3A.1 Size & Surface Area


3A.1 Size & Surface Area

Exchange of Substances with the Environment

  • Every organism needs to exchange substances with it's environment, no matter what size it is.
  • Every organism needs oxygen and nutrients for respiration and create waste products such as carbon dioxide and urea, that need to be removed.
  • Most organisms need to remain at the same temperature, so heat is exchanged as well.

Surface Area : Volume Ratio

  • Surface area : volume ratio affects how quickly substances are exchanged.
  • Smaller organisms have a higher surface are : volume ratio than larger organisms.

Exchange Organs in Single-celled Organisms

  • All organisms need to be supplied with substances for respiration and have waste substances removed.
  • These substances can diffuse directly into & out of the cell across the cell membrane.
  • Diffusion rate is quick because of a short diffusion pathway (distance needed to travel).

Energy Organs in Multicellular Organisms

  • Diffusion across the outer membranes is too slow  because:
  • 1. Cells are too deep in the body, there is a large distance between them and


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