Variables & Controls Practice Test

PASSAGE II

BIOLOGY: This passage is adapted from a study on the factors affecting the rate of photosynthesis in aquatic plants.

Introduction

Photosynthesis is the process by which green plants use sunlight to synthesize nutrients from carbon dioxide ($CO_2$) and water ($H_2O$). The process releases oxygen ($O_2$) as a byproduct according to the following chemical equation: $$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$$ Students conducted three studies to investigate how different environmental factors affect the rate of photosynthesis in Elodea, an aquatic plant. The rate was measured by counting the number of oxygen bubbles produced by a cut stem of Elodea submerged in water over a 5-minute period.

Study 1

To test the effect of light intensity, students placed a 10 cm sprig of Elodea into a test tube filled with a 0.5% sodium bicarbonate ($NaHCO_3$) solution (a source of $CO_2$). A light source was placed at various distances from the test tube. The temperature was maintained at 25°C. The number of bubbles produced in 5 minutes was recorded.

Study 2

To test the effect of light color (wavelength), students used the same setup as in Study 1. The light source was kept at a constant distance of 10 cm. Colored filters were placed between the light and the plant to isolate specific wavelengths. Clear cellophane was used as a control.

Study 3

To test the effect of $CO_2$ availability, students prepared five test tubes with different concentrations of sodium bicarbonate ($NaHCO_3$). A 10 cm sprig of Elodea was placed in each. The light source was kept constant at 10 cm (white light).

PASSAGE II

BIOLOGY: This passage is adapted from a study on the factors affecting the rate of photosynthesis in aquatic plants.

Introduction

Photosynthesis is the process by which green plants use sunlight to synthesize nutrients from carbon dioxide ($CO_2$) and water ($H_2O$). The process releases oxygen ($O_2$) as a byproduct according to the following chemical equation: $$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$$ Students conducted three studies to investigate how different environmental factors affect the rate of photosynthesis in Elodea, an aquatic plant. The rate was measured by counting the number of oxygen bubbles produced by a cut stem of Elodea submerged in water over a 5-minute period.

Study 1

To test the effect of light intensity, students placed a 10 cm sprig of Elodea into a test tube filled with a 0.5% sodium bicarbonate ($NaHCO_3$) solution (a source of $CO_2$). A light source was placed at various distances from the test tube. The temperature was maintained at 25°C. The number of bubbles produced in 5 minutes was recorded.

Study 2

To test the effect of light color (wavelength), students used the same setup as in Study 1. The light source was kept at a constant distance of 10 cm. Colored filters were placed between the light and the plant to isolate specific wavelengths. Clear cellophane was used as a control.

Study 3

To test the effect of $CO_2$ availability, students prepared five test tubes with different concentrations of sodium bicarbonate ($NaHCO_3$). A 10 cm sprig of Elodea was placed in each. The light source was kept constant at 10 cm (white light).