Plants Don’t Convert CO2 into O2

881400078_5dd598fbbf.jpgArticles about photosynthesis in the popular press or online often make me cringe.


Because sometimes they lead people to think that the oxygen (O2) produced by photosynthesis is derived from carbon dioxide (CO2).

Some even further compound their mistake by stating that plants actually convert CO2 into O2 at night!


This is simply NOT true!!

Please allow me to explain…

The Oxygen You Breathe Comes from Water

Yes, that’s correct, water… H2O

chloroplastsfigure1.jpgHere are how, where, and when this works in green plants:

How: Photosynthesis is basically a two-step process, and the first step is when water is converted into oxygen.

The first step directly requires light energy, which is captured by the photosynthetic pigments, mainly chlorophyll. The chlorophyll converts light energy (photons) into chemical energy, in the form of high-energy electrons.

This chemical energy is used in the photosynthetic reaction centers to split 2 water molecules, producing 4 electrons, 4 protons, and 2 oxygen atoms, which combine to form oxygen gas (O2).

2H20 –> 4 e + 4 H+ + O2

Where: In green plants, photosynthesis occurs in chloroplasts, about two to four dozen of which float around in the cytoplasm of photosynthetic plant cells.


The first step, described above, takes place in the thylakoid membranes (see Figure 1 above).

When: Since the splitting of water to form oxygen requires light energy, this only occurs naturally during the daytime.

Where Does the CO2 Come In?

The chemical energy captured in step one above is used in step two of photosynthesis, that is, to convert CO2 into carbohydrates (sugars). This is called carbon fixation, a.k.a., the Calvin cycle, which takes place in the chloroplast stroma. (see Figure 1 above)

What is the scientific evidence that O2 isn’t derived from CO2 in photosynthesis?

Well, one way to test this is to use water or CO2 containing an isotope of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H2O18 or CO218, produces O218.

In 1941, Ruben, et al. (see Ref. 1 below) reported that they used an isotope of oxygen, O18, to find out where the oxygen atoms went in photosynthesis. They fed plants water containing O18, but because O18 is not a radioactive isotope of the most common form of oxygen, O16, they used a mass spectrometer to determine the fate of the O18.

The O18 was found in the oxygen gas produced by the plant, but was NOT found in the sugars formed during photosynthesis.

This and other scientific experiments have provided clear evidence that the oxygen produced by photosynthesis is derived from water.

Cyanobacteria, Green Algae and Plants All Do This

All of the photosynthetic organisms – plants, green algae (e.g., phytoplankton in the oceans), and cyanobacteria – that use water as an electron source do this.

So, where does the oxygen you enjoy breathing mostly come from?

For a probable answer, see here.

Bottom line: Green plants DO NOT convert carbon dioxide (CO2) into oxygen (O2). The oxygen produced during photosynthesis comes from water. During photosynthesis, green plants DO, however, convert atmospheric CO2 into sugars. And we know now that one half the oxygen atoms in the CO2 wind up in the sugars (e.g., glucose = C6H12O6) and the other half wind up in phosphate byproducts of the Calvin Cycle (see Addendum below).


1. Ruben, S., M. Randall, M. D. Kamen, and J. L. Hyde. (1941) “Heavy oxygen (O18) as a tracer in the study of photosynthesis.” Journal of the American Chemical Society, Vol. 63, pp. 877–879. (PDF)

Addendum: During the so-called “Reduction Phase” of the Calvin Cycle, ATP is used to phosphorylate the 3-PGAs, and then NADPH is used to reduce (add electrons) the 3-carbon compound to produce GAP. In doing so, one of the phosphate (PO43-) groups is removed, containing one of the original oxygens from the CO2.

HowPlantsWork © 2008-2015 All Rights Reserved.


  1. This post is great – I’ve never really understood it before. Thanks, it actually makes sense now !!

  2. hi it ws nice….i got it now ..but plz tell how they converts co2 into sugar….from which tissue…how it works

    • Thanks for the comment.
      You ask: How do plants convert CO2 to sugars?
      It happens in the chloroplasts (any green part of a plant).
      It is such complex chemistry that it took very smart people nearly 50 years to solve it.
      Melvin Calvin led the team that solved it. (That’s why it’s often called the “Calvin cycle”.)
      You can Google “Calvin cycle” to find out more. (A fun site: )

  3. Oh, please.

    You can say, with just as much truth, that green plants are black boxes that turn water and CO2 into more biomass and O2.

    The gas throughput is simply CO2->green plant->O2.

    That’s no more an oversimplification than saying gasoline makes your car go. The popular press has to express things truthfully without losing its intelligent (but non-technical) audience in a blizzard of details or technical esoterica. Describing the (incomplete) inputs and outputs but not the underlying mechanism is a useful technique for imparting important non-technical knowledge.

    That said, i do appreciate the clarity of the explanation and the beautiful illustrations. Really excellent work!

    • Thanks for the comment.

    • Are you are arguing simplification when teaching science is ok? Sure I don’t expect any truth in a newspaper, or any popular media. But Entertainment Tonight is much different than American Scientific. I don’t question if simplification is true in the media, but this shouldn’t be taught out in Schools, which it has been.

  4. Is it possible to seed the atmosphere of Venus with Photosynthesis based plant spores and eventually convert the atmosphere into free Oxygen and other biological or organic molecules?

  5. What happens to the Hydrogen? Please can you tell about that.

  6. What is the time that the plant would not take in or give out oxygen and carbon dioxide???

  7. I have a question: your first equation is balanced (2H20 –> 4 e- + 4 H+ + O2), but there is no way to balance the second equation (4e-+4H++CO2 -> C6H12O6), you end up with 6 extra oxygens (12e-+12H++6CO2-> C6H12O6+O6) what happens to those? are they 3O2?

