4.1 - Structure of cell-surface membrane

• What are membranes around and within a cell known as ?


  Plasma membranes
• What component of the bilayer points outwards ?


  Phospholipid hydrophilic phosphate heads
• Why do the phosphate heads face outwards ?


  They are attracted to water on both sides of the membrane
• What component points inwards in the membrane ?


  The hydrophobic fatty acid tails
• Why do the hydrophobic tails point inwards ?


  They are repelled by the water on both sides of the membrane
• 3 functions of the phospholipids in the membrane ?


  allow lipid-soluble substances to enter and leave the cell, prevent water-soluble substances entering and leaving the cell, make the membrane flexible and self-sealing
• Difference between plasma and cell-surface membrane ?


  2 plasma membranes surround organelles whereas cell-surface is only around the cell
• What’s the function of channel proteins ?


  They form water-filled tubes to allow water-soluble ions to diffuse across the membrane
• What’s the function of carrier proteins ?


  They bind to ions or molecules then change shape in order to move them across the membrane
• 5 functions of proteins in the membrane ?


  Structural support, transport channels, cell-surface receptors to identify cells, help cells adhere together, allow active transport via carrier proteins
• What 3 properties does a molecule have to have to pass through the membrane without a transmembrane protein ?

  Small, nonpolar, lipid-soluble.
• 3 functions of cholesterol in the membrane ?


  reduce lateral movement of other molecules, reduces fluidity at high temperature, prevents leakage of water and dissolved ions
• 3 functions of glycolipids in the membrane ?


  Act as recognition sites, help maintain stability, helps cells attach to one another to form tissues
• 3 functions of glycoproteins ?

  Recognition sites, helps cells attach to each other to form tissues, allow cells to recognise each other (eg lymphocytes)
• What 3 qualities does a molecule have to have to pass through the phospholipid bilayer ?


  Lipid-soluble, small, non-polar
• What model describes the cell-surface membrane ?

  Fluid-mosaic model
• Why is the membrane described as fluid ?


  phospholipid molecules can move, making the structure flexible
• Why is the membrane described as mosaic ?


  Proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern like a mosaic
• What can the fatty acid tails be ?


  Can be saturated or unsaturated
• Why does being unsaturated change the phospholipid ?


  Contains C=C bonds, creates kinks, pushes phospholipids sort so the membrane is more fluid
• What’s the role of transmembrane proteins ?


  They transport molecules that can’t pass through the bilayer (large, polar, lipid insoluble)
• By what processes do molecules move through transmembrane proteins ?


  Facilitated diffusion or active transport
• What does the number of transmembrane proteins determine ?


  The rate of transport
• 4 functions of membranes ?


  Isolate enzymes from rest of cell, provide surfaces for reactions, separate organelles from cytoplasm, controls exit + entry of materials from the cells and organelles