2.1.5 biological membranes

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  • plasma membrane
    • Roles within the cells and at the surface
      • PM is partially permeable
        • It only allows some substances to pass through not all
        • small molecules diffuse through the PM
        • some substances dissolve in the lipid layer to pass through
        • larger substances pass through protein channels or are carried by carrier proteins
      • Roles of the plasma membrane (PM) and outside the  cells
        • Separates cell content from the external environment
        • regulate the transport of materials into and out of the cell
        • can contain specific enzymes for metabolic reactions
        • contains antigens  so immune system can recognise it as itself and not attack it.
        • can release chemicals which signal to other cells
          • cells communicate with each other using messenger molecules ( MM) eg. hormones
            • 1) cell releases a      MM                2) the molec. travels to another cell 3)MM binds to a receptor on the cell membrane
        • may be the site of chemical reactions
      • Roles of membrane within the cell
        • the cristae of the mitochondria provides a large surface area for aerobic respiration
        • the thylakoid of the chloroplast  is the house of chlorophyll . on these membranes some reactions of photosynthesis occur
        • the small intestine  the PM of epithelial cells which line ti contain digestive enzymes which break down  some sugars
    • fluid mosaic structure
      • B y singer and Nikelson -  the FMM consists of a phospholipid bilayer with proteins floating in it making it a mosaic pattern
      • the bilayer is fluid as some lipids and proteins in the layer can change places with each other
      • the proteins have a polsaccheride chain attached ( carbohydrates) these are called glyco proteins
      • some lipids have polysaccheride  chains attached and are called proteins
    • phospholipids
      • have a head and tail
      • the head is hydrophilic - it attracts water ( has contact with the exterior and interior  of the cytoplasm
      • the tails are hydrophobic and are found in the centre of the bilayer ( they are in the interrior hidden from water )
    • cholesterol
      • is a type of lipid and it gives the PM stability
      • present in all PM  ( not bacteria )
      • cholesterol  molec . fit between phospholipids  and they bind to the hydrophilic tails  making them move closer. it makes the membrane less fluid and more rigid
    • proteins
      • control what enters and leaves the cell
      • some proteins are channels - they allow ions with a charge or small particles to pass through
      • some are protein carriers- they change shape to carry specific molec. across the membrane
        • proteins can atttach to carrier proteins and function as - enzymes , antigens , receptor sites for chemical signalling
    • Not all  cells have the same composition
      • neurons          ( nerve cells )
        • axon is covered in protein channels and carriers which allows transport of ions and allow electrical impulses along the axon
        • they have a mylenial sheath of flattened cells which are wrapped around several time to give more membrane layer and insulates the electrical impulses - m.s- is 20% proteins and 76% lipid
      • PM of white blood cells
        • they have special protein receptor which enables antigens to detect  invading pathogens
      • the root hair cell
        • it has many carrier proteins to transport nitrate ions from the soil and into the cell
      • cristae in mitochondria
        • inner membrane contains electron carriers made from proteins
        • hydrogen ion channels which are associated with ATP synthesis
    • glycolipids and proteins
      • they are the site where hormones , drugs and antigens bind
      • they act as receptors for cell signalling
        • in drugs they bind to a receptor and trigger a response or block the receptor and prevent it from working
      • they stabilise the membrane by forming HB with water molecules


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