    • Rodigo,

      Take a deep breath.
      Now you’ve just demonstrated where some of those extra oxygen (O2) molecules go. From the chloroplasts, into the plant cells, out of the cells, out of the leaves through the stomates, and ultimately into the atmosphere.
      The three O2 molecules you’re concerned about (produced in the so-called “light reactions”) are not involved in any way with the CO2-fixation reactions (Calvin cycle).
      Hoped this answered your question.

      P.S. If you are concerned about the accounting for the oxygen in the Calvin Cycle, this is more complicated due to the complexity of this metabolic pathway. A simple summary of the Calvin Cycle is:

      3 CO2 + 9 ATP + 6 NADPH –> glyceraldehyde-3-phosphate (C3H6O6P) + 9 ADP + 8 Pi + 6 NADP+

      The high-energy compounds ATP and NADPH are generated during the “light reactions”. The glyceraldehyde-3-phosphate (PGA) is the main stable product of carbon fixation during the Calvin cycle. Subsequent metabolic reactions covert 2 PGAs into glucose + two phosphates (2 PO3). The “extra” oxygens in this case wind up in the phosphates.

      Yes, it’s complicated. That’s why it was so difficult to figure out in the 1940’s and 1950’s. And why Calvin (along with two other colleagues) received the Nobel prize.

      • Hi,

        Thanks for the post.
        My question relates most closely to Rodigo’s post, so that’s why I’m posting here.
        The article posits that no oxygen output from the plant is “derived” from the CO2 intake.
        This raises an issue in that the overall balanced equation for photosynthesis does not work – You can only get 3 moles of 02 output for every 6 moles of H2O input, Whereas there is a 1 mole to 1 mole ratio; We’re missing oxygen! This can’t happen and we must conserve mass.
        What I’m not saying is that the O2 output is a direct product of the break up of CO2. But three moles of O2 output must come from the (6 moles input of) CO2 molecules . I’m assuming the exact mechanism of how this arises is part of the complex process during / after the carbon fixation in the Calvin cycle. In some way every other Oxygen atom which was previously part of a CO2 must become part of a H2O which is subsequently split in Photo-system II (If this is the only mechanism for release of O2 from the system?)
        I believe the following is correct for the overall process of Oxygenic Photosynthesis:
        2H2O → 4e− + 4H+ + O2
        CO2 + 4e− + 4H+ → CH2O + H2O
        Combining these allows the general equation for photosynthesis to balance. The implication of this, however, is that every other Oxygen atom from CO2 would end up being split from H2O and released as O2.
        I don’t believe this would contradict the assertions made by Ruben, et al as they have been stated, as by this mechanism it is still true that no oxygen in the Glucose molecule would have come from the H2O.
        Is it not true that half of all Oxygen atoms released must have been derived from a CO2 molecule?
        Any help in clearing this up would be much appreciated!

        • Sam J,
          Thanks for the comment and question.
          In addition to the seminal Ruben, et al. paper published in 1941, there have been other studies re. the question of photosynthetic oxygen production using stable O isotopes. For example, please see the following:

          To my knowledge, there is no published scientific evidence supporting the notion that a measurable amount of O2 produced via photosynthesis is derived from the oxygen atoms in CO2.
          (But, if you can find some evidence to the contrary, please enlighten me.)
          One final note: I’ve always had a problem with rendering down the entire process of the complex biochemistry involved in photosynthesis into a simple arithmetic equation. In reality, the factors involved – both known and unknown – makes this ultimately a futile endeavor. (But teachers and textbooks love to do it because it gives something the students can memorize for multiple-choice tests.)

          • Hi,

            Thanks for your prompt reply.

            What’s certainly clear here is that the 6 mole to 6 mole “classic” over simplified net equation is confusing and introduces misconceptions. However I would say that it should not be a problem to create an overall equation for a complex set of reactions which balances. In fact, if you cannot do it then you are violating some basic scientific principles (conservation of mass). Those atoms HAVE TO end up somewhere and this a continuous process so the compounds they form (regardless of mechanism) will be being continuously produced. This means that they are either output from the plant or concentrations within the plant of this product would accumulate throughout its life (unlikely!). Both of these should be quite measurable so I’m assuming one of them is known (to those more knowledgeable than myself!) to happen.

            So, if we were going to double the number of moles of water in the “classic” equation (in order to reconcile it with the clear evidence that each mole of O2 produced comes from 2 moles of water) it would look something like this:

            6 CO2 + 12H2O –> C6H12O6 + 6O2 + ? (6xO & 12XH)

            However we have to take account of the extra products. It seems to be screaming out for 6H2O to be a product (which by no means stipulates that it has to be.) It would also tally with the justification to simplify it to 6 mol. H2O on the left

            I have found from a few sources that water is a net product of the Calvin Cycle:

            3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
            (Raven PH, Evert RF, Eichhorn SE (2005). Biology of Plants…)
            Although I haven’t found this consistently, or the mechanism in which it is generated (any help here would be appreciated) but it would seem from this equation that the Oxygen forming the H2O would have to come from the CO2.

            In your addendum, you’ve state that the second (non glucose) oxygen from CO2 becomes part of the the phosphate group during the reduction of the 3-PGA, however this can’t be the end of the story WRT that oxygen, as if this was a net product plants would have to output this phosphate, or it would accumulate, i’m assuming they don’t (I’d also assume, the phosphate is reattached to an ADP in PSII and therefore is not a net product at any point, but the attached oxygen would have to be a product as it keeps being added to the system)
            Assuming that the above equation is correct then H2O produced in the Calvin cycle could be split in PSII and O2 from the CO2 which made its way into H2O would be released as O2. However, this does not tally with experiment. I think I’ve realised why that would not show in experiments such as the pretty conclusive Stephens, et al. You linked in your reply (thanks for that!) They stated that they could pick up anything above 10% of O2 from CO2. What I had not thought of when commenting before was that very little water which passes through a plant is ever used for photosynthesis (less than 1% – Shah, Janat, (1998), Plant Physiology…) As such, even if this hypothesis is correct, then fewer than 1 in 100 of the H2O made from the CO2 would ever be split in PSII, which would be insignificant and possibly unmeasurable.
            I think this reconciles my differences with the net equation for photosynthesis (which has to balance) and the conclusion that the O2 output from plants comes from the H2O.
            I’d be very interested to hear your thoughts on this, and wonder if you could shed any further light on the issue, particularly WRT water production in / subsequent to the Calvin cycle.

