surface area to volume ratio

• Why don’t most organisms use simple diffusion?

  Most cells are too far away from exchange surfaces and from each other for simple diffusion alone to maintain the composition of tissue fluid within a suitable metabolic range
• As the size of an object increases what happens to the sa:v ratio 


  Decreases
• Why is it beneficial that smaller organisms have more surface area per unit of volume

  It’s easier for them to exchange enough substances across their cell surface membrane for metabolism
• Why is increased body mass associated with a reduction in metabolic rate per kg of body mass?

  - many metabolic reactions release heat energy

  - large organisms have a low sa:v ratio so find it difficult to lose heat

  - overheating will denature proteins by the breaking of ionic and hydrogen bonds

  - to avoid overheating, large organisms reduce their metabolic rate per kg of body mass
• Why do smaller organisms have higher metabolic rates?

  - they have high surface area to volume ratios

  - they have high rates of heat loss

  - so they have high metabolic rates to release enough heat to provide enough kinetic energy for enzyme-substrate complexes to form rapidly enough

  - this means hey also need higher feeding rates to get enough substrates (like glucose) to respire