Cell membranes

• What are the roles of a membrane ?

  • Control the passage of substances in and out of the cells and organelles
  • Separate the content of cells from their environment
  • Separate different areas within organelles
  • Provide a surface for attachment of enzymes and receptors
• What do all membranes within cells have ?

  A phospholipid bilayer, proteins and cholestrol
• What is a phospholipid bilayer ?

  A thin polar membrane made up of two layers of phospholipid molecules.
• Is the phospholipid bilayer polar or non poar ?

  Polar
• Is water polar or non polar ?

  Polar
• Describe the structure of a phospholipid molecule.

  +

  • polar head = hydrophillic (attracts water)
  • two non-polar tails = hydrophobic (repels water)
• What is cholestrol a type of ?

  Lipid
• What is the function of cholesterol ?

  • Make the membrane less fluid at high temperatures
  • Prevent leakage of water and dissolved ions from the cell
  • Pull together the fatty acid tails of the phospholipids to prevent them from moving
• Describe 3 roles of membranes inside cells.

  • Control the passage of substances into and out of organelles
  • Separate different areas within organelles – matrix and intermembrane space in mitochondria
  • Provide a reaction site for attached proteins, such as enzymes and receptors
• Explain how saturated fatty acids affect the fluidity of the phospholipid membrane.

  Saturated fatty acids contain single carbon to carbon bonds so they are able to fit together compactly and regularly in the membrane, meaning there are less spaces between the molecules and the membrane becomes less fluid.
• Explain how unsaturated fatty acids affect the fluidity of the phospholipid membrane.

  Unsaturated fatty acids contain double carbon to carbon bonds which creates kinks, so they are irregular in shape and they don’t fit together well, meaning there are more spaces between the molecules and the membrane becomes more fluid.
• What is the function of phospholipids in the membrane ?

  • Allow lipid soluble molecules to enter and leave the cell
  • Prevent water soluble substances entering and leaving the cell
  • Make the membrane flexible
• Name three types of proteins in the membrane ?

  Channel proteins, Carrier proteins and glycoproteins
• Describe the function of a channel protein.

  To provide a channel for passive movement of substances through facilitated diffusion. They do not change shape.
• Describe the function of a carrier protein.

  Transport substances across membranes by changing their shape. This can be passive (facilitated diffusion) or active transport.
• Describe the function of glycoproteins.

  +

  • Act as receptors
  • Helps cells to attach to one another (adhesion)
  • Allows cells to recognise one another (self recognition)
• Are receptors proteins ?

  Yes
• What are the functions of proteins in the membrane ?

  • provide structural support
  • allow active transport across the membrane via carrier proteins
  • act as channels to transport water soluble molecules across membranes
  • act as a receptor
• What are intrinsic proteins ?

  Proteins which pass through both layers of the plasma membrane
• What are extrinsic proteins ?

  Proteins which are present on only one side of the membrane
• What are the similarities in structure between channel proteins and carrier proteins ?

  Both are intrinsic and both are made up of proteins with a specific shape and therefore only allow a molecule with a complementary shape to move through.
• What are the differences in structure between a channel protein and a carrier protein ?

  Channel proteins cannot change shape but carrier proteins can
• What are the similarities in function between channel proteins and carrier proteins ?

  Both allow passive movement of specific molecules by facilitated diffusion
• What are the differences in function between channel proteins and carrier proteins ?

  Carrier proteins allow movement of molecules by active transport (requires ATP) but channel proteins cannot.
• What do glycoproteins have attached to them ?

  Carbohydrate chains
• What are glycoproteins involved in ?

  1. Cell adhesion = hold cells together in a tissue
  2. Cell signalling = communication between cells
• What do glycolipids have attached to them ?

  Carbohydrate chains
• What is the function of a glycolipid ?

  Cell recognition
• What can both glycoproteins and glycolipids act as ?

  antigens
• What does fluid mosaic mean ?


  Fluid: parts of the membrane can move around freely, if they are not attached to other parts of the cell, making it flexible

  Mosaic: proteins that are embedded into the membrane vary in size and shape which interlock with each other like tiles on a mosaic floor
• What can affect the permeabillity of a membrane ?

  Temperature and solvent concentration
• What factors affect the ability of molecules to move across membranes ?

  Size: small molecules move across easily through gaps between phospholipids

  Type of molecules: lipid based molecules move across easily because the membrane is made up of phospholipids

  Charge: uncharged molecules move across easily than polar molecules, and these move across more easily than ions
• What type of molecules usually require a channel or carrier protein ?

  Large, charged or protein based molecules
• How does water move across a membrane ?

  Osmosis
• Define diffusion.

  The net movement of molecules from a high concentration to a low concentration, down the concentration gradient until equilibrium has been reached.
• What is simple diffusion caused by ?

  random movement of molecules due to their kinetic energy
• What is the difference between simple diffusion and facilitated diffusion ?

  Simple diffusion: passive movement of molecules across a membrane without the need of a protein . Facilitated diffusion: passive movement of molecules across a membrane with a protein.
• What type of molecules does simple diffusiom happen with ?

  small, uncharged or lipid-based molecules
• What type of diffusion involves membrane proteins ?

  Facilitated diffusion
• Define facilitated diffusion.

  The movement of molecules from a high concentration to a low concentration, across a membrane using a channel or carrier protein until equilibrium is reached.