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So where DOES the mass of a tree come from?

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Yesterday, I asked where the mass comes from in a tree.

The answer is actually really simple: Carbon.  Photosynthesis is the act of converting CO2 from the air into O2 and a bit of H2O. 

It turns out that if you ask new Harvard graduates this question, the vast majority of them answer some variant of "It comes from the soil".  When people think of photosynthesis, they don't think about the carbon that's left behind.  They'll usually be puzzled as they answer, because in their hearts, they realize that "the soil" doesn't actually work as an answer, but they can't quite put all the pieces together.

If you ask 7th graders the same question right after they've finished their photosynthesis unit, they end up coming up with a variant of the same answer.  Or they say it comes from the water the plant absorbs. Soil and water have mass, and that seems to the determining factor in their answer.

You see, 7th graders don't seem to get the idea that air contains mass - it's just the stuff that's around them, it doesn't "weigh" anything.  Since they have always been exposed to the weight of air, it doesn't occur to them that it has any real properties at all.  If you show them a block of dry ice, and ask them what it is, they'll say "It's ice". If you ask them to weigh it, they get that part.  It's not until you follow that up and ask "So what's happening to the ice?" and they realize that the fog it's generating is disappearing into the air that they'll start figuring out what's happening - that the mass of the dry ice is being absorbed into the air.  The very smartest of the kids will then put the pieces together and realize that air DOES have mass.

Yesterday's "quiz" netted over 70 correct answers and about 15 incorrect answers, I've got to say that I'm pretty impressed. The reality is that since I only let the incorrect answers through, it biased the sample towards correctness (several people mentioned that they'd read the other comments and realized that their first thought was wrong).

 

Valorie had one more question: Could those of you who got the answer right, and who are under 30, and who were educated under the American education system post a comment below?

Comments

  • Anonymous
    August 02, 2006
    "Could those of you who got the answer right, and who are under 30, and who were educated under the American education system post a comment below?"

    Sure thing. (18yr old)

  • Anonymous
    August 02, 2006
    A related question: when you fill a balloon with helium, does the mass increase or decrease?

  • Anonymous
    August 02, 2006
    I got it right and I'm 26 years old and went to US schools my whole life.

  • Anonymous
    August 02, 2006
    Well, I don't see my comment from yesterday posted yet, but that means I got it at least partially right.  I learned in 9th grade biology, in 1994, that most of the mass comes from water.  Mr. Anderson was not completely correct it appears.

    Then I thought about it as I typed, and realized that we were burning trees at the camping trip I was on this weekend and producing carbon dioxide.  This means they need a lot of carbon, which is what makes them organic anyway.  That could only come from the air.

    I'd be interested to know what percentage of a tree is water and how much is carbon though.  Here's how much of Soylent Green is water, although it depends on gender and fat/lean tissue ratios:
    http://ga.water.usgs.gov/edu/propertyyou.html
    http://www.funtrivia.com/ask.cfm?action=details&qnid=18791

  • Anonymous
    August 02, 2006
    I was educated in the Cambridge, MA public schools and am under 30 (just barely)

  • Anonymous
    August 02, 2006
    I fit those criteria, even though I didn't respond yesterday.

    I realized this fact after letting plants sit in a bowl of water and watching them grow, and realizing that the added matter must be coming from somewhere other than the soil.  And the fact that earth's life is "carbon-based".

  • Anonymous
    August 02, 2006
    "A related question: when you fill a balloon with helium, does the mass increase or decrease?"

    The mass increases, but the density decreases. Eventually buoyancy causes it to rise.

  • Anonymous
    August 02, 2006
    Here's a fun question:

    You're in a boat in a lake. There's a rock in the boat. You pick up the rock and drop it into the lake, and the rock sinks.

    1) (easy) How does the boat move relative to the water (up or down)?
    2) (moderate) How does the level of the water change relative to the shore (does it go up or down)?
    3) (hard) How does the boat move relative to the shore (does it go up or down)?

    For bonus points, in each case, how big is the change?

  • Anonymous
    August 02, 2006
    I got it right, I'm 26.  Educated in the US.

  • Anonymous
    August 02, 2006
    I'm 10 years beyond 30, but I remember doing an experiment in HS - or maybe it was JH where we proved that mass does not come from water (experiment involved starting with "dried" dirt, weighing a plant, watering the plant with exactly measured amounts of water and caring for the plant for a month or so, then extracting the plant, drying the dirt and weighing the plant again).
    I can't remember if we took evaporation into account, but the overall growth of the plant was less than 1% of the watered amount - ymmv.