            Many thanks


          • Sam,

            You’ve gone way deeper into this than I have for many years. So, I’ll recommend a couple of books that may answer your questions much better than I can:
            1. A classic is “Biochemistry of Photosynthesis” by RPF Gregory, which I’ve seen in libraries and used bookstores.
            2. Another more recent one is “Biochemical Models of Leaf Photosynthesis” by S. von Caemmerer, which tends to get more into the “real world” photosynthesis.

            What I mean by “real world” is that it’s “in vivo”, rather than “in vitro”, where most of the information regarding the stoichometry of isolated chemical reactions arises. I agree with you that conservation of mass must be adhered to in biochemical reactions, and simple “accounting” of a process even as complex as the CO2-fixation reactions (a.k.a., Calvin cycle) can be determined — “in vitro”, that is, in a test tube, under controlled conditions of temperature, pressure, etc.

            What I’m saying is that “in vivo” – like a living leaf – things are obviously a lot more complicated. (Consider, for example, that inside most leaves are colonies of microbial endophytes that are also engaged in metabolism.) And, thus, such simple accounting of biochemical “input = output” is rendered irrelevant because of too many unknown variables.

            Anyway, back to the original question….I wrote this post because I saw online that many people assumed that the photosynthetic production of O2 came from the breakdown of CO2 (you can see why this would seem to make sense by simply looking at the two molecules.) Anyway, my experience has been that students paid way too much attention memorizing biochemical pathways and missed the “big picture”, such as “where, ultimately, does the oxygen you breathe actually come from?” The answer, of course, is water. (And there is no scientific evidence that I’m aware of – either “in vivo” and “in vitro” – that refutes this conclusion.)

            Finally, is it possible that a minuscule amount of photosynthetically-generated O2 is derived from water molecules produced somewhere along the along the line during CO2 fixation, or subsequent carbohydrate respiration for that matter? Possibly. But it’s likely so minute an amount that it’s undetectable when using 18O-labelled CO2 in experiments.

          • Thanks for that!

            I’ll give those books a look.

            That’s a great point about the real world vs lab realities. As a non-biologist who has no practical experience of this, that’s invaluable.

            Your willingness to respond in order to help out is much appreciated!

  8. This was really helpful! Just wanted to say thanks!

  9. Hm. Interesting. Must say I always had the idea that the oxygen came from CO2. So you are saying that all carbon in green plants takes the form of sugars (or is derived from sugars)?

    Im interested too in the question of which plants fix more CO2 in a given time (eg forests or agriculture).

  10. So really what you mean is:

    “Fools! Imbeciles! You are all idiots!

    Plants don’t convert CO2 to O2! They convert CO2 and H2O into O2 and sugar!

    How could you all be such dimwits as to say the former instead of the latter? Morons! Cretins!”

    • No, I prefer to consider them ignorant, not stupid.
      Why does such ignorance frustrate me?
      Photosynthesis is the primary reason we (indeed all animals) exist.
      Because of its literal “existential” importance to us, you’d think that so-called “educated” people would have a basic understanding of where the oxygen they breathe and the food they eat ultimately come from.
      Most don’t.
      This little blog post is a humble attempt to help make them a bit less ignorant on the subject.
      Cheers, and thanks for your comments.

    • I suspect it’s lack of space in a column of an newspaper. Ha Ha.

  11. Since Mars’ atmosphere is 95.32% CO2, how can photosynthesis be used to transform it to a 21% O2 atmosphere as on Earth?

    • The simplest answer may be that, unless a large amount of water is available, there’s no way that photosynthesis can be used to generate significant amounts of O2 on Mars.
      Also, the Martian atmosphere is much thinner than Earth’s. That is, there’s much less of it compared to Earth’s.
      From what I’ve read regarding your question, it’s likely that the chemical conversion of CO2, rather than photosynthesis, will be used to generate O2 on Mars.
      Thanks for a very interesting question.

  12. ddeeswxadelvinpplliokj

    what is co2 and o2.

    a primary school guy getting research info.

    • CO2 is carbon dioxide, and O2 is oxygen. Both are gases at temperatures that normally occur on the Earth’s surface.
      Oxygen is about 21% of the air you breathe and carbon dioxide is only about 0.04%. That is, if all the parts of the air you breathe = $100, then oxygen accounts for $21 and carbon dioxide accounts for only about 4 pennies (4 cents).

  13. Very interesting explanation. I stumbled on your site because I was wondering why folks are talking about carbon sequestration through deep drilling supercritical fluid to store excess carbon, rather than just plant a whole lot more trees. Is the carbon that is converted to carbohydrates then stored in the tree, and later released when the tree dies? What happens to the carbon, and why can’t we just plant more trees to help alleviate the problem of greenhouse gases? Are they too inefficient

    • Planting a lot more trees would, indeed, photosynthetically “fix” and “lock up” CO2 for the life of the tree. Some have advocated the use of biochar as a way of “permanently” sequestering the CO2.
      The problem is that the increasing number of humans need food and fuel and are continuing to engage in deforestation, which is one of the chief reasons for increasing levels of atmospheric CO2.

