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 O[sub]2[/sub].
If the man didn’t need food, he’d still die. The CO[sub]2[/sub] he exhales comes from the food fuels stored in his body. The plant uses the CO[sub]2[/sub] 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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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.
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?
God, eight year-olds sure ask the damndest questions! 
Now my curiosity is really piqued!
“Teaching without words and work without doing are understood by very few.”
-Tao Te Ching
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 C0[sub]2[/sub], 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
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.
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.
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.
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.
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.
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.
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.
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.
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.
I think it boils down to:
- How much oxygen does the average human use in a day?
- How much oxygen does a plant give off in a day?
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.
[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
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.
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.
They don’t call me the colonel because I’m some dumb ass army guy.