Several scientists considered some different environmental factors and their influence on the growth of certain bacteria. The following experiments used Salmonella bacteria to measure the effect of pH levels, nutrients, and temperature on the number of bacteria produced within a given time period.
1. According to Table 1, what might best contribute to the growth of Salmonella bacteria?
A. A pH level above 9
B. A pH level below 5
C. A pH level near 7
D. A pH level near 5
2. According to the results of the three experiments, which combination of the three factors studied would be expected to produce the highest percent growth?
F. pH level of 5, organic compound in Dish 2, temperature of 40◦C
G. pH level of 7, organic compound in Dish 2, temper- ature of 10◦C
H. pH level of 5, organic compound in Dish 1, temper- ature of 40◦C
J. pH level of 9, organic compound in Dish 1, temper- ature of 90◦C
3. Which of the following conclusions is strengthened by the results of Experiment 1?
A. Salmonella bacteria reproduce most efficiently in an acidic environment.
B. Salmonella bacteria reproduce most efficiently in a neutral environment.
C. Salmonella bacteria cannot reproduce in a basic environment.
D. Salmonella bacteria cannot reproduce in an acidic environment.
4. Bacteria will generally reproduce until all of the nutrients available have been depleted. How could the experiment be altered to maximize the length of time that bacteria will reproduce?
F. Change the observation time from 6 hours to 12 hours.
G. Regularly re-supply each group of bacteria with unlimited nutrients.
H. Increase the rate of growth by decreasing the pH levels.
J. Do not test the effect of different nutrient combinations on growth.
5. Which of the following was the independent variable in Experiment 3?
A. pH level
C. organic compound
D. percent growth
6. The experiments recorded the percent growth that occurred over a 6-hour period. Bacteria often repro- duce at a rate that drastically varies from one stage to the next. The best way to study the different stages of growth would be to record the percent growth:
F. after 2 hours only.
G. after 4 hours, then again after 6 hours.
H. after 8 hours only.
J. every 15 minutes for 3 hours.
1. The best answer is D. According to Table 1, Salmonella bacteria had the greatest percent growth at a pH level of 5. This information suggests a pH level near 5 might best contribute to the growth of the bacteria.
2. The best answer is H. To answer this question you must look at the results shown in all of the tables and choose the conditions that create the greatest percent growth of bacteria. In Table 1, a pH level of 5 created the highest percent growth (81%) of the bacteria. In Table 2, the organic compound in Dish 1 created the greatest percent growth (37%) of the bacteria. In Table 3, a temperature of 40◦C created the greatest percent growth (83%) bacteria. By combining these three conditions you would expect to get the greatest percent growth.
3. The best answer is A. Table 1 shows that the Salmonella bacteria reproduced most efficiently in a pH of 5 (acidic). A conclusion stating that Salmonella bacteria reproduce most efficiently in an acidic environment would reaffirm the results of Experiment 1.
4. The best answer is G. To maximize the length of time that bacteria will reproduce, the bacteria should be provided an unlimited supply of nutrients. In order to make sure the nutrients are not depleted, the bacteria groups must be regularly re- supplied with unlimited nutrients, answer choice G.
5. The best answer is B. In a scientific experiment, the independent variable is the factor that is changed by the experimenter during the experiment. In Experiment 3, the temperature was changed; each dish had a different temperature. The factor being studied or observed, in this case the percent of the growth, is the dependent variable. Whether it changes depends on the manipulation of one or more independent variables.
6. The best answer is J. Answer choice J is the best answer because it is the only answer choice that tests the bacteria at intervals. Because bacteria reproduce at a rate that varies from one stage to the next, by testing at 15-min intervals you could study the different stages of growth most effectively.
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