      • Thanks Plantguy — it does always concern me when public policies don’t take even small steps (such as planting trees) even while they are reasonable, progressive, economical and can be implemented locally, because it is not the *best* way to solve a problem. That whole making the perfect the enemy of the good thing. The fact is that there is plenty of land that is underutilized for all sorts of purposes — like most pressing global issues (poverty, water scarcity, hunger, etc) it is often a matter of resource management and distribution of goods more than real lack of goods.

        I enjoyed your contribution to the issue.

    • CO2 is not a problem … Its plant food … CO2 at 100 ppm would kill the planet … CO2 at 1400 ppm + the planet thrived …

      • Though this post is mainly about the photosynthetic production of oxygen gas, it does involve the fate of carbon dioxide in photosynthesis. And, indeed, you raise a valid point, that at high levels of atmospheric (atm) CO2 (above 1000 ppm) green plants probably would (and HAVE, during the Carboniferous age, for example) thrive. I doubt, however, that 100ppm CO2 would “kill the planet”, since at the end of the Carboniferous atmospheric CO2 may have approached this level. (But that’s a debate for another day, especially since this historically low value of atm CO2 is unlikely to occur.)
        Anyway, I presume from your comment that the current rapid rise in atm CO2 may be a good thing for green plants. And that, alone, may be.
        But, unfortunately, this geologically unprecedented spike in atm CO2 (and also methane) will likely also result in an unprecedented spike in global average temperature. (They call CO2 and methane “greenhouse” gases for valid reasons, based on physics and chemistry, which I won’t go into here.) This is the main reason that the vast majority atmospheric scientists (and most plant biologists, I might add) are worried about “global warming”.

  14. Don’t we have a CO2 problem with global warming or something. And plants turn CO2 into something less harmful? Cant we just play more plants to solve global warming?

  15. your explanation is exactly what i used to tell my folks and every other person who shoved a few type of plants in my face and tell me that they convert CO2 to O2. Later on i didn’t even bother to explain because neither did they know enough science nor were they going to accept that they were wrong. I am happy and fulfilled to see i was not just making up theories. 🙂

  16. Hello,

    I am glad I read through your comments section because I was wondering *why* a plant converted H2O to O2 (so… where did the H go?) but it got answered, thankfully, in the comments. You might want to consider adding that to your original explanation:) That the H is used to make ATP. I actually have a biology degree, but I really don’t think we ever covered *this*. Yes, I also assumed it was CO2 -> O2. Although farther down in your explanation you say that the 6 extra O go to making 3x O2 for breathing, but then doesn’t that means that SOME of the CO2 goes to O2?

    This might be a far out question, but how is it possible that we have more carbon now than we did 10,000 years ago? We aren’t ‘creating’ carbon, so where is it coming from? Is it because we are burning fossil fuels, so we are ‘freeing’ that carbon? If that is the case, then it would be wonderful to create a device that converts H2O and CO2 into O2 and just C (which could be made into graphite, or graphene, or carbon nanotubes, etc). Carbon is such an excellent super conductor, that it seems as though we should spend more time isolating it out of CO2. If we figure out how to make diamonds out of it directly, that would be helpful too.

    So… the reason I am so interested in this all of a sudden is because of Mars. If we could make a large dome of some sort, and figure out how to effectively use solar panels to convert enough CO2 to O2, then we could start planting trees in the dome to convert more. (short version of my thought) But clearly all that would do it simply lock up the carbon in carbohydrates:( Is there any chemical process that will release the O2 from the carbohydrates? (I am totally okay with you editing this down, or omitting it from the comments if it is too unwieldy, etc:) )

    Thanks! -Isabelle

  17. I like your website..thanks! I am an architect related to sustainable architecture. I am in need of an average tree or trees required to generate enough oxygen for x number of people. I am also in need of a “rule of thumb” for crop land corn, beans, etc. and how much oxygen would be generated. I am tying to plan for a “closed loop development” and I want to to be as correct as possible. If you could help me that would be so kind.


    • Bruce,
      These, as one would expect, are very difficult questions to precisely answer, because of the variabilities caused by plant species, climate, amount of direct sunlight available, etc. Here’s a quote from the following link:

      “Trees freshen the air we breathe by releasing oxygen as a byproduct of photosynthesis. Net annual oxygen production varies, depending on tree species, size, health, and location. For example, a healthy 32-foot tall ash tree produces about 260 pounds of oxygen annually. A typical person consumes 386 pounds of oxygen per year. Therefore, two medium-sized, healthy ash trees can supply the oxygen required for one person over the course of a year.”

      You pose very interesting questions, especially considering that increased atmospheric CO2 usually stimulates photosynthesis in most plants. Does that mean that as humans release increasing CO2 (e.g., from burning fossil fuels) into the atmosphere that plants, in turn, will have higher rates of photosynthetic oxygen production? And will it be enough of an increase to raise the oxygen levels globally?

  18. Carlos Davarre Salazar


  19. Hi,
    I still have a question. Is there any link between O2 and CO2?
    So if you know how much CO2 there is in an specific room, could you calculate the O2 the plant produces? If so, what formula could be used?

