ACT Science Practice Test 4

Directions: The science passage below is followed by several questions. After reading the passage, choose the best answer to each of the ACT Science practice questions. You may refer to the passage as often as necessary. Calculators may NOT be used.

Passage IV

A group of scientists were studying the growth of bacteria. It is their hope that they will be able to induce the bacteria to grow and metabolize oil as a food source. They have taken three samples of Escherichia coli and are growing them on nutrient agar plates. The scientists used three conditions to test the E. coli bacteria. The first group was grown at 37°C on plain nutrient agar plates. The second group was grown at 37°C on plain nutrient agar plates with a 5% oil solution. The third group was grown at 37°C on plain nutrient agar plates with a 20% oil solution. The results of the experiment are listed in the graph below.
oil graph

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Question 1
What is the independent variable in the experiment above?

A
The type of bacteria used.
B
The nutrient agar.
C
The nutrient agar with oil.
D
The number of days the bacteria grew.
Question 1 Explanation: 
The correct answer is (C). The independent variable is percentage of oil solution on the agar plates. The scientists are hoping to induce the metabolism of a bacteria to eat and survive on an oil plate. That is the “new” change that is introduced in this experiment.
Question 2
What is the dependent variable in the experiment above?

A
The number of days the bacteria grew.
B
The nutrient agar.
C
The nutrient agar with the oil.
D
The amount of growth of the bacteria.
Question 2 Explanation: 
The correct answer is (D). The dependent variable is always what occurs in response to the independent variable. In this case, the amount of growth displayed is the response.
Question 3
Which bacterial culture had the greatest rate of growth throughout the 8 day period?

A
Only the culture grown on nutrient agar had significant growth.
B
The nutrient agar plate and the 5% oil plate had growth.
C
The nutrient agar plate and the 20% oil plate had growth.
D
The 5% oil and 20% oil plates had growth.
Question 3 Explanation: 
The correct answer is (A). The nutrient agar plate is the only plate that showed any significant growth. Although there was a small amount of growth on the 5% oil plate it was very insignificant.
Question 4
What is the best explanation for why the E.coli grew on the 5% oil plate but not the 20% oil plate?

A
The bacteria are used to an environment with 5% oil and not 20% oil.
B
The bacteria were able to mutate to tolerate a slightly oily environment of 5% oil, providing it had other nutrients available and the 20% had far too much oil.
C
The 5% plate had conditions that are similar to the natural environment they are accustomed to.
D
The 5% plate allowed more oxygen to interface with the bacteria because less oil was present.
Question 4 Explanation: 
The correct answer is (B). The bacteria had to mutate in order to survive in a hostile environment. As a result, there must have been some bacteria there were able to mutate and survive on slightly oily conditions like the 5%. However, the 20% may have been far too toxic an environment for the bacteria to survive on.
Question 5
Why did the nutrient agar bacteria plateau and then drop in numbers as they approached Day 8?

A
The bacteria died because they had reached their mature age.
B
The bacteria ran out of nutrients and had reached the carrying capacity of the plate.
C
The bacteria always die after Day 6 in culture.
D
There is no definitive reason why the bacteria died off after Day 6.
Question 5 Explanation: 
The correct answer is (B). In a closed, small environment, all organisms will eventually reach carrying capacity. Carrying capacity is that point at which food resources have been depleted and wastes have accumulated. As a result, there is a massive die off of the organisms as the environment can no longer sustain growth.
Question 6
An excellent way to demonstrate that the scientists have induced E.coli to survive and metabolize oil would be to:

A
Repeat the experiment several times with 5% oil on a nutrient agar plate.
B
Induce the bacteria on the 5% oil plate to grow on nutrient agar.
C
Take the bacteria from the 5% oil plate and test them on a new 20% oil plate.
D
Try new bacteria with the same experiment to see if that is the only bacteria that can accomplish this.
Question 6 Explanation: 
The correct answer is (A). One way to validate the results of any experiment is to show that the results are reproducible. A reproducible result demonstrates that there is proof of the experiment working each time. This means the results were not a one time event and can be considered a norm.
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