Part 1: Introduction

The seed of a Douglas fir tree is only a few millimeters (mm) long and about 1 mm in diameter. This tiny seed can develop into a tree 100 meters tall weighing more than 1000 tons (more than half a million kilograms).

1. Where do you think that all this mass came from?

2. How could a scientist investigate where the mass of a Douglas fir tree comes from?

This figure shows the processes that plant cells use to provide the energy needed for many of the activities of life.

First, photosynthesis converts the energy in sunlight to chemical energy (stored in glucose molecules) using the reactants carbon dioxide and water.

Then, cellular respiration uses glucose and oxygen as inputs for glycolysis and the Krebs cycle, a series of reactions that release energy, which cellular respiration uses to make ATP from ADP and P.

Finally, hydrolysis of ATP provides an energy currency that can be utilized in numerous biological processes throughout the cell.

3. Photosynthesis produces glucose and oxygen. These molecules are inputs for:

4. Cellular respiration produces carbon dioxide and water. These molecules are inputs for:

5. Notice that photosynthesis and cellular respiration make a cycle where the products of each process are inputs for the other process. Draw an oval around the part of the figure that shows this cycle.

6. Cellular respiration produces ATP and H₂O. These molecules are the inputs for:

7. The hydrolysis of ATP produces ADP and P. ADP and P are inputs for:

8. Cellular respiration and hydrolysis of ATP make a cycle where the products of each

9. Using the inputs and outputs shown in the diagram above, write one equation representing cellular respiration and one equation representing photosynthesis.

10. In your own words, explain why plants need to carry out both photosynthesis and cellular respiration.

11. In your own words, explain why animals need to carry out cellular respiration, but not photosynthesis.

Predicting and Measuring Changes in Biomass

To understand where plants’ mass comes from, it is important to know what types of molecules are in plants. The main types of molecules in plants are water and organic molecules. To grow and gain mass, plants take up water and inorganic molecules from the soil and plants make organic molecules.

12. What are the four types of organic molecules?

To grow, plants add more organic molecules and water. In the introduction, you created a diagram to show:

• the process plants use to make the organic molecules needed to grow and
• the sources of the carbon dioxide and water needed for photosynthesis and growth.

Biomass is the weight of the organic molecules in an organism, after the water has been removed.

biomass = an organism’s weight  ̶  the weight of the water in the organism

13. This table shows data for a typical small plant. What is the biomass of this plant?

	Mass
Water	4.2 g
Organic molecules	1.4 g
Total	g

14. A plant is made up primarily of:

• _____________  like cellulose and proteins
• _____________  molecules.

15. Which process can contribute to increased biomass for a plant? cellular respiration / photosynthesis

16. Where does the increased biomass come from? 

17. Which process can result in decreased biomass for a plant? cellular respiration / photosynthesis

18. How does this process result in decreased biomass? Where does the mass go? 

19. A typical seed contains many starch and/or fat molecules. When a seed germinates and a plant first begins to grow, these starch and fat molecules are broken down to provide glucose and fatty acids, which can be used as inputs for cellular respiration to produce ATP.  When a seed begins growing

underground in the dark, the plant cannot carry out _________________________________ because

 (cellular respiration/photosynthesis)

there is no light. A plant growing in the dark will only carry out _______________________________

(cellular respiration/photosynthesis)

and the plant will _________ biomass.

                              (gain/lose)

20. In this figure, add arrows and labels showing the inputs needed for a Douglas fir tree to gain biomass.

21. Explain what process(es) the Douglas fir tree used to create biomass.

22. Which molecule was the source of most of the biomass of the Douglas fir tree?

23. In the same figure, add arrows and labels showing the outputs when a Douglas fir tree loses biomass.

24. Explain what process(es) in the Douglas fir tree could cause it to lose biomass.

25. Which molecule is lost by the Douglas fir tree that causes decreases in biomass?