AP Biology: Notes and Practice
Anaerobic Respiration
When oxygen is not present, cells can still extract energy from glucose. Many cells, including human muscle cells
can use the anaerobic (without oxygen) pathway known as lactic acid fermentation. Other organisms, such as
yeast, can use alcoholic fermentation. These pathways produce much less ATP than cellular respiration, even
though fermentation itself does not produce any ATP. In animals, fermentation can only supply momentary energy
and cannot meet long term energy demands.
Figure 1: Equations for Fermentation
Fermentation
in yeast and
bacteria
Fermentation
in muscle
Pyruvate + NADH  CO2 + Ethyl Alcohol + NAD+
Pyruvate + NADH  Lactic Acid + NAD+
1. Explain why the above reactions displayed in Figure 1 are considered anaerobic.
2. Refer to the reactions in Figure 1 to answer the following questions:
a. Based on the reactants for both fermentation reactions, what cellular process occurs BEFORE
these reactions take place in the cell? Explain your reasoning.
b. If you start with one glucose molecule, how many ATPs are made during a fermentation reaction?
Explain your reasoning.
c. Where in the cell can you predict that fermentation takes place? Explain your reasoning.
AP Biology: Notes and Practice
3. The efficiency of a cell is dependent on the cyclic nature of NAD + and NADH. Building NAD+ molecules
from raw materials each time one was needed would require a huge amount of free energy and
resources. Even though the fermentation steps shown in Figure 1 do not provide any ATP, they are
critical to the energy production of the cell. Predict what would happen to the energy supply in a cell if
fermentation did not happen under anaerobic conditions. Explain your reasoning.
4. Compare Cellular (Aerobic) respiration to Anaerobic respiration based upon how much ATP can be
produced from a single glucose molecule.
5. Using your answer to question 4, assuming that one molecule of glucose is broken down every hour for
a 6-hour period; create a table depicting how much ATP is produced cumulatively via both methods
6. Notice that lactic acid fermentation can occur in human muscle cells. If humans can do fermentation,
why would a person die from lack of oxygen?
AP Biology: Notes and Practice
7. Under laboratory conditions, muscle cells were broken up and separated into fractions of mitochondria
and cytoplasm in an attempt to learn more about cellular respiration. Each fraction was incubated
with glucose or pyruvate. No oxygen was provided to any sample. Tests were carried out during
incubation for the presence of either carbon dioxide or lactic acid. The results are shown below.
Cell Fraction
CO2
Lactic Acid
Mitochondria + Glucose
Absent
Absent
Mitochondria + Pyruvate
Present
Absent
Cytoplasm + Glucose
Absent
Present
Cytoplasm + Pyruvate
Absent
Present
a. What is glycolysis and where does it take place?
b. What is the Krebs Cycle and where does it take place?
c. What does the presence of CO2 in a sample indicate about the processes that were able to
occur? Explain your answer.
d. What does the presence of Lactic Acid in a sample indicate about the processes that were able
to occur? Explain your answer.
e. Based on both the results given and your answers to parts A-D, explain the presence or absence
of CO2 and lactic acid for each cell fraction tested. Provide reasoning for your claim.