surface area to volume ratio

• Surface area to volume ratio

  Measure of how big the surface area is compared to the volume of an organism
• Small organisms like single-celled bacteria can rely on diffusion through their surface to exchange substances with their environment
• Large multicellular organisms like humans require specialized exchange surfaces like the lungs and intestines to get things in and out of their body
• Large organisms require specialized transport systems like the heart and blood vessels to transport substances around the body
• How a single-celled organism survives

  1. Absorb resources like oxygen, glucose, amino acids

  2. Get rid of waste products like carbon dioxide

  3. Carry out chemical reactions to build up and break down molecules
• As organisms get larger

  Their surface area to volume ratio decreases
• Cubes of different sizes

  • Small cube: Surface area 6 cm^2, Volume 1 cm^3, Ratio 6:1
  • Medium cube: Surface area 24 cm^2, Volume 8 cm^3, Ratio 3:1
  • Large cube: Surface area 54 cm^2, Volume 27 cm^3, Ratio 2:1
• As cubes get larger, their volume increases much more quickly than their surface area
• Bacteria


  • Have a high surface area to volume ratio, can rely on diffusion across their surface
• Humans


  • Have a low surface area to volume ratio, require specialized exchange surfaces like lungs and intestines, require transport systems like circulatory system
• Diffusion distance


  Distance molecules have to diffuse from outside to inside of an organism
• Diffusion distance increases massively as organisms get larger, e.g. 1 micrometer for bacteria vs 5 cm for humans
• Larger organisms cannot rely on diffusion alone, require transport systems to move substances around</b>
• Plants also have exchange surfaces (roots, leaves) and transport systems (phloem, xylem)
• When referring to "large organisms", this includes anything big enough to see with the naked eye, not just large animals