GCE Biology CIE 5090
5.1 Enzyme action
CONTENTS
1 Describe a catalyst as a substance that increases the rate of a chemical
reaction and is not changed by the reaction
2 Describe enzymes as proteins that function as biological catalysts and are
involved in all metabolic reactions
3 Explain enzyme action with reference to the substrate, active site, enzyme-
substrate complex, and product
4 Explain the specificity of enzymes in terms of the complementary shape and
fit of the active site with the substrate (‘lock and key’ hypothesis)
5.1 Enzyme action
5.1.1 What are Enzymes?
Catalyst: A substance that increases the rate of a chemical
reaction and is not changed by the reaction.
Enzyme: A protein that functions as a biological catalyst. Enzymes
are essential for all metabolic reactions in living organisms.
5.1.2 The 'Lock and Key' Hypothesis
This model explains how enzymes work with specificity.
1. Substrate: The molecule that the enzyme acts upon.
2. Active Site: A specific region on the surface of the enzyme with a
shape that is complementary to the substrate.
3. Enzyme-Substrate Complex: The substrate fits perfectly into the
active site, like a key into a lock, forming an enzyme-substrate
complex.
4. Product: The reaction occurs at the active site, and the substrate is
converted into product(s), which are then released. The enzyme
remains unchanged and can be reused.
Specificity: Each enzyme has a uniquely shaped active site that is
complementary to only one type of substrate. This means each
enzyme usually catalyses only one specific reaction.
Paper 2 Tip: When explaining enzyme specificity, you must use the
terms active site, substrate, and complementary shape. Stating that
they have the "same shape" is less precise than saying their shapes
are "complementary."
5.1.3 Investigating Enzyme Reactions
The progress of an enzyme-catalysed reaction can be tracked by
measuring either:
The rate at which the product is formed.
The rate at which the substrate is used up.
5.2 Effects of temperature and pH
CONTENTS
1 Understand that the progress of enzyme-catalysed reactions can be
followed by measuring the concentrations of reactants and products
2 Investigate and describe the effects of temperature and pH on enzyme
activity
3 Explain the effect of changes in temperature and pH on enzyme activity in
terms of kinetic energy, shape and fit, denaturation and the frequency of
effective collisions
5.2 Effects of temperature and pH
5.2.1 Effect of Temperature
Low Temperatures: Enzymes have low activity because both
enzyme and substrate molecules have low kinetic energy. This
results in fewer and less energetic collisions, so the frequency of
effective collisions between substrate and active site is low.
Optimum Temperature: As temperature increases towards the
optimum, molecules gain kinetic energy. This increases the speed of
movement, leading to more frequent and energetic collisions, and
thus a higher rate of reaction. Most human enzymes have an
optimum temperature of around 37°C.
High Temperatures: Beyond the optimum, the high temperature
breaks the bonds holding the enzyme in its specific 3D shape.
The active site changes shape and is no longer complementary to
the substrate. The enzyme is said to be denatured. This change is
irreversible.
5.2.2 Effect of pH
Optimum pH: Each enzyme has a specific pH at which it works
most effectively.
Changes from Optimum pH: If the pH is too high or too low, the
bonds holding the enzyme's 3D shape are disrupted. This causes
the active site to change shape, so the substrate can no longer fit.
The enzyme is denatured.
5.2.3 Denaturation
Denaturation: An irreversible change in the specific three-
dimensional shape of an enzyme (particularly its active site), caused
by extreme temperatures or pH.
When an enzyme is denatured, it can no longer bind to its substrate
and loses its catalytic function permanently.
Paper 4 Tip: When designing an experiment to investigate
temperature or pH, you must state all other variables that need to be
kept constant. These include enzyme concentration, substrate
concentration, and the factor you are not investigating (e.g., pH if
investigating temperature).
Past Paper Practice Questions & Solutions
Paper 1 (MCQ) Style Questions
Question 1: What is a catalyst?
A. A substance that decreases the rate of a chemical reaction.
B. A protein that is used up during a chemical reaction.
C. A substance that increases the rate of a chemical reaction and remains
unchanged.
D. A biological molecule that works best at high temperatures.
Solution: C
o Explanation: This is the definition of a catalyst. They speed
up reactions without being consumed.
o Why C is correct: A is incorrect (it increases the rate). B is
incorrect (catalysts are not used up). D is incorrect (enzymes
are denatured by high temperatures).
Question 2: The 'lock and key' hypothesis of enzyme action describes
the specific fit between which two molecules?
A. Product and active site
B. Substrate and active site
C. Enzyme and product
D. Substrate and product
Solution: B
o Explanation: The hypothesis describes the specific,
complementary fit of the substrate into the enzyme's active
site.
o Why B is correct: This is the core of the 'lock and key'
model.
Question 3: What happens to an enzyme when it is denatured?
A. Its active site changes shape.
B. It is used up by the reaction.
C. It works more effectively.
D. Its substrate changes shape.
Solution: A
o Explanation: Denaturation is the irreversible change in the
3D shape of the enzyme, specifically the active site, which
prevents it from functioning.
o Why A is correct: This is the definition of denaturation.
Paper 2 (Theory) & Paper 4 (ATP) Style Questions
Question 4 (Paper 2): Explain why high temperatures stop enzymes
from working. [3]
Solution:
1. High temperatures cause the bonds within the enzyme to
break.
2. This causes the specific 3D shape of the active site to change.
3. The enzyme is denatured, and the substrate can no longer fit
into the active site.
Examiner's View (P2): The examiner is looking for these three key
points:
4. Mention of bonds breaking.
5. The effect this has on the active site (its shape changes).
6. The use of the correct term denatured.
Question 5 (Paper 4): A student investigates the effect of pH on the
activity of the enzyme amylase, which breaks down starch. She uses
iodine solution to test for the presence of starch.
(a) Describe the results the student would expect to see in the test-tube
with the fastest reaction rate. [2]
(b) Apart from temperature, state one other variable that should be kept
constant in this investigation. [1]
Solution:
(a) The solution will turn from blue-black to brown/yellow the
quickest. This is because the starch is broken down most rapidly at
the optimum pH.
(b) Any one from: Concentration of the amylase
solution, concentration of the starch solution, volume of the
solutions used.
Examiner's View (P4):
(a) For the result, you must describe the colour change and link it to the
speed of the reaction. Stating the initial and final colours of the iodine test
is crucial.
(b) This tests your understanding of controlling variables. You must
identify a factor that could affect the rate of reaction other than the one
being investigated (pH) or the one being kept constant
(temperature). Concentration is the most common correct answer.
