What is life is a valid question to some because they would simply like to make some. Yet others, although they don't know what it is, search for life in other planets like Mars. How do you find life if you don't know what it is?
Ken Nealson, Director of Center for Life Detection in Jet Propulsion Laboratory (NASA/Caltech) informs: "What we've discovered is that virtually any environment that allows liquid water to be there, will be compatible with life... that life has a way of inhabiting every place that has liquid water on this planet." He is obviously responding to: where is life?
Life seems to be a planetary occurrence. As far as we know there is no entirely airborne life either. In fact, life exists in a deep biosphere on the planet Earth. We say "deep" because life can be present miles underground, below the surface of our home planet. Biosphere is that part of the Earth and its atmosphere in which living organisms exist or that is capable of supporting life. The term can also imply the living organisms and their environment composing the biosphere in toto.
How is life? Let's look into that for a bit. A process known as oxygen activation helps convert food into energy in animals, plants, fungi, and in some bacteria. Flavoproteins are the proteins that catalyze many biochemical reactions, including oxygen activation that use molecular oxygen (O2) . Biophysicist Allen Orville at Brookhaven National Laboratory (DOE/BNL) explains his team's new research results: “Flavoproteins represent one of only a handful of ways that nature activates molecular oxygen, a process that’s important for all life on the planet. We’ve determined structures of some oxygen intermediates involved in several important enzymes that assist in this process. But no one has ever seen an oxygen intermediate attached to the flavin. Until now.”
The O2 of life for life like us! Asymptotic thinking helps here for approximation and interpretation. Earth contains 49.2% oxygen while a human body, 65%. A higher percentage of oxygen in humans suggests its essential quality for human life. Recently, Mount Everest climbers were sampled for blood oxygen levels, the lowest being at 19.1 millimeters of mercury (mm Hg). The sample group is very small but those four individuals in the group were under extreme climbing conditions and high altitude (266 mm Hg). At sea level (760 mm Hg) the oxygen content varies between 90 and 105 mm Hg. If the blood oxygen falls below 60 mm Hg, a human is considered critically ill.
When you compare the partial pressure percentages for the lowest at the Everest expedition and for the critically ill at sea level, 7.1 % and 7.8 %, respectively, you can appreciate the nearness of the last breath at these values. If the critically ill is at 60 % of the healthy, the Everest lowest is just below it with a value at 57.1 %. We know that their lungs had accumulated water to measure such low values.
I mentioned liquid water earlier. There are underwater insects who live underwater indefinitely with air bubbles on their bodies that function as external lungs. When these insects breathe underwater, oxygen decreases in the bubble and carbon dioxide (CO2) increases. The diffusion rate of dissolved oxygen from the ambient water into the respiratory bubble meets the metabolic requirements and the requirements for the interfacial exchange of O2 and CO2 with the ambient.
Yes, the O2 of life must be present in a form that the living organism can use to live. Recall the climbing or diving gear or the astronaut suits that are designed to provide the necessary respiratory support and protection outside the usual human environment. You are as safe as your bubble safety.
There is another bubble to consider for human presence in the universe, the sun's bubble in between the stars, the solar system's bubble, or heliosphere. While oxygen is the most abundant chemical element in our biosphere, it is the third most abundant in the universe. The sun consists of only 0.9 % oxygen. NASA's Interstellar Boundary Explorer (IBEX) satellite is currently measuring the size and shape of the sun's bubble in space. This means mapping is underway of the largest bubble in which we live.