Where does a plant's mass come from?1

1. For each hypothesis in the cartoon, write a T if you think the hypothesis is true or an F if you think the hypothesis is false.

In this activity, you will learn more about each of these hypotheses.

2. Plants are made up of two types of molecules: water and organic molecules (e.g. sugars, cellulose, starch and proteins). So, our first question is how much of a plant’s mass is water and how much is organic molecules. To find out, you can weigh a plant, then dry the plant in an oven to remove all the water, and then weigh the dried plant. This table shows representative results for a small plant. Complete the second and third columns of the table.

Weight % of plant’s massTotal mass of plant 5.6 grams 100%Mass of dried plant = mass of organic molecules in plant = biomass

1.4 grams 25%__ or 75%__

Mass of water in plant before it was dried 25%__ or 75%__

To understand how plants make their organic molecules, we need to begin with photosynthesis. This chemical equation summarizes how photosynthesis makes the sugar, glucose.

sunlight6 CO2 + 6 H2O C6H12O6 + 6 O2

3a. Can the energy in sunlight be converted to the C, H or O atoms in the glucose molecules that are made by photosynthesis? yes ___ no ___ How do you know?

3b. Where do the C, H and O atoms in the glucose molecules come from?

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.

4. Circle a glucose monomer in the cellulose molecule.

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.

This figure shows that: Plants get CO2 from the air and H2O from the soil. Plants use CO2 and H2O for photosynthesis, which occurs in chloroplasts inside plant cells. Most of the mass of the sugar molecules produced by photosynthesis comes from CO2. The sugar molecules 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.

6a. Helmont carried out an early 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. At the beginning and end of his experiment, he recorded the weight of the tree and the weight of the dried soil in the pot. 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 pounds1647 169 pounds, 3 ounces 199 pounds, 14 ouncesChange in Weight

6b. 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.

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6c. If Helmont's conclusion is not justified by the results of his experiment, state a more valid conclusion.

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Scientists have found that: Proteins contain carbon, oxygen, hydrogen and nitrogen atoms. Many plants can be grown with their roots in water instead of soil. However, plants do

not grow well or survive long unless the water has a small amount of soil or fertilizer that contains nitrogen and other minerals.

7a. Where do most plants get the nitrogen atoms that are contained in their proteins? Explain your reasoning.

7b. Carbon and oxygen atoms are responsible for most of the mass of a protein molecule. Where do these carbon and oxygen atoms come from?

8. The weight of all the organic molecules in a plant is called the biomass. A plant’s mass is the sum of the biomass and the mass of the water contained in the plant. Complete this table to summarize your conclusions about where a plant’s mass and biomass come from.

For each statement, indicate whether it is true or false.

Explain the evidence or scientific principlethat supports your conclusion.

A large part of a plant’s mass comes from water taken up by the plant’s roots.

___ true___ false

A plant’s biomass comes mainly from molecules in the air that enter the leaves.

___ true___ false

A plant’s biomass comes mainly from nutrients in the soil that are taken up by the plant’s roots.

___ true___ false

A plant’s biomass comes mainly from the sun’s energy.

___ true___ false

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