CHAPTER 8: HOW CELLS HARVEST ENERGY
WHERE DOES IT ALL FIT IN?
Chapter 8 builds upon the cell anatomy coverage of Chapter 4 and information about cellular energy
in Chapter 8. It provides students with details of cellular metabolism using cell respiration as a model
pathway. All of the principles covered in Chapters 2 through 7 culminated in this chapter and should
be gently reviewed as concepts in this chapter are covered. The information in Chapter 8 is crucial
for students to understand other concepts that integrate cell function to homeostasis and
development.
SYNOPSIS
Biological endergonic reactions do not occur spontaneously and are generally coupled with
reactions that split energy-carrying molecules like ATP. ATP is not a long-term energy storage
molecule, it is made only when needed. It is an extremely valuable molecule because it is used to
do most of the work in a cell and is used to drive endergonic reactions. Cells generate ATP
through two different processes, substrate level phosphorylation and chemiosmosis. The
substrate level phosphorylation produces ATP from ADP and phosphate by association with an
exergonic reaction and is the more ancient process. Chemiosmosis occurs when protons pumped
out through specific transmembrane channels re-enter through other channels coupled to ATP
synthesis. Most biological ATP is produced in this manner.
Glycolysis occurs in the cytoplasm of a cell and is catalyzed by enzymes not associated with any
membranes or organelles. Glucose is converted to two glyceraldehyde-3-phosphate (G3P)
molecules in a reaction that costs two ATP molecules. G3P is then converted to pyruvate and
produces four ATPs via substrate level phosphorylation, a process that occurs with or without
oxygen. In addition, a pair of electrons and one proton are removed from G3P reducing the
coenzyme NAD+ to NADH. The net energy yield at this point is two ATPs per glucose.
Glycolysis continues as long as there is a fresh supply of glucose and there is sufficient NAD+. It
is advantageous for a cell to do something with its NADH other than allowing it to build up
because its supply of NAD+ is generally limited. NADH returns to NAD+ through aerobic
respiration.
The process of aerobic respiration includes the oxidation of pyruvate to acetyl-CoA and the
Krebs cycle. Pyruvate is oxidized to a two-carbon molecule, acetyl-CoA, one NAD+ is reduced
to NADH and one molecule of CO2 is given off. This reaction occurs within the mitochondria of
eukaryotes or on special membranes in a few bacteria. The Krebs cycle is a complex set of
reactions in which a four-carbon molecule is added to the acetyl-CoA from pyruvate oxidation.
During the cycle, two molecules of CO2 are given off and three NADH, one FADH2, and one
ATP are produced. These quantities are, of course, for a single molecule of pyruvate. The
degradation of a whole molecule of glucose produces twice the quantity of each substance.
Oxidative respiration in itself produces no more ATP than glycolysis, but it becomes highly
efficient only when it is coupled to the fourth stage, the chemiosmotic generation of ATP via an
electron transport chain. This process occurs on the inner mitochondrial membrane, requires
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oxygen as a final electron acceptor, and therefore occurs only in aerobic organisms. In theory,
each NADH from the oxidative respiration (a total of eight per glucose) activates three pumps
and produces three ATPs (a total of 24). Each FADH2 from the Krebs cycle (two per glucose)
activates two pumps and generates two ATPs (a total of four). The cell uses one ATP to get the
NADH from glycolysis (a total of two) into the mitochondrion, thus the net value of each is only
two ATPs (a total of four). Overall, glycolysis plus complete oxidative respiration produces 32
ATPs via chemiosmosis and four ATPs by substrate level phosphorylation. In actuality, the
mitochondrial membrane is leaky, and only 2.5 ATPs are produced per NADH and 1.5 per
FADH2. Thus, on average, closer to 30 ATP are produced by chemiosmosis in the electron
transport chain.
