • Other proteins (carrier proteins) transport molecules and ions across membrane by active transport and facilitated diffusion
  • Proteins also act as receptors for molecules in cell signalling (whena molecule sbinds to the protein, a chemical reaction is triggered inside cell)
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Role of Glycolipids and Glycoproteins

  • Glycolipids and glycoproteins act as receptors for messenger molecules
  • They stabalise membrane bgy forming hydrogen bonds with water molecules 
  • Also site where drugs, hormones and antibodies bind
  • Act as receptors for cell signalling 
  • Also anitgens
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Effect of temperature on Membrane Permeability

  • Increasing temp. = increase membrane permeablity
  • Temp below 0 'C:

> phospholipids have little engergy = cant move alot. Packed closely, membrane is rigid, channel and carrier proteins deform (increasing permeability). Ice crystals form and pierce membrane 

  • Temp between 0-45 'C:

>phospholipds can move and arent packed as tight, partially permable. As temp increases, phospholipids move more

  • Temp above 45 'C:

>phospholipd bilayer melts = membrane is permable, water inside cell expands putting pressure on membrane. Chanel and carrier proteins deform so cant control what eneters and leaves 

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Effect of Solvent on Membrane Permeability

  • Surrounding cells in solvents + permeability; solvents dissolve lipids in membrane so it loses its structure 
  • Some solvents + permeability morethan others (ethanol more than methanol)
  • Can investigate this by using the beetroot experiment
  • + concentration of solvent will also +  permeablilty
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