While a plant is alive, it is taking up water. Some of that water is stored in the plant itself and the rest is returned to the atmosphere via transpiration. When a plant dies, whatever water that is stored within the plant itself is going to be (1) returned to the atmosphere directly via evaporation as the plant biomass breaks down, (2) consumed by an organism eating the plant biomass, or (3) buried and contribute to soil moisture (or some mixture thereof). None of this water is "lost", though it may be transferred to a different part of the hydrologic cycle.

Iirc some of it is also used in sugar production with the CO2 they absorb, as hydrogen

Eventually though the molecules will end up back in the atmosphere or other ways to recombine into water.

Agreed on all of your 3 points. But Apart from all the water it releases back into the atmosphere , it actually has taken some water to grow and that water is not present in the plant when it die only a percent of what it has taken to grow is present when it dies.

if we eat plants , then it has to be lost forever.

Also i think there is always some water lost when converting from one form of it to another.

Water in plants consumed by organisms will be respired, excreted in waste, or ultimately also returned to the environment when the organism dies. There is always some amount of water locked up in the biosphere, but this water is not lost in a real sense.

I was kind of lumping that in with water being stored in the plant (either as water or as a part of biomass), but you are definitely correct.

Water is made of hydrogen and oxygen atoms. There's not exactly a constant amount of these, because for instance radiation can convert nitrogen atoms into oxygen at some very slow pace, but it doesn't change very much even over a long time. Chemical reactions generally can't change what element an atom is, but they can take particles apart to build something else out of them. So as long as water particles have their hydrogens taken off to build hydrocarbons out of them, that reduces the amount of water, but the amount of hydrogens and oxygens remains the same.

And, contrarywise, when a hydrocarbon burns, the hydrogens from it rejoin with oxygen to make water again.

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>if we eat plants , then it has to be lost forever.

It is not lost "forever" the molecules just breakdown and get combined into different things which eventually breakdown themselves or converted into something else, eventually the moluces (hydrogen and oxygen) get back to being water and so the cycle continues.

Nothing is "lost forever" because it has to go somewhere but it'll come back eventually in some shape or form.

When plants grow, they don't destroy the atoms that make up water (and carbon dioxide), they build new cells using them. Those cells break down when they die because the process that was supplying them with energy for homeostasis has ended. When the cells break down, those atoms are still there. If something eats them and there's water inside, they pee it out. It's always still there, somewhere.

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To be precise, plants convert water and carbon dioxide into carbohydrates through the process of photosynthesis. Those carbohydrates are not just used for energy - they are made into structural molecules that make up the plants themselves. For instance, cell walls in plants are essentially sugar polymers.

The lost water is therefore "captured" by the structure of the plant in an equal ratio to the carbon dioxide captured. This process is commonly referred to as "carbon capture" because people care more about atmospheric carbon dioxide.

Edit: This got some attention so let me add two more specific examples of water capture. The cellulose that makes up virtually every tree is a sugar polymer, so trees themselves are literally made of sugar. All of that involves captured water molecules that will only ever be released when the tree decays, burns down, or is eaten by termites.

A very different example is the fat in a camel's hump. When fatty acids are made, a great deal of water molecules are stripped of their hydrogens and the oxygen is released. Those water molecules are gone in a real sense, as the oxygen in them is gone. However, digesting that fatty acid requires adding the oxygen atoms back, and water and carbon dioxide is released. This is how camels "store" water in a form that is highly compact and actually devoid of the oxygen atom needed to make the water.

Water is created (and destroyed) in a huge number of chemical reactions. Acid+Base reactions are a notable example. For instance, Hydrochloric Acid and Sodium Hydroxide react to make Water and Salt. (HCl + NaOH -> NaCl + H2O)

So, plants using water as part of their biological processes are countered by other natural processes that create it.

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Whatever water the plant doesn't cycle back into the atmosphere gets turned into more plant.

Like... look at a tiny little tree sprout. Now look at a huge tree, hundreds of years old. Huge difference in size, right? Where does all that tree come from?

It's the water that didn't get cycled back. Turned into tree. (Also some other things aside from water, that also got turned into tree.)

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Plants turn water and carbon dioxide into glucose and oxygen. Forgive me for using superscript instead of subscript as I don't know how to do subscript.

Carbon Dioxide is CO^(2).

Water is H^(2)O.

Glucose is C^(6)H^(12)O^(6).

Oxygen is O^(2).

6CO^(2) + 6H^(2)O -> C^(6)H^(12)O^(6) + 6O^(2)

So, for the moment, the water is 'gone.' It's now a part of a sugar molecule. Plants also make more complicated molecules, various starches and proteins, etc. But the core idea is the same.

Now, when the plant gets eaten by an animal, the reverse happens. The glucose and other molecules get combined with oxygen, and water and carbon dioxide are released.

