familiar to us as it is the molecule/pigment that we use. It exists in a number of forms and is found throughout the animal kingdom in many (but not all) phyla, including vertebrates, echinoderms, flatworms, mollusks, insects, crustaceans, annelids, nematodes, and ciliates. Other pigments include hemocyanin, a copper-containing pigment found in gastropods, crustaceans, cephalopods, and chelicerates; and hemerythrin, the iron-containing pigment found in sipunculans, polychaetes, and priapulans.
All of these pigments bind oxygen more strongly when oxygen levels are high (in lungs or gills) and release it when oxygen levels are low (in respiring tissues). This occurs because in areas of low oxygen the chemical bond holding oxygen to the respiratory pigment molecule breaks more easily than it would in high-oxygen concentrations. Moreover, oxygen uptake is further facilitated in the respiratory structures where levels are low (and thus alkaline pH), and oxygen release is facilitated at actively respiring tissues where there is excess carbon dioxide and thus an acid pH.
We’ve seen that oxygen is critical, from a chemical perspective, in four necessary/useful functions of animal life. Indeed, we do not see animal life in oxygen-free zones.
So how important is oxygen to animal and plant life on our planet? This question is perhaps best answered by looking at where animals do—and don’t—live on Earth.
One of our most powerful methods in deducing the biology of any organism is in looking at the extremes that limit its life, for instance the upper and lower temperatures or chemical compositions an organism can withstand. In particular: at what level of oxygen are organisms never found?
It is not hard to find such places. On any beach, for instance, digging down into the sand with a shovel takes one quickly from the upper sandy layers, usually populated by a diversity of invertebrate animals, to a deeper, dark stratum that is both foul smelling and almost devoid of animal life. Usually it is necessary to dig no more than a