Discuss the importance of oxygen to all living organisms, and describe how three different organisms—human, flatworm, and fern—obtain the oxygen they need.

Importance:
                    Oxygen is the third most abundant element and constitute about 21% of the gases in earth's atmosphere. It is a key for generating energy in cellular respiration. In aerobic respiration, electron release in electron transport chain is culminated by oxygen, result into the production of chemical energy.
Oxygen obtained by human:
                                               Human obtain oxygen through respiratory system by applying partial law of diffusion.The partial pressure of oxygen in atmosphere is higher than inside lungs, so oxygen move into lungs in exchange for carbon dioxide.
Oxygen obtained by flatworms:

Oxygen obtained by ferns: