How Life Avoids Oxygen
Avoid? Am I kidding myself? Oxygen means so much to us. Yet in the larger context of biosphere, it not only is harmful, but is rather toxic to many bacteria — the dominant life form on Earth. To survive, life must avoid any toxic substance.
The earliest bacteria lived deep in the ocean floor. The Earth was free from oxygen molecules back then. Atmosphere was a mixture of nitrogen and carbon dioxide. It was all peaceful some three billion years ago. Then some adventurous bacteria invented a machinery to harness sunlight for making stored energy. We know the process by the name of photosynthesis. Cyanobacteria were the pioneers of the technique long before plants came about. In fact plants acquired photosynthesis from the cyanobacteria via a sort of technology transfer. Photosynthesis was quite an invention. It splits water with the help of sunlight and uses hydrogen for energy generation and storage. With the help of this technological revolution cyanobacteria flourished and began to dominate the near-surface ocean. And the world had to deal with a new element — oxygen — the byproduct of the process of photosynthesis. Oxygen rose substantially in the atmosphere as a result of waste removal by cyanobacteria. That was some 2.5 billion years ago. The arrival of oxygen wasn’t quite a good news for life. Oxygen easily gives rise to some chemical entities collectively known as reactive oxygen species that are toxic to life. They damage DNA — the very core of life.
Life had this dilemma. Either face the wrath of the deleterious oxygen or shut down the photosynthesis factories. Gaining extra energy from the free Sun was too attractive a proposition to forego. And it did not make sense to not use a technology for which life had invested millions of years in R&D. Executive decision was made — photosynthesis will continue. Let’s find a solution to the problem created by oxygen. A group of bacteria took up the challenge.
While that group was working on the solution, photosynthesis was gaining popularity among microorganisms and some bigger organisms that arose 650 million years ago. This pumped more waste oxygen into the atmosphere and even oxygenated near surface ocean waters. Now there was no escape. The oxygen problem had become too big to ignore.
Fortunately just about that time the solution was ready. Life has this knack for turning threats into opportunities. It made use of one special virtue of oxygen, its hunger for electrons.
The earlier energy generation processes that life had at its disposal were all dependent on maintaining a pool of hydrogen ions (protons) on the other side of a membrane dam. Like a hydroelectric dam, release of protons is regulated via a channel which rotates a mechanical ATP generator to produce the life’s currency of energy. This pretty much is the fundamental process of energy generation in organisms. Now all that a cell needs to do is maintain the proton concentration in the reservoir behind the membrane. This is where a series of electron acceptors come into play. They serially accept high-energy electrons to a lower energy regime and the energy thus made available is used to pump protons across the membrane. Oxygen, because of its strong affinity for electrons, does this job more efficiently than other electron acceptors. Therefore when added to the series of electron acceptors, it makes a big difference to the cell’s ability to pump protons. Presence of oxygen enables organisms to make 15 times more ATP than primitive cells. One such energy generating entity is mitochondrion which lives inside our cells.
Relegated to the status of an organelle inside cell, mitochondria are now universally regarded as progeny of once free living bacteria. We know them as the power house of the cell. That is precisely because of their role in respiration via consumption of oxygen as per the method described above. Our body then can be considered as a colony of bacteria. The same applies to an ocean dwelling animal or any animal for that matter.
Oceans have a much less amount of dissolved oxygen compared to atmosphere. Remember, fundamentally life does not require oxygen. And it is toxic too as we have seen above. Yet once life had tasted the blood of oxygen via respiration, it didn’t look back. When animals came out of water onto the land, they were faced with extraordinary quantity of oxygen in the atmosphere.
Too much oxygen is like a flammable gas. You cannot have it in a quantity that would set your house on fire. Yet you need it regularly at a lower pressure to be able to sustain the flame for cooking food. Think of the supply of natural gas from producing region to your home. As it flows down the chain from processing plant through distribution mains and finally to your home, there is a cascading drop in pressure of the gas. Life does a similar balancing act to handle atmospheric oxygen. From atmosphere-body interface through lungs, blood and cell membrane, there is a cascading drop of oxygen pressure at every interface. When the oxygen finally enters the mitochondria, its pressure has dropped significantly to the level that a free living aerobic bacterium can handle. Our entire body then seems like an infrastructure that helps make oxygen available at the right pressure to each of the mitochondria inside trillions of cells. The cumulative surface area of skins of all land animals thus becomes a defence wall against the toxicity of oxygen with nasal and mouth openings being the permissible channels for controlled entry of oxygen inside for further distribution.
So effectively by breathing in air we are allowing oxygen inside in a calculated manner for mitochondria to generate energy required for life activities. But more importantly, we are protecting mitochondria from “burning” by presenting several barriers in the path of oxygen — first by humidifying air after intake, then by mixing with carbon dioxide inside lung sacs, then mixing with blood and finally by diffusion into tissue cells and diffusion into mitochondria. See, we are letting our cells avoid oxygen rush. As does a fish which presents the first barrier in the form of gills.
And deep inside some big organisms, the infrastructure is so designed that there are few chambers where oxygen is completely shut off. These protected chambers allow some bacteria to thrive using the “old technology” of fermentation in the absence of oxygen. Which is what some deep ocean bacteria do too. So what such animals like cow are effectively doing is creating the deep ocean like environment on land for bacteria to do their cooking the primitive way. That we oxygen breathing creatures do not like the waste methane - generated in the process and ejected from their mouth and anus - is another matter.