ACT Science Practice Test 88

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.


Simple diffusion (SD) is the process by which an uncharged solute in water migrates directly across an uncharged membrane, while facilitated diffusion (FD) is the process by which a charged or polar solute travels through a channel or transporter that crosses the membrane. Figure 1 illustrates how two solutes can diffuse, one by SD and one by FD.


Figure 1

Solutes that cross a membrane by SD or by FD show different rates of flow across a membrane, also known as flux. As a solute crosses a membrane by SD, the flux follows a linear pattern over time, with smaller solutes having the greatest increase in flux over time. As a solute crosses a membrane by FD, the flux follows a logarithmic pattern, leveling off at a maximum flux since there are only a limited number of channels or transporters through which the solute can travel.

Experiment 1

One scientist introduced five different solutes of the same concentration to similar membranes at a constant temperature. The molecular masses of these solutes are shown in Table 1.

Table 1
SoluteMolecular mass (amu)
#1160
#2800
#32,000
#410,000
#540,000

This scientist then measured the time it took for the solute to reach equilibrium, which is a state of equal concentration of the solute on both sides of the membrane. The results are shown in Figure 2.


Figure 2

Experiment 2

Mixtures of solutes are subsequently introduced near three different membranes with different properties. The results of these three trials are presented in Figure 3.


Figure 3

1. Based on the results of Experiments 1 and 2, Mixture #3 is likely to consist of which solutes from Experiment 1 ?

A. Solute #1 only
B. Solutes #1 and #3 only
C. Solutes #3 and #5 only
D. Solutes #2, #4, and #5 only

2. In Experiment 1, which solute spends the least amount of time flowing across the membrane before reaching equilibrium?

F. Solute #1
G. Solute #2
H. Solute #3
J. Solute #4

3. Based on the results of Experiments 1 and 2, which of the following ranks Solute #3, Solute #4, and Mixture #2 in order of smallest to largest average molecular mass?

A. Solute #3, Solute #4, Mixture #2
B. Solute #4, Mixture #2, Solute #3
C. Mixture #2, Solute #3, Solute #4
D. Mixture #2, Solute #4, Solute #3

4. In Experiment 1, on average, did molecules of Solute #3 or molecules of Solute #4 more easily diffuse across the membrane?

F. Solute #3, because it has a larger molecular mass.
G. Solute #3, because it has a smaller molecular mass
H. Solute #4, because it has a larger molecular mass.
J. Solute #4, because it has a smaller molecular mass.

5. In which mixture is the molecular mass most likely less than 160 amu ?

A. Mixture 1
B. Mixture 2
C. Mixture 3
D. Neither Mixture 1, 2, or 3

6. How does the number of molecules in 1 gram of Solute #1 compare with the number of molecules in 1 gram of Solute #5 ? The number of molecules in 1 gram of Solute #1 is:

F. less, because Solute #1 has a larger molecular mass than Solute #5.
G. less, because Solute #1 has a smaller molecular mass than Solute #5.
H. more, because Solute #1 has a larger molecular mass than Solute #5.
J. more, because Solute #1 has a smaller molecular mass than Solute #5.

The term "evolution" is often used in the context of biological changes in organism populations over time, but it can also be applied to the change in the chemical composition of the Earth's atmosphere. The hypotheses of two studies claim that this chemical evolution has altered the types of chemicals found in the atmosphere between the early stages of Earth's existence and the present day.

Study 1

Based on the hypothesis that volcanic eruptions were the source of gases in the early Earth's atmosphere, scientists recreated four model volcanic eruptions in closed chambers, each containing different percentages of the same volcanic particulate matter. They then observed the gases in the air above this model over time. The percent composition of this air after 1 day, when the air achieved a steady state of constant gas concentrations, is represented in Table 1.

Since the experiment provided only a suggestion of the gas levels in the early Earth's atmosphere, the scientists then analyzed the amount of trapped gases in sediment layers, which indicate the changing atmospheric levels of gases over billions of years. The data collected on O2 and H2O vapor are presented in Figure 1.

Study 2

A separate study used the same volcanic models as in Study 1, but it hypothesized that the scientists in Study 1 underestimated the amount of H2 in the early Earth atmosphere. They proposed a different composition of gases, highlighting an increased H2 level in the atmosphere, also represented in Table 1. Based on these new data, the scientists proposed an alternative graph for the changing atmospheric levels of O2 and H2O vapor, also shown in Figure 1.



Figure 1

7. According to the results of Study 2, between 4 and 3 billion years before the present day, the percent composition of O2 in the atmosphere:

A. increased only.
B. increased, then decreased.
C. decreased only.
D. decreased, then increased.

8. According to the results of Study 1, the percent composition of H2O vapor in the atmosphere decreased most rapidly over what period of time?

F. Between 2.5 and 2 billion years ago
G. Between 2 and 1.5 billion years ago
H. Between 1.5 and 1 billion years ago
J. Between 1 and 0.5 billion years ago

9. Suppose that the actual early Earth atmosphere had a high H2 composition of 42%. Based on Study 2, is it likely that the corresponding H2S and N2 compositions of this atmosphere were each 3%?

3% H2S 3% N2

A. Yes Yes
B. Yes No
C. No Yes
D. No No

10. Suppose that in a new trial in Study 2, the percent composition of H2 in the atmosphere was set at 33%, and the percent composition of N2 was found to be 2%. The percent composition of H2O vapor in this trial would most likely be:

F. greater than 40%.
G. greater than 35% and less than 40%.
H. exactly 35%.
J. greater than 30% and less than 35%.

11. Consider an early Earth environment that featured microorganisms. Based on the results of Study 2, is it more likely that aerobic organisms (those that require O2 to survive) or anaerobic organisms (those that do not require O2 to survive) would have existed on Earth 4 billion years ago?

A. Aerobic organisms, because of the high H2O level 4 billion years ago
B. Aerobic organisms, because of the low O2 level 4 billion years ago
C. Anaerobic organisms, because of the high H2O level 4 billion years ago
D. Anaerobic organisms, because of the low O2 level 4 billion years ago

12. According to Study 2, how long did it take the H2O vapor level to decrease to 75% of its composition 4 billion years before the present day?

F. 500 million years
G. 1 billion years
H. 1.5 billion years
J. 2 billion years