Week 2 – Review Sheet

Exercise 4: Pure bacterial colonies

1. When an agar plate is inoculated, why is the loop sterilized after the initial inoculum is put on?

2. Distinguish between a pure culture and a mixed culture.

3. Define a bacterial colony. List four characteristics by which bacterial colonies may

be distinguished. 4. Why should a Petri dish not be left open for any extended period?

5. Why does the streaking method you used to inoculate your plates result in isolated colonies?

Exercise 5: Pour plate and streaking technique to obtain pure cultures

1. Discuss the relative convenience of pour- and streak-plate techniques in culturing clinical

specimens.

2. How do you decide which colonies should be picked from a plate culture of a mixed flora?

3. Why is it necessary to make pure subcultures of organisms grown from clinical specimens?

4. What kinds of clinical specimens may yield a mixed flora in bacterial cultures?

5. When more than one colony type appears in pure culture, what are the most likely sources of extraneous contamination?

Exercise 3: Primary media for isolation of microorganisms

1. Define a differential medium and discuss its purpose.

2. Define a selective medium and describe its uses.

3. Why is MacConkey agar selective as well as differential?

4. Why is blood agar useful as a primary isolation medium?

5. What is the major difference between Modified Thayer-Martin (MTM) and chocolate agar? When would you use MTM rather than chocolate agar?

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• Read through the introductory materials below.
• Open the Unit 1 Experiment Answer Sheet and complete the following Experiment exercises this unit:
  • Experiment 1 Exercise 1 – The Scientific Method (~30-45 min)
  • Experiment 1 Exercise 2A – pH of Common Materials (~30-45 min)
  • Experiment 1 Exercise 2B – pH and Buffers (~45-60 min)
• Save your completed Unit 1 Experiment Answer Sheet and submit it no later than Sunday midnight CT.

The Scientific Method – Introduction

The Scientific Method is the basis for almost all scientific research. If you click on the Unit 1 Overview page, you can read about how the Scientific Method is conducted. You can also read about the process in your book on pp 14-17. One area of confusion often involves the difference between a hypothesis and a prediction. This is because many people use these terms interchangeably, but in fact, they are different. Here is how your book discerns the two:

Hypothesis – an answer to a question or explanation of an observation (p 14).

Prediction – an expected outcome if our hypothesis is correct; often worded as “if…then” (p15).

The purpose of this first exercise is to have you use the Scientific Method yourself. We will use the following web site. Be sure that you can access it and use it:

Bowers, N. 2013. Scientific Method Exercise (Links to an external site.)
When you are ready to begin, use the instructions in the Unit 1 Experiment Answer Sheet and work through the exercise.

pH of Common Materials – Introduction

This unit we are also learning about some of the chemistry that is important in biological systems, such as pH. Be sure you have read pp 32-33 in your book and our online lecture this unit before beginning this exercise. The pH scale ranges from 0 to 14; a pH less than 7 is considered acidic and a pH greater than 7 is basic. The pH scale is logarithmic, which means that a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4 and a hundred times more acidic than a solution with a pH of 5.

Acids and bases are not necessarily a bad thing. Many of the materials that we handle and eat and drink everyday vary in pH. Some of these materials are safe to handle, such as “weak” acids (e.g., soda, coffee). Stronger acids (e.g., battery acid) and bases (e.g., ammonia) can be quite caustic and damaging. One way to measure the pH of liquids is to use pH indicator paper; paper that turns a particular color depending on the pH of the solution. Anyone with a swimming pool or hot tub is probably familiar with such paper.

When you are ready to begin, open the Unit 1 Experiment Answer Sheet and follow the instructions to complete this exercise.

Buffers – Introduction

As you saw in the previous exercise, the pHs of common solutions vary across the pH scale! Yet our body is constrained to work within a very narrow pH range. Small changes in pH can alter the function of biologically important molecules such as enzymes, by breaking hydrogen bonds and denaturing these proteins. For this reason, in most organisms (such as ourselves), pH is very closely regulated. pH can be kept relatively constant by the use of buffers, chemicals which can absorb or release hydrogen ions to maintain a relatively steady pH.

In most vertebrate animals, blood pH must be maintained between 7.35 and 7.45. There are several biological buffers that work to maintain this pH; one of the more important being the carbonic acid – bicarbonate system:

H2O + CO2 <–> H2CO3 <–> H+ + HCO3-

In the reactions above, the double headed arrows indicate that each step is reversible. If carbon dioxide (CO2) levels increase in our blood, it can combine with water to form carbonic acid (H2CO3), which can break down to form bicarbonate (HCO3-) and hydrogen ions. This would shift the pH towards the acidic end. If the acidity levels become too high, the whole process will reverse, such that hydrogen ions are removed and carbon dioxide is produced; thereby shifting the pH towards the alkaline end. This is only one example of a biological buffer; there are several other systems involved, but they all operate in a similar manner.

The purpose of this exercise is to help you understand the chemistry of buffers. Be sure that you have read through the material on pp 32-33 in your book and this unit’s online lecture on The Chemistry of Life. For this exercise, you will use the following website (be sure your speakers are on):

McGraw-Hill Education. No date. Buffers Animation. (Links to an external site.)

You may need to download and install a plugin to use this simulation, so test this simulation early in the unit in case you run into problems. When you are ready, open the Unit 1 Experiment Answer Sheet and follow the instructions there to complete this exercise.

WEEK 1 EXPERIMENT ANSWER SHEET Please submit to the Week 1 Experiment dropbox no later than Sunday midnight.

SUMMARY OF ACTIVITIES FOR WEEK 1 EXPERIMENT ASSIGNMENT

· Experiment 1 Exercise 1 – The Scientific Method

· Experiment 1 Exercise 2A – pH of Common Solutions

· Experiment 1 Exercise 2B — pH and Buffers

Experiment 1 Exercise 1: The Scientific Method

After viewing following video, answer the questions that follow:

Bowers, N. 2013. Scientific Method Exercise http://www.youtube.com/watch?v=8S7N_1xnYEE&feature=youtu.be

1. Write out four biological observations you made as you watched the video. Be specific about what you saw. These observations might deal with the habitat, season, flora, fauna, behaviors and interactions (2 pts).

2. Choose one or two of the observations above and write two questions you have regarding what you observed (2 pts).

3. Based on your observations or questions write two possible hypotheses that might explain your observation and/or answer your question (2 pts).

4. Write one prediction based on each hypothesis listed in Q#3 (2 pts).

5. What additional information might you need in order to design and conduct an experiment to test that one of your hypotheses (2 pts)?

Experiment 1 Exercise 2A: pH of Common Solutions

Be sure that you have completed your text book readings, have read through the online lecture and have read the introductory material for the Week 1 Experiment before starting. First, answer the following questions:

Questions

1. What is the definition of an acid? Your definition should include more than just a pH range. Provide one example of an acid. Cite your sources. (2 pts).

2. What is the definition of a base? Your definition should include more than just a pH range. Provide one example of a base. Cite your sources. (2 pts).

Procedure

A. Using your textbook, online lecture or other source, fill in Table 1 below. Be sure to complete your predictions BEFORE you look up the actual pH values.

B. Be sure to provide complete citations for the sources used to determine the actual pH values.

Table 1. Predicted and actual pH values and your explanations. You are only required to complete the first six; the others are optional. Use your textbook, online lecture or other source to determine the actual pH values (6 pts).

Questions

1. Which of your substances tested are considered an acid (1 pts)?

2. Which of your substances tested are considered a base (1 pts)?

3. What surprised you most about your results in this activity (1 pts)?

Experiment 1 Exercise 2B: Buffers

Before beginning, answer the following question:

Question

1. What is a buffer and briefly, how do they work? Cite your source (2 pts)?

Procedure

Watch the following simulation and answer the questions after watching.

https://www.youtube.com/watch?v=ZLKEjXbCU30

Questions

2. Why does the green bar in the graph drop? Why does the purple bar in the graph rise? Explain what is occurring chemically (4 pts).

3. In the simulation shown, what happens to the pH in the beaker when HCl is added? How do you know this based on what you see in the graph (2 pts)?

4. What will happen to the pH if HCl is added after all of the acetate is used up? (1 pts)?

5. What is formed when sodium hydroxide is added and how does this affect the pH (4 pts)?

Week 1 Experiment Grading Rubric

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