Proteins and fats are also metabolized. Proteins provide the same efficiency as glucose as
constituent amino acids are converted to participants in the Krebs cycle. Fats are metabolized via
b-oxidation during which two carbon chunks are converted to acetyl-CoA, NADH, and FADH2
molecules. A six carbon fatty acid molecule produces 36 actual ATP compared to 30 actual from
a six carbon sugar. The NADH produced in glycolysis also returns to NAD+ through various
anaerobic fermentations. A carbohydrate serves as the final electron acceptor in most
fermentations. Products of familiar eukaryotic fermentations include ethyl alcohol and carbon
dioxide by yeast and lactic acid by overworked muscle cells.
The stages of cellular respiration have evolved over time through 6 major events, each building
on what had come before.
LEARNING OUTCOMES
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Understand the value of ATP in biological metabolic reactions.
Describe two ways in which cells generate ATP and indicate which is a more efficient process.
Know the location of glycolysis in a generalized eukaryotic cell.
Understand the differences in the glycolytic reaction under anaerobic and aerobic conditions.
Describe glycolysis in general terms, including the molecules that exist at its start and its end, as
well as its net versus total ATP production.
Understand the mechanisms that recycle postglycolytic NADH molecules under anaerobic and
aerobic conditions.
Explain why the electron-carrier molecule produced in glycolysis and the two kinds of molecules
produced in the Krebs cycle differ from one other in the number of ATP each generates.
Compare the overall energy efficiency of the complete aerobic degradation of one molecule of
glucose with the efficiency of the glycolytic process alone.
Understand how proteins and fats are metabolized.
Compare the number of ATPs produced in the degradation of carbohydrates, proteins, and fats.
Describe alcoholic fermentation in terms of electron acceptors, ATP yield, and end products.
Explain the process of anaerobic respiration as it occurs in human muscle.
Describe the 6 major innovations that highlight the evolution of metabolism.
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COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 8 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students believe respiration is the same as breathing
Students believe food is anything that goes into the organism including minerals, water,
carbon dioxide
Carbon dioxide is formed from oxygen during cellular respiration
Water is not a product of cellular respiration
Oxygen burns the food molecules during cellular respiration
Glycolysis is the same as fermentation
Cellular respiration always breaks down glucose completely and always makes 36 ATP
per molecule
All of the energy produced by mitochondria is converted to ATP
The Krebs cycle takes place in the cytoplasm
Each glucose molecule undergoes one Krebs Cycle and ETC event
ATP gives energy to a cell
ATP is produced in the body and not in the cell
Only carbohydrates are involved in ATP production
Glucose is the only molecule that fuels ATP
ATP production increases with calories taken in the diet
Lipids are solely consumed during weight loss
Enzymes are so highly selective that they only bind to one type of substrate
Enzymes cannot bind to products of their reaction
Dietary calories (kilocalories) are the same as the standard calorie unit
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Another analogy to help explain the value of controlled pathways is jumping off a skyscraper.
Without controlled pathways, i.e., stairs, you go splat! With a controlled pathway, you may be a
little out of breath and have tired legs, but you get down in one piece. Even if you stumble or fall
down a few, at least you are not dead! Also, unless you are Superman, you’re not going to be
able to leap tall buildings in a single bound and do the pathway in reverse. It is possible, albeit
exhausting, to climb the stairs.
There must be a clear differentiation between the generation of ATP via substrate-level
phosphorylation and chemiosmosis. The former occurs directly in the cytoplasm. The latter
requires a membrane bound proton pump. In eukaryotes, chemiosmosis is associated with the
mitochondria, the electron transport chain and oxygen as the final electron acceptor. If any of
these are not present, NADH will not be converted to ATP. It is important to stress where each
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of the stages of cellular respiration occur, how ATP from each is generated, and whether it relies
on NAD+ or FAD+ as electron carriers. Figures 9.7 and 9.12 are especially valuable in presenting
“the big picture.”
Remind students that the products of the Krebs cycle must be doubled for each molecule of
glucose degraded. However, the products of glycolysis are not doubled. Also remind them to be
careful in converting NADH to ATP. Those produced in the cytoplasm (through glycolysis) are
worth only 2 theoretical (or 1.5 actual) ATP, those produced in the mitochondria are worth 3
theoretical (or 2.5 actual) ATPs. You may find it valuable to color-code or specially label the
energy products of each stage as you go through. Instruct the students to do the same in their
class notes.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with
the tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 8.
