Plants, oxygen, and us

[Soup]

If a man were locked inside an airtight box, how big of a plant would have to be locked in the box with him to keep him supplied with enough oxygen to keep him alive?

Good question! I hope UC picks this to answer personally, a la "Can man live on bread alone?"

My WAG: a pretty big plant.

The box would also have to simulate daylight/nighttime. It couldn't be completely dark, or the plant wouldn't make any O2.

If the man didn't need food, he'd still die. The CO2 he exhales comes from the food fuels stored in his body. The plant uses the CO2 to build its own structure. He'd slowly be depleted of carbon.

If the plant grew food that the man could eat so that the carbon would be recycled. Presumably, he'd excrete into the plant's soil so nitrogen and other nutrients could be recycled too.

Like I said initially, this is a big question that would require Cecil's intellect to resolve completely.

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Everybody got to elevate from the norm - Rush

[Lumpy]

Also remember that plants don't always absorb carbon dioxide. During the day, they use sunlight to convert water and carbon dioxide into sugars. At night, the plant usually burns those sugars again to power it's life processes. Cellulose, the main ingredient of plant fibers, is made by polymerizing sugars. So plants only show a net uptake of carbon dioxide when they're gaining mass. So as a WAG I'd say you'd need enough plants that they increase their mass by five pounds a day or so.

[Soup]

Good answers, thanks. But now I need specifics, put in terms that an eight-year-old would be satisfied with (that's who originally asked me this question). So, are we talking about one really big plant, like a redwood tree? Or a large volume of plants, like an acre of rainforest? Would a billion Chia Pets do the trick?

[Demo]

God, eight year-olds sure ask the damndest questions! ;-) Now my curiosity is really piqued! ;-)

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"Teaching without words and work without doing are understood by very few."
-Tao Te Ching

[Lord Jim]

Soup
I thought this was a gimme. If I looked around, someone would say a certain size plant would support a human. I didn't find it. I learned a lot over the last couple of days but didn't find much help. So, I decided an alternate method was needed.
I read (god knows where) that maybe half of the earth's oxygen is produced in the oceans, that would mean that the other half was produced on land. A little arithmetic means that about 750 million square mile are producing 1/2 the needed oxygen.
Now, generally, all living cells consume oxygen and give off C02, the only oxygen producers are the green portion through photosynthesis. Therefore, I tried to figure out how much of the land was green on the average, year round. Taking out all the tundra, deserts, mountains, cities, highways, I figured that maybe 30% was green. Of which, the tropics might be green year round while outside of the tropics would probably only be green 1/2 the year. Averaged over the year, would mean that 20% might be producing all the oxygen for land mass. Therefore, 150 million square miles are producing 1/2 the needed oxygen. So, 300 million square mile would be needed to produce enough oxygen without the oceans.
Now, since plants in general are oxygen producers and animals are oxygen consumers, the next step was to guess what percent of the total animal mass was human. God only knows. So, I decided to use 1%. Therefore, 3 million square miles of green plants should handle humans. Divide by 6 billion humans and you end up with about 14,000 square feet per human, which is about 1/3 acre.
Now, what plants? I think that large trees are probably less efficient than other plants. The percentage of green being higher in grasses and other plants. I would guess that vines, like ivy, and shrubs, and long bladed grasses, like monkey grass, have a high percentage of oxygen production than trees.
So, I'm going with a 1/3 acre of shrubs, monkey grass and ivy, until someone comes along and fixes all my wild assumptions.

