Cellular Respiration—An Overview

What are the phases of cellular respiration?

Why?
All cells need energy all the time, and their primary source of energy is ATP. The methods cells use to
make ATP vary depending on the availability of oxygen and their biological make-up. In many cases the
cells are in an oxygen-rich environment. For example, as you sit and read this sentence, you are breathing
in oxygen, which is then carried throughout your body by red blood cells. But, some cells grow in envi-
ronments without oxygen (yeast in wine-making or the bacteria that cause botulism in canned food),
and occasionally animal cells must function without sufficient oxygen (as in running sprints). In this
activity you will begin to look at the aerobic and anaerobic processes that are used by all organisms to
produce ATP.

Model 1 – Cellular Respiration

Cell Membrane

Oxidative
Phosphorylation
Cytoplasm 34 ATP
6 O2
6 H2O

Krebs
Glucose 2 Pyruvate 2 Acetyl- Cycle
Glycolysis Link CoA ×2

Mitochondrion
2 ATP 2 CO2 4 CO2 2 ATP Inner
Membrane
Inner
Matrix

2 NADH 2 NADH 6 NADH

2 FADH2

C6H12O6 + 6O2  → 6CO2 + 6H2O + Energy

  1. According to Model 1, what are the reactants of cellular respiration?

Glucose and Oxygen

  2. According to Model 1, what are the products of cellular respiration?

Carbon dioxide, water, and energy(ATP)

Cellular Respiration—An Overview 1

   3. Cellular respiration occurs in four phases: glycolysis, the link reaction, the Krebs cycle, and
oxidative phosphorylation.
a. Which phase of cellular respiration occurs in the cytoplasm of the cell?
Glycolysis

b. Which phases of cellular respiration occur in the mitochondria?

The Krebs cycle

c. Which of the four phases of cellular respiration require oxygen?

Oxidative Phosphoralation

d. Which of the four phases of cellular respiration produce carbon dioxide?

The Krebs cycle

e. Which of the four phases of cellular respiration produce water?

Oxidative Phosphorylation
  4. The goal of cellular respiration is to provide the cell with energy in the form of ATP.
a. Which of the four phases of cellular respiration result in the production of ATP?
Glycolysis

b. How many ATPs (total) are produced for every glucose molecule that undergoes cellular
respiration?
38 ATP Molecules

c. What reactants of ATP must be available in the cell in order to produce ATP?
ADP and P

d. Brainstorm several cellular processes for which energy or ATP is necessary.

Chemical reactions within the body, muscle movement, etc.

Read This!
Glucose, or any carbon-based molecule, can be burned in oxygen (oxidized) to produce carbon
dioxide and water. Combustion reactions release large amounts of energy. However, the energy release is
uncontrolled. An organism would not be able to handle all that energy at once to do the work of the cell.
Cellular respiration is essentially the same reaction as combustion, but the oxidation of glucose occurs in
several controlled steps. The same amount of energy is ultimately released, but it is gradually released in
small, controlled amounts. High potential energy molecules of ATP are produced while the carbon atoms
are used to form various other molecules of lower potential energy. Each of these steps is catalyzed by an
enzyme specific to that step. Model 1 illustrates the ideal circumstances for cellular respiration. In some
situations, however, one glucose molecule may not result in 38 ATP molecules being produced.
  5. Consider Model 1. Besides ATP, what other molecules appear to be high potential energy
molecules (free energy carriers) during cellular respiration?
NADH and NADPH

2 POGIL™ Activities for AP* Biology

Model 2 – Electron Acceptor Molecules
2e-
______ + NAD+ +  H+ ⎯→ NADH
2e-
______ + FAD + 2H+ ⎯→ FADH2
2e-
______ + ½O + 2H+ ⎯→ H O 2 2

  6. Nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) are
coenzymes used in cellular respiration to transport high potential energy electrons to the electron
transport chain (a step in oxidative phosphorylation) in the mitochondria. At the conclusion of
cellular respiration, oxygen is the final electron acceptor. The reactions in Model 2 show these
electron acceptors in the process of picking up an electron.
a. How many electrons are in a hydrogen ion (H+)?
None

b. Is a hydrogen ion with its positive charge likely to be attracted to NAD+, FAD or O2 without
an input of free energy? Explain your reasoning.
No, it would only be attracted to negativly charged particles

c. Consider the charges in the first reaction in Model 2. Can two positive particles combine to
form a neutral particle? Explain your reasoning.
No, it would results in a 2+ charge ion instead of a neutral charge ion

  7. The reactions in Model 2 are missing very important particles—electrons.

a. What is the charge on an electron?
Negative

b. For each reaction in Model 2, predict the number of electrons that must be involved in the
reaction to make the charges in the reaction balance. 2 electrons
c. Add electrons to each reaction in Model 2 on either the reactant or product side of the
equation to complete the reactions.

Read This!
Oxidation is a loss of electrons. Reduction is a gain of electrons. The two processes must go hand-in-hand.
In other words, electrons cannot be added to something from thin air, they must have been taken off of
something first.
  8. Are the reactions in Model 2 oxidation or reduction reactions?

