Investigation
34
Introduction
Purpose
Materials
and
Equipment
Procedure
Enzymes
One of the distinguishing features of living organisms is the presence of organic catalysts
called enzymes. Those enzymes isolated to date have been found to be proteins; most
are soluble in water or a dilute salt solution. However, enzymes found in mitochondria
are bound together by lipoprotein (a phospholipid-protein complex) and this makes
them insoluble in water. Some enzymes consist solely of protein. Others consist of two
parts, one of which is a protein, an apoenzyme, the other being made of a smaller
organic molecule, a coenzyme. In some cases the second portion is a metallic ion and is
known as a cofactor. Separately, the apoenzyme and the coenzyme or cofactor remain
inactive, but combined they form an active functional unit known as a holoenzyme.
To compare the action of catalase to a non-protein catalyst under different conditions.
Materials
3% hydrogen peroxide
manganese dioxide
fresh or frozen liver
potato
ice
Equipment
fine clean sand
stirring rod
Bunsen burner or hotplate
250 mL beaker
test tubes
scalpel
Catalytic reactions
Add 2 mL of hydrogen peroxide to two test tubes. Place 0.1 g of sand in one test tube
and add 0.1 g of manganese dioxide to the second tube. Observe and record the
rates of reaction. What gas evolves?
B. The effect of an enzyme
Add 2 mL of hydrogen peroxide to each of two clean test tubes. In one place a small piece
of liver and in the other a small piece of potato. Record the rates of reaction and
compare these results to those with manganese dioxide. Do not discard these
materials.
C. Re-using an enzyme
Divide the liquid portion of the previous tube in B containing the liver into two test
tubes. Cut the liver from procedure B into two equal portions and add these to the
two test tubes. To the first add a fresh piece of liver and to the second add 1 mL of
hydrogen peroxide. Record your observations and explain the reaction in the test
tube containing the fresh liver. What would happen if additional hydrogen peroxide
were added to the second tube?
D. Effect of particle size
Place a small piece of liver in one test tube and a small piece of potato in a second. Add a
pinch of sand to each tube and crush these materials with separate stirring rods.
Add 2 mL of hydrogen peroxide to each tube, then observe and record the rates of
these reactions. Compare the results with those of the uncrushed liver and potato in
part B.
tt.1
2 mL H 20 2
liver
Record
Results
tt.2
2 mL H202
potato
Record
Results
B. Effect of an enzyme ----------C. Re-using an enzyme
E. Effect of temperature
Place a small piece of liver in a test tube and heat it for 5 minutes in a boiling water
bath. Add 2 mL of hydrogen peroxide to the boiled liver and record the results.
Place a small piece of liver into each of two test tubes. Place one test tube in a 37'C
water bath for 5 minutes and the second test tube in an ice-water bath for the same
length of time. Remove both test tubes from the water baths and add 2 mL of hydrogen
peroxide to each tube. Record the rates of the reactions.
Observations
Record your results in a chart similar to the one shown below. Rates of reaction
can be designated as follows: 0 = no reaction, + = slow, ++ = moderate,
+++ = fast.
Reaction Rates
Observations
A
B
Rate of
Reaction
Interpretations
Sand
MnO 2
Liver
-
Potato
Used liver
+ fresh
liver
C
Used liver
+ H2O2
Crushed
liver
D
Crushed
potato
Boiled
liver
E
Liver at
37°C
Liver at
0°C
Questions
1. How do you account for the differences in the rates?
2. Can H 20 2 be broken down by catalysts other than those found in living
systems? Explain your answer.
3. Describe the effect of temperature and particle size on the rate of enzyme
action.
4. The body temperature of a dog is approximately 40°C. Would your results be
different if you had used pieces of dog liver for this investigation?
5. Explain the difference between the rates of reaction with potato as compar ed
to liver.