The fluid mosaic model of membrane structure

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The fluid mosaic model of membrane structure

  1. 1. CELL MEMBRANES AND TRANSPORT The fluid mosaic model of membrane structure ALBIO9700/2006JK
  2. 2. Phospholipids• Form a single layer with heads in water and tails projecting out of the water• Can form stable structures called micelles if shaken up with water• Two-layered structures can from in sheets (bilayers) ALBIO9700/2006JK
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  5. 5. Structure of membranes• The phospholipid bilayer is visible using the electron microscope (> x100 000)• 7nm wide• Also contains proteins• Model for the structure is called fluid mosaic model• ‘fluid’ – the individual phospholipid and protein molecules move around within their layer• ‘mosaic’ – describes the pattern produced by the scattered protein molecules when the surface of the membrane is viewed from above ALBIO9700/2006JK
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  7. 7. Features of the fluid mosaic model• Membrane is bilayer of phospholipid molecules that move about by diffusion within their own monolayer• Phospholipid tails point inwards and form a non-polar hydrophobic interior. Heads face the aqueous medium surrounding membrane• Some phospholipid tails are saturated and some unsaturated. The more unsaturated, the more fluid the membrane• Most protein molecules float in phospholipid layers• Some proteins are embedded in the outer layer, some in the inner layer and some span the whole membrane• Thickness = 7nm• Many proteins and lipids have short , branching carbohydrate chains attached to the external surface, forming glycoproteins and glycolipids• presence of cholesterol ALBIO9700/2006JK
  8. 8. Features of the fluid mosaic model ALBIO9700/2006JK
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  10. 10. Roles of components of cell membranes• Phospholipids – Form bilayer (basic structure of membrane) – Act as a barrier to most water-soluble substances• Cholesterol – Have hydrophilic heads and hydrophobic tails – Regulates the fluidity of the membrane, preventing it from becoming too fluid or too rigid – Important for the mechanical stability of membranes – Hydrophobic regions help to prevent ions or polar molecules from passing through the membrane (myelin sheath around nerve cells – leakage of ions would slow down nerve impulses) ALBIO9700/2006JK
  11. 11. • Proteins – Transport proteins: provide hydrophilic channels or passageways for ions and polar molecules to pass through the membrane – Enzymes: catalyse the hydrolysis of molecules such disaccharides (small intestines) – Protein in membranes of organelles (mitochondria and chloroplasts): involved in the processes of respiration and photosynthesis• Glycolipids and glycoprotein – Protein and lipid molecules with short carbohydrate chains – Form hydrogen bonds with the water molecules and so help to stabilise the membrane structure – Act as receptor molecules, binding with hormones or neurotransmitters (chemicals that enable nerve impulses to pass from one nerve cell to another) ALBIO9700/2006JK
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