Cell Membrane Structure

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  • Cell Membrane Structure
    • Basics
      • All cells surrounded by membranes
      • In eukaryotic cells, all organelles surrounded
      • Barrier between cell and environment. Controls what enters/leaves cell
      • Partially permeable
      • Substances move across CSM by diffusion, osmosis or active transport
    • Fluid Mosaic Structure
      • 1972- suggested to describe arrangement of molecules in membrane
      • Phospholipids form bilayer
      • 'Fluid' because lipids constantly moving
      • Cholesterol molecules present in bilayer
      • Channel/carrier proteins scattered through bilayer like mosaic- allow large molecules/ions to pass through membrane. Receptor proteins allow detection of chemicals.
      • Proteins able to move side ways thorough bilayer, some fixed in position
      • Contains glycoproteins/glycolipids- proteins and lipids w/ polysaccharide chain attached
    • Components
      • Phospholipids: hydrophilic head, hydrophobic tail. Heads face out towards water. Centre hydrophobic- doesn't allow water soluble substances through
      • Cholesterol: Molecules fit between phospholipids. Bind to tails causing them to pack closely together- restricts movement , membrane less fluid, more rigid. Maintain shape of animal cells
    • Permeability practical
      • 1: Using a scalpel, cut 5 equal sized pieces of beetroot. Rinse to remove pigment.
      • 2: Add pieces to different test tubes containing 5cm3 water (measured w/ pipette)
      • 3: Place each test tube in water bath at different temp e.g. 10,20,30,40,50 degrees for same length of time
      • 4: Remove beetroot from liquid
      • 5: Using a colorimeter, measure absorbance. Higher = more permeable
    • Increasing Temp
      • Below 0- phospholipids have low energy, packed closely together, rigid membrane. Channel/carrier proteins deform, increasing permeability. Ice crystals may form and pierce membrane, highly permeable when it thaws
      • 0-45: phospholipids can move, not as closely packed, partially permeable membrane. As temp increases phospholipids move more, increasing permeability of membrane
      • Above 45: bilayer starts to melt, increases permeability. Water in cell expands, pressure on membrane. Channel/carrier proteins deform, more permeable membrane


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