Cell Membranes

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  • Cell membranes
    • Small molecules such as oxygen and carbon dioxide can cross the membrane by diffusion
    • Has a 'Fluid Mosaic' structure
      • Made mainly of lipids, proteins and carbohydrates
      • 1972, Fluid Mosaic model was said to be an arrangement of molecules in the membrane
      • Phospholipid molecules form a bilayer
        • This bilayer is 'fluid' as the phospholipids are constantly moving and proteins can move around within it
        • The molecules have a head and tail
          • Head is hydrophilic, loves water, heads face out towards water
          • Tail is made of two fatty acids, hydrophobic and hates water, tail are on the inside, centre of the bilayer is hydrophobic
            • Ions and other water-soluble substances cant pass through
          • Molecules automatically arrange into the bilayer
        • Protein molecules are scattered through the bilayer, like tiles in a mosaic
          • Some proteins have a polysaccharide (a carbohydrate) chain attached - called glycoproteins
          • Lipids also have a polysaccharide chain attached - called glycolypids
          • Cholesterol is present in the membrane, fitting between the phospholipids, forming bonds and making them rigid
    • Scientific evidence
      • Before the 1970s, scientists believed cell membranes were made of a phospholipid layer between two continuous layers of protein
        • This was due to Electron microscopes showed three layers
        • EM techniques improved over time and showed a bi layer of phospholipids, new methods showed randomly distributed proteins

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