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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 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 comes from water. Green plants DO, however, convert atmospheric CO2 into sugars. So, the oxygen atoms in the CO2 wind up in the sugars (e.g., glucose = C6H12O6).


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. (Abstract)

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59 Responses to “Plants Don’t Convert CO2 into O2”

  1. Mike says:

    Just passing by.Btw, you website have great content!

    Making Money $150 An Hour

  2. Louise says:

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

  3. akash says:

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

    • howplantswork says:

      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: http://www.youtube.com/watch?v=Q1XMSNvN-OI )

  4. dan says:

    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!

    • howplantswork says:

      Thanks for the comment.

    • Mike Rowley says:

      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.

  5. David Grant says:

    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?

  6. ponnani says:

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

  7. loanjipan says:

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

  8. Rodrigo says:

    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?

    • plantguy says:


      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.

  9. Audrey says:

    This was really helpful! Just wanted to say thanks!

  10. Joe Flood says:

    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).

  11. Amity says:

    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!”

    • plantguy says:

      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.

    • Mike Rowley says:

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

  12. Robb says:

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

    • plantguy says:

      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.

  13. ddeeswxadelvinpplliokj says:

    what is co2 and o2.

    a primary school guy getting research info.

    • plantguy says:

      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).

  14. Rocky says:

    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

    • plantguy says:

      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.

      • Rocky says:

        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.

    • KaiserDerden says:

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

      • plantguy says:

        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”.

  15. ChuckTesta says:

    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?

  16. Avik says:

    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. :)

  17. Isabelle says:


    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

  18. Bruce says:

    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.


    • plantguy says:

      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: http://www.itreeservice.com/pdfs/environmentalbenefitsoftreesinurbanareas.pdf

      “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?

  19. Carlos Davarre Salazar says:


  20. Mizuka says:

    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?

    • plantguy says:

      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.

  21. Anna says:

    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

    • plantguy says:

      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?)

  22. Nathan says:

    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.


  23. Tamadite says:

    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.

    • plantguy says:

      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., http://www.licor.com/env/products/photosynthesis/ ).
      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., http://www.eurekalert.org/pub_releases/2014-08/ajob-mym081414.php ).

  24. Janakiraman says:

    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.?

    • plantguy says:

      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!

  25. GName says:

    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!)

    • plantguy says:

      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. Aaron says:


  27. happy says:

    nice explanation

  28. Marty says:

    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.


    • plantguy says:

      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) https://youtu.be/xRMKiLlpATk – 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 (https://en.wikipedia.org/wiki/Transposable_element), 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.)

  29. SBB says:

    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?

  30. Evan says:

    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.

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