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Confessions of an Old Fogey
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So where DOES the mass of a tree come from?

So where DOES the mass of a tree come from?

  • Comments 51

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?

 

  • "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)
  • A related question: when you fill a balloon with helium, does the mass increase or decrease?
  • I got it right and I'm 26 years old and went to US schools my whole life.
  • 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
  • I was educated in the Cambridge, MA public schools and am under 30 (just barely)
  • 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".
  • "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.
  • 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?
  • I got it right, I'm 26.  Educated in the US.
  • 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.
  • 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...
  • I was educated in the US and under 30, though I spent my high school years in a magnet school.

    "when you fill a balloon with helium, does the mass increase or decrease?"

    Its mass increases, but the balloon + helium becomes less dense as the helium is less dense than the original ballon. Once this density drops below the density of the surrounding air it lifts off. This is also why a deflating helium balloon drops back to earth -- it's becoming more dense as it looses the less dense helium.
  • "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.
  • 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.
  • I have Google to thank for my correct answer yesterday rather than a specific educatorial moment.
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