why we breathe oxygen and not something like carbon dioxide or nitrogen?
In: 40
Because for 2 billion years that is what stromatolites crapped into the environment. If they had released something else into the environment we would have evolved to breath whatever it was, assuming we evolved at all.
Different molecules have different tendencies to react with other molecules. Oxygen is a fairly high-energy substance that is happy to break apart and attach to other things. Carbon dioxide is a lower-energy substance that doesn’t usually want to break apart, you need to put energy *into* it to convince carbon dioxide to break apart.
So the other comment is partly right – this is because we evolved to use oxygen. But it’s also because oxygen is inherently more suitable to *burn for energy* in the body, just like oxygen is necessary if you want a fire to burn, and spraying a bunch of carbon dioxide at a fire is actually a strategy that can be used to extinguish a fire.
Could we have evolved to function without oxygen? Probably not in our current form, but bacteria and other forms of life *do* function without needing oxygen. But would we “breathe” carbon dioxide or nitrogen? Probably not. We would have evolved to get energy without “breathing” the way we do now.
We don’t breathe carbon dioxide because we evolved well after the CO2 was depleted from the environment, so there really isn’t much in the air to work with.
We don’t breathe nitrogen because nitrogen is extremely stable, and you’d have to put in a large amount of energy to get it to do anything that might be useful for biological life.
When the first bacteria started producing oxygen as a byproduct of photosynthesis and thus causing the mass extinction of anaerobic organisms. Our ancestors were the ones to utilise this waste product as an energy source. The carbon dioxide they produced was then quickly utilised by the photosynthesisers giving a feedback loop and a kick start to new options for predator/prey and environmental utilisation.
I’ll just caveat, none of these answers are really ‘correct’ it’s just trying to explain something logically despite it being impossible to have an answer for this question.
Oxygen: can be used to generate energy because it loves to react with other elements (for example when burning) easily
Carbon Dioxide: can’t be used to generate energy because you need more energy to make it react with something
People have written whole books about this: https://www.amazon.com/Oxygen-molecule-Oxford-Landmark-Science/dp/0198784937
Three hundred million years ago, in Carboniferous times, dragonflies grew as big as seagulls, with wingspans of
nearly a metre. Researchers claim they could have flown only if the air had contained more oxygen than today –
probably as much as 35 per cent. Giant spiders, tree-ferns, marine rock formations and fossil charcoals
all tell the same story. High oxygen levels may also explain the global firestorm that contributed to the
demise of the dinosaurs after the asteroid impact.
The strange and profound effects that oxygen has had on the evolution of life pose a riddle, which this book
sets out to answer. Oxygen is a toxic gas. Divers breathing pure oxygen at depth suffer from convulsions
and lung injury. Fruit flies raised at twice normal atmospheric levels of oxygen live half as long as their
siblings. Reactive forms of oxygen, known as free radicals, are thought to cause ageing in people.
Because we don’t have photosynthesis.
We break Carbohydrate chains and it makes Energy.
Creating new chains requires energy.
We can’t use more energy then we are making because if you run out of energy you are dead.
If we wanted to breath in CO2 we would need to use a lot of energy to turn it into C6-H12-O6 (glucose) because it has a lot of bonds and we would require an additional energy source.
Like billion years ago, with the evolution of photosynthesis, that is, take CO2 from air (in CO2 rich atmosphere, where there was never any O2) and use sunlight and water to change it into sugar and release O2.
Happened for millions of years, now there’s a lot of O2 in the atmosphere but since there was never any O2 before this time, all the life forms (mostly bacteria) depended on CO2 for “food” and survival.
O2 can turn into a different forms as it is relatively unstable and form free radicles(which is harmful and causes ageing and mutations, etc), hence anaerobic(that depended on CO2 and not O2) life forms slowly died out.
Now this one very very ancient bacteria might have survived in these conditions and mutated might have even started to use this abundant O2 in air for food. By basically using O2 to make H2O it made a molecule called ATP which had a lot more energy (around 15 times more) than the previous bacteria which couldn’t use O2.
When you have extra energy(more money, can buy more expensive things), you can carry out more energy consuming life processes. And this bacteria might have been incorporated into a larger cell in a symbiotic relationship with it, and today we call it the mitochondria.
And then, evolution kept things going, structures and chemicals sensitive to O2 start developing and the rest is history.
Hope it helps, feel free to ask questions.
Edit why couldn’t they use nitrogen? Because carbon and nitrogen are relatively stable forms and don’t readily react with stuff, as opposed to oxygen which needs to 2 more electrons to be reach stable form, hence it has tendency to accept electrons more easily and hence react more easily.
One of the main drivers in chemistry is the way the electrons group up in an atom. They don’t just go random, they fill in zones. For various reasons, the zones form stable groups (have “magic” numbers, in a way, 2, 10 and 18 are the number most small elements most want, if they can get it, and getting to the closest magic number of electrons is pretty much what makes reactions happen). having a particular number of electrons (“magic” numbers) is more stable than having one or two more, or less, than the “magic” number.
So, what happens when elements react and form compounds, is some elements will become more stable (get to a “magic” number) by grabbing an electron or more if they can find one, whereas others have only a weak hold on the one or two (or more) electrons that put them beyond the ideal stable number, so the elements with a big need GRAB an electron or more from elements that only have a weak hold, and both elements become more stable in the process. For example, oxygen has 8 to begin with, so it kind of really would prefer to have 10 if it could. Carbon has 6, so it could go either way, taking 4 to get to 10, or losing 4 to get to 2.
In the big mix of the world of all elements, there is a fight for electrons. Strongest ones win. Oxygen is really strong at attracting electrons (only fluorine, the next element over in the periodic table, with one more electron that oxygen, pulls harder, but it is a fairly uncommon element).
So, oxygen, because it is very common and wants to react with almost anything it can, is the basis for a lot of chemistry (we even name a major type of chemical reaction, “oxidation” on what happens when oxygen reacts with other elements by stealing electrons).
A lot of other things come into play, of course, but the fight for electrons is a big reason for a lot of chemical reactions. You need to know, though, that once an element has stolen (or lost) electrons and moved into its most stable condition, it won’t fight anymore, so no longer will react.
So we breath in oxygen as O2 (has not stolen electrons from anywhere yet) and it is used to steal electrons from, well, mostly carbon. Carbon’s electron number is plop in the middle of the stable range so will benefit by either adding 4 electrons or losing 4 electrons, so can go either way. It does not have a good strength for stealing electrons and doesn’t have a good strength for keeping electrons, so which way it will go depends on what else is around that is fighting for electrons. Carbon is an average fighter, not a weak fighter like sodium, say, but not a brute like oxygen either.
Life chemistry uses this to its advantage, making sugars using sunlight (so carbon is acting more like an electron thief in sugar, with the help of solar energy) but getting energy back by “burning” carbon with oxygen. When oxygen combines with carbon, the end result is carbon dioxide. Carbon dioxide is pretty stable and won’t react more without putting in energy (going back to C and O2 is the wrong way on the energy hill so something has to push it back up, and plants do the pushing using energy from the sun).
Nitrogen is sort of in between carbon and oxygen in how strong it is at stealing electrons (nitrogen “wants” 3 electrons but oxygen “wants” only 2) , so can be a source of energy through oxidation of something else, but it is less energy that you get by doing the same thing with oxygen. There are also issues with the shapes of structures made using nitrogen that make it less useful (compare ammonia NH3, a triangle in shape with nitrogen in the middle so not really polarized, to water H2O which has oxygen in the middle and is polarized by having extra electrons on one side and H on the other). Nitrogen is still a very important element in organic chemistry (chemistry of living things) but it is not a major energy source.
So, we breath in oxygen as O2 (wants more electrons) and breath out oxygen as CO2 and water (H2O) both of which are as stable as they will get – all the elements involved have the best number of electrons for their make-up; C gave 4 electrons to get down to 2, which each oxygen took 2 to get to 10, making CO2, or with water/H2O, each H gave one electron (the only one it had, and went to 0) and the O took 2 going to 10. Everybody is as happy as they can be, until plants come along and zap some solar energy back into the mix and undo all the work.
Latest Answers