Application
Analysis
Synthesis
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Have students explain why a person does not need carbohydrates as a
source of ATP.
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Have students describe a possible mechanism for the observation that
high levels of carbon dioxide cause yeast to switch from aerobic to
anaerobic respiration.
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Ask students explain why long-term high protein diets can result in
kidney damage.
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Ask students to explain the activity of the glycolysis in organisms that
obtain energy from diets consisting of lipids and proteins.
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Ask students to explain the activities of cellular respiration if oxygen
uptake is block by carbon monoxide poisoning.
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Ask students to hypothesize why it is normal for ATP to inhibit the Krebs
cycle.
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Ask students predict the commercial applications of a technique that
causes mitochondria to transport electrons to a metal ion instead of
oxygen.
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Have students explain agricultural researchers have evidence that almost
all of the metabolic rate of offspring comes from the female parent.
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Ask students to explain the changes in how cellular respiration operates in
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diabetics who are not able to shuttle glucose rapidly into cells.
Evaluation
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Ask students to evaluate the benefits an athlete may derive by resting and
eating lots of carbohydrates a day before a high performance athletic
event.
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Ask students to refute the claim that consuming sports drinks containing
phosphorus increases the body’s ability to produce ATP.
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Have the students evaluate the claim the diets high in fructose are just as
effective at providing energy for the body as high glucose diets.
VISUAL RESOURCES
Prepare bunches of colored cards, a different color to represent ADP, ATP, NAD+, NADH,
FAD+, and FADH2. This currency can be distributed as you present each stage of cellular
respiration. For example, a bacteria cell ready for fermentation starts with two ATPs and two
NAD+. Take the two ATPs to start glycolysis. Then take the two NAD+ and give out two NADH
and four ATPs. When you try to run through glycolysis again, there are no NAD+. After
alcoholic fermentation occurs, trade the two NADH for two NAD+ and do another glycolysis
reaction. Aerobic respiration can be done in a similar manner.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
Name That Molecule Game
Introduction
A mock game show is a fun and effective way to review the pathways of cellular
respiration. Encourage the students to come up with questions that would test the knowledge of
their classmates
Materials
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Large index cards
Thick black marker
Student volunteers
Procedure & Inquiry
1. Label the index cards to represent one concept of cellular respiration. Possible labels are:
a. Glucose
b. Fructose-6-phosphate
c. ATP synthase
d. Mitochondrial inner membrane
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2.
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5.
6.
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e. Pyruvate
Announce to the class that a student will be called down to class to receive an index card
containing the name of a molecule or structure associated with cellular respiration. The
label cannot be seen by the class.
Then tell the class that they have to figure out the molecule or structure based on a
statement of fact made by the student holding the card.
Ask a student to come down to the room and hand them a labeled index card so that the
card is not facing the class
Tell the student that they must give the class some fact about the molecule or structure
without giving away its name.
The class must guess the answer based on the student’s facts. Inaccurate hints should be
brought up and corrected by other students.
A correct answer results in another student coming down to pick up a different labeled
card.
USEFUL INTERNET RESOURCES
1. Case studies are excellent for reinforcing scientific concepts. The University of Buffalo
produced a study using anemia as a way of understanding the signs of metabolic disease.
This case study can be done in class or be given as a take-home. The case study can be
found at http://www.sciencecases.org/ida/ida.asp.
2. Students need to be reminded that the mitochondria, which are responsible for much of
the metabolism covered in this chapter, are “little creatures” living within the cell. A case
study called “Little Mito”, produced for the University of Buffalo gives a humor account
of the origins of mitochondria. The website is at
http://www.sciencecases.org/mito/mito.asp.
3. Animations are effective ways of reinforcing lecture content. Virtual Cell Animation
Collection, by North Dakota State University has a collection of animations that model
cellular metabolism. The website can be found at:
http://vcell.ndsu.nodak.edu/animations/.
4. Cellular metabolism can be an overwhelming topