ACT Science: Research Summaries (Drill 1)

Question 1. According to Table 1, what was the photosynthesis rate when light intensity was 50%?

• A) 6 bubbles/min
• B) 14 bubbles/min
• C) 20 bubbles/min ✓
• D) 24 bubbles/min

Explanation: Table 1 directly shows that at 50% light intensity, the photosynthesis rate was 20 bubbles/min. This is a straightforward table reading, locate the row for 50% light and read across to the bubbles/min column. The other values correspond to different light intensities: 6 bubbles/min at 10%, 14 at 25%, and 24 at 75%.

Question 2. Which of the following variables was held constant in both Experiment 2 and Experiment 3?

• A) CO₂ concentration
• B) Water temperature
• C) Light intensity ✓
• D) Bubble count

Explanation: The passage states that in Experiment 2, "light intensity was held at 75%," and in Experiment 3, "light intensity was held at 75%." Light intensity was therefore the common controlled variable in both experiments. CO₂ concentration was the independent variable in Experiment 2 (varied, not held constant). Temperature was the independent variable in Experiment 3 (varied, not held constant). Bubble count is the dependent variable, what was measured, not something that was controlled.

Question 3. Which of the following best describes the difference between the result patterns in Experiment 2 and Experiment 3?

• A) In Experiment 2, rate increased then leveled off; in Experiment 3, rate increased then decreased ✓
• B) In Experiment 2, rate decreased as the variable increased; in Experiment 3, rate increased steadily
• C) Both experiments showed rate increasing steadily with no plateau or decline
• D) Both experiments showed rate peaking then falling to zero

Explanation: Table 2 shows rate rising from 0 to 32 bubbles/min as CO₂ increases, then holding steady at 32 between 1.00% and 1.50%, a plateau indicating that light has now become the limiting factor. Table 3 shows rate rising from 4 bubbles/min at 5°C to a peak of 26 at 35°C, then dropping sharply to 8 at 45°C and 0 at 55°C, a bell curve with a clear decline caused by enzyme denaturation at high temperatures. These two patterns are different: CO₂ produces a rising plateau while temperature produces a rise-then-fall.

Question 4. Suppose Experiment 1 were repeated with CO₂ concentration increased from 0.10% to 0.50%, while all other conditions remained the same. Compared to the original results in Table 1, the photosynthesis rate at 100% light intensity would most likely be:

• A) lower, because higher CO₂ inhibits photosynthesis at high light levels
• B) the same, because light intensity is the only factor that affects photosynthesis rate
• C) higher, because more CO₂ available allows the plant to sustain a greater rate ✓
• D) higher, because CO₂ directly increases the amount of light the plant absorbs

Explanation: Table 2 shows that at constant 75% light, raising CO₂ from 0.10% to 0.50% increases the rate from 24 to 30 bubbles/min; CO₂ is still a limiting factor at 0.10%. In Experiment 1, the rate approaches a plateau near 100% light because CO₂ (held at 0.10%) is limiting how fast the plant can process additional light energy. If CO₂ is raised to 0.50%, that bottleneck is eased and the plant can use the higher light intensity to produce more oxygen. The rate at 100% light would therefore increase above 26 bubbles/min. Option D is incorrect; CO₂ does not affect light absorption by the plant.

Question 5. The students held light intensity and CO₂ concentration constant throughout Experiment 3. What was the most likely reason for this design choice?

• A) To ensure that any change in photosynthesis rate could be attributed specifically to temperature ✓
• B) To prevent the plant from producing oxygen at low temperatures
• C) To make the results of Experiment 3 identical to those of Experiment 1
• D) To maximize the photosynthesis rate at every temperature tested

Explanation: The fundamental principle of controlled experimentation is that only one variable should change at a time. By holding light intensity and CO₂ constant, the students ensured that any observed change in photosynthesis rate was caused by temperature alone, not by unintended shifts in light or CO₂. If multiple factors changed simultaneously, it would be impossible to determine which one caused a given result. This is the definition of a controlled variable: it is held constant so that the independent variable (temperature) can be studied in isolation.