the cell surface membrane

• phospholipid bilayer:

  • hydrophilic phosphate head point to outside of the cell-surface membrane as they attracted by water on both sides
  • hydrophobic tails point into centre of the membrane as they are repelled by water
  • allows lipid-soluble substances to enter and leave the cell
  • makes the membrane flexible and send healing
• proteins:

  • channel proteins - span the bilayer from one end to the other, allows for movement of polar substances in and out of the cell
  • carrier proteins - change shape when ions or molecules bind to them
  • allow active transport
  • form cell-surface receptors fro identifying cells
  • help cells to adhere to each other
• cholesterol:

  • add strength to bilayer
  • are very hydrophobic
  • pull fatty acid tails of phospholipids together to limit movement and make the cell less fluid
  • prevents leakage of water and dissolved ions
• glycolipids:

  • acts as recognition sites
  • maintains stability of membrane
  • help cells to attach to one another to form tissues
• glycoproteins:

  • act as recognition sites
  • help cells attach to each other forming tissues
  • allows cells to recognise one another
• phospholipid bilayer prevents movement of:

  • substances that are not water soluble
  • that are water soluble and too large to pass through channel proteins
  • polar molecules