ACT science practice test 14

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 pH at which a protein is uncharged is called its isoelectric point (pI). As the surrounding pH decreases, proteins gain an increasing positive charge. As the surrounding pH increases, proteins gain an increasingly negative charge. In gel electrophoresis, a mixture of proteins can be separated based on their relative charge. The proteins are first dissolved in a solvent and then placed at the starting point of an agarose gel. A current is applied to the gel and the proteins migrate different distances according to their charge (see Figure 1).

Figure 1

The following experiments were done to determine how varying the pH of a solvent affects the separation of proteins with gel electrophoresis. Table 1 shows the isoelectric points of the proteins and the pH values of the solvents used. The pH scale is logarithmic. Solutions with a pH less than 7.0 are acidic, while those with a pH more than 7.0 are basic.

Table 1
ProteinpI
A
B
C
D
8.2
7.4
6.8
5.9
SolventpH
1
2
3
8.9
9.6
10.2

Experiment 1

A special paper 150 mm long is treated with an agarose gel. Electrodes were attached on each end and wired to a 100-volt source. A 150 μg mixture of proteins A-D was added to Solvent 1 to make a 200 μL solution. The solution was placed at the starting point of the gel and allowed to separate for 60 minutes. The density of the separated proteins was plotted as a percentage over their distance traveled. The procedure was repeated for Solvents 2 and 3 and the results presented in Figure 2.

Figure 2

Experiment 2

The procedures of Experiment 1 were repeated after reversing the electrode attachments on the voltage source. Results are shown in Figure 3.

Figure 3

1. In Experiment 2, when Solvent 2 was used, the majority of Protein D migrated a distance from the starting point closest to:

F. 15 mm.
G. 35 mm.
H. 50 mm.
J. 65 mm.

2. Suppose that Experiment 1 were repeated using a solvent with a pH of 8.4. The migration distance of Protein A would most likely peak at:

A. less than 10 mm.
B. between 10 mm and 20 mm.
C. between 20 mm and 30 mm.
D. greater than 30 mm.

3. Protein L has an isoelectric point (pI) of 6.6. The results of Experiments 1 and 2 would be most similar to the plots shown in Figures 1 and 2 if, in each trial, Protein L were added to the protein mixture after removing:

F. Protein A.
G. Protein B.
H. Protein C.
J. Protein D.

4. The resolution of gel electrophoresis decreases as the overall distance between the peaks on the density plot decreases. Based on the results of Experiments 1 and 2, which of the following sets of conditions had the lowest resolution for the separation?

Experiment 1

A. Solvent 1
B. Solvent 3
C. Solvent 2
D. Solvent 3

5. Suppose that Experiment 1 will be repeated using Solvent 2, but Protein Y (pI = 7.1) is added to the overall mixture. Which of the following best predicts the order of migration distances of the 5 proteins, from shortest to longest?

F. D, C, Y, B, A
G. D, Y, C, B, A
H. A, B, Y, C, D
J. A, Y, B, C, D

6. In Experiment 2, for Solvent 2, at the migration distance where Protein B returned to its 0% migration detection, the percent of Protein A that migrated using Solvent 3 was closest to:

A. 0%.
B. 25%.
C. 50%.
D. 75%.

Students studying gravity and motion were given the following information:

o Gravity is an attractive force between two bodies that is directly related to their mass and indirectly related to the square of the distance between their centers.

o Acceleration due to gravity is the acceleration of an object that results from the force of gravity.

o Weight is the force on an object that results from gravity, and is not the same as mass.

o Drag is a force directly related to the velocity of a moving object and which results from air resistance and acts to slow an object down.

o When the drag on a free falling object is equivalent to the weight of that object, the object maintains a constant velocity called terminal velocity.

The students' teacher then described the following experiment:

The experimenter dropped a ball from a known height and recorded the time it took to hit the ground. In a second location, a second ball was dropped from the same height and the experimenter observed that it took a longer time to fall to the ground.

Providing no additional information, the teacher asked her three students to provide an explanation of the experimental conditions that would account for the different times it took the two balls to fall.

Student 1

Both trials were conducted in air with the same atmospheric properties. The balls had the same mass and weight, but the second ball had a larger radius and surface area. Therefore, the second ball was subjected to more drag and reached a lower terminal velocity than the first. This resulted in an increased fall time.

Student 2

Each ball had identical dimensions, but the first ball was made of a denser material giving it both greater mass and weight. Each ball was dropped through air with the same atmospheric properties. Since the second ball was subjected to less gravitational force and weighed less, it reached a lower terminal velocity compared to the first. Therefore, the second ball took more time to hit the ground.

Student 3

Both balls had the same dimensions and mass. The first ball was dropped above the Earth, while the second ball was dropped above the Moon. The first ball reached terminal velocity in the Earth's atmosphere. The second ball was not subjected to any atmosphere or air resistance. However, there was substantially less gravitational force on the second ball and subsequently it weighed less than the first ball. The overall net result was that the second ball fell more slowly and took longer to hit the ground.

7. Based on Student 1's explanation, the velocity of the first ball as it landed most likely equaled:

F. the product of acceleration of gravity and the time it took to fall.
G. the product of one-half the acceleration of gravity and the time it took to fall squared.
H. the velocity of the ball directly before it landed.
J. zero.

8. The teacher added another question to the students' assignment: Suppose the experimenter repeated the experiment by dropping two balls at the same time from the same height in a single vacuum, where no air resistance was present. The balls have different dimensions but identical weights, and they hit the ground at the same time. This new result is consistent with the explanations of which student(s)?

A. Student 1 only
B. Student 2 only
C. Students 1 and 2 only
D. Students 1, 2, and 3

9. According to Student 1, which of the following graphs demonstrates the velocity of the two balls as time increases?

F.
G.
H.
J.

10. According to Student 1, did the surface area of the second ball have an effect on its terminal velocity?

A. Yes; as the surface area of a ball decreases, its terminal velocity decreases only.
B. Yes; as the surface area of a ball increases, its terminal velocity decreases only.
C. No; as the surface area of a ball increases, its terminal velocity decreases, then increases.
D. No; as the surface area of a ball increases, its terminal velocity is not affected.

11. Assuming that Student 3's explanation is correct, once the second ball starts falling, does it reach terminal velocity?

F. Yes, because the weight of the ball was constant and drag force increased.
G. Yes, because the weight of the ball decreased and no drag force was present.
H. No, because the weight of the ball decreased and drag force was constant.
J. No, because the weight of the ball was constant and no drag force was present.

12. The 3 explanations of the motion of the balls are similar to each other in that all 3 explanations suggest that:

A. differences in the gravitational force are responsible for the change in falling times.
B. increases in velocity result from gravity.
C. drag plays only a small part in determining how long it takes an object to fall.
D. a lead ball would have fallen faster.

13. Based on the explanations of the 3 students, what did all 3 students assume about the first ball?

F. The velocity did not change.
G. The velocity increased only.
H. The velocity decreased only.
J. The velocity increased for a time, and then reached terminal velocity.