  • Anonymous
    August 02, 2006
    I didn't bother replying, but had it right. Something else to think about: with all the talk about hybrid vehicles, ethanol, and other potential sources of fuel these days, i'd expect a more wide-spread understanding of the carbon cycle to emerge...

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    "It turns out that if you ask new Harvard graduates this question, the vast majority of them answer some variant of "It comes from the soil".  When people think of photosynthesis, they don't think about the carbon that's left behind.  They'll usually be puzzled as they answer, because in their hearts, they realize that "the soil" doesn't actually work as an answer, but they can't quite put all the pieces together."

    There's another reason this wouldn't work. Trees don't grow as quickly or really at all during the winter months... why?... no leaves.

  • Anonymous
    August 02, 2006
    Larry, I'm confused!

    The question was: Where did the 999,999 grams of mass come from?

    Your answer: Carbon.

    The mass comes from carbon ? A tree is 100% carbon ?
    I have a question then: What's the role of the roots (obviously besides preventing the tree from falling) ? How do they fit in the picture ?

    I realize I may have thoroughly under-estimated the importance of photo-synthesis but aren't you over-estimating it ?

    In all cases, thanks for raising this interesting topic.

  • Anonymous
    August 02, 2006
    I have Google to thank for my correct answer yesterday rather than a specific educatorial moment.

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    Thanks for all the feedback everyone. Needless to say you are reassuring me about the science education system. So many of the presentations are talking about how little science understanding US citizens have these days. It's always a little hard to figure out just how inflated their numbers are. Yes, I know Larry's blog gets read by really smart people, but this means that some people are getting it :-)

    Valorie

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    I'm 22 and from the state of Canada in the Americas, so I'm an American.

    I asked a plains grain farmer about fertilizer once (trying to get an answer to this for myself), and he said they didn't use any. I assumed crop rotation accounted for some of it, irrigation must bring in some scant dissolved minerals, and that carbon dioxide and water must make up the bulk. Fascinating stuff.

  • Anonymous
    August 02, 2006
    I was about to answer when I realized I turned thirty a month ago :( Nonetheless, I did get the question right and was educated in public schools in Wisconsin and California. Of course, my wife studies carbon uptake and sequestration in photosynthetic marine grasses, so I'd be in trouble if I didn't get it right!

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    Rob Leitman,

    if you fill an empty balloon with helium the mass increases as the balloon has helium added to its inside. Adding the gas increases the mass. However the density of the helium-filled balloon is less than the air surrounding it ending up that it wants to go up into the air.

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    Another interesting thing is that most of the O2 produced by photosynthesis comes from the water, not the fixation of CO2.

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    The comment has been removed

  • Anonymous
    August 02, 2006
    I got it! I got it!
    It comes from dead poets.
    For example, the poet that Knuth quoted:
     "I think that I shall never see
      a poem lovely as a tree."
    Knuth's logs were published on dead trees,
    so it all balances out.

    New question:
    When you delete root from a tree,
    why do you still find bugs in it?

  • Anonymous
    August 02, 2006
    So Photosynthesis is:
       CO2 (from air) + H2O (from ground through roots) --> CH (carbohydrates) + H2O (in the tree) + O2 (to the air)
    Right? If so, then some of the mass comes from the CO2 in the air, and some from the water in the soil.

    BTW, I'm 30 (non-American), and I did not get this. Shame on me.

    As for the validity of the survey, I think it is kind of tilted because it includes:
    1. Only readers of this weblog, which should be more technically-inclined than the general population
    2. Only people who responded. I suppose people who are certain of their answer will have a higher tendency to post.

  • Anonymous
    August 03, 2006
    I'm 28 and I was educated in US schools, I've always been geeked about science though so I read a lot on my own.

  • Anonymous
    August 03, 2006
    I got the correct answer, was educated in American public schools, but I'm over 30. I think I first did photosynthesis in 4th grade, around 1975.

  • Anonymous
    August 03, 2006
    The comment has been removed

  • Anonymous
    August 03, 2006
    so what has more mass 1 cubic foot of dry air or moist air?

    surprising but understandable it is dry air.... molecular weight of an air molecule is more than a water molecule......so as water displaces the air molecules in humid air its mass per unit of volume decreases..


    i realized this after the latest .NET app im writing for a fan manufacturer,

    ;-)

  • Anonymous
    August 03, 2006
    So, larry,

    what conclusion is your wife coming to from the answers above? I am interested to know.

  • Anonymous
    August 03, 2006
    The comment has been removed

  • Anonymous
    August 04, 2006
    > 1) (easy) How does the boat move relative to the water (up or down)?
    > 2) (moderate) How does the level of the water change relative to the shore (does it go up or down)?
    > 3) (hard) How does the boat move relative to the shore (does it go up or down)?

    Well... let's see...

    A floating object displaces its mass; an object submerged in water displaces its volume.

    Therefore the rock displaces more water in the boat than it does in the lake.

    So...

    1) The boat weighs less than the boat+rock.  It floats before and after.  It displaces less water, so it moves UP.

    2) The level of water relative to the shore goes DOWN.

    3) The lake goes down less than the boat goes up, so the boat goes UP relative to the shore.  No proof given until later, in the bonus round.

    > For bonus points, in each case, how big is the change?

    I will take MR to be the mass of the rock; VR to be the volume of the rock; and VWR to be the volume of an amount of water with the same mass as the rock.  VWR > VR.
    I will take MB to be the mass of the boat, not counting the rock; VWB to be the volume of an amount of water with the same mass as the boat.  VB > VWB.

    1) The boat displaces VWR less water, so it will go up by "that much."  The vertical change depends heavily on the shape of the boat.

    2) Before, the boat displaced VWB + VWR.  After, the boat displaced VWB and the rock displaced VR.  The net change is VWR - VR.

    3) The boat goes up VWR relative to the lake; the lake goes down VWR - VR relative to the ground.  Net result for the boat: UP by VWR - (VWR - VR) = VR relative to the ground.

  • Anonymous
    August 05, 2006
    Ah, Larry, a live tree is roughly 50% water by mass, so it seems unlikely to me that most of the mass comes from the air. Except in the sense that rain comes out of the air... more likely the dry weight is mostly carbon, but since you get the dry weight by killing and heating the tree, that's not a good measure.

    If you're searching, the key term is "dry weight", as used by every treekiller from firewood sellers to the US Congress.

  • Anonymous
    August 07, 2006
    I'm under 30, educated in the US and got it wrong. That said, I eschewed biology and chemistry in favor of physics while I was in school.

  • Anonymous
    August 19, 2006
    Larry,
    I related this to my wife who doesn't take anything I say at face value, so I looked into it a bit more and I don't think carbon is the correct answer.  Keep in mind that chemistry wasn't my best subject many years ago and most of the brain cells that understood it have been reallocated.  None the less I think the mass of a tree is mostly Oxygen, not Carbon.

    I think trees are mostly cellulose.  Here's a string of cellulose molecules:
    http://pslc.ws/mactest/cell.htm
    It looks like each cellulose molecule has 5 Oxygen atoms and 1 Carbon atom and some bits of Hydrogen.  Oxygen has a higher atomic weight than carbon so I think the mass of a tree is mostly Oxygen.  
    This makes sense because when I go camping and burn tree parts, what is left in the bottom of the fire pit is the carbon but the majority of the tree is gone.

  • Anonymous
    August 21, 2006
    Missed out on reading the original tree question but, if I could "branch" out a little here, the point about air having mass is fundamental to understanding weather as well. The amount of water air can hold changes at different temperatures.  As the air rises and cools off, it can hold less water. Until finally, it can't hold it anymore and it rains.

  • Anonymous
    October 22, 2006
    HI I am an old (66years) professional Illinois Licensed electrical engineer with a masters degree in nuclear physics. The question is a simple one using simple reasoning and balanced equasions. If we look at trees we see that the surface of the ajacent land is flat (even) so how do you solve this equasion? The only answer is derived by answering a few questions. If the tree mass came from the soil there would be a trough by the trunk of the tree. If the tree mass came from the air (ONLY) there would be a hill by the trunk of the tree (the tree would be on a hill). Therefore the only answer is that the mass of the above ground and below ground comes both form the air and soil and must be pretty close to a balance combination of 50/50 percent (because the adjacent surface is pretty flat). You could say that the top of the tree (above ground) comes from the air and the below ground mass (root system) comes from the soil since supposedly the mass of the root system equals the mass of the above ground system (I have heard). Seriously though the answer 50/50 fits best. Probably most of the carbon comes from air CO2 and the minerals and neutriments come from the soil. Enough said since I am not a biologist. I hope this answers the questions for you young guys. Steve (betstevled1@aol.com)

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