    • You ask a very interesting questions, indeed.
      Let me try to answer your first one – Is there any link between O2 and CO2 – first.
      At the chloroplast level, there are two basic links between light-driven oxygen production and CO2 fixation in photosynthesis. The first link is that CO2 fixation reactions (a.k.a., the Calvin cycle) are dependent on the energy produced during the light-driven oxygen production. This energy is in the form of two high-energy chemical compounds, namely, NADPH and ATP. Sometimes it’s convenient to think of NADPH and ATP as two different types of fully-charged, rechargeable batteries, and when they have been spent, let’s call them NADP- and ADP. So, the CO2 fixation reactions take in NADPH and ATP and “spit out” NADP- and ADP, which are recharged using ” solar power” during light-driven O2 production. This, as it turns out, is the second link between O2 and CO2, because the rate of light-driven O2 production is dependent on the availability of the “spent batteries” NADP- and ADP.
      And it turns out that the CO2 fixation reactions are slower than light-driven oxygen production, so the rate-limiting step in photosynthesis is the second step, namely, the Calvin cycle.
      Think of this like a story about two escaped convicts – an old, slow guy (CO2 fixation) and a young, fast guy (light-driven O2 production) – shackled to one another. They’re only going to run as fast as the slower guy.
      So to your second question…. As you might expect, if you increase the amount of CO2, the rate of CO2 fixation increases, thus allowing the light-driven O2 production to go faster, too. So, in theory, it might be possible to develop a formula relating CO2 concentration to the rate of oxygen production, under specific conditions.
      I haven’t been able to find one, and I think I know the answer why. It’s probably because there are simply too many variables – plant species, temperature, light intensity, water availability, relative humidity, and CO2 concentration – that can affect the rate of photosynthesis, and thus O2 production. For example, when CO2 levels increase, this causes the stomates on the leaves of most plants to close (to conserve water loss from leaves). This would confound a simple CO2-versus-O2 formula.
      I can imagine, however, that under very tightly-controlled conditions in the lab, one could possibly come up with a formula linking relative CO2 concentration and photosynthetic oxygen production for an individual leaf, for example.
      Sorry about the length of this response, but you ask tough (but fair) questions, without easy answers. Reality is complex, especially when biology is involved.

  20. Thank you for your clear explanation. I am wondering where the O2 comes from in the production of CO2? The atmosphere? If so, is the level of oxygen in our atmosphere being reduced and at what point will it have an impact on animal life? The question applies for our oceans, as they become increasingly acidified. Thank you. Anna

    • Sugars contain oxygen atoms. And that’s where the oxygen in CO2 comes from when sugars are metabolized during respiration, for example. Indeed, oxygen gas (O2) is used during respiration, but these oxygen atoms end up in water (H2O), not CO2.
      Also, keep in mind that the level of oxygen (O2) currently in the Earth’s atmosphere equals about 20% and the level of carbon dioxide is much, much lower (about 0.04%). Another way of thinking about this is that if all the gases in the atmosphere equals $100, then oxygen (O2) would be equal to $20, but carbon dioxide would only be equal to about four pennies. Thus, there is currently about 500 times more oxygen (O2) in the Earth’s atmosphere than CO2.
      (Do you know which gas is most abundant in Earth’s atmosphere?)

  21. Plantguy, thank you for the great explanation. I was looking up how to convert CO to CO2 in relation to detoxing humans who have decided to attempt to kill themselves via CO. But I’m really glad I did. I’m not sure how old Amity is, but I didn’t appreciate his simplification of your explanation. Sometimes we hear an explanation, model it in our heads and never have time/reason to go back and revise it. It has nothing to do with being a moron, etc. Simplification of what you’ve mentioned for Middle schoolers wouldn’t be that difficult and would still be true and lead them to, what I consider, to be a high-school level explanation that you provided.

    Unfortunately, plant cycles aren’t the only thing that textbooks screw up. We always talk about the lack of education in American children. What about the mis-education like this?

    Thank you for the great explanation. My kids are teens now, but at least I can revise what I told them.


  22. Hi,
    If I want to measure how my plant is performing, what could be the best indicator to do that? O2, CO2, humidity level around the plant, etc. Just in normal conditions, like at home.

    • By “performing”, I presume you mean your plant’s relative rate of photosynthesis. Over what period of time? An hour, day, week, month….?
      Over short time periods (hours), measuring the rates of O2 production, CO2 consumption, or both, are ways people determine relative rates of photosynthesis, using small plants or individual leaves. The problem, of course, is that to do this accurately typically requires expensive equipment (see, e.g., ).
      Over the long term, weeks to months, you could try estimating relative increase in plant size – this may also be a challenge for an individual plant. Maybe a good way would be to measure leaf area of the plant over time. There’s new smartphone app for that (see e.g., ).

  23. I plan to do research by converting CO2 /CO to oxygen from vehicle emissions by using Engine temperature for breaking bonds. Although is there any method for using photons in this process. Could you help on this research to get clear idea.?

    • This sounds like an interesting idea, but the chemistry you’re engaged in is certainly way beyond my knowledge, so I’m sorry that I’d be no help to you.

      Good luck with this project!

  24. Unless I missed it, no one seems to have noticed the statement that O18 is radioactive. It is NOT. It’s true that you can use O18 labelled CO2 and H2O to figure out which of these molecules provides the oxygen atoms that end up producing an O2 molecule. But it’s done by measuring the oxygen isotopes (the three naturally occurring, stable isotopes of oxygen have masses of 16, 17, and 18 atomic mass units) using stable isotope mass spectrometry, not by radioactive decay of O18 (O18 is NOT radioactive!)

    • Thanks for noting the error regarding O18.
      Because of your comment, I have revised this post to in order to correct my mistake.
      I feel that this is now a much better explanation regarding the evidence for the path of oxygen from H2O to O2 in photosynthesis.
      My hat’s off to you.


  26. nice explanation

  27. I have an unusual question.

    I am a writer, and I am writing a fiction novel about evolutionary changes from land to water, and I am looking for a plant that could allow its user to extend their time underwater using it like a regulator. Plus, this plant, (probably aquatic in nature) would impart small genetic changes that would accelerate the evolutionary changes in the user, especially pre-pubescent subjects. Any/all advice will be appreciated.


