Question

1. A group of high school students set up an experiment to determine where plants get the materials they need to make glucose. They planted three identical bean plants in separate pots, each containing 200 grams of dry soil.

Plant A was placed in a sunny window and watered every two days.
Plant B was placed in a dark closet but watered the same way.
Plant C was placed in the sunlight but not watered at all.
After three weeks, the students measured the mass of each plant and the remaining soil.

Plant A had grown taller and heavier, while the soil’s mass decreased by only 0.2 grams.
Plant B had wilted and died.
Plant C showed no growth and had dry, brittle leaves.
The students initially concluded that soil must be the source of most of the plant’s mass, but their teacher asked them to reconsider based on earlier lessons about photosynthesis.


Which statement best explains the main source of the plant’s increased mass?

Responses

A. The mass came from nutrients absorbed from the soil through the roots.
A. The mass came from nutrients absorbed from the soil through the roots.

B. The plant gained mass from the water that was absorbed and stored in its roots.
B. The plant gained mass from the water that was absorbed and stored in its roots.

C. The fertilizer and minerals provided in the soil directly increased the plant’s total mass.
C. The fertilizer and minerals provided in the soil directly increased the plant’s total mass.

D. The increase in mass came from sunlight energy being stored inside the plant’s leaves.
D. The increase in mass came from sunlight energy being stored inside the plant’s leaves.

E. The mass came primarily from carbon dioxide taken in from the air and incorporated into glucose.

Answer 1

E. The mass came primarily from carbon dioxide taken in from the air and incorporated into glucose.

Photosynthesis uses CO2 (carbon) and water to build sugars; most of the added dry mass of the plant is carbon fixed from the air. Sunlight supplies energy for the reaction but not mass, and the soil lost only ~0.2 g so soil nutrients were not the main source of the plant’s mass.