Articles about photosynthesis in the popular press or online often make me cringe.
Why?
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!
Ack!
This is simply NOT true!!
Please allow me to explain…
The Oxygen You Breathe Comes from Water
Yes, that’s correct, water… H2O
Here 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 the radionuclide, a.k.a., radioactive isotope, of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H2O18 or CO218, produces radioactive O218. Turns out, it’s the 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).
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Just passing by.Btw, you website have great content!
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This post is great – I’ve never really understood it before. Thanks, it actually makes sense now !!
You’re welcome! (That’s primarily what this site is here for.)
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: http://www.youtube.com/watch?v=Q1XMSNvN-OI )
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.
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?
Probably too hot (no liquid water to germinate spores) and too dark (clouds too thick).
See http://en.wikipedia.org/wiki/Venus#Atmosphere_and_climate .
What happens to the Hydrogen? Please can you tell about that.
Thanks
The hydrogens are used to make ATP.
See here, e.g. – http://en.wikipedia.org/wiki/Chemiosmosis#In_plants
Simply, hydrogen ions produced during the “light reactions” build up inside the thylakoid. This “hydrogen pressure” is used to make ATP (think water behind dam used to drive turbines to generate electricity).
What is the time that the plant would not take in or give out oxygen and carbon dioxide???
I don’t know.
That would be very difficult to determine.
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.
This was really helpful! Just wanted to say thanks!
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).
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.
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.
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).
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.
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?
Good idea….others have thought of it, too.
Several problems, however.
More people -> chop down more trees to clear land for agriculture & for fuel to burn -> fewer trees -> more CO2 -> more global warming -> higher temps -> kill even more trees. ( see, for example: http://www.climatecentral.org/news/climate-change-stress-killing-forests-and-why-it-matters-14960 )
Still a good idea to plant more trees, though. It may slow global warming down a bit.
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.
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