    • Most interesting question in a while…
      Re. Oxygen (O2) production, first: The person would need a fair amount of green plant biomass and a way to capture the O2 “exhaled” by the plants (and be near the water surface to capture sunlight, of course). I suggest a “cape” of vine-like aquatic plants – use Elodea (pronounced el-O-deeaah) – streaming behind the person (think Superman). The cape of plants would be enclosed by thin plastic film to capture the O2, with a snorkel-like breathing tube attached at the neck. The person would breathe in the O2 from the cape and then exhale CO2 into the cape for the plants to “breathe”.
      Re. accelerating genetic changes? = a much tougher nut…As the plants in the cape grow, the person can occasionally snack on the Elodea (e.g., through small zip-lock opening in cape). Evolutionary change is typically based on the accumulation of many genetic mutations (although a single gene mutation can have a large effect). Genetic mutations may arise from DNA replication error, transposition (, or DNA damage, the last of which might be spontaneous or induced by chemicals or radiation. Anyway, let’s say this Elodea contains a substance that increases the rate of genetic transpositions in humans –> more genetic mutations –> faster evolution rate? But keep in mind not all genetic mutations lead to positive outcomes..
      …this was a fun thought exercise.
      Thanks for the question. (And you can mention HowPlantsWork in your acknowledgements.)

  28. People always talk about trees to absorb CO2. I am interested in green vegetables.
    Do green vegetables consume CO2? Do they release O2? In both cases I would expect the answer to be yes. How much of that CO2 is released when the vegetable is eaten?

  29. I found it interesting that you presented this article from the angle of CO2 does not produce the “all-important” O2. Arguably, The process of the Calvin Cycle through which CO2 is fixed is exceedingly more important, as the essentially creates life (organic molecules that can be metabolized) from non-living matter.

  30. Umm…but don’t plants respire at night? I’ve searched elsewhere and was told that at night they take in oxygen and give out carbon dioxide, and that it was so as to use the sugar that was made from the photosynthesis earlier in the day. So if this were true then would that not mean that the amount of O2 and CO2 is at a standstill?(assuming that night and day have the same amount of hours that is) Does that mean that the plant literally just uses the oxygen that it produced that day and therefore we aren’t acctually gaining any oxygen here? I asked my science teachers before of this matter and somehow or another they just go uhhhhh then dissapear as if I hadn’t asked the question.
    Hoping that someone might reply one day

  31. Excellent question!
    Yes, plants respire all the time, day and night. But they only produce oxygen during the day. So an important question is: How much of the oxygen that plants produce by photosynthesis is consumed by respiration?
    You may remember Joseph Priestly’s (discoverer of oxygen) experiments (1774) in which he placed a mouse and a candle and a mint plant under an air-tight bell jar (Google it).
    He clearly demonstrated that, in this case at least, the plant produced more oxygen than it consumed. That is, there was a NET production of oxygen. But what about natural ecosystems?
    As you can imagine, this would be very difficult to measure for trees in a forest, for example.
    Plant ecologists have determined that net primary production (plant growth) is proportional to net oxygen production.
    “If carbon dioxide uptake during photosynthesis exceeds carbon dioxide release by respiration during the year, the tree will accumulate carbon (carbon sequestration). Thus, a tree that has a net accumulation of carbon during a year (tree growth) also has a net production of oxygen. The amount of oxygen produced is estimated from carbon sequestration based on atomic weights:
    net O2 release (kg yr) = net C sequestration (kg yr) × 32 12”
    (from: )
    It turns out that aquatic plants (algae, phytoplankton, etc.) in the oceans and lakes and rivers likely produce more net oxygen than land plants (think respiring root systems).
    Bottom Line: Yes, plants do consume oxygen in respiration – both night and day. But usually plants produce more oxygen via photosynthesis than they consume through respiration. And, so, we animals benefit.
    Take a deep breath….

    • Thanks for replying!☆*:.。. o(≧▽≦)o .。.:*☆
      However if there was also a candle in the jar…then wouldn’t the test be unfair(do not know if that’s what you’d called it) as there was a candle wouldn’t the plant continue to photosynthesis even at night considering that the candle would give off light energy? Therefore it wouldn’t be a fair experiment?(idk?)
      Also I tried to read the uhhh article? About the urban forest, carbon sequestrian and all that but I couldn’t quite understand the important part of it cause I’ve never heard all those stuff before let alone learnt it.
      They were also kind of talking abt plant growth,which I guess would,affect a ton of stuff and also this leads to a new question which is wether or not even if it grows bigger produces more oxygen during photosynthesis does that mean that it’ll also take in more O2 and give out more CO2 during respiration? Thus bring us back to the start-middle?

      • Good point about the candle light, although a candle doesn’t produce much light. But keep in mind that Priestley also did the experiment with the mouse alone, showing that the plant was a net producer of O2.
        I think, however, you may have missed the point about plant growth.
        It’s simply a way to estimate the amount of excess photosynthetic O2 released by plants into the air. Put another way:
        (Amount of photosynthetic O2 produced) – (Amount of O2 used by respiration) = Net amount of O2 released by the plant = plant biomass production (growth)
        This presumption has been experimentally supported by many different studies over many years.
        The fact is that – over days, weeks, months, and years – most photosynthetic organisms produce significantly more O2 than they themselves consume. The evidence for this is both obvious (you’re breathing, right?) and experimentally overwhelming.

  32. Hi, Can you tell me if this is correct as Sky News appear to think so!

    “Plants don’t absorb carbon dioxide very well”

  33. No, this Sky News article seems to have little basis in scientific evidence, at least when it comes to plants. (I couldn’t find an author name for this story.)
    Stating that an increase in global average temperature of “two degrees” (F or C? – doesn’t say) will stop plant growth and cause plants to emit CO2 is preposterous, in my opinion.
    Why? Because both greenhouse experiments over the years, as well as paleobotanical studies, tend to support just the opposite effects on most plants.
    Regarding the paleobotany data, please investigate the “Paleocene–Eocene Thermal Maximum (PETM)”, about 56 million years ago. Also, I recommend a great book “The Emerald Planet” (
    Don’t get me wrong, however. I think the unprecedented rapid increase in atmospheric greenhouse gases caused be humans will likely have very upsetting effects on the climate, the oceans, and terrestrial ecosystems and should be a very serious concern to everyone.
    There has been much scientific research over the past 20 to 30 years to try to predict the consequences.
    Unfortunately, the person at SKY News who wrote the story in question is apparently largely ignorant of this scientific evidence.

