Name
Period
Title: Spit It Out- Evolution of Enzymes
Unit: Evolution
Introduction: Although there is a diversity of life on Earth, all organisms depend upon
similar foods. Living things need starches, lipids, proteins, and nucleic acids to survive. It
follows, then, that there are common traits among organisms. This investigation
demonstrates a similarity in proteins (enzymes) between members of Monera (Eubacteria)
and a member of Animalia.
A genetic similarity between bacteria and human beings can be demonstrated using an
inexpensive qualitative test. This activity is an introduction to comparative biochemistry,
a field of study that is used when classifying organisms.
The discovery of the enzymes studied in this lab allows manufacturers to make
sweeteners from starch without producing toxic byproducts. Amyloglucosidase and
amylase are commercially used to make glucose from cornstarch.
Purpose: Do soil bacteria and humans produce similar enzymes for the digestion of
starch? What does this infer concerning their evolution?
Materials:
2 ml Lugol’s iodine (0.3%)
2 cotton swabs
400 ml beaker or 9 oz cup
soil sample
2 starch agar plates
100 ml graduated cylinder
1 ml pipet or dropping pipet
Procedure:
1. Put a clean cotton swab in your mouth until it is completely soaked with saliva. You
are collecting amylase, an enzyme that is in saliva.
2. Make a pattern on your starch agar plate with the cotton swab. Write your name, draw
a picture—anything to spread the liquid around. Allow the liquid to soak into the plate
for five minutes.
3. You are going to perform a qualitative test for starch by using iodine. A qualitative
test determines if something is present (or not); a quantitative test is one that
determines how much is present.
4. Cover the surface of the agar with iodine. Iodine is an indicator that turns purple-black
in the presence of starch. The areas where the starch is broken down by amylase do
not react with iodine and do not change color. If the iodine turns purple-black it is a
positive (+) result, meaning that starch is present. It is a negative (-) result if the iodine
does not change color. Record your observations and interpretations in Table 1.
5. Is it possible that organisms found in soil also produce amylase? You need to collect
a tablespoon of dirt for this part of the lab.
6. Pour 100 ml of water into a cup or beaker and add your soil sample to it. Swirl your
mixture and remove 1 ml while the soil is well suspended.
7. Place the 1 ml sample on a fresh starch agar plate and spread it around with a clean
cotton swab. Allow the liquid to soak in. Cover the surface with iodine and observe
the plate. Record your observations and interpretations in Table 1.
Biology 6-27
Observations:
Table 1: Results of tests for the presence of starch.
Result of
Substance
IKI test
Observations
used
(+/-)
Saliva
Soil
organisms
Analysis:
1. What lab evidence supports the statement, “Living organisms produce enzymes that
break down starch”?
2. Did you observe that enzymes break down starch or did you infer that enzymes break
down starch? Use observations from your lab to defend your answer.
Connections/*Textbook Research:
1. Suppose you added more starch to the agar plates after you added the iodine. Should
you expect the added starch to be broken down? Defend your answer.
2. A possible explanation for soil bacteria having enzymes similar to those of human
beings is that bacteria decompose plants and “eat” the same starches that humans do.
Is this explanation an example of divergent evolution or convergent evolution?
3. How does the presence of a similar enzyme in different organisms suggest that there is
a genetic similarity between the organisms?
Conclusions:
1. What is it called when different organisms develop similar traits because they live in
similar environments?
2. What is the difference between an observation and an inference?
3. What is a qualitative test?
4. How does comparative biochemistry help to determine genetic similarities between
different organisms?
Biology 6-28