First, let’s look at energy production. All cells on Earth need energy to run their cellular machinery. This energy comes from the chemical reaction that changes the molecule adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and a free phosphorus atom through a pathway involving water (part of the water molecule is required to allow the chemical reaction to run). The splitting off of this single phosphorus atom releases energy that is utilized by the cell. There is no life on earth without ATP, and cells on Earth cannot live without a constant splitting of ATP into ADP. But organisms do not take in ATP. ATP has to be made, by bonding the phosphorus atom back on to ADP to make ATP. Organisms cannot either find or take ADP or ATP from other organisms by eating them. This involves an oxidation-reduction reaction. The ADP gains an electron and thus receives stored energy. At the same time, the electron donor is oxidized. While there is a wide range of chemical electron acceptors, the one that is most energetically favorable is oxygen and the species that use oxygen in this way. For animals the energy needed to start this ball rolling is the sugar called glucose. In the presence of oxygen, glucose is split, and the eventual product from several chemical reactions down the road is ATP. Organisms using this glucose plus oxygen chemical reaction to produce ATP from ADP (by splitting oxygen-hydrogen bonds) are said to use aerobic respiration. There is a chemical waste product of all of this—carbon dioxide.

Cells using this chemical pathway can make much more ATP than cells that do not over the same time period. For example, an animal that “burns” glucose by using oxygen makes more ATP per unit of time than does a bacterium using fermentation. All animals on Earth use this oxygen-mediated kind of cellular respiration. All animal cells thus need a constant supply of oxygen; without it they quickly die. Every body plan has its own way of getting the life-giving molecule, and this acquisition process is part of the foundation of any animal’s design.

Now, let’s look at other molecules of life that require oxygen, particularly those involved in oxygen and carbon dioxide movement to cells, called respiratory pigments. Respiratory pigments are used to help acquire oxygen from either air or water. They are formed from metal ions attached to organic compounds. Hemoglobin is the most



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