Notes to Instructors
Chapter 8 An Introduction to Metabolism
What is the focus of these activities?
Many students study biology as if it were a foreign language. For them, each new topic or
idea is distilled into separate definitions. Each is then written on a flash card to be
What are the particular activities designed to do?
Activity 8.1 What factors affect chemical reaction in cells?
This activity asks the students to construct a concept map that integrates their
Activity 8.2 How can changes in experimental conditions affect enzyme-mediated
reactions?
This activity asks students to apply their general understanding from Activity 8.1 to
What misconceptions or difficulties can these activities reveal?
Activity 8.1
If done in small groups in class, the concept mapping activity can be used to reveal both
36 Notes to Instructors
Question 1: Because they tend to memorize terms and definitions separately, many
students don’t recognize that a single reaction can simultaneously be spontaneous,
catabolic, and exergonic and have a negative G.
Question 2: For years the dogma held that all enzymes were proteins. This question can
be used to point out that we now know that some enzymes are made of RNA.
Question 3: This question points out the types of factors that can affect enzyme function.
Students need to make the logical leap that for cells to maintain their metabolism at
optimal levels, they must be able to regulate their internal environment. Many students do
not make this connection without additional help or suggestions.
Activity 8.2
This set of experimental situations gives students practice in making the connection
Answers
Activity 8.1 What factors affect chemical reactions in cells?
Construct a concept map of general metabolism using the terms in the list below. Keep in
mind that there are many ways to construct a concept map.
• Begin by writing each term on a separate sticky note or piece of paper.
Here is an example:
Activity 8.1 37
Sun
provides
energy for which
occurs in
Photosynthesis Plants
Terms
38 Activity 8.1
peptide bonds
proteins
helix
activation energy
G/ free energy
endergonic
activator
four-step enzyme-mediated
reaction sequence or
Use the understanding you gained from doing the concept map to answer the
questions.
1. Reduced organic compounds tend to contain stored energy in C—H bonds. As a
general rule, the greater the number of C—H bonds, the greater the amount of
potential energy stored in the molecule. Answer each question in the chart as it
relates to the two reactions shown at the top. Be sure to explain the reasoning behind
your answers.
Reaction 1:
CH42 O2 → H2O CO2
(methane)
Reaction 2:
6 CO26 H2O → C6H12O6 6 O2
a. Is the reaction
exergonic or
endergonic?
Exergonic
This reaction releases
energy.
Endergonic
This reaction requires
energy.
2. All metabolic reactions in living organisms are enzyme mediated. Each enzyme is
specific for one (or only a very few similar types of) reaction. Given this, there are
approximately as many different kinds of enzymes as there are reactions.
a. What characteristics do all enzymes share?
b. What characteristics can differ among enzymes?
Before ribozymes were discovered, we thought that all enzymes were composed
3. How can enzyme function be mediated or modified? To answer, complete a and b
below.
Activity 8.1 39
a. What factors can modify enzyme
function?
b. What effect(s) can each of these
factors have on enzyme function?
Among other factors, the temperature
and pH at which an enzyme is active can
vary.
Each enzyme has a range of
temperatures in which it functions.
(Refer to Figure 8.16.)
c. What role(s) can modification of enzyme function play in the cell?
To maintain optimal levels of enzyme activity and therefore metabolism, cells
must maintain a relatively constant internal environment (homeostasis). On the
Activity 8.2 How can changes in experimental conditions
affect enzyme-mediated reactions?
1. You set up a series of experiments to monitor the rates of a reaction. The reaction is
an enzyme-mediated reaction in which A →B + C. For each experiment in this
series, you continuously add the reactant A and monitor its concentration so that the
amount of A remains constant over time.
For each group of experiments, explain how the differences in experimental
conditions could affect the reaction.
a. You compare two side-by-side experiments. In experiment 1, you use Xamount
of the enzyme. In experiment 2, you use 2Xamount of the same enzyme.
If you double the amount of enzyme present, you double the rate of accumulation
b. You compare two side-by-side experiments. In both you use equal amounts of the
enzyme. In experiment 3, you allow the products to accumulate over time. In
experiment 4, you remove the products from the system as they are produced.
Recall that most metabolic reactions are reversible, and for these reactions, the
c. In the next two experiments, you use equal amounts of the enzyme. You run
experiment 5 at 20°C and experiment 6 at 25°C.
As noted in Figure 8.16, each type of enzyme is functional within a set range of
d. In two final experiments, you use equal amounts of the enzyme. You run
experiment 7 at pH 6 and experiment 8 at pH 8.
Similar to part c, each enzyme has an optimal pH and a range of pH in which it is
2. Enzyme function can be inhibited or regulated by the presence of chemicals that
mimic either the reactants or the products.
a. How do competitive and noncompetitive inhibition of an enzyme differ?
Refer to Figure 8.17. A competitive inhibitor can bind at the active site of an
b. What are allosteric enzymes? What function(s) can they serve in reaction
sequences?
Refer to Figure 8.19. Allosteric enzymes are usually made up of more than one
Activity 8.2 41
3. An enzyme catalyzes the reaction X →Y + Z. In a series of experiments, it was
found that substance A inhibits the enzyme.
• When the concentration of X is high and A is low, the reaction proceeds rapidly
and Y and Z are formed.
• As the concentration of A increases, the reaction slows regardless of whether X is
present in high or low concentration.
• If the concentration of A is high (relative to X), the reaction stops.
• If the concentration of A again decreases, the reaction will ultimately resume.
What type of enzyme regulation is described here? Explain or justify your answer.
The enzyme in the reaction described, X →Y + Z, is most likely an enzyme that has
4. In an enzymatic pathway, A, B and C are intermediates required to make D and 1, 2,
and 3 are enzymes that catalyze the designated reactions:
123
A →B →C →D
E
This is analogous to what happens in a factory. In a leather goods factory, for example,
the leather (A) is cut (1) into the parts needed for shoes (B). The shoe parts are sewn
(2) together (making C), and C is packaged (3) for shipping as D. Now shoe sales are
dropping and backpack sales (E) are increasing. As a result, the manager of the factory
decides to switch production from shoes to leather backpacks (E).
a. Where should the shoe-making process be shut down: step 1, 2, or 3? Explain.
The shoe-making process should be shut down at step 1. It makes no sense to
b. In a cell, if an excess of a chemical product D arises, where should this synthetic
pathway be shut down in the cell?
If we use the same logic as in part a, then the synthetic pathway A →B →C →D
c. What type(s) of enzyme regulation is/are most likely to occur in the cell in this
type of feedback system?
42 Activity 8.2
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