Case Study 1: The Discovery of Penicillin
In 1928, Dr. Alexander Fleming was investigating the biology of Staphylococcus aureus, a common bacteria on the surface of human skin that causes several diseases. After a summer break away from his lab he returned, only to discover a few culture plates of S. aureus had been contaminated by a blue-green mold. In plates where this mold, Penicillium, was present there were clear areas that contained no bacteria. In culture plates without mold these clear areas were not present (Figure 2).
Given this observation, Fleming hypothesized that the mold must produce a chemical capable of killing S. aureus. To prepare his experiment he first isolated the mold. Dr. Fleming then exposed the mold isolates to a new culture of S. aureus. After exposure, Fleming observed that all the bacteria in the new culture died. Having identified this as an anti-bacterial chemical, Fleming named the substance penicillin. However, it would take another decade before penicillin could be fully developed into the wonder drug it came to be known as.
1a. What was Dr. Fleming investigating?
1b. What was Dr. Fleming’s hypothesis?
1c. Rewrite Dr. Fleming’s prediction using the if/then format.
1d. What was the independent variable for this experiment? What was the dependent variable?
1e. Do the results indicate that the hypothesis should be rejected or retained? Explain your reasoning.
Case Study 2: Improving Plant Growth
You are interested starting a roadside produce business and wish to improve the quality of your peanut plants. Nitrogen is an essential macronutrient for protein synthesis and photosynthesis in plants. However, it is often considered to be a limiting nutrient – available in short supply in nature. In agriculture, a common way to increase plant yield is to apply fertilizers that contain essential nutrients.
You design an experiment that compares the height (growth) of two groups of plants after exposure to fertilizer. Group A has 50 peanut plants that receive fertilizer. Group B has 50 peanut plants that do not receive fertilizer. After two weeks you measure the changes in height and observe that plants from Group A had an average height of 12.43 cm and plants from Group B had an average height of 12.38 cm.
2a. From this experiment, identify the following:
· the control group?
· experimental group?
· independent variable?
· dependent variable?
2b. Based on these results, what is your conclusion? How does fertilizer affect the growth of the peanut plants?
Peanuts belong to the legume family and are noted for their association with symbiotic nitrogen-fixing bacteria (Rhizobia). Living within the root nodules of the peanut plants, these bacteria can convert (or fix) inorganic forms of nitrogen from the air and soil into a nitrogen source that is useable by the plant. Because peanuts can easily obtain fixed nitrogen, they do not require additional nitrogen sources.
2c. Given this information, explain your experimental results.
2d. How could this experiment be improved considering the information above?
Case Study 3: New Drug Development – Positive & Negative Controls
Some controlled experiments may also incorporate positive and negative controls into the experimental design. A positive control provides a baseline, or sets the standard, for what should happen if the experiment is successful. On the other hand, a negative control is used to indicate what should happen if the outcome of the experiment is unsuccessful (no change due to the tested variable).
Consider our previously introduced drug trial example. In this case, you are involved in developing a new drug for the treatment of chronic headaches. To gain FDA approval for your drug, you designate three trial groups each consisting of 100 individuals. Prior to the drug treatment, each participant was monitored for a month and the number of headaches lasting more than 5 hours were recorded. Each group was subsequently exposed to a month-long trial of one of three treatments:
Group A: Received a dosage of the new drug (Drug X).
Group B: Received a dosage of Naproxen (Aleve). This is a drug that is currently available for the prevention and treatment of headaches.
Group C: Received a dosage of a placebo (sugar pill) that contains no medication.
After the trial period, the number of headaches reported by Group A decreased by an average of 3.5 headaches per month. Group B reported an average decrease of 3.2 headaches per month and Group C reported an average increase of 0.3 headaches per month.
3a. Identify the following:
· independent variable
· Group ____ was the experimental group
· Group ____ was the positive control
· Group ____ was the negative control
3b. What is the importance of positive and negative controls within a scientific experiment?
Activity 2: Performing an Experiment and Collecting Data
Cardiovascular fitness is a measurement of how well your body takes in oxygen and supplies that oxygen throughout the body during sustained physical activity. A simple way to determine cardiovascular fitness is by either measuring the heart rate or respiration rate of an individual after prolonged physical exercise. A person who is more fit (has higher cardiovascular endurance) may have a comparatively slower pulse or a lower respiratory rate after exercise.
The World Health Organization (WHO) suggests adults aged 18-64 years of age participate in at least 150 minutes of intentional exercise per week to avoid the negative health effects associated with a sedentary lifestyle. Additionally, the type of aerobic exercise can have a different impact on cardiovascular function; brisk walking, running, swimming, or even dancing is often associated with more vigorous, heart-pumping results. In this exercise, you will work with your classmates to investigate the impact activity level on cardiovascular fitness, as measured by heart rate.
Below is the in-person protocol for this experiment. Fill out the following information based on your own change in heart rate:
1. Students will be split into two groups and will participate in an activity for 60 seconds:
Group A. This will the low intensity activity group. Students will walk in place at a steady, sustained rate.
Group B. This will be the high intensity activity group. Students will repeat a sit-stand motion. Students sit on a chair with feet shoulder width apart and arms crossed across their chest. From this position, the student will stand up (without pushing off with arms) and sit down repeatedly.
2. Prior to starting the experiment, we need to define the hypothesis:
If _______________________ ( choose: low intensity / high intensity) exercise is associated with greater aerobic activity, then the _____________________ ( choose: low intensity / high intensity) activity group will have a greater change in heart rate after exercise.
3. Your instructor will act as timekeeper and instructor you when to start and stop each exercise. Prior to the exercise, you should first practice taking your heart rate and to find your resting heart rate.
· While sitting quietly use two fingers (pointer and index fingers) to find the pulse in the radial artery (Figure 3).
· On your instructor’s command, count the number of beats per 15 seconds. Multiply this number by 4 to determine beats per minute (bpm).
Resting heart rate = __________________ bpm
4. On your instructor’s command, begin your designated activity. After 60 seconds, stop and immediately take your pulse following the procedure in Step 3.
Trial 1: Post-activity heart rate = __________________ bpm
5. Calculate your change in heart rate: Post-activity Heart Rate – Resting heart rate. Input your data below as part of the class dataset in Table 1.
Trial 1: Change in heart rate = __________________ bpm
6. Wait at least 30 minutes before repeating Steps 3-5. On your instructor’s command, begin the same designated activity as Trial 1. Input your data below and as a part of the class dataset in Table 1.
Trial 2: Post-activity heart rate = __________________ bpm
Trial 2: Change in heart rate = __________________ bpm
7. Calculate the average pulse rate for both low intensity activity and high intensity activity groups. To find the average, take the total sum of all numbers (change in heart rate) for each activity group and divide that sum by the number of data points collected (for example, 12 students = 12 data points). Add your results to Table 1.
8. Use Table 1 to create a bar graph comparing the average change in heart (pulse) rates after low intensity activity and high intensity activity exercises. Answer the questions on the end of lab worksheet.
· In this experiment, what was the independent variable? __________________________________
· What was the dependent variable? ___________________________________________________
· Use the data from Activity 2 (Table 1) to make a bar graph. On the x-axis (horizontal axis) place the independent variable and on the y-axis (horizontal axis) place the dependent variable. Label both axes.
· Based on these results, was your hypothesis rejected or retained? Explain your reasoning.
· Were there any weaknesses in this experimental design?
· What additional variables could be controlled for?
· What is an additional or modified hypothesis that you could test to determine other variables that may impact cardiovascular fitness?
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