Cell membrane and transport

• the fluid mosaic model helps to explain:

  • passive and active movement between cells and their surroundings
  • cell-to-cell interactions
  • cell signalling
• structure of phospholipid

  Phosphate head that is polar (hydrophilic) and therefore soluble in water

  lipid tail is non-polar (hydrophobic) and insoluble in water
• Cell membranes contain several types of molecules:

  • Three types of lipid: phospholipids, cholesterol, glycolipids (also containing carbohydrates)
  • Two types of proteins: glycoproteins (also containing carbohydrates), other proteins (e.g., transport proteins)
• Cholesterol molecules have hydrophobic tails and hydrophilic heads
• Cholesterol molecules fit between phospholipid molecules and are orientated the same way (head out, tail in)
• Cholesterol molecules are absent in prokaryotic membranes
• Structure of cholesterol

  Cholesterol

  • Cholesterol molecules also have hydrophobic tails and hydrophilic heads
  • Fit between phospholipid molecules and orientated the same way (head out, tail in)
  • Are absent in prokaryotes membranes
• Cholesterol regulates the fluidity of the membrane by:

  • Sitting between phospholipids, preventing them from packing too closely together in low temperatures to avoid freezing and fracturing
  • Stabilizing the cell membrane at higher temperatures by interacting with phospholipid tails to prevent excessive fluidity
  • Binding to the hydrophobic tails of phospholipids, stabilizing them and causing phospholipids to pack more closely together
• Cholesterol contributes to the impermeability of the membrane to ions and increases mechanical strength and stability of membranes
• Without cholesterol, membranes would break down and cells would burst