Biology - Phospholipids and Fluid Mosaic Model


Why are membranes important?

  • Cells have many membranes: plasma membrane; tonoplast; outer and inner membranes of mitochondria; outer membranes of chloroplasts and the nuclear envelope
  • They are flexible, and are able to break and fuse many times. They act as a barrier between cell and environment
  • They allow cellular components to have different conditions e.g. lysosomes have a pH of around 4. The cytosol has a pH of around 7. The membrane acts has a barrier between them
  • They are partially permeable so they let some molecules through but not others. Substances move across membranes by diffusion, osmosis or active transport
  • They are also around organelles, dividing them into different parts.
  • They allow fat soluble molecules through the bilayer but polar molecules require channel proteins
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What is the phospholipid bilayer made of?

  • Phospholipids (see other card)
  • Extrinsic proteins (on the surface of the bilyaer)
  • Intrinsic proteins (spans the whole layer)
  • Channel proteins (spans layer, and lets molecules and ions through)
  • Carbohydrates attached to the proteins (glycoproteins) - acts as recognition site for other cells
  • Glycolipids attached to lipids - acts as recognition site e.g. for ABO blood system
  • Cholesterol, a type of lipid. Fits between phospholipids and bind to the tails so they're more closely packed together, restricting movement and making the membrane less fluid and more rigid. Maintains shape of animal membranes (no cell wall) and cells unsupported by other cells e.g. red blood cells
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What is the history of the Fluid Mosaic Model?

  • 1925: Gorter and Grendel suggested it forms a bilayer with hydrophobic tails and hydrophilic heads
  • Initial studies showed the membrane had layers. 1935: Davson-Danielli proposed a model of a bilayer with proteins on either side of the heads after it was discovered there were proteins in the bilayer
  • The electron microscope proved this to be incorrect. Early 1970s: Singer and Nicholson used techniques like freeze-etching to prove the bilayer. They showed the proteins were arranged in a mosaic pattern throughout the membrane. Found the membrane to be fluid due to considerable lateral movement of molecules in it. Proposed the Fluid-Mosaic Model as it was based on experimental and chemical evidence.
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What is the role of phospholipids in the membrane?

  • Membrane is predominantly phospholipids.
  • They have a hydrophilic head and a hydrophobic tail. This means that because the cytoplasm and surroundings are aqueous, the taills face inwards to avoid external water, and the heads face outwards. This makes it a polar molecule.
  • This also makes the centre of the bilayer hydrophobic so the membrane doesn't allow water-soluble substances, like ions, through and acts as a barrier against dissolved substances
  • The phospholipid is made of: phosphate group, a phsophoester bond (hydrophilic head), a glycerol molecule, and ester bond and two fatty acid groups (hydrophobic tail)
  • These hydrophobic tails and hydrophilic heads form micelles (droplets) when in water.
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