Reduction reaction because electrons were being gained within the reaction

  9. Which form of nicotinamide adenine dinucleotide is the “reduced” form, NAD+ or NADH?
Explain your reasoning.

NADH because it is being reduced down to NAD+ for the extra energy required

Cellular Respiration—An Overview 3

 10. According to Model 1, glucose undergoes the following changes during cellular respiration. In
each step NADH or FADH2 is produced. Is glucose being oxidized or reduced during cellular
respiration? Explain your reasoning.
Glucose 
⎯→ pyruvate ⎯→ acetyl-CoA ⎯→  carbon dioxide
Glucose is oxidized in this reaction due to it losing electrons to NADH

11. Historically, the term “oxidation” was derived from the combination of a substance with oxygen.
Question 10 addressed which molecule is the “more oxidized form.” Validate your answer to
Question 10 by comparing the ratio of carbon atoms to oxygen atoms in the chemical formulas
of glucose (C6H12O6) versus carbon dioxide (CO2).
Since carbon dioxide contains more oxygen per atom, it is the oxidized form of oxygen, resulting in an
oxidized reaction

12. Model 1 shows ten NADH molecules being produced during cellular respiration. What reactants
must be available in the cell for these molecules to be produced?
NAD+ and H+

13. The hydrogen ions and electrons that were carried by NADH and FADH2 are used in oxidative
phosphorylation.
a. What molecules are produced as the hydrogen ions and electrons are removed from NADH
and FADH2?
NAD+ and FAD

b. Is the removal of hydrogen ions and electrons from NADH and FADH2 oxidation or reduc-
tion? Explain your reasoning.

An oxidation reaction because NADH is losing electrons to the reaction

c. Predict what happens to those product molecules after they “drop off ” their hydrogen ions
and electrons.
They get recycled back into the cycle to be used in the Krebs and Glycolysis reactions

14. Cells can survive for short periods without oxygen. Only the glycolysis phase of cellular respira-
tion occurs in those circumstances.
a. Predict the number of ATP molecules that could be produced from one glucose molecule if
oxygen were not available.
2 ATP molecules

b. Since oxidative phosphorylation no longer occurs when oxygen is not available, predict what
would eventually happen to the supply of NAD+ in the cell if only glycolysis were occurring?
If NAD+ was the only source f energy, soon after glycolysis were to occur, it would run
out of free energy to use and glycolysis would no longer be able to occur

4 POGIL™ Activities for AP* Biology

Model 3 – Energy in Anaerobic Environments

Glucose 2 Pyruvate 2 Alcohol Glucose 2 Pyruvate 2 Lactic Acid

Glycolysis Fermentation Glycolysis Fermentation

2 ATP 2 NAD+ 2 CO2 2 ATP 2 NAD+

2 NADH 2 NADH

15. Examine the two anaerobic processes in Model 3. Is oxygen used in either process?
No

16. What is the definition of anaerobic?

Without Oxygen

17. Compare the energy output (in the form of ATP) for a single glucose molecule that undergoes
glycolysis and fermentation to that of a glucose molecule undergoing cellular respiration.

Compared to cellular respiration, glycolysis does not produce nearly enough ATP. Glycolysis makes about 2 ATP molecules,
where as cellular respiration produces abut 38 ATP molecules.

18. 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 Model 3 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.

If NADH was not recycled back into the cycle, then a shortage of NAD+ would occur and and glycolysis would no longer be
able to happen.

Cellular Respiration—An Overview 5

Extension Questions
19. The muscle “burn” that you feel when doing strenuous activity (sprints for example) is caused
by a buildup of lactic acid in the muscle tissue of your body. Explain this phenomenon in the
context of cellular respiration and fermentation.
When you are doing strenuous activity, instead of a proper supply of oxygen being there for the muscle to use with cellular
respiration, lactic acid fermentation occurs

20. The evolution of photosynthesizing organisms on Earth and the development of an oxygen-rich
environment led to a rapid diversification of life. Explain why there is an evolutionary advantage
to an organism that requires oxygen to live compared to one that does not require oxygen.
With an organism that can metabolize glucose, they are gong to get far more energy in a more efficient way per
molecule of glucose, compared to organisms that use anaerobic processes to gain energy.

21. Under laboratory conditions muscle cells were broken up and separated into fractions of mito-
chondria and cytoplasm in an attempt to learn more about cellular respiration. Each fraction was
incubated with glucose or pyruvate. 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 incubated with glucose Absent Absent
Mitochondria incubated with pyruvate Present Absent
Cytoplasm incubated with glucose Absent Present
Cytoplasm incubated with pyruvate Absent Present
a. What does the presence of lactic acid in a sample indicate about what process is occurring in
each cell fraction?
Fermentation

b. Explain why lactic acid was produced by the cytoplasm fraction incubated with glucose, but
not the mitochondrial fraction.
Since cytoplasm is the site of ocurance for glycolysis and cellular respiration, this is where lactic acid is produced and
synthesized there.

c. Why was no carbon dioxide produced by either fraction incubated with glucose?

CO2 cannot be used by the mitochondria and cannot be converted into glucose within there.

6 POGIL™ Activities for AP* Biology

 d. Why did the cytoplasm fraction produce lactic acid in the presence of both glucose and
pyruvate?
Since lactic acid is a product of fermentation, and this start with a pyruvate, this leads to pyruvic acid. Pyruvic
acid can allow for the synthesis of glucose, showing how no matter what is present, lactic acid can be produces
from both base molecules.

e. Why did the mitochondria produce carbon dioxide in the presence of pyruvate but not in the
presence of glucose?

When the mitochondria, in the presence of pyruvate allowed for the krebs cycle to take place, it produced CO2 as a
product, but not as a reactant. The mitochondria used the free glucose instead of CO2 for energy because it is more
efficient at lowering activation energy.

Cellular Respiration—An Overview 7