Oxygen MiraCosta Biology

We're going to start here with a little biography of oxygen, O2, the gas that we depend on so
heavily—literally with every breath—and that figured so significantly in the previous
videos on mitochondria and oxidative phosphorylation.
First, it’s interesting to note that O2 has not always been a part of the atmosphere.
Prior to around 2.5 billion years ago (mention “BYA means billion years ago”)—with
the origin of photosynthetic prokaryotes like the cyanobacteria—the earth’s atmosphere
had no oxygen. Oxygen is produced by photosynthetic green cells—we’ll talk about that later.
Now the earth is 4.6 billion years old, and therefore for the first 2.1 billion years,
there was no O2. Photosynthesis must have evolved not too long before the fist noticeable
appearance of oxygen in the evidence left by the earth’s geologic record. Starting
from the state of “no oxygen” 2.5 billion years ago and trickling in O2 as the result
of photosynthesis by the planet’s microbial life, it took a really long time before there
was any appreciable amount of oxygen. At first most of the oxygen was removed immediately
by reacting with iron, and one of the reasons we know that oxygen wasn’t around before
this time is that iron oxide—or as you know it “rust”—wasn’t around in mineral
deposits dating before 2.5 billion years.
The evolution of the aerobic metabolism we described in previous videos must have occurred
well after the evolution of photosynthesis—that is after oxygen was first introduced into
the earth’s atmosphere. Before this time, there would have been no use for a Krebs cycle
or oxidative phosphorylation. There was a long history of life before the origin of
oxygen, all living cells would have been anaerobic—meaning that they would be getting their ATP energy
through the non-oxygen-dependent fueling reactions—glycolysis is one such reaction.
There are a lot of anaerobic organisms still around today. The yeasts that we use to bake
bread and ferment fruit juice into wine and digested barley juice into beer are good examples
of anaerobic organisms. They get all of their ATP energy from the glycolytic conversion
of glucose into two molecules of pyruvic acid. The “leftover” pyruvic acids undergo fermentation
resulting in the production of ethyl alcohol and carbon dioxide—the intoxicating stuff
of alcoholic drinks as well as the bubbles that are sometimes captured to make beverages
like beer or champagne fizzy. It’s the same carbon dioxide that inflates the tiny spaces
within the dough to make breads light and airy.
In addition to yeast, there are lots of bacteria and archaea, as well as several types of eukaryotic
organisms that are incapable of using oxygen for ATP synthesis. We generally refer to such
cells as “anaerobes,” while the cells that do use oxygen are “aerobes.”
The initial introduction of oxygen into the atmosphere by photosynthetic cells 2.5 billion
years ago could also be thought of as enormously disastrous for most of the earth’s anaerobic
life forms. Oxygen is a very powerful oxidizing agent and it’s for this reason that we can
use it to peck away and degrade the food molecule fragments completely, extracting all of the
potential energy from them. A good question to ask at this point is, “if oxygen is so
destructive of an oxidizing agent, why doesn’t it also degrade our cells’ carbohydrates
and proteins” or in other words , what’s there to keep oxygen from pecking away at
the organic molecules in our cells?
In order to live in the presence of oxygen like our cells do, we must have a pretty substantial
set of defenses against the destructive effects of oxygen. When we are sold products that
“contain anti-oxidants,” the main presumed benefit of these anti-oxidant chemicals is
that they will enhance the natural defenses we have against oxygen’s harmful effects.
Hydrogen Peroxide—the stuff in the familiar brown bottle from the drug store—is basically
a solution containing the oxidizing, destructive effects of oxygen. If you have ever applied
hydrogen peroxide to a cut or other wound you are familiar with one of our principal
defenses—the bubbles that you see are actually made by the catalase enzymes of your cells,
it’s your enzymes that are breaking apart the peroxide before they can harm your proteins.
Hydrogen peroxide is effective as an antibacterial agent because any infecting bacteria that
might be there in the cut is probably far less-protected against the oxygen, as compared
with your cells. Most kinds of dangerous bacteria would be killed by the oxidizing effect of
the hydrogen peroxide.
Getting back to our history lesson now, at the time when photosynthetic bacteria were
first releasing oxygen, none of the cells at the time—except for, perhaps, the photosynthetic
bacteria themselves—would have been very well-protected against oxygen’s destructive
effects. The photosynthetic guys would have been guilty of a global mass destruction of
life through their release of this incredibly toxic gas.
It was only after the introduction of oxygen that cells would have had any reason to use
oxygen for aerobic metabolism in the way we have seen in previous videos. Bacteria were
the first cells capable of using oxygen’s destructive potential to peck away at glucose
bits to derive more energy from food. And it was only after we had aerobic bacterial
cells that we could have had the evolution of eukaryotic cells with mitochondria, which
themselves derive from these same aerobic bacteria that were among the first living
things to use oxygen.
Oxygen, O2, presently accounts for about 21% of the air that we breathe, and with only
a couple of very bizarre exceptions, all animals are completely dependent on aerobic metabolism—and
therefore oxygen—for meeting their ATP energy needs. We get our oxygen from the air, while
most fish use oxygen that is dissolved in the water, though many fish are also able
to use oxygen in the air. When the water’s content of oxygen becomes too low, some fish
will swim to the surface and gulp air and hold this pocket of air inside their bodies
until their circulatory system can pick up some of the oxygen from the gulped air.
In the next video, we’re going to think about oxygen in a more practical (and less
historical) way, as we look at how our bodies respond to physical activity—I’ll give
you a hint: it’s going to focus on the amount of oxygen that’s needed and the amount of
oxygen that we can provide.