C^(6)H^(12)O^(6) + 6O^(2) -> 6CO^(2) + 6H^(2)O

The water is now 'back' in the environment. The same chemical reaction occurs not only when the plant is eaten, but also when the plant dies and rots. When a plant rots, it's basically being eaten by bacteria and fungi. The same chemical reaction will also happen when a plant burns, like in a forest fire.

So, generally speaking, the water and carbon dioxide are only bound up temporarily. The only way that the water and carbon dioxide stay locked away is if the biomatter gets buried in a way that it doesn't rot. This is the origin of fossil fuels.

Technically, the amount of water on the surface of the planet went down gradually over millions of years with the gradual deposition of fossil fuels, and up with recent mass burning. However the amount of water in fossil fuels is relatively inconsequential, when compared to the amount of water in the ocean.

So far as I know, aside from a vanishingly small number of atoms that have escaped Earth's atmosphere, all the atoms that have ver been on Earth are still on Earth. But of course, the atoms may not comprise the same molecule they once did.

When we eat the plants, the matter ultimately undergoes cellular respiration, which is the opposite reaction of photosynthesis, so all the water “captured” by the plants is converted back into water by consumers, and is eventually either excreted or released upon the organism’s death so the water cycle continues. The overall reaction is C6H12O6 + 6O2 -> 6H2O + 6CO2, and has water as a product.

Excuse me, exactly which nuclear reaction do you think is taking place to convert nitrogen into oxygen?

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Uhh, being hit with an alpha particle randomly somewhere where something radioactive is exposed to air? I think Patrick Blackett proved that's what happens when that happens, back in 1925 or so.

It absolutely is an incredibly insignificant amount, but I didn't want to say "the number of oxygens on Earth is perfectly fixed and they're just cycled through being part of different particles", because even the number of oxygen atoms on Earth goes up or down. Probably more down as we send it into space? I don't know.

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When we eat plants we are eating lots of water. Consider a fresh apricot compared with a dried apricot.

The dried apricot is chewy and wrinkled because much of the water has been removed during the dehydration process. That water was turned to water vapour and went into the air.

If you eat a lot of watermelon you may need to pee more. Because there was a lot of water in it.

I forgot to water a plant of mine. The leaves went crispy and the whole pant shriveled up. Not only did it not get any new water to the point that it died… some of the water which was in the leaves and body of the plant was lost and is no longer in the plant anymore.

Dried herbs are dryer than fresh herbs. Because some of the water that was in the plant is no longer in the plant. It went somewhere else (likely into the air as water vapour).

Of course some of the water was used to build the plant itself as it didn’t come from nowhere.

The plant uses water and carbon dioxide to build sugars.

When you breathe in your breathe in oxygen (plus other gassed) When you breathe out you breathe out carbon dioxide (plus other gasses).

Where does the carbon come from? From the plants you eat.

The sugars in the plant are turned back into water and carbon dioxide.

Like animals, plants are aerobic organisms, and must also consume the sugar they make via aerobic respiration (C6H12O6 + 6O2 -> 6CO2 + 6H2O). So from just making and "burning" food, the mass of water is conserved by plants, less any sugar they store for later use.

(Backing up a bit, in photosynthesis, the oxygen atoms that go into the sugar come from the CO2, while the oxygen from the water (that makes 89% of H2O's mass) is released as oxygen into the surrounding air.)

On average the mass of water in and outside of a plant that isn't growing is in dynamic equilibrium, except for the changes in water temporarily stored in or moving through the plant (e.g., in sap). But for a plant that is growing, including just storing food, its overall mass increases. Most of this mass is carbon and oxygen from CO2, but also some is hydrogen from "destroyed"/"lost" water. (The rest of the water molecule is released as oxygen into the surrounding air.)

The bulk of a plant is composed of carbon, oxygen, and a bit of hydrogen. Some of this is sugar (food) that is temporarily stored for later use. Most of this is cellulose and hemicellulose, which are polymers (long, chemically bound chains) of sugar molecules, which comprise the structure of the plant. (Cellulose has the chemical formula (C6H5O10)n, where n is some big number of the C6H5O10 units. Note that C6H5O10 is a simple sugar, minus 2 H's and an O, or H2O. The combination of simple sugars to make cellulose actually releases water, so that somewhat reduces the net water consumed by a growing plant. But I digress.)

Switching gears entirely, there are many other non-biological factors that affect the amount of water on or above Earth's surface through goelogic time. It's not at all a trivial matter of whether the amount of water is increasing or decreasing through time, or at over a given time peirod. Volcanoes release water from the interior. Chemical weathering of rocks puts some of the water into the chemical structure of minerals. Subduction returns some of the water and "water"-containing minerals to the interior. Some water vapor is broken down into H and OH by ultraviolet sunlight, and some of those (especially the H) escape into space. (Comets and asteroids also deliver a bit of water and hydrated minerals, but beyond the very early Earth, this is negligible.)

