a. The amoeba which is a single celled organism does not have a specialised system for gaseous exchange. Explain why this organism has no need of such a system and why humans need a complex system involving specialised surfaces and mechanisms of ventilation  
  Single celled organisms  
  • have a large S/A to volume ratio
  • which increases rate of diffusion
  • and they have short diffusion distances to all parts of the organism
  • have a small S/A to volume ratio
  • have long diffusion pathways
  • they have a waterproof/gastight skin
  • therefore they need a moist internal gas exchange surface with a large S/A
b. How does a molecule of oxygen reach and then enter ther cells of the spongy mesophyll in a dicotoledonous leaf  
  • oxygen diffuses onto the leaf down a diffusion gradient
  • through the stoma
  • into the air spaces in the leaf
  • along a diffusion gradient
  • it then dissolves in the moisture layer on cell walls
  • oxygen now in solution enters the cells by simple diffusion over the cell membrane
  • respiration in the cells maintains the diffusion gradient