Exchange surfaces.

• When can cells use diffusion?


  to take in substances they need and get rid of waste products.
• Examples of cells using diffusion.

  Oxygen and carbon dioxide transfered between cells and the environment in gas exchange.

  In humans, urea diffuses from cells, into blood plasma for removal by the kidneys.
• What does it depend on for the ease of exchange substances?


  The surface area to volume ratio.
• Whats the equation for surface Area.


  Length x width.
• What’s the equation for volume? 


  Length x width x height.
• Why can gasses and dissolved substances diffuse across the cell membrane?


  Because they have a large surface area compared to their volume.
• What’s the difference between single-celled and multi- celled organisms? 


  Multicellular organisms have smaller surface areas compared to their volume, not enough substances can diffuse.
• How are exchange surfaces adapted to maximise effectiveness?


  They have thin membranes - substances only have short distance to diffuse.

  They have large surface area - lots of substance can diffuse at once.

  Exchange substances in animals have lots of blood vessels, can get stuff into and out of the blood quickly.

  Gas exchange surfaces in animals are often ventilated.