passive transport – movement without requiring an external source of energy 
  diffusion – movement of molecules down a concentration gradient 
  osmosis – diffusion of water 
  facilitated diffusion – diffusion where a carrier molecule is used for the transport to occur across a membrane 
  electrostatic attraction – attraction between an electrically positive and an electrically negative substance for one                                            another 
active transport  – cell uses energy to move molecules against the concentration gradient 
  endocytosis - movement against the concentration gradient, from the environment into the cell 
  pinocytosis – cell drinking; endocytosis of dissolved molecules and ions in small amounts of fluid 
  phagocytosis – cell eating; endocytosis of large particles and cells 
  exocytosis – movement of materials (usually wastes) from the cell to the environment that travels against a                         concentration gradient 

semipermeable membrane – due to its structure, the cell membrane allows some molecules to pass through, but denies access to other molecules