Diffusion

• Define diffusion:

  The net movement of particles from a region of higher concentration to a region of lower concentration until they’re equally distributed (down a concentration gradient)
• What are the feature of diffusion?

  It’s a passive process meaning it doesn’t require energy from respiration
• What key factors increases the rate of diffusion?

  Higher concentration gradient

  Larger SA

  Shorter diffusion pathway

  Temperature
• What is the concentration gradient?

  Where there’s a large different in concentration between 2 areas
• How is equilibrium reached for diffusion?

  Where there is no net movement of molecules meaning that there’s an equal concentration
• What is a net movement?

  Overall movement (big movement in one direction)
• Why can’t all substance cross over by diffusion?

  some substances cannot cross over the cell surface membrane at the same rate due to the centre of the membrane being hydrophobic (fatty acid tails)

  Hydrophilic substances e.g ions, polar molecules cannot diffuse across excepts from H2O because despite it being a polar molecule it is very small
• What is facilitated diffusion?

  Where hydrophilic molecules diffuse across via protein molecules without the interaction with the hydrophobic centre of the phospholipid bilayer
• How is facilitated diffusion done?

  Through carrier proteins and channels which are intrinsic and so, they completely span across the membrane from one side to the other
• How does a carrier protein work?

  Possess a specific tertiary structure which is complementary to the polar molecules

  When they bind, it causes the tertiary structure of the carrier protein to change

  This pushes the molecule through the membrane and releases it
• What is active transport?

  The movement of molecules or ions, from a region of lower concentration to a region of higher concentration against a concentration gradient
• What does active transport require?

  energy from ATP which is produced by respiration making it an active process

  Carrier molecules
• What factors decrease the rate of active transport due to the decrease in the rate of respiration? 


  Lower temperatures (less kinetic energy)

  Lack of oxygen (reactant for respiration)

  Respiratory inhibitors (can bind and inhibit enzymes involved in catalysing reactions of aerobic respiration)