  34. Hello,
    This was an amazing post, really enjoyed everything here, i had a question which has probably been asked before. First, if i understand this correctly, simply put, there are two processes involved, the first being the generating of power via sun energy into the splitting of H2O, the second being the converting of Co2 into carbohydrate sugars via power supply of the first process. From what i read in one of your replies to another post, the second process is a bottle neck in throughput. The input or energy supply exceeds capacity of the output. Now this is extremely over simplifying things, but would it be possible to engineer a green plant species or crop to increase capacity of the output to match the input of the first process in order to maximize the Co2 intake of a single plant, say an aver ash tree? Then in turn reduce the amount of ” total trees” needing to be planted to create an equilibrium with human growth rate and Co2 emissions. This could in turn reduce the issue of increased population by needing less area per square ft of ambient plant life in say rural areas to stabilize our atmosphere. Obviously reducing Co2 omissions is a dynamic problem with many sides, but this if possible seems like a step in a good direction. Apologize, not a simple question… and thank you again for all that you have presented.

    • Yes, many molecular biologists are focusing on the key photosynthetic enzyme RuBisCo as a way to improve photosynthetic efficiency in most crop plants.
      For example:
      “A major target for improvement is the enzyme Rubisco [ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase] whose deficiencies in CO2-fixing speed and efficiency pose a key limitation to photosynthetic CO2 capture (6, 7).”
      (from: – and see especially refs 6 & 7 therein)

  35. Awesome post. It seems as if the cycle slowly loses water when its split from its oxygen. Is there a natural process that creates water?

    • Yes, many metabolic oxidation reactions result in the production of H2O from H+ and OH- (H+ accepts electron).

      For more, Google “metabolic redox reactions”, for example.

  36. Tree planting, while laudable, will not solve the greenhouse problem. Trees will make their sugars, etc., live for a century or so, die, rot and bacteria & fungi will exhale that carbon back into the atmosphere and we will be right back where we started.
    The problem lies in geologic time spans. Three hundred million years ago there was a fifty million year period (the Carboniferous) when high atmospheric carbon dioxide levels caused tremendous plant growth. That C, H & O, buried by geologic processes decomposed over the next 250 million years. Oxygen went leaving hydrocarbons; followed by hydrogen leaving coal – almost pure carbon.

    In less than a century, we have liberated millions of years’ worth of this stable carbon which had been sequestered underground, and converted it back to carbon dioxide by burning it.
    I am pessimistic that this process is reversible in human time spans. The best to hope for is to stop making the situation worse.

  37. Thank you for this informative post.

    I have a question:

    Is the oxygen in sugars and in phosphate by-products of the Calvin cycle depleted in 18O with respect to CO2 ?

    In other words: is there an isotopic discrimination against 18O-containing CO2 during photosynthetic CO2 uptake ?

    Can you suggest any experimental evidence for or against this effect ?

    Thank you very much,


  38. I presume this question has been addressed, especially in the plant ecology literature, but this is certainly outside of my realm of knowledge.

    Here is, perhaps, a helpful reference:
    and be sure to scroll down to view additional papers citing this article. This should help answer your questions.

  39. Does plants can do photosynthesis if just Co2 is provide in Atmosphere ?

  40. Nice,
    no wonder people believe things that aren’t true.

  41. Thank you for your explanation ..
    But, I have an inquiry
    we all know that in photosynthesis, O2 and glucos are produced
    Ok, but in cellular respiration this Oxygen is used for Oxidation glucos for producing energy

    So, Do that mean there is no oxygen released to the atmosphere ?! because it’s used in the cellular respiration

    My question in another way .. What’s the difference between the oxygen which we breathe and the oxygen using in the cellular respiration?

    Or , Do the oxygen which is produced from photosynthesis release to the atmosphere ?or it’s used directly into the respiration process in mitochondria?

    • Yes, plants do consume oxygen in respiration – both night and day.
      But plants typically produce more oxygen via photosynthesis than they consume through respiration. So, there is a net productions of oxygen, which is released into the atmosphere.
      And, so, we animals benefit.

  42. Thankyou! this post was a great source of inspiration for me by its help I realized that we can actually build artrificial leaves that can convert water into oxygen.
    I will inform you after project is completed.

  43. Is it possible to build a artificial chloroplastic system?How?

  44. In this article it was mentioned that “The chlorophyll converts light energy (photons) into chemical energy, in the form of high-energy electrons.” I was wondering if this could be elaborated on. I’m conducting a small experiment for school to explore organic batteries and sources of electricity.


  45. Shikhar Srivastava

    What happens at night? I have read that plants inhale O2 and exhale CO2 at night?
    Is it true? And What reaction happens at night?

    • Yes, plants do consume oxygen during the metabolic process collectively known as Respiration – all of the time, both night and day.
      But green plants typically produce more oxygen via Photosynthesis than they consume through Respiration.
      Therefore, green plants yield a net production of oxygen, which is released into the atmosphere.
      And, so, we animals benefit.
      (Next time you see a green plant say thanks.)

  46. We now have about 20% of O2 in the atmosphere. According to your article, this O2 originated from water (H20). Now, I am trying to figure out how much C02 is needed to produce 20% O2 by photosynthesis?
    Can photosynthesis somehow re-use the same C02 to release ever more O2 from water or do we need exactly as much C02 to create the same amount of O2?