Hope this helps

[kanicbird]

great estimation, but I think there is no answer if your goal is a life support closed system.
Plants take in CO2 and give off O2 basically (net +O2 net - CO2). Animals do the reverse, no problems yet. Now lets look into the plants do with it, They convert the C in CO2 to some food or structure inside the plant. the plant eventually dies (decays), is eaten, or is burned, the end result is ALL the C is eventually released back into the atmosphere as CO2 (some CH4 in some cases, which eventually become CO2 anyway) (with one possible exception below).
now if the forest is expanding you will have an increased rate of CO2 to O2 conversion, but this is temperary as the forest will eventually reach its limits and the rate of growth will equal the rate of death. in this case the rate of co2 to o2 will basically equal. therefore trees act more as a co2 to o2 capacitor then a sink. now it is possible that some trees fall in a place that gets burried and doesn't decay, and acording to conventional wisdom will become fossil fuel one day, efectivelly removing some carbon from the atmoshere for a great deal of time, but this case is very small at best.
Looking at a diagram of the carbon cycle there is only one way to get carbon out of the atmos. and it occurs in the ocean in the formation of a rock. Plants may have helped moderate CO2 spikes but they are not long term solutions, so feel free to cut down that achr of jungle (rainforest) just don't mess up the oceans.

[Lord Jim]

Well, k2dave, I'm not sure that the idea was to support life forever, but simply to produce enough oxygen to support a person for a short period of time, maybe the life of the person. That doesn't leave time for the decay of the plants to be much of a consideration.
My scenario was to produce a short term environment that could be maintained. Bioshere II failed by not taking into considerations things like the concrete absorbing oxygen. I don't think this is what an 8 year old is asking. And I sure don't know how to take all those things into consideration.

[Jois]

Did the little dumpling bring his science papers or book home?

If that is what he's been doing in school you might get a near answer in his school work.
The EB (on-line) might say something under plants or Krebs cycle? or something that would say how much O2 a plant might give off.
And then you could use an empty gallon milk bottle and have him determine lung capacy from blowing into the bottle? inhaling air from the milk bottle???

Let him decide if he half filled or half emptied and then say 1 quart of air per gallon and you breathe so many time a minute, hour, day, week-

And then--go back to the amount of O2 tossed off by plant and add enough plants to last as long as he'd like.

That's as close as he'll want to know.

[Lord Jim]

Soup try forgeting the redwood, maybe a 1/2 acre of rain forest type plants. I have no idea how many chia pets the translates to.

[Soup]

JimB and k2dave, thanks for your efforts thus far! I was discussing this with my brother (The Great Brain) and he also mentioned Biosphere. Maybe if any of the former inhabitants read this, they can shed some light.Or perhaps a botanist or biologist can pick up where Jois left off.

[Lord Jim]

Jois:
My trouble was finding the amount of oxygen given off by x amount of surface area of a green plant or any simular information. How many times you can re-breath your own exhaled air into a gallon jug doesn't seem to help much in determining the size of the plant needed to replenish the oxygen.

I may be missing your point, (entirely posible) but the question was how much plant was needed to create the needed oxygen.

[Jois]

You're dealing with a question from an 8 year old.

You find out how much air/oxygen he'd use in a breath by making a guess against a breath into a gallon milk jug - "Well, do you think you could fill up this jug with a breath?"
(Nice time to talk - a sentence - about half, more than half, less than half. It's always fun to watch kids think.)

You'd only do it once or twice so you would both get a visible concept of a breath.

Don't let the little guy go huffing into the milk jug, you'll both get arrested.

[Jois]

I.e., it lets you know how much air the kid needs, what would it matter how much the plant gives off if you don't know what's needed.... Better?

It's a kid's question and it needs a kid sized answer.

[funneefarmer]

Testing the size of the child's lung capacity should be easy. Have him take a normal breath then blow it into a balloon. Measure the diameter and plug it into the sphere volume equation.

[Soup]

I think it boils down to:
1. How much oxygen does the average human use in a day?
2. How much oxygen does a plant give off in a day?

[Lumpy]

In a closed system, any plant growth that doesn't result in humanly edible food will ultimately be non-contributary to the goal of maintaining human life. As pointed out earlier, forests will reach a steady state where rotting, consumption by insects, etc., will return as much CO2 as the plants absorb. Although in the short term you could presumably dehydrate and bag lawn clippings or some such to get the carbon out of the atmosphere.
That's probably one reason why Biosphere II failed, and why proposed space colonies filled with grass and trees wouldn't work either. If it ain't growing food for humans directly or indirectly, it's a waste of space.

[kanicbird]

[Well, k2dave, I'm not sure that the idea was to support life forever, but simply to produce enough oxygen to support a person for a short period of time, maybe the life of the person]
Maby it was ment to substain life for a space flight or a lunar or martian colony. In which case you might be able get away with growing plants and dumping them outside your sealed enviroment when they die(ignoring the soil/water depleation issues). We are trained to think that plants produce our O2 and we need them for that reason, I once believed this but looking deeper into this, it doesn't make sense.
If you were to jetison the plants when they died then you could calculate the plants neeeded by finding out the total increase in plant mass per day needed to offset the C02 as mentioned before.
Also spacecraft use C02 scrubbers to remove the co2 from the breathable air. anyone know how they work? might be a better solution then bringing a jungle with you

[funneefarmer]

http://www.fitnessworld.com/library/aerobic/aerofacts0894.html

This site states that the average human breathes in 3500 gallons of air a day. This doesn't give you an idea of oxygen use but might impress your 8 yr. old.

[Padeye]

Sorry I didn't reply to this thread earlier. I worked at Biosphere 2 for nearly seven years, including the original 2 year mission. I did not live inside except for a couple of overnight stays after the first mission.

Two of the biggest overlooked problems were concrete and microbes. The bare concrete absorbed and sequestered massive amounts of C02, robbing a lot of the O2 from the atmosphere. Most of the soil used came from a cattle tank - a pond where range cattle get water. It was chock full of cow shit so very fertile but also with lots of microbes that took a lot of the Oxygen.

The design wasn't sustainable for that an many other reasons but I won't call it a total failure. It certainly didn't go as planned but it's the biggest test tube of its kind anywhere. I've been away nearly two years though and last inside scoop I've gotten is that the new management, Columbia University, has virtually gutted the apparatus.

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They don't call me the colonel because I'm some dumb ass army guy.

[funneefarmer]

Another site with many links relating to the problem. Incidently, is this your child's homework ? This site seems to be one that proposes science experiments for students.
http://www.gallaudet.edu/~tsapsoar/tsanov.html

[Lord Jim]

So, padeyed, what was the basic formula for amount of plant life to produce enough oxygen for a human? Surely y'all had a good idea before the concrete started eating all the oxygen.

[Lumpy]

Also spacecraft use C02 scrubbers to remove the co2 from the breathable air. anyone know how they work? might be a better solution then bringing a jungle with youI believe they work by using a chemical (lithium oxide?) that absorbs CO2 out of the atmosphere. I don't know if once the chemical is saturated the scrubbers have to be chucked, or if they have ones that can be cycled to drive off the CO2 for expulsion. I wonder what system is used onboard nuclear submarines?
In any event, while scrubbers keep the atmosphere breathable they don't address the problem that a consumptable- oxygen- is being used up. Unfortunately there currently isn't any practical way to reduce CO2 into carbon and oxygen. If there was, it would be great to have aboard a spacecraft where dumping the carbon would be an acceptable medium-term solution.

[Lumpy]

Also spacecraft use C02 scrubbers to remove the co2 from the breathable air. anyone know how they work? might be a better solution then bringing a jungle with youI believe they work by using a chemical (lithium oxide?) that absorbs CO2 out of the atmosphere. I don't know if once the chemical is saturated the scrubbers have to be chucked, or if they have ones that can be cycled to drive off the CO2 for expulsion. I wonder what system is used onboard nuclear submarines?
In any event, while scrubbers keep the atmosphere breathable they don't address the problem that a consumptable- oxygen- is being used up. Unfortunately there currently isn't any practical way to reduce CO2 into carbon and oxygen. If there was, it would be great to have aboard a spacecraft where dumping the carbon would be an acceptable medium-term solution.

