Name: ______Block: ___
Where does a plant's mass come from?[1]
1. Which of these opinions do you agree with?
2. Almost all of a plant's mass consists of H2O and organic molecules. How does a plant get or make these molecules?
Major Type of Moleculein Plants / How does the plant get or make these molecules?
H2O
Organic molecules
(e.g. cellulose which is
a polymer of the sugar
glucose)
During photosynthesis, light energy is converted to chemical energy stored in the bonds of sugar molecules. The atoms in these sugar molecules come from CO2 and H2O, as shown in the following chemical equation which summarizes the process of photosynthesis.
sunlight
6 CO2 + 6 H2O ----> ----> ----> 6 O2 + C6H12O6
3. Use the information in this chemical equation to complete the following table.
Atom in sugar moleculeproduced by photosynthesis / What molecule or molecules could this atom come from?
C
H
O
4a. In 1642-47, Helmont carried out a classic experiment to evaluate where a plant’s mass came from. He grew a willow tree in a pot and added only water during the five-year experiment. He recorded the weight of the tree and the weight of the soil in the pot at the beginning and end of his experiment. Complete the following table to show the change in weight of the tree and the dried soil.
Weight of Tree / Weight of Dried Soil1642 / 5 pounds / 200 pounds
1647 / 169 pounds, 3 ounces / 199 pounds, 14 ounces
Change in Weight
4b. Helmont concluded from his experiment that, for all plants, almost all of their weight comes from water. Is this conclusion justified by the results of his experiment? Explain why or why not. If Helmont's conclusion is not valid, state a more valid conclusion based on the results of his experiment.
5. Summarize your understanding of the hypotheses in the cartoon by completing this table, using the information already presented plus these additional research findings:
· Scientists have shown that most of the mass of the sugar molecules produced by photosynthesis comes from CO2 and very little from H2O.
· Many plants can be grown in water instead of soil, but growth and survival are limited unless a little bit of soil or fertilizer is added. (This is the basis of hydroponic agriculture.)
How much of a plant's massdo you think comes from
each of the following? / Explain the evidence and reasoning
that supports your conclusion.
Nutrients in the soil that are taken up by the plant’s roots
__a lot __a tiny bit __none
The sun's energy
__a lot __a tiny bit __none
Molecules in the air that come
in through holes in the plant’s leaves
__a lot __a tiny bit __none
Water taken up by the plant’s roots
__a lot __a tiny bit __none
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