Ch 4 Enzymes and metabolism
Practical 4.1
Demonstration of the breaking-down action
of enzymes
Results (p. 4-3)
Sample
Glowing splint relights
hydrogen peroxide + liver extract
+
distilled water + liver extract
–
hydrogen peroxide + distilled water
–
Questions (p. 4-3)
1
This increases the surface area for reactions.
2
Grinding action produces heat. The high temperature resulted may denature any
enzyme present in the tissues.
3
The gas given off is oxygen.
4
a
It is a control to show that no oxygen is given off from the liver extract.
b
It is a control to show that no oxygen is given off from the hydrogen peroxide.
5
Liver extract reacts with hydrogen peroxide to produce oxygen.
6
No. This experiment only shows that the breakdown of hydrogen peroxide is
speeded up by the liver extract. Boiled liver extract, instead of fresh liver extract,
can be used in a further experiment. If boiled liver extract has no catalytic action, it
is more likely that the reaction is catalysed by an enzyme.
7
Yes. For the three test tubes, only one variable (the sample) is changed at a time,
other variables (e.g. the volume and temperature of hydrogen peroxide, liver
extract and distilled water) are kept constant.
Conclusion (p. 4-4)
The breakdown of hydrogen peroxide is catalysed by the liver extract, probably by an
enzyme in the liver tissues. Nevertheless, further experiments should be done to
confirm this.
Practical 4.2
Investigation of the effect of temperature on
enzyme activity
Results (p. 4-7)
Temperature (°C) Time for disappearance of blue-black colour (min)
0
The blue-black colour does not disappear.
20
40
(Results vary with the origin of amylase.)
60
80
100
The blue-black colour does not disappear.
Questions (p. 4-7)
1
To ensure that the amylase and starch solutions inside the tubes reach the
respective temperatures before the reaction starts.
2
To prevent the changing of the condition of a mixture by any residue in the
dropper.
3
Amylase is inactive at low temperatures. Its activity increases with temperature
and is the highest at 60°C. Afterwards the activity decreases and stops at 100°C.
With a rise in temperature, the kinetic energy of amylase and starch molecules
increases. They collide and react more frequently. As the temperature increases
further, the active sites of amylase become distorted (i.e. the enzyme is denatured)
and the reaction rate decreases. At 100°C, all amylase is denatured and no reaction
takes place.
4
5
a
Starch will be digested and blue-black colour will disappear. This is because
the inactive amylase will resume its activity with an increase in temperature.
b
Starch will not be digested and blue-black colour will remain. This is because
the activity of the denatured amylase will not restore even when it is cooled.
Measuring the rate of appearance of maltose molecules.
Conclusion (p. 4-8)
Amylase is inactive at low temperatures. Its activity increases with temperature until
it reaches a maximum. Afterwards the activity decreases and stops.
Practical 4.3
Investigation of the effect of pH on enzyme
activity
Results (p. 4-10)
Tube
pH
A
3.2
B
4.0
C
5.2
D
6.0
E
7.0
F
8.0
Depth of brick-red precipitate settled
(mm)
(Results vary with the origin of invertase.)
Questions (p. 4-11)
1
To ensure the invertase has sufficient time to catalyse the breakdown of sucrose
into glucose and fructose.
2
(Answer depends on results.)
3
a
No or less brick-red precipitate will be formed. This is because extremely
low pH will denature the invertase and reduce the enzyme activity.
b
No or less brick-red precipitate will be formed. This is because extremely
high pH will denature the invertase and reduce the enzyme activity.
4
Weighing the precipitate formed. / Using an arbitrary system of ‘+’ to denote the
relative amount of precipitate.
Conclusion (p. 4-11)
Invertase is most active in the acidic medium (pH 5.2) and less active in the neutral
and alkaline medium.
Practical 4.4
Investigation of the effect of inhibitors on
enzyme activity
Results (p. 4-13)
Tube
Sample that
contains
A
copper(II) sulphate
solution
B
silver nitrate solution
C
distilled water
Depth of brick-red precipitate settled
(mm)
(Answer varies with Ss.)
Questions (p. 4-14)
1
It is a control to show that the activity of invertase is slowed down or stopped by
the inhibitor.
2
No or less brick-red precipitate is formed in tubes A and B because copper(II) ions
and silver ions are inhibitors of invertase. They slow down or stop the activity of
invertase.
Brick-red precipitate is formed in tube C because the activity of invertase is not
affected by any inhibitor.
3
Whether an inhibitor is competitive or non-competitive can be found out by
increasing the substrate concentration of the reaction medium. The reaction rate
will be increased in case of competitive inhibition, but not in case of
non-competitive inhibition.
Conclusion (p. 4-14)
Copper(II) ions and silver ions are inhibitors of invertase. They slow down or stop the
activity of invertase.
Practical 4.5
Investigation of protease activities in
different fruit juices
Results (p. 4-17)
Well
Diameter of clear zone
(number of squares on graph paper)
Sample
A
Pineapple juice
B
Kiwi fruit juice
C
Papaya juice
D
Guava juice
E
Distilled water
(Results vary with Ss.)
Questions (p. 4-17)
1
It is a control to show that the formation of the clear zones is due to the fruit juices.
2
Proteases in the fruit juices break down the white milk protein nearby. Therefore,
the white colour of the milk disappears and the clear colour of the agar is shown
around the wells containing fruit juices.
3
(Answer depends on results.)
4
It is because the proteases in pineapple are denatured by the high temperature
during the canning process.
5
The proteases in fresh pineapple can break down the proteins in beef steak.
Leaving beef steak in contact with slices of pineapple for half an hour allows
enough time for the enzymes to work.
Conclusion (p. 4-18)
Pineapple, kiwi fruit, papaya and guava contain proteases that can break down
proteins, but their activities differ from one another.