[Lord Jim]

Come on y'all. The question isn't "how to build Bioshere 3." And not how much microbes in cow shit or in decaying plants or absorbed by the concrete effect the Bioshere (earth). I think Soup wanted to know, you got one "X" human being here, you got "Y" plant there. The Oxygen from "Y" minus the Oxygen used by "X" equals Zero. "X" is a half human,... no wait, a half-sized human, ... ok, "X" is a human. What is "Y"?
Or, in other words, can a kid plant a christmas tree. And he and his tree equal each other (oxygenwise) and therefore are not taking oxygen from the environment. Or does he need a whole greenhouse to equal out.
Soup, if I'm off track, just tell me to shut up. I'm tough, I can take it.

[Soup]

No, JimB, you're pretty much on track. And thanks to the website funeefarmer provided, it appears that a normal human at rest needs about 360,000 ml of oxygen a day delivered to the tissues in his body. The trick now is to find out what plant (or plants) can pump out that much oxygen in a day.

[Soup]

So, are there any botanists or biologists out there who can tell us how much oxygen plants produce in a day?

[Lord Jim]

Hey, Soup, I'm going to push this to the top one more time. I can't believe that no one in this vast expanse of knowledge wants to answer your question. I would if I could, but I haven't found an answer. I'm really interested myself in the answer.

[Soup]

Thanks, Jim. We'll crack this one yet. C'mon people! There's an little eight-year-old counting on us!

[scr4]

Let's see if I understand it correctly. Plants convert CO2 and H2O into, well, big organic molecules. (They do the opposite as well, but this is the net effect.) Animals convert big organic molecules into CO2 and H2O. The two makes a closed loop. So if there was a single plant and a single animal in a closed system, if the animal eats enough of the plant so that the plant doesn't get bigger or smaller, the plant is also releasing just enough oxygen to support the animal.

Then the original question is equivalent to "How big a plant is needed to feed a man on a long-term basis?" So we just need to figure out how big a farm is needed to support a single person. A human needs, what, 2000 Calories per day? According to one on-line reference (which is in Japanese so I won't bother to mention it), that's equivalent of 600 grams of wheat a day, or 200 kilograms (400 pounds) per year. (Does that sound right?)

So how big a field is needed to grow 200 kg per year? Apparently the maximum yield of a wheat farm is about 80 Bushels per acre, or 3 cubic meters per acre. I have no idea what the density of wheat is, but as a wild guess I'd say about 0.5 grams per cm^3. So 3 cubic meters weighs 1500 kg. To produce 200 kg/year you need about 1/7 of an acre, or
600 square meters. That's an area 20x30 meters, or 60x100 feet.

But this area of wheat farm doesn't just feed people, it also feeds - and thus supplies oxygen for - other animals such as microbes and maybe cows. So the fraction of oxygen we use from this farm is the fraction of the plants we eat. Not a lot in terms of weight, but much of the energy is probably in the parts we do eat, so I'd guess about half. So, the answer to the original question is that we need a 50x50 feet area of some type of fast-growing crop to produce enough oxygen to keep a man alive.

Now, I have no feel about if this answer is correct, so it may be way off. Let us know if someone finds any big mistakes.

[Soup]

Thanks scr4.
I'd still like to know the actual daily oxygen output of a plant. I suppose this is usually measured in milliliters? Then we can try to match it up with a person's daily oxygen requirement (the number I'm working with right now is 360,000 ml).

[Padeye]

JimB sez:what was the basic formula for amount of plant life to produce enough oxygen for a human?

They never bothered with stuff like that. It was all based on the so-called Gaia (sp?) hypothesis that the earth is a self-regulating entity. They did some work with a single person in a house size test module but AFAIK a lot of was wild ass guesses.

They overlooked a lot of simple things. The glass cut out all UV light but it never occurred to them that it would effect the primary pollinators like bees. They all died. The bats died. The hummingbirds died. One of the galagos (bush babies) crawled into an electric bus, was electrocuted and died. They took the rest out. The cockroaches and ants thrived. You did not want to be the first one to turn on the light in the kitchen in that place.

