ACT science practice test 29

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


The leaves of green plants use the energy in sunlight to convert atmospheric carbon into organic carbon through the reactions of photosynthesis. These reactions can be summarized by the following equation:

Gas exchange between the leaf and the environment is an integral part of the photosynthesis reactions. As carbon dioxide enters the leaf, the oxygen produced as a by-product of photosynthesis is released into the environment in a 1:1 ratio. Enclosing a leaf within a lighted chamber allows for the rate of this exchange, and therefore the rate of photosynthesis, to be measured.

Students in a biology class used lighted chambers to measure the photosynthetic rate of leaves from four common plant species: sunflower, water hyacinth, rhoeo, and pothos. A leaf was placed inside the chamber, and a flow of air was introduced. Sensors within the chamber recorded data on light intensity (LED irradiance), carbon dioxide concentration, air temperature, and relative humidity.

The leaf was initially exposed to a constant light intensity of 300 μE/m2/s to stimulate photosynthesis. After this initial period, students incrementally increased the light intensity to investigate the relationship between light intensity and photosynthetic rate.

Figure 2.5 shows the light intensity (LED irradiance) over time for a chamber containing a water hyacinth.

Figure 2.5

Source: "BISC 111/113: Introductory Organismal Biology," by Jocelyne Dolce, Jeff Hughes, Janet McDonough, Simone Helluy, Andrea Sequeira, and Emily A. Bucholtz. http://openwetware.org/wiki/Lab_5:_Measurement_of_Chlorophyll_Concentrations_and_Rates_of_Photosynthesis_in_Response_to_Increasing_Light_Intensity.

Figure 2.6 shows the change in carbon dioxide concentration over time for a chamber containing a water hyacinth.

Figure 2.6

Source: "BISC 111/113: Introductory Organismal Biology," by Jocelyne Dolce, Jeff Hughes, Janet McDonough, Simone Helluy, Andrea Sequeira, and Emily A. Bucholtz. http://openwetware.org/wiki/Lab_5:_Measurement_of_Chlorophyll_Concentrations_and_Rates_of_Photosynthesis_in_Response_to_Increasing_Light_Intensity.

Figure 2.7 shows the change in air temperature over time for a chamber containing a water hyacinth.

Figure 2.7

Source: "BISC 111/113: Introductory Organismal Biology," by Jocelyne Dolce, Jeff Hughes, Janet McDonough, Simone Helluy, Andrea Sequeira, and Emily A. Bucholtz. http://openwetware.org/wiki/Lab_5:_Measurement_of_Chlorophyll_Concentrations_and_Rates_of_Photosynthesis_in_Response_to_Increasing_Light_Intensity.

Figure 2.8 shows the change in relative humidity (RH) over time for a chamber containing a water hyacinth.

Figure 2.8

Source: "BISC 111/113: Introductory Organismal Biology," by Jocelyne Dolce, Jeff Hughes, Janet McDonough, Simone Helluy, Andrea Sequeira, and Emily A. Bucholtz. http://openwetware.org/wiki/Lab_5:_Measurement_of_Chlorophyll_Concentrations_and_Rates_of_Photosynthesis_in_Response_to_Increasing_Light_Intensity.

Students performed 10 light-chamber trials with leaves from each of the four plant species. The carbon dioxide concentration data was then used to calculate the maximum carbon dioxide exchange rate for each leaf.

Table 2.2 shows the calculated and mean carbon dioxide exchange rates for each of the four plant species.

TABLE 2.2 Carbon Dioxide Exchange Rates

Source: http://openwetware.org/wiki/Lab_5:_Measurement_of_Chlorophyll_Concentrations_and_Rates_of_Photosynthesis_in_Response_to_ Increasing_Light_Intensity.

1. The atmospheric carbon absorbed by green plants is in the form of:

A. carbon monoxide.
B. carbon dioxide.
C. carbohydrates.
D. water.

2. Which molecule is formed as a by-product of the photosynthesis reactions?

A. Carbon dioxide
B. Glucose
C. Water
D. Oxygen

3. According to Figure 2.5, the initial photosynthesis-stimulating period lasted approximately:

A. 5 minutes.
B. 20 minutes.
C. 50 minutes.
D. 80 minutes.

4. The slight increase in air temperature indicated in Figure 2.7 is most likely related to the:

A. increasing light intensity as the study progressed.
B. peak in relative humidity at the 50-minute mark.
C. increase in CO2 concentration at the end of the study.
D. heat generated by the sensors in the light chamber.

5. Sensors within the lighted chamber monitor the presence of which chemical reactant of the photosynthesis reactions?

A. Carbon dioxide
B. Oxygen
C. Sunlight
D. Glucose

6. Which graph represents the independent variable in the students' study?

A. Figure 2.6
B. Figure 2.7
C. Figure 2.5
D. Figure 2.8

7. Based on the data in Figures 2.5 and 2.6, which light intensity causes a water hyacinth leaf to absorb carbon dioxide at the fastest rate?

A. 0 μE/m2/s
B. 300 μE/m2
C. 100 μE/m2
D. 1,000 E/m2/s

8. The data in the table would best support the assertion that sunflower plants:

A. require less intense light than the other three species.
B. release more oxygen than the other three species.
C. are the fastest growing of the four species studied.
D. have the shortest life cycle of the four species studied.

9. According to Table 2.2, which plant showed the least variability across trials?

A. Water hyacinth
B. Pothos
C. Rhoeo
D. Sunflower

10. Based on the information in the passage, if the oxygen concentration within the chamber had been recorded, its graph would most closely resemble which figure?

A. Figure 2.6
B. Figure 2.7
C. Figure 2.5
D. Figure 2.8

11. According to the data in Table 2.2, which plant species perform(s) photosynthesis at a faster rate than pothos?

A. Sunflower only
B. Sunflower and water hyacinth
C. Rhoeo only
D. Rhoeo and water hyacinth

12. Which of the following generalizations is supported by the data in Figures 2.5 through 2.8?

A. Photosynthesis occurs at a faster rate in a highly humid environment.
B. The rate of photosynthesis varies directly with air temperature.
C. The greater the light intensity, the faster the rate of photosynthesis.
D. The rate of photosynthesis depends on the level of carbon dioxide available.

13. Which of the following statements is supported by the data in Table 2.2?

A. The single leaf with the fastest gas exchange rate was from a sunflower plant.
B. The single leaf with the slowest gas exchange rate was from a water hyacinth plant.
C. No two leaves from different species exhibited the same gas exchange rate.
D. No two leaves from the same species exhibited the same gas exchange rate.

14. Based on the data in Table 2.2, which plant could be expected to be most tolerant of a low-light environment?

A. Rhoeo
B. Pothos
C. Sunflower
D. Water hyacinth

15. The passage states that the rates recorded in Table 2.2 represent the maximum carbon dioxide exchange rates observed for each trial. Assuming that light intensity was increased at the same intervals for each trial, at approximately which point during each trial were the exchange rates recorded in the table most likely observed?

A. 30 minutes
B. 70 minutes
C. 10 minutes
D. 50 minutes