    There currently is very little C02 in the atmosphere as it is stored in living organisms and buried dead organisms. Is the amount of living bio-mass enough to restore O2 levels to 20% when – through some kind of catastrophy – all the O2 would be taken out of the atmosphere?

    In the early earth history, most of the produced O2 disappeared in oxydizing iron and other chemicals that react with O2. How much C02 did photosynthesys require to produce all that O2?

  47. I know this is not about plants but just wanted to find out: Can we convert CO, CO2 and O3 to oxygen using electricity? I think we can do that to O3, I looked it somewhere, but what about other two? I am doing this for a science project

  48. While I appreciate your quite accurate article, it fails to answer the primary question…that being how much oxygen does a plant produce in relationship to the CO2 consumed. I realize that there are many variables such as the particular type of plant (everything from common grasses to a sequoia), amount of atmospheric CO2 available in ppm, and how much water the plant consumes (optimally).

    The anaolgy I use is an engine. As you point out, plants break down the water molecules to produce the oxygen, but the CO2 is the fuel used for that process, just as my engine produces torque by using a combination of fuel (gasoline or diesel) and air. The injectors can pump all of the fuel into the engine it wants to, but without the air to support combustion it won’t work at all, hence no torque.

    Given the above, is there a “standard” formula one can use to determine a “global average” rate of oxygen production based on “optimal” water consumption (optimal being a variable also based on CO2 levels since increasing CO2 reduces the amount of water needed) and a CO2 variable, given the relatively low levels of CO2 in our atmosphere for optimal plant growth?

    e.g., in a sealed atmospherically controlled greenhouse situation (given optimal water for the plants), increasing CO2 levels from the current atmospheric level of approx. 400 ppm, what would be the increase in O2 levels be per 100 ppm step increase in CO2?

  49. Thanks for your questions.
    First off, the “fuel” or energy that is used to break down water into oxygen, electrons and hydrogen ions is sunlight, not CO2. (Think solar-powered donut factory, where CO2 is used to make the “donuts”.)
    The plants “burn” the “donut fuel” as convenient sources of energy (somewhat analogous to storing solar-generated electricity in batteries) using the process we call respiration. And, yes, oxygen, is consumed in respiration.
    So, as you say, the big question is: What is the net oxygen production on Earth provided by photosynthetic organisms (e.g., plants, algae, Cyanobacteria).
    As you have also noted, this is very difficult to measure…even with a single tree, over the course of a single day, let alone on a global scale.
    If you’re thinking that increasing atmospheric CO2 may lead to a net increase in photosynthetic oxygen production, you may indeed be correct. (There is some evidence for this in the geologic record.)
    But just as complicated as figuring out the current net O2 production, predicting how increased atmospheric CO2 will affect atmospheric O2 levels is extremely complicated, due mainly to many unknown variables.
    Only time will tell…..

  50. Thanks for letting me know the answer which I was never found in my text book. Now it is clear toto me. It is really very helpful.

  51. Thank you for answering a seemingly complex concept in an understandable way, and for diligently answering the questions of so many people of over all these years. This is definitely a valuable, timeless and and informative resource.

  52. I realize this post is nearly eight years old, but I just wanted to say thank you for your concise explanation! I’m a university bio student, and as I was studying the Calvin cycle I realized that my text didn’t really explain where the O2 from CO2 ended up. A google search brought me here, and your post clarified everything! I had a bit of a facepalm moment when I finally thought about what G3P was actually made of, and found my missing O2 molecules. Thank you for taking the time to write this post, it was exactly what I needed! 🙂

  53. You’re welcome, Liz.
    And thanks.
    Such comments help to motivate me in keeping this blog going.

  54. Joke only: From all this; I would have thought that the CO2, Light and H20 was the food of the plant. It gets converted to sugar which the plant uses to grow… and Oxygen was the “poop” of the plant. I certainly enjoy plant poop…

    On the Real: I was thinking about cleaning up the environment and trying to figure out how to clean up the air using plants/photosynthesis but from what I’ve been reading, it’s efficiency at it’s best is 8%? Has anyone been able to replicate the Calvin Cycle outside of plant/bacteria based solutions?

    Side Note: I do appreciate that you still keep up to date on comments. 7 year old article and still responding!

  55. Mitchell Brown

    Most of Earth’s oxygen comes from tiny ocean plants – called phytoplankton – that live near the water’s surface and drift with the currents. Scientists agree that there’s oxygen from ocean plants in every breath we take. Most of this oxygen comes from tiny ocean plants – called phytoplankton – that live near the water’s surface and drift with the currents. Like all plants, they photosynthesize – that is, they use sunlight and carbon dioxide to make food. A byproduct of photosynthesis is oxygen.

    Scientists believe that phytoplankton contribute between 50 to 85 percent of the oxygen in Earth’s atmosphere. They aren’t sure because it’s a tough thing to calculate. In the lab, scientists can determine how much oxygen is produced by a single phytoplankton cell. The hard part is figuring out the total number of these microscopic plants throughout Earth’s oceans. Phytoplankton wax and wane with the seasons. Phytoplankton blooms happen in spring when there’s more available light and nutrients.

    Content in this section supports the concept of growing crops in space and the symbiotic relationship between plants and space travelers. Plants in space are beneficial for a number of reasons. They provide nourishment for the body when eaten as food, and they improve the quality of indoor air. Plants take the carbon dioxide from air to produce oxygen that humans can breathe. Find information about how plants, people, microbes and machines work together in self-contained space vehicles.

  56. I am curious to know whether it is possible to produce CO2 with the O18 isotope, or is there something special about H2O with the O18 isotope that make it easier to produce. If “heavy CO2” could be produced then perhaps the complimentary experiment can be run to see if any O18 is found in the atmosphere where the plant grows. i.e. there is a transfer of O18 from CO2 in the atmosphere to O2 in the atmosphere, through the plant.

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