Where does a plant's mass come from?[1]
(from "Hard-to-Teach Biology Concepts" by Susan Koba with Anne Tweed, NSTA Press)
1. Which of the four hypotheses in the cartoon do you agree with?
The following information and questions will help you to evaluate these four hypotheses.
Plants are made up of two types of molecules: water and organic molecules such as cellulose, starch and proteins. The weight of all the organic molecules is called the biomass. Biomass is ~30% of the total mass of a typical plant.2. In the pie chart, label the area that represents the proportion of a plant’s total mass that is organic molecules. Label the other area in the pie chart water.
In plants, photosynthesis produces sugar molecules which are used to make organic molecules.
This equation summarizes how photosynthesis in plants produces the sugar, glucose.
sunlight
6 CO2 + 6 H2O 6 C6H12O6 + O2
3a. Photosynthesis makes sugar molecules using ______from the air and ______from the soil.
3b. Can the energy in sunlight be converted to the C, H or O atoms in glucose? ___ yes ___ no
Some of the sugar molecules produced by photosynthesis are used to make other organic molecules. For example, multiple glucose molecules are joined together to make the polymer, cellulose. Cellulose molecules are the main molecule in plant cell walls.
4a. Circle a glucose monomer in the cellulose molecule.4b. Why do plant cells need to make cellulose?
This figure shows how photosynthesis in chloroplasts inside plant cells produces sugars, and these sugars are transformed into the different types of organic molecules that become part of growing plant cells.
5. Explain how a carbon atom from a CO2 molecule in the air can end up as a carbon atom in a protein in a plant cell.
6. Most of the mass of the sugar molecules produced by photosynthesis comes from CO2. Most of the mass of plant organic molecules comes from these sugar molecules. Therefore, most of the mass of plant organic molecules comes from CO2, which enters the plant through the ______.
(leaves/roots)
7a. In 1642-1647, 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 dried soil in the pot at the beginning and end of his experiment. Complete this table to show the changes in weight for the tree and for 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
7b. Helmont concluded that almost none of the weight of the tree came from the dry soil, so almost all of the weight of plants comes from water. Is his conclusion justified by the findings from his experiment? yes___ no___ Explain why or why not.
7c. If Helmont's conclusion is not justified by the results of his experiment, state a more valid conclusion.
8. Complete the table below to summarize your evaluation of four hypotheses about where a plant’s mass comes from. Use the information already presented and this additional research finding:
Many plants can be grown with their roots in water instead of soil. However, growth and survival are limited unless a small amount of soil or fertilizer is added to the water.
How much of a plant's masscomes from each of the following? / Explain the evidence and reasoning
that supports your conclusion.
The sun's energy
__a substantial amount
__a very small amount
__none
Molecules in the air that come
into the plant’s leaves
__a substantial amount
__a very small amount
__none
Water taken up by the plant’s roots
__a substantial amount
__a very small amount
__none
Nutrients in the soil that are taken up by the plant’s roots
__a substantial amount
__a very small amount
__none
2
[1] By Dr. Ingrid Waldron, Dept Biology, Univ Pennsylvania, © 2017. This Student Handout (can be copied for classroom use) and Teacher Notes (with background information and instructional suggestions) are available at http://serendip.brynmawr.edu/exchange/bioactivities/plantmass.