ACT science practice test 33

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.


Organic molecules (carbohydrates, lipids, proteins, and nucleic acids) compose and are produced by living organisms. Scientists believe that simple organic molecules originally formed from inorganic molecules on primitive Earth. This step is considered a key precursor to the development of life on our planet. Two leading theories on the origin of the first organic molecules are described here.

Primordial Soup

The theory that organic molecules formed in the atmosphere of primitive Earth using energy from lightning is often called the "primordial soup theory." Evidence for this theory includes the Miller-Urey experiment, in which the conditions believed to exist in the primitive atmosphere were reproduced to create organic molecules.

The major components of the primitive atmosphere were believed to be methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). These gases were put into a closed system and exposed to a continuous electrical charge to simulate lightning storms. After one week, samples taken from the apparatus contained a variety of organic compounds, including some amino acids (components of proteins). Figure 4.1 is a diagram of the apparatus used in the Miller-Urey experiment.

Figure 4.1

Hydrothermal Vents

The theory that organic molecules originally formed in the deep oceans using energy from inside the earth focuses on the existence of hydrothermal vents. Evidence for this theory includes the fact that ecosystems of diverse organisms have been found to exist around hydrothermal vents in the deep ocean. These ecosystems thrive without any energy input from the sun.

Organic molecules are only stable within a very narrow temperature range. Hydrothermal vents release hot (300°C) gases originating from inside the earth into the otherwise cold (4°C) water of the deep ocean. This release of gases causes a temperature gradient to exist around deep-sea vents. Scientists believe that within this temperature gradient exist the optimal conditions to support the formation of stable organic compounds. Figure 4.2 shows a diagram of the gradient produced by deep-sea vents.

Figure 4.2

1. Which of the following is not an organic molecule?

A. Carbohydrates
B. Water
C. Lipids
D. Nucleic acids

2. Both theories on the origin of organic molecules are based on the assumption that those molecules:

A. contain different atoms than inorganic molecules.
B. only exist in the atmosphere and deep ocean.
C. have not yet been produced in the laboratory.
D. can be produced from inorganic molecules.

3. In Figure 4.1, the purpose of the heat source is to:

A. produce water vapor for the simulated atmosphere.
B. simulate hydrothermal vents in the deep ocean.
C. reduce inorganic compounds to organic compounds.
D. generate an electrical charge to stimulate the reaction.

4. Ammonia (NH3) is an:

A. inorganic compound.
B. element.
C. amino acid.
D. organic compound.

5. In Figure 4.1, the reaction that produces organic molecules occurs in which part of the Miller-Urey apparatus?

A. Condenser
B. Large sphere
C. Trap
D. Small sphere

6. Based on the hydrothermal vents theory, which of the following would most likely be the optimal temperature range for organic molecule formation?

A. Between 0°C and 4°C
B. Higher than 300°C
C. Lower than 300°C
D. Between 4°C and 25°C

7. Which of the following statements would scientists supporting either theory most likely agree on?

A. At least some organic compounds on Earth likely originated in meteorites from space.
B. The production of amino acids requires the existence of a temperature gradient.
C. The existence of water on Earth was essential to the original formation of organic compounds.
D. A single method most likely produced the original versions of all organic molecules.

8. The specific source of energy used to form simple organic molecules is:

A. addressed in the primordial soup theory only.
B. not discussed in either of the two theories.
C. a major difference between the two theories.
D. the only similarity between the two theories.

9. According to the primordial soup theory, which of the following gases is not believed to have been a major component of the primitive atmosphere?

A. Methane
B. Hydrogen
C. Water vapor
D. Helium

10. The greatest limitation in the design of the Miller-Urey experiment is the:

A. use of a condenser to cool water vapor.
B. production of a variety of organic compounds.
C. presence of a constant electrical charge.
D. recycling of water throughout the apparatus.

11. The hydrothermal vents theory states that organic molecules originally formed:

A. inside the earth.
B. in the earth's atmosphere.
C. in the deep ocean.
D. within volcanoes.

12. Scientists consider the outcome of the Miller-Urey experiment to be evidence:

A. that refutes the primordial soup theory.
B. in support of the primordial soup theory.
C. that refutes both the primordial soup and the hydrothermal vents theories.
D. in support of the hydrothermal vents theory.

13. According to the passage, temperature gradients exist in the deep ocean due to the:

A. constant release of hot gases into cold water.
B. decreased availability of sunlight at greater depths.
C. existence of ecosystems made up of diverse organisms.
D. reactions that produce organic molecules.

14. Which of the following is a key assumption of the primordial soup theory?

A. Sunlight provided the energy needed to convert inorganic compounds to organic compounds.
B. The composition of the primitive atmosphere was different than that of the current atmosphere.
C. Amino acids can be produced from inorganic compounds in the laboratory.
D. Organic compounds can only be produced by the reaction of other organic compounds.