Chapter 5
Enzymes
Questions
1. Explain why enzymes are called biological catalysts.
ANS. Biological because they are made by organisms. Catalysts because they increase
the rate of chemical reactions.
2. You read about the enzyme lactase at the start of this chapter. a What type of
enzyme is lactase? b Name the substrate of lactase.
ANS. a. carbohydrase
b. lactose
3. Look at Figure
Describe what is shown on the graph. (‘Describe’ means state the main points in
words; give main features that you can see. When you are describing a graph it is a
good idea to quote figures, with units.)
ANS. As temperature increases from 0 to 37 °C, the rate of reaction increases. The
increase is exponential. As temperature increases above 37°C, the rate of reaction
decreases and reaches 0 at 58°C. This decrease is steeper than the increase at lower
temperatures.
4. Look at Figure 5.5.
a. What is the optimum pH of the protease in the stomach?
b. What is the optimum pH of most enzymes?
c. What is the range of pH values at which most enzymes are able to work?
ANS. a. 2
b. 7
c. between pH 5.6 and 10.4
5. Look at the graph that you drew from your results in Experimental skills 5.2
Investigating the effect of temperature on the activity of lipase. In your own words,
explain the shape of the graph. (‘Explain’ means using your biological knowledge and
understanding to give reasons for the shape of the graph.)
ANS. Explanations should refer to:
• the increase in kinetic energy of enzyme and substrate molecules as temperature
increases
• therefore, an increase in the frequency of collisions
• the loss of shape of the active site of the enzyme as temperature increases above
optimum, so that the substrate no longer fits into it
• use of the term denaturation.
6. Use your own words to explain the shapes of the curve for most enzymes, in
Figure 5.5.
ANS. Explanations should refer to:
• the active site of the enzyme only having its ‘correct’ shape within a narrow range
around pH 7
• the loss of shape of the active site of the enzyme as pH decreases or increases from
the optimum, so that the substrate no longer fits into it
• use of the term denaturation.
EXAM-STYLE QUESTIONS
1. Which statement is not true for all catalysts?
A. They are enzymes.
B. They are not used up in the reaction.
C. They increase the rate of a chemical reaction.
D. They remain unchanged at the end of a reaction.
ANS. A
2. What are the smaller molecules from which an enzyme molecule is made?
A. amino acids
B. fatty acids
C. glucose
D. glycerol
ANS. A
3. A solution containing an enzyme and no other substance was tested. Which test
would give a positive result?
A. Benedict’s test
B. biuret test
C. ethanol test
D. iodine test
ANS. B
4. What explains why a reaction controlled by an enzyme happens more slowly as
temperature decreases?
A. The enzyme and substrate molecules have less kinetic energy.
B. The enzyme is denatured.
C. The enzyme is killed by the low temperature.
D. The substrate changes shape and cannot fit into the active site.
ANS. A
5. a. Copy and complete the sentences about enzymes. Use words from the list. You
may use each word once, more than once or not at all.
Active, carbohydrate, catalysts, chemical, complementary, identical, metabolic,
proteins
Enzymes are _________________ that function as biological _________________.
They are involved in all _____________________ reactions. The part of an enzyme
where a substrate binds is called its _____________________ site. The shape of this
site, and the shape of the substrate, are __________________.
b. Describe why enzymes are important to all living organisms.
ANS. A. Enzymes are proteins ; that function as biological catalysts . ; They are
involved in all metabolic reactions . ; The part of an enzyme where a substrate binds
is called its active site . ; The shape of this site, and the shape of the substrate, are
complementary . ;
b. they increase the reaction rate of metabolic reactions ; so that these work well
enough to sustain life ;
6 The figure below shows some stages in an enzyme-controlled reaction. What are P,
Q, R and S?
ANS. C
7. Amylase is an enzyme that breaks down starch to maltose.
a. A student put some starch solution into a test-tube. He took a small sample of
starch from the tube and added it to a drop of iodine solution on a white tile.
Next, he added some amylase solution to the starch solution. After five minutes, he
took another sample from the tube, and tested it with iodine solution. The table
shows his result.
Explain the student’s results.
ANS. the iodine went blue-black at the start because the solution contained starch ;
it stayed brown after adding amylase because the solution did not contain starch ;
because it had been broken down by amylase ;
b. The student investigated the effect of pH on the activity of amylase. He placed the
same volume of starch solution in five tubes. He added different buffer solutions to
each tube. He then added amylase solution to each tube. He took samples of the
mixture from each tube and measured the time for all the starch to disappear. His
results are shown in the table.
i. Use the student’s results to describe the effect of pH on the activity of amylase.
ii. The student kept the volume of starch solution the same in each tube. State three
other variables that the student should have kept the same in his experiment.
ANS. i. amylase works most quickly at pH 7 / its optimum is pH 7 ; 4 minutes is the
shortest time ; it does not work at all at pH 4 or 11;
ii. Any three from: • the concentration of amylase solution ; • the concentration of
starch solution ; • the volume of amylase solution ; • the temperature ;
8. The graph below shows how the activity of two enzymes, A and B, is affected by
temperature.
a. Compare the effect of temperature on enzyme A and enzyme B.
b. Explain the effect of temperature on enzyme B from:
i. 0 to 60 °C
ii .70 to 90 °C
ANS. a Any five from:
• the optimum temperature of enzyme B is higher than enzyme A ;
• lowest temperature at which activity begins is higher for B than A ;
• highest temperature at which activity ends is higher for B than A ;
• figures quoted for both (e.g. optimum is 70 °C for B and 40 °C for A) ;
• for both, increase in temperature below optimum causes an exponential increase
in activity ;
• for both, increase in temperature above optimum causes a steeper decrease in
activity ;
b. i. Any three from:
• kinetic energy of enzyme and substrate molecules increases as temperature
increases ;
• so the frequency of collisions increases ;
• frequency of formation of enzyme–substrate complexes increases ;
• so, rate of reaction increases ;
ii. Any four from:
• enzyme molecule begins to lose its shape ;
• enzyme is denatured ;
• so active site no longer complementary shape to substrate ;
• enzyme–substrate complexes cannot form ;
• so, rate of reaction decreases ; idea that increasing temperature over this range
results in a greater degree of denaturation ;