A lot of it was apparently done just to be exotic. They developed a hybrid of a Japanese silky chicken and jungle fowl for eggs but it didn't work out. After much research and analysis they determined that common leghorns would be better. Any farmer in America could have told them that.


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"Popeye? Hm? He's not much of a judge of women!" King Blozo

[Arjuna34]

Since O2 concentration in normal air is about 21%, and in exhaled breath is about 19% (depending on how much you're working, etc.), about 2% of the 3500 gallons you breathe in is used. This gives about 270,000 ml, which is pretty close to your 360,000 ml (close for this kind of "back of the napkin" calculation).

My WAG is about 100 square feet of leaves per human, or maybe 10 trees.

Arjuna34

[Soup]

Everyday we creep closer to The Answer! Thanks Arjuna34. I'm stll looking for the actual oxygen output of a plant. It may be different for different species. I've yet to see hard numbers for this.
Just to recap. Our educated guesses as to how much plant is needed to supply a man with enough oxygen to live are: 1/3 acre of shrubs and monkey grass, 1/2 acre of rainforest, a 50x50 plot of fast-growing crops, and 10 trees (I'm dropping my guess of a billion Chia Pets). We're getting there!

[Padeye]

The calculation issues are much more complex than anyone generally accounts for. If you use a single figure for human's oxygen uptake what happens when they all have to do strenuous labor to farm enough food to eat? What happens when the leaf material dies out? the rotting leaves release carbon back into the system. How about the glass and steel structure that blocks out a large percentage of sunlight?

One of the biggest problems with a small sealed system is that variations in O2 and CO2 don't have much of a buffer. On biosphere 1 (the earth) CO2 is rising but it isn't a big problem right this second though it will be in the future. When CO2 starts rising in a small system people can be dead in a very short time.

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"Popeye? Hm? He's not much of a judge of women!" King Blozo

[Soup]

Padeye-
While I'll admit our eight-year-old is bright, I don't think the original question went much beyond "How much oxygen does a plant produce?" How did you figure out how much plant life was needed in Biosphere 2?

Remember, this is a hypothetical man and a hypothetical plant locked in a hypothetical box. Ballpark figures are gladly accepted.

[Soup]

Padeye-
While I'll admit our eight-year-old is bright, I don't think the original question went much beyond "How much oxygen does a plant produce?" How did you figure out how much plant life was needed in Biosphere 2?

Remember, this is a hypothetical man and a hypothetical plant locked in a hypothetical box. Ballpark figures are gladly accepted.

[Soup]

Looks like I found some hard numbers! According to the website below, 1kg of organic matter will release 1.5 kg of oxygen.
http://www.infodienst-mlr.bwl.de/la/lap/ergebnisse/agraroekologie/allgemeine_oekologie/CO2-Bindung_E.htm

This guy even goes into what plants produce how much oxygen. I'm not sure what unit of measurement he's using in that chart, but as soon as I figure out how to convert kg into ml, we may have something to work with! Feel free to crunch some numbers!

[Padeye]

Soup-
Sounds like you have a very bright child asking good questions. I was just trying to point out that in a tiny closed system the fluctuations of Oxygen and CO2 become critical long before the long term averages do.

I wondered the same thing a a child, if you could seal a terrarium with a few mice and plants and it would sustain itself. I was told by more than one teacher that it couldn't be done, no reason, just that it could not be done. I was also told we would never see an atom but thanks to electron microscopes we have pictures of individual atoms. I found Bio2 to be fascinating because of this and it's one of the reasons I took the job they offered me. That plusx the fact that they offered me more money than Computerland was paying.

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"Popeye? Hm? He's not much of a judge of women!" King Blozo

[Soup]

Padeye-
She's a very bright kid, and has more questions like this. Thank goodness for The Straight Dope! We're having alot of fun watching the thought process involved in answering this question.