Eli5: If water is 2 thirds oxygen, why do we need lungs?


Eli5: If water is 2 thirds oxygen, why do we need lungs?

In: Chemistry

We need oxygen in the form of two free atoms of oxygen bonded only to themselves, not chemically bonded to other things. Elements bond to form substances with dramatically different qualities and behaviors, so just because something contains an element doesn’t mean it is equivalent to that element alone or other substances which also contain that element.

The study of how those elements behave, bond, and interact is called “Chemistry”.

Because water is chemically bound oxygen.

We need raw oxygen to produce energy in our cells, by burning Carbonhydrates (sugars, fats, proteins) with Oxygen to Water and CO². Energy is released because water and CO² are more stable than molecular oxygen and the carbonhydrates.

Fish also don’t use the bound oxygen for that, their gills filter out the molecular O² that is dissolved in water

Because we breathe molecular oxygen in gaseous form that’s available in the atmosphere, and do not use electrolysis to break down H/O bonds in water. Gill breathing animals like fish do not break down water for oxygen, they also breathe gaseous O2 that’s dissolved in water.

The oxygen atom in water is already “reduced” – it has reacted with hydrogen at some point and is now settled into a low-energy state that can’t react again.

You need oxygen gas to live because it’s still highly reactive. Your body uses pure oxygen’s powerful reactivity to drive complex chemical processes.

The waste product is more reduced oxygen in carbon dioxide and water.

The oxygen in water isn’t available to us in it’s current form.

Each oxygen atom in water is stuck (very strongly) to two hydrogen atom to form a water molecule. Our bodies can’t rip the oxygen atoms off the hydrogen into a form we can use to fuel ourselves. We need “free oxygen”, oxygen atoms that aren’t attached to anything else besides oxygen.

You can split the hydrogen and oxygen in water but it takes a lot of energy to do so (more than you can get back from using the oxygen to fuel your body) and you generally do it with lots of electricity, which we can’t generate inside ourselves.

Water is Hydrogen and Oxygen that are chemically bound.

If you looked at a balanced equation, you would see something like…

2x H2O + Energy = 2xH2 + O2

That is, the separate Hydrogen and Oxygen molecules have more stored energy than when they are combined into water. The only way to split water into hydrogen and oxygen is to add that extra energy back.

Even fish don’t actually breath the Oxygen that makes up the water. Instead, they breath the Oxygen that is *dissolved* in the water.

The energy required to split H2O is so much more than we get ourselves. It isn’t actually sustainable.

In addition to everyone else’s comments. Water is never 2/3 oxygen, it’s either 1/3 by Molarity or 7/9 by mass. Each water molecule only had one oxygen

Regardless of what breathing structures you use, gills or lungs, you are taking advantage of a process much like a magnet pulls on pieces of metal.

Imagine this, you have a smallish magnet that is ready to pull on some tiny metal fragments (blood cell with no bound oxygen), the metal floating around the magnet eventually gets pulled into it (oxygen pulled into res blood cell), the magnet moves in the blood until it finds an even bigger magnet (the cells in the body, combined with chemicals such as 23-bpg pull off the oxygen with more strength than that blood cell has), this then repeats.

(Outside of ELI5 material below)
Now that above scenario was done using very little energy (23bpg is a byproduct of regular metabolism and red blood cells can be reused and require very little energy to stay alive). Now imagine you wanted to use oxygen from water particles alone, you would first need to split the hydrogen from the water, a process which takes 260kj/mol, the equivalent of about one third of the energy you get from a single glucose molecule.

If a person needs about 1kg of oxygen per day (roughly 2E25 molecules/day of O2) that equates to about 16,000 kj/day of energy required, which requires 3-4 kg of glucose needed to fuel it.

In other words, you would need 5 cups of straight table sugar to be able to separate oxygen from water to use instead. Not to mention all of the heat generated in the process.

It is muchhh easier to just take the oxygen already in the air, already separate from any hydrogen.