The Scientific Method: Observation
How do I start?
is the key tool of a scientist. Observation helps a researcher to identify
promising aspects of natural phenomena that are worth knowing about. The scientist
is specifically looking for causal relationships in nature that (taken together
with other knowledge) will help to explain in the broadest terms how natural
systems work. The outcome of any observational process will be the formulation
of one or more research questions (RQ’s). RQ’s are (at some level) guesses
about the biological principles operating in a given situation.
exercise will be about the observation of Beta vulgaris L., the common
garden beet. It contains a reddish pigment called betacyanin. Betacyanin is
found deep in the cells of the beetroot in a cellular organelle called the
- Examine the beet that you
have available at your table. Cut some cylinders of beet tissue using the
cork borer. What was the result? Can you think of other types of
treatments that might also cause betacyanin to leak from the root? Try out as many of these ideas as you
have time to do. Make notes about your observations. As a hint to get
started, we have available in the lab some solutions of ethanol in water.
You may observe that certain treatments will cause betacyanin to leak from
the beetroot, whereas others will not. You may also note different rates
of leakage with specific treatments.
- As you pose your RQ’s,
remember that you are looking for ideas that lead to an understanding of
causal relationships in nature. Put another way, you are trying to learn
something about why betacyanin normally stays in the root by
considering the results of treatments that will cause it to leak out!
- Avoid posing RQ’s such as,
“Will blows with a sledge hammer cause betacyanin to leak from the
beetroot?” Only RQ’s with sound reasoning behind them (what scientists
call the rationale) are worth pursuing. A much better question might be,
“Will ethanol cause betacyanin to leak from the beetroot faster than water
estimates of betacyanin leakage from beetroots, although useful, are only the
first step in our investigation. We also need to quantitatively measure
how much of this pigment is present in a given sample. Our method for doing
this is spectrophotometry, a technique that estimates how much betacyanin is
present by determining how much light it absorbs. Before we can do quantitative
measurements with the spectrophotometer, we first need to know what wavelength
of visible light betacyanin absorbs most strongly. This wavelength is called
Directions for Measuring Betacyanin with a
Spectrophotometer and Determining λmax
- Turn on your
spectrophotometer using the left hand knob on the front.
- Wait 10 minutes
- Get two cuvettes. Label one
tube “S” for sample, and the other “B” for blank.
- Put 5 ml of ethanol in the
“B” tube and 5 ml of betacyanin solution in the “S” tube.
- Using the left knob adjust
the needle until it reads infinity on the lower scale. This will be toward
the left of the bottom scale on the display.
- Set the wavelength knob to
- Get the “B” tube and place
it in the cuvette holder. Using the right hand knob, adjust the absorbance
to zero. This will be towards the right of the display.
- Remove the “B” tube and put
in the “S” tube. Record in your notebook the indicated absorbance on the
- Take out the “S” tube and
recheck that the needle still is on infinity (step 5 above).
- Adjust the wavelength to
- Repeat steps 7-9 until you
reach 650 nm.
- Plot the results on graph
paper with absorbance (dependent variable) on the “Y” axis and wavelength (independent
variable) on the “X” axis.
- Identify the wavelength
that exhibits the highest absorbance. This is λmax for