Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Carbon= Mass?

  1. Oct 4, 2007 #1
    I learned in Ap Bio that a tree's mass comes from carbon.It makes sense, but is it the whole tree or just the trunk of the tree? Cuz I took a Practice Ap test and there involved a question with a piece of potatoe in Water. At first I assumed that it was diffusion or osmosis that occured but it said it's mass increased. Wouldn't absorbing water increase just it's weight? How did it's mass increase? Cuz Potatoe is a type of root is it built upon carbon like the trunk of a tree? And since it was in water, not to mention it's a root, how would the potatoe take in the carbon? It's only the beginning of the year so maybe I haven't learned enough yet to comprehend but could someone explain this to me? Thanks in Advance
  2. jcsd
  3. Oct 4, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    What do you mean by this:
    The weight of an object is defined as its mass multiplied by a gravitational field strength constant. This is the same everywhere on earth, and does not change in time. So, a change in an objects weight implies a change in the objects mass.
  4. Oct 4, 2007 #3
    I phrased that wrong. What I meant was for both of them to be separate variables. When I read the problem i assumed they meant the mass of the potatoe alone. If water was absorbed by the slice of potatoe it wouldn't be considered part of the potatoes mass would it?
  5. Oct 5, 2007 #4

    jim mcnamara

    User Avatar

    Staff: Mentor

    You are confusing biomass, concept of mass as cristo describes, and fixed carbon.

    Duirect answer: potato water counts for the mass of the potato. A potato does not have "biomass", just fixed carbon. When you get on the bathroom scale just after drinking two glasses of water, you count the mass of the water, too.

    Biomass is the total mass of living organisms in a biotic system. Includes biological water, minerals, etc. Not just carbon compounds.

    Fixed carbon is carbon derived from CO2 found as simple sugars and their metabolites. The energy to fix carbon dioxide comes from sunlight, or in ocean trenches from oxidizing metals from deep sea vents. Further metabolizing creates other carbon compounds like protein or fat or .....

    The mass of fixed carbon is the dry weight of an organism minus any mineralization. In plants this could be lithocysts, like silicates in horsetails. Because of minerals what early researchers did was:
    1. get a oven dry weight of a piece of tissue, or the whole plant
    2. oxidize it to remove all carbon
    3. weigh the ash.

    Then total fixed carbon = dry weight - ash

    In a potato, there is no biomass in the strict sense of the word, just fixed carbon.
    Trees contain huge amounts of water, as much as 70% of a live tree's mass is water, depending on species.
    Last edited: Oct 5, 2007
  6. Oct 9, 2007 #5
    Oh Ok Thanks Cristo and Mcnamara ^_^"
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook