Diffusion is one form of passive transport that doesn't require the expenditure of cellular energy. A molecule can diffuse passively through the cell membrane if it's lipid-soluble, uncharged, and very small, or if a carrier molecule can assist it. ... The assisted process is known as facilitated diffusion.
Got this off google but still hope it helps :)
A Respiration by animals CO2 is being added.
B trees burning same
C tree leaves removed
D oil added
