This is What Happens to Your Dopamine Level on the ACT

---

Dopamines serve as enhancers or catalysts (a substance that initiates or increases the rate of impulses during a chemical reaction, but is not depleted during the process) to certain reactions involved in the activity of human thought. The dopamine intropin is involved in the stimulation of the neurotransmitters in the brain when thought is initiated. A student investigated the effects of dopamine activity on a specific neurotransmitter. 

Experiment 1

To each of 10 test tubes, 7 milliliters (mL) of a peptide (a neurotransmitter) solution was added. Two mL of an intropin solution was added to each of Tubes 1–9. Tube 10 received 2 mL of water without intropin. The tubes were then stirred at a constant rate in water baths at various temperatures and incubated (heated) from 0 to 15 minutes (min). At the end of the incubation period, 0.3 mL of NaCl solution was added to each tube. The NaCl stopped the reaction between the intropin and the peptide. The precipitates, solids formed in a solution during a chemical reaction, which in this case were caused by the reaction of NaCl and the pep- tide, were removed from the tubes and dried. The masses of the precipitates, in milligrams (mg), were measured to determine the relative amount of enhancer that remained in the tube. The results are shown in Table 1. 

Experiment 2

Peptide solution (8 mL) was added to an additional 8 test tubes to which 2 mL of intropin solution was then added. The tubes were incubated at 10 degrees Celsius and stirred at a constant rate for 15 min. The effect of acidity on the neurotransmitter was observed by varying the acidity levels (using the pH scale). The relative amount of neurotransmitter present in each tube was determined in the same manner as Experiment 1, by adding NaCl solution to each test tube. The results are in shown in Table 2. 

Questions

1. In Experiment 1, which of the following conditions allowed for the large amount of precipitate in Tube 1?
​
A. Lack of intropin.
B. Higher temperature.
C. Lack of water.
D. Shorter incubation period.​

2. In which of the following ways did the designs of Experiments 1 and 2 differ?

F. A larger volume of the peptide solution was used in Experiment 2 than in Experiment 1.
G. The temperature was held constant in Experiment 1 and varied in Experiment 2.
H. No NaCl was added after incubation in Experiment2, but it was in Experiment 1.
​J. The remaining fluid level was measured in Experiment 1 but not in Experiment 2. 

3. Which of the following hypotheses about the effects of pH on intropin activity is best supported by the results of Experiment 2? As the pH of the solutions increases from 2 to 13, the effectiveness of intropin:

A. increases only.
B. decreases only.
C. increases, then decreases.
D. remains the same. 

4. Suppose that NaCl had been added immediately to Tube 5 with no incubation period. Based on the results from Experiment 1, the best prediction about the amount of precipitate (in mg) formed would be:

F. 4.1
G. 3.5
H. 2.1
​J. 1.4 

5. According to Table 1, which of the following combinations of water bath temperature and incubation time yielded the greatest amount of precipitate?
​
A. 25◦C, 5 min 
B. 25◦C, 10 min
C. 35◦C, 5 min
​D. 35◦C, 10 min 

6. According to the results of both experiments, one can predict that the LEAST amount of precipitate would be formed if tubes were incubated for 15 min under which of the following conditions?

F. 20◦C at pH of 2.0
G. 20◦C at pH of 6.0
H. 30◦C at pH of 2.0
​J. 30◦C at pH of 6.0 

Answers:

1. The best answer is D.

According to Table 1, the mass of the precipitate in Test Tube 1 was 4.3 mg. The difference between this test tube and the other test tubes is that it had a shorter incuba- tion time (0 min) than other test tubes at the same temperature water bath. 

2. The best answer is F.

​In Experiment 1, “7 milliliters of a peptide (a neurotransmitter) solution was added.” In Experiment 2 “8 mL of peptide solution was added” to the test tubes. One difference between the two experiments is the amount of peptide solution used. A larger volume of the peptide solution was used in Experiment 2, answer choice F. 

3. The best answer is C.

​According to Table 2, as the pH level increases (going down the table), the mass of the precipitate first increases from 2.5 mg to 6.2 mg and then starts to decrease. This information best supports answer choice C. Answer choice D can be eliminated because the masses of the precipitates in Table 2 do not remain constant. 

4. The best answer is F.

According to Table 1, the masses of the precipitate for Test Tube 1 (incubation period 0 min) and Test Tube 2 (incubation period 5 min) were 4.3 mg and 3.9 mg, respectively. If Tube 5 is incubated for 5 min and has a mass precipitate of 3.6 mg, the mass is just below that of Tube 2 for the same incubation period. It could be predicted that if Tube 5 were not incubated, the mass of its precipitate would equal 4.1 mg, just below that of Tube 1’s mass. 

5. The best answer is A.
​According to Table 1, 3.9 mg of precipitate was collected from Test tube 2 at a temperature of 25◦C after a 5-minute incubation period. This is the greatest amount of precipitate yielded among the combinations shown in the answer choices. 

6. The best answer is H.

​To answer this question, you must look at the results in Tables 1 and 2. Accord- ing to Table 1, the mass of the precipitate is the least for test tubes with an incubation period of 15 min for 30◦C (1.4 mg). According to Table 2, the mass of the precipitate is least for the test tube with a pH of 2.0. Therefore, if combined, these two conditions would likely produce the lease amount of precipitate.