2.51 understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell

Explanation:
-unicellular organisms: large surface area to volume ratio because they only have one cell so there is efficient rate of diffusion.
*Remember: as size of organism increases, surface area to volume ratio decreases.

Common features of exchange surfaces for diffusion:
1. Thin cell walls:
   -short diffusion distance
2. Large surface area to volume ratio:
   -efficient rate of diffusion
3. Moist
   -substances can easily diffuse through membrane.