3.2.3 Revision

• How does facilitated diffusion work?

  Molecules diffuse across the plasma membrane with assistance from channel and carrier proteins
• What is osmosis?

  The movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane until equilibrium is reached
• How do large molecules cross membranes?

  By facilitated diffusion through channels created by pore proteins in the membrane
• What molecules/particles can't pass through the phospholipid bilayer?

  Large molecules, charged particles and polar particles
• What is the use of cholesterol In the fluid mosaic model?

  Stabilises the model, reducing fluidity to give membrane more strength, and makes membranes less permeable
• What is diffusion?

  The movement of particles from an area of high concentration to an area of low concentration
• What does the rate of diffusion depend on?

  Steepness of concentration gradient, temperature, size of molecules, nature of membrane and whether molecules are polar
• Name three properties of water that is important in biology?

  It's a solvent, high heat capacity and high heat of vaporisation
• Suggest one advantage of milk-producing cells containing large numbers of mitochondria?

  Mitochondria releases ATP which is used for active transport
• The movement of substances across cell membranes is affected by membrane structure. Explain how.

  The phospholipid bilayer allows movement of non-polar particles. It also prevents movement of polar, charged and large particles. Carrier proteins allow active transport and facilitated diffusion