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GCSE level biology exam revision notes on
Enzymes: 3.
The effect of changing substrate or
enzyme concentration on the rate of an enzyme catalysed chemical reaction
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[Key
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Sub-index
of biology notes on enzymes and digestion
3.
Factors
that affect an enzyme's performance - concentration
Enzymes perform best in their 'optimum' ambient conditions
BUT, first consider a basic view of enzyme activity and collision theory.
1.
Suppose you mix the reactant molecules without the appropriate
enzyme - the reaction is very slow.
2. The
appropriate enzyme is added and the reaction speeds up - the activation is
lowered by the enzyme and the rate of fruitful collisions increases
producing more product.
3.
From 2. to 3. the reaction proceeds at a steady rate, enzymes molecules
fully active and ample substrate molecules, plenty of fruitful collisions.
4. Eventually the enzyme is running out of substrate
molecules, less fruitful collisions possible, less product per unit time, so
the rate of reaction steadily decreases to zero when all the substrate is
used up.
The levelling off in rate can also be caused by high
concentration of substrate in which the active sites on the enzyme
are all filled in a given instant of time.
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1. What is the
effect of changing concentration of the substrate for an enzyme catalysed reaction?
When
investigating the effect of concentration on enzyme activity, three
factors must be kept constant, (i) either the concentration of substrate
or enzyme, (ii) the temperature and (iii) the pH of the solution (see
experimental methods).
Concentration:
If the substrate reactant e.g. sugar, concentration is increased, the rate of reaction
increases in a simple proportional way as long as the enzyme
concentration is constant.
Kinetic
particle theory:
The greater the concentration of substrate (or enzyme,
see below) the greater the probability of a fruitful collision leading
to the formation of products.
For more details see
Effect on
rate of
changing reactant concentration
Assume the grey background is the water solvent.
The graph on the above-left shows what
happens as you gradually increase the substrate molecule concentration
for a fixed constant concentration of enzyme.
Initially the rate of
reaction steadily increases, in fact, usually, it rises proportionately with
increase in substrate concentration, but ....
What happens if the enzyme gets
overloaded with substrate molecules?
However, if the concentration of
enzyme is low but the substrate concentration becomes very high, the
rate of reaction rises to a maximum and then stays constant.
This is because the maximum
number of catalyst active sites for the 'key and lock' mechanism are
all in use and the maximum rate of reaction depends on the rate of
diffusion of substrate in, and diffusion of product out from the
active sites.
This shows up as the later horizontal portion on the graph
- meaning the speed of the reaction has become constant because there
are no more active sites to bring into operation.
What happens if you change the enzyme concentration?
At constant substrate concentration, the rate of reaction (usually)
increases linearly with increase in enzyme concentration.
The greater the concentration of enzyme, the greater
the probability of a fruitful collision leading to the formation of
products. Hence the initial linear part of the graph.
However, if the enzyme concentration is very high and
the substrate concentration low, the rate tails off and becomes
constant. This is because there are lots of 'active sites' available, in
excess of those needed, and adding more of the enzyme makes no
difference.
See a
'decay' investigation using milk and lipase gcse
biology revision notes
Part of the section
Carbon cycle,
nitrogen cycle, water cycle, decomposition - decay investigation
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Key points
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Key ideas on changing concentrations in an
enzyme catalysed reaction
Enzyme-Catalysed Reactions:
Key Concepts
Enzymes are biological catalysts
that speed up chemical reactions by lowering the activation energy.
Their
activity depends on several factors, two of which are substrate
concentration and enzyme concentration.
1.
Effect of
Changing Substrate Concentration
Description:
As the substrate
concentration increases, the rate of reaction also
increases — up to a point.
Explanation:
-
At low substrate
concentration:
-
At medium
substrate concentration:
-
At high substrate
concentration:
-
All enzyme active
sites are occupied — enzymes are working at their maximum
rate.
-
The reaction reaches a
plateau (called Vₘₐₓ).
-
Adding more
substrate has no effect
on the rate of reaction.
Limiting Factor: At high
substrate concentration, the enzyme concentration becomes the
limiting factor.
Graph Shape:
A rising curve
that levels off to a plateau.
2.
Effect of
Changing Enzyme Concentration
Description:
As enzyme
concentration increases, the rate of reaction
also increases — assuming there is an excess of substrate.
Explanation:
-
More enzymes = more
available active sites.
-
This allows more substrate
molecules to be converted into products per unit time.
-
Reaction rate increases
proportionally with enzyme concentration (linear
relationship) — but only if substrate is not limiting.
-
If the substrate
concentration is low, then increasing enzyme
concentration has limited effect, as there aren’t
enough substrate molecules to bind to the extra enzymes.
⚠ Limiting Factor: At low
substrate concentration, substrate becomes the limiting factor
even if enzyme levels are high.
Graph Shape:
A linear increase
that may level off if substrate is insufficient.
Quick Summary Table
| Factor Changed |
Effect on Reaction Rate |
Limiting Factor (at high
levels) |
| Substrate concentration |
Increases, then levels off (Vₘₐₓ) |
Enzyme concentration |
| Enzyme concentration |
Increases proportionally (if
substrate excess) |
Substrate concentration |
Summary of learning objectives and key words or phrases
Be able to explain the theory of the effect of changing enzyme
or substrate concentrations
on rate of enzyme catalysed reaction.
be able to explain and interpret graphs of
reaction rate versus concentration of substrate or enzyme.
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