4.1.3.1 diffusion

• How do substances move in and out of cells across the cell membrane?

  Via diffusion
• what is diffusion?

  a net movement from an area of higher concentration to an area of lower concentration
• does diffusion require energy?

  no - it’s a passive process
• Where does diffusion occur?

  Across a semi-permeable membrane
• what are some of the substances transported in and out of cells by diffusion?

  oxygen and carbon dioxide in gas exchange and the waste product urea from cells into the blood plasma
• what are the factors that affect the rate of diffusion?

  
  

  • The difference in concentrations (concentration gradient)
  • The temperature
  • The surface area of the membrane
• how does temperature affect the rate of diffusion?

  the hotter it is, the more kinetic energy the particles have which means they spread out faster
• how does the surface area affect the rate of diffusion?

  the greater the surface area, the more opportunities for particles of substances to be exchanged
• how does the concentration gradient affect the rate of diffusion?

  the larger the difference in concentration between two areas, the faster the rate of diffusion occurs to reach equilibrium
• what does a single-celled organism have and what does this allow?

  a large surface area to volume ratio which allows sufficient transport of molecules into and out of the fell to meet the needs of the organism
• what happens when the surface area to volume ratio is small?

  not much surface area for substances to diffuse across but lots of cells inside need substances
• what do multicellular organisms require due to their surface area to volume ratio?

  a transport system
• what are surfaces and organisms specialised for in multicellular organisms and why?

  for exchanging materials to allow sufficient molecules to be transported into and out of the cells for the organism’s needs
• how does the effectiveness of an exchange surface increase?

  by:

  • having a large surface area
  • A membrane that is thin to provide a short diffusion path
  • An efficient blood supply
  • being ventilated (in animals for gaseous exchange)