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Scientific Investigation
(Lab 1)
Scientists ask questions and test hypotheses in order to gain insight into the biological
world that surrounds us. In order to gain the most knowledge possible, a creative and
curious scientist must participate in the process of science which includes the
development and utilization of critical thinking skills.
(Hypothesis)
(Observation)
The scientific method consists of a universally accepted sequence of steps for
conducting scientific research. Scientists from all disciplines in every country must
follow the scientific method in order for their research to be accepted by the scientific
community. Scientific inquiry begins with an observation of an event. The researcher
then formulates a question and translates the question into a hypothesis. The four
general parts of the scientific method include (1) formulating a hypothesis, (2)
designing an experiment to evaluate the hypothesis using appropriate materials and
methods, (3) evaluating the results of the experiment, and (4) formulating logical
conclusions. Scientific investigations are reported at scientific meetings and in
professional publications where they are closely scrutinized by the scientific community.
Critiques (peer reviews) are essential to maintaining high standards in procedures and
ethics of scientific research. Scientific findings, in turn, steer the direction of future
research efforts, affect public policy, influence funding decisions for environmental
preservation and the implementation of new technologies, etc. Our society and the
global community at large are affected by scientific research in far too many ways to list
here.
Every researcher begins with a careful observation which leads to a question concerning
the area of science observed. The researcher formulates his or her “best guess” as to the
correct answer to that question. This guess, or possible explanation for what has been
observed, is called a hypothesis. All scientific experiments begin with a hypothesis, or a
prediction about how something will turn out (even Calvin’s “experiment” illustrated above).
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In order to answer a scientific question, it must first be converted into a hypothesis.
Write a hypothesis for each of the following observations. Recall that the hypothesis is a
statement that explains the phenomenon you are interested in investigating. Hypotheses
are often formulated as “If… , then…” statements.
1. Individuals who use cell phones appear to have poor hearing.
2. Children whose mothers run each day appear to be smarter than children with
mothers who do not run.
Researchers often propose and reject several hypotheses before they design an
experimental procedure. Discuss the following statements with your class and decide
which of these would be useful as scientific hypotheses and could be investigated using
the scientific method. Explain your reasoning. State whether or not each statement could
possibly be falsified. What factors are measurable and controllable?
1. Toxic fungi growing on grass eaten by mares is causing the death of unborn horses
in Kentucky.
2. During the full moon, crime rates surge.
3. People who think positively live a longer life than most.
4. Contact with small amounts of pesticides enhances the possibility of acquiring
Parkinson’s disease.
Next, the experimental procedure is designed to test the hypothesis to see if it falsifiable.
A hypothesis may be rejected if experimental findings fail to support it. However, a
hypothesis can never be proven to be true. This portion of the scientific method is
called the materials and methods when reported to peers in the scientific community.
The following are some important components of the procedure.
 Constants: All factors which are kept the same during the experiment are called
constants. It is important that all the constants are identified so that every effort can be
made to insure they do not vary between experimental units. Experimental units may
be set up in containers such as test tubes, microtubes, petri plates, flasks, etc.
 Independent variables: The variable that is purposely changed by the experimenter is
called the independent variable. This is the factor which is being evaluated during the
experiment. If using a chart (or graph) to communicate experimental results, the
independent variable is represented by the x-axis.
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 Dependent variables: The variable that responds to the independent variable is the
dependent variable. It is important to choose appropriate dependent variables. The
researcher must ask questions such as, “Is this really the best way to measure a
response to the independent variable? Is this type of measurement too time
consuming or too expensive?” There are two types of dependent variables; (1)
Quantitative = measured variables to which a numerical value can be assigned, and
(2) Qualitative = observed variables. Obviously both types of dependent variable
involve some form of observation, but quantitative variables can be expressed as a
measurable quantity (i.e. a number) and qualitative variables cannot. For example, the
speed of a runner is quantitative because it can be measured by timing with a
stopwatch and recording data with numbers and units. The form and style of a
competitive ice skater or diver is qualitative because it cannot be measured (no
numerical data can be recorded). Human senses, judgment, and descriptive terms are
involved in qualitative observations. Therefore, qualitative data is subjective.
Quantitative data is objective.
 Control groups: The controls provide a standard for comparing experimental effects.
Controls are used to ensure that the constants are not affecting the dependent variable
measurements (the data/results). The researcher must be confident that the
independent variable, and not some other unexpected factor, is properly correlated to
the type of data being collected. Controls are used to eliminate the effects of
everything except the one variable being tested.
 Repeated trials: The number of objects or organisms undergoing treatment for each
value of the independent variable, or the number of times the experiment is repeated.
A sufficient number of trials must be conducted to give the researcher confidence in the
results and in order for data to be statistically significant.
 Following completion of the experiment, the results are reported. In the
conclusions the researcher states whether the hypothesis was rejected (i.e.
falsified) or tentatively accepted, and gives possible reasons and explanations for
the results.
Laboratory Objectives:
After completing this laboratory exercise, you should be able to:
1. Formulate questions that can be answered through scientific investigation
utilizing definable, measurable, and controllable components.
2. Define: hypothesis and communicate features of an appropriate scientific
hypothesis.
3. List the various components of a scientific experiment.
4. Differentiate dependent and independent variables.
5. Differentiate qualitative and quantitative dependent variables.
6. Understand the importance and use of controls.
7. Discuss results with peers and evaluate experiments.
8. Interpret and communicate results.
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EXERCISE:
Materials (for a 2-student team):
Sarcina lutea culture
3 nutrient agar (NA) petri plates
Incinerator or Bunsen burner (optional)
sterile cotton swabs
UV light chamber with cardboard masks
incubator set to 37ºC
The Effect of Ultraviolet (UV) Radiation on Sarcina lutea
In this experiment you will examine the effect of ultraviolet (UV) radiation on the
bacterium Sarcina lutea. UV light is a known mutagen; it causes changes in an
organism’s DNA which are called mutations. Mutations may also occur spontaneously,
but spontaneous mutations typically occur at a much slower rate than those induced by
mutagens such as UV light. The effect of a mutation on an organism can vary from
virtually no change (a silent mutation) to causing death (a lethal mutation). In most
cases, induced mutations to the bacterial DNA will be lethal. Germicidal lamps that emit
UV radiation are commonly used in medical settings (e.g. hospital rooms, operating
rooms and dental offices) as well as in areas where food is prepared (e.g. food
processing plants and slaughterhouses) to prevent disease transmission.
Question:
Write a question in reference to UV light exposure and bacterial survival/growth. Your
question should include observable and measurable components
Hypothesis:
Convert your question (above) into a hypothesis. This should be a clear statement.
Formulate a hypothesis about the effect of ultraviolet light on S. lutea growth.
Procedure:
Since you will be working with a concentrated bacterial culture, you must wear a
lab coat, eye protection, and gloves for this exercise. Work in pairs.
1. Label each petri plate on the base of the plate. Label the plates A, B, and C.
Labels should also include your initials, the date, the medium, and the
sample/treatment for each plate.
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2. Using a sterile cotton swab, dip into the tube containing the Sarcina lutea culture
and transfer the bacteria to plate A (i.e. inoculate the plate) by completely
covering the surface of the agar three times. Refer to Figure 1.1 below. Repeat
with a second plate (plate B). [Your instructor may ask you to work near an
incinerator or Bunsen burner during these steps in order to decrease the
probability of contamination by unwanted microbes.] Do not open the third plate
yet; plate C does not get bacteria.
Figure 1.1 Procedural diagram for inoculating a single
plate (repeat these three steps for plates A and B):
1. Dip swab in
bacteria and run
the swab along
the surface of
the agar in one
direction.
2. Rotate the
plate 90 degrees
in your hand,
and run the
swab along the
surface of the
agar a second
time.
3. Rotate the
plate 45 degrees
and run the
swab along the
surface of the
agar a third time.
3. Remove the lid from plate A containing S. lutea, and place a cardboard mask (i.e.
stencil) over it. The mask will allow UV radiation to strike those bacteria under
the opening, while preventing the UV light from reaching bacteria under the
cardboard. Place the petri plate into the UV light chamber and expose it to UV
radiation for 60 seconds. Following UV exposure, remove the cardboard mask
and return the lid to the petri plate.
4. Remove the lids from plates B and C, place a stencil over each plate, and place
both plates in the UV light chamber for 60 seconds. DO NOT turn the UV lamp
on. (These plates should NOT get exposed to UV light.) Remove the cardboard
mask and return the lids to the petri plates.
5. Incubate all three plates at 37oC (inverted) until the next lab period.
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RESULTS:
Table 1.1
Plate
Sample
Treatment
A
S. lutea
exposed to UV
light
B
S. lutea
no UV
C
no bacteria
no UV
Observations
Questions
Answer the following questions based on your experimental results (Table 1.1) and your
hypothesis.
1. Which plate(s) is(are) the control group(s)? What did each control group
control for?
2. Which plate(s) is(are) the treatment group(s)? What did each treatment group
test for?
3. What is the independent variable? In other words, what purposefully changed
to test your specific hypothesis?
4. What is the dependent variable? In your experiment, was the dependent
variable qualitative or quantitative? Explain.
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5. List all the constants.
6. Conclusions. Did you reject your hypothesis? Did you fail to reject (i.e.
tentatively accept) your hypothesis? Give possible reasons for your results to
support your conclusion.
7. Based on the previous discussion of the scientific method, could we improve
this experiment? If so, explain.