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Yes, in fact about a hundred tons of atmosphere are lost to space every day, mostly on the trailing side as earth moves through space. https://www.sciencealert.com/earth-loses-hundreds-of-tons-atmosphere-to-space-every-day/amp

> Rutherford had found out that the nucleus of the nitrogen atom could be disintegrated by firing fast alpha particles into nitrogen. He asked Blackett to use a cloud chamber to find visible tracks of this disintegration, and by 1925, he had taken 23,000 photographs showing 415,000 tracks of ionized particles. Eight of these were forked, and this showed that the nitrogen atom-alpha particle combination had formed an atom of fluorine, which then disintegrated into an isotope of oxygen 17 and a proton.

https://en.m.wikipedia.org/wiki/Patrick_Blackett

It is 100% possible but is an extremely, very very rare occurrence.

This is one of those things that sounds concerning until you remember it’s been happening for billions of years so clearly it’s not an issue

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"Water" is not elemental. It's made from two hydrogens and an oxygen (H2O).

The H and O elemental atoms are conserved no matter what the earth's biochemistry does. But they can be separated from each other easily, and different Hs and Os can come together from other sources and processes to make "new" water.

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That water isn't permanently lost though. When the structure of the plant oxidizes either slowly (decomposition or animal digestion) or rapidly (fire) most of the hydrogen combines with oxygen from the atmosphere to form new water molecules.

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While it is clearly true that the water is not “lost” and much of the water sequestered in this way is released through various degradation processes, some water is captured, much the way that some carbon is captured. This captured water makes up not only the organic molecules present in all fertile soil but also the bulk of all biomass.

However, since the entire biomass of every living thing on earth is about 10 million times less than the mass of all water on earth, the water captured in this way will never be more than a negligible amount.

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Oxygen isn't a relevant process (although technically possible: Nitrogen-15 + neutron can become nitrogen-16 which decays to oxygen-16), but carbon is: Nitrogen-14 + neutron -> carbon-14 + proton. That's the dominant way carbon-14 is produced. It decays back to nitrogen over thousands of years, and we use that process for radiocarbon dating.

/u/Minnakht /u/ArcherofFire

So would that water ever come back? As in would it ever become h2o again or does it get split up and become other stuff

The earth has the same amount of water it had billions of years ago. All of it has just been constantly recycled back into the atmosphere, then into rivers and oceans as rainfall, then back into the atmosphere, etc. Known as the water cycle.

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When you breathe you create water. You take oxygen from the air and it reacts with hydrogen in your mitochondria to produce water.

This is very overly simplified but the takeaway point is that plants aren't the only things that can create or disassemble water.

If you burn hydrogen gas, you make water. If you run an electric current through water, you generate hydrogen and oxygen gases.

Take a step back. Matter cannot be created or destroyed (let's ignore E=mc² for now). So you must know the water doesn't just vanish. You've presumably seen the difference between a green leaf and a dead one (hint: the latter is dry).

Trees don't grow forever nor do they live forever, so I don't understand why you thought the water was trapped in them forever.

This is in addition to the chemical processes others have described which convert the water to and from plant matter via photosynthesis and respiration.

Your technically right. That Water (H2O) itself is lost. However living things also create water (vapor) to move, grow, think, or do anything really. Also burning things creates water vapor. The vapor will become water eventually.

Furthermore plants also use water to grow. this 5% you are talking about probably refers to the water that plant used to create new cells. This water can eventually become water by burning it.

Also, even if some water is lost forever and never becomes water again. The percentage is small. So small i fact that it does not really matter

Well like when it becomes tree-stuff it is often split up, the tree will retain some water as part of its inventory of healthy-tree biomass but also some of the H and some of the O in the H₂O gets turned into sugar and starch and structural carbohydrates and proteins and stuff.

You can regard those other substances as a reservoir for water, in a sense, because over time as they are metabolized or whatever happens to them they may break back down into water again. As part of the tree's life cycle.

But in terms of where it ultimately goes? Sure absolutely, someday the tree will die, and when it dies the tree will decompose, and some part of the tree's biomass will be eaten by bacteria, fungi, and the other usual suspects. During that process much of the tree will be turned back into water again. And then go be part of the soil, from which new sprouts will sprout again, and so on and so forth.

Some statistically minded biologist might be able to give an estimate of how long your average water molecule remains water continuously over a timeframe like that (let's say several hundred years). It might be that a lot of the water a tree consumes just always remains water. Or it might be that most of it changes form! That is an interesting question.

When wood is burned, water is a byproduct?

Chemically speaking, wood contains mostly carbon, hydrogen and oxygen atoms. When it is burned, the hydrogen atoms end up as water vapour.

Something I haven't seen discussed here is the answer to your title when taken as the whole question; a not insignificant amount of water is "lost" to space every second in the form of H2 leaving our atmosphere. Said H2 can't be combined into watermolecules anymore, at least not on earth. Some molecules get enough kinetic energy (temperature) to simply shoot out of our orbit, others get taken away by solar winds.