AP Biology: Notes and Practice
Metabolism
Chemical reactions in living systems involve changes in energy. These can be expressed as changes in available energy,
called free energy (ΔG). This change in energy can be calculated simply, by determining the amount of energy contained
in the products and reactants and finding their difference (Gproducts – Greactants). Reactions are classified as exergonic or
endergonic, depending on the change in energy. The diagram below displays an example of each type of reaction.
EXERGONIC REACTIONS
1. Based on the diagram provided, which molecules (reactants or products) contain more energy? Based on your
answer, generalize if the ΔG value for an exergonic reaction will be positive or negative. Explain your answer.
2. Both synthesis (building polymers) and decomposition (breaking down polymers) reactions occur in living organisms.
According to the figure, which of these reactions is most likely to be exergonic? Explain how you know.
ENDERGONIC REACTIONS
3. Based on the diagram provided, which molecules (reactants or products) contain more energy? Based on your
answer, generalize if the ΔG value for an endergonic reaction will be positive or negative. Explain your answer.
4. Both synthesis and decomposition reactions occur in living organisms. According to the figure, which of these
reactions is most likely to be endergonic?
AP Biology: Notes and Practice
CONNECTING ENDERGONIC WITH EXERGONIC
5. Large molecules containing many bonds tend to contain more energy in them than small molecules with few bonds.
For example, a starch molecule (a carbohydrate polymer) contains much more energy than a glucose molecule (a
single sugar monomer). Is this information supported by the data contained in the figures? Explain your answer.
6. Based on the figures and the information provided in question 7, in which type of reaction does energy serve as a
reactant, meaning it must be added during the reaction? Explain how you know.
7. Based on the figures and the information provided in question 7, in which type of reaction does energy serve as a
product, meaning it can be let out of the reaction? Explain how you know.
8. In general, which type of reaction tends to increase order or organization of molecules? Which type of reaction
results in less order or disorganization of molecules? Explain how you know.
9. Based on the information provided in question 7 and your answers to questions 8, 9 and 10, complete the following
tasks for the reactions listed below:
1. Determine if the reaction is endergonic or exergonic and explain your reasoning
2. Write energy into the reaction as a reactant or product

C12H22O11 + H2O  C6H12O6 + C6H12O6

C3H8O3 + C8H16O2  C11H22O4 + H2O

6CO2 + 6H2O  C6H12O6 + 6O2
AP Biology: Notes and Practice
Metabolism and Enzymes
To fuel the processes of life, living organisms are dependent on the molecule adenosine triphosphate (ATP). ATP is
decomposed into ADP and inorganic phosphate (Pi) throught the process of hydrolysis which releases energy. This
energy can be used by the living organism. An energy diagram for the decomposition of ATP is displayed below. The xaxis indicates the course of the reaction over time, and the y-axis indicates the amount of energy in the reactants, products,
and intermediate structure (when the reactants are transitioning to products).
1. Is the decomposition of ATP into ADP and Pi endergonic
or exergonic? Explain what this means and how you
know.
Activation Energy (Ea):
The amount of energy
required to initiate a
biochemical reaction
2. Organisms depend on two “energy rules” in order to
sustain life. First, organisms must maintain order or
organization. An increase in order occurs when organisms
build polymers, organizing monomers into a long chain.
Second, organisms must NOT release more energy than
they consume. What about the decomposition of ATP
poses a problem for these dependancies?
3. Fortunately, biochemical reactions rarely occur in isolation; rather, the energy released from one reaction can be used
to fuel another. For example, the amino acid glutamine (C5H10N2O3) must be synthesized in the body on a regular
basis for use in building proteins. The reaction by which this synthesis occurs follows below:
C5H9NO4 + NH3  C5H10N2O3 + H2O
ΔG = +14.2 kJ/mol
a. Is glutamine synthesis endergonic or exergonic? Give at least 2 reasons.
b. If one mole of ATP decomposes, releasing 30.5 kJ of energy, this released energy can be used to produce
glutamine. How many moles of glutamine can be synthesized if one mole of ATP decomposes? Explain your
reasoning.
AP Biology: Notes and Practice
c. How can decomposition reactions (like ATP decomposition), which usually create disorder, still promote
order in living systems?
d. Though an organism is constantly using ATP, this molecule is renewable, meaning it can be regenerated by
the synthesis of ADP and Pi. Predict whether this synthesis reaction is endergonic or exergonic and explain
why.
4. The diagram displayed on the previous page displays the
reaction process when ATP breaks down into ADP and Pi. In
reality, ATP does this with the help of an enzyme called
ATPase. The energy diagram to the right displays the course
of the decomposition reaction in both the presence and
absence of ATPase.
a. What changes does the presence of ATPase cause to
the course of this reaction? What aspects of this
reaction are the same whether or not ATPase is
present? Justify your answer by referencing the
figure.
ATPase absent
ATPase present
b. Based on this figure, what can you conclude is the function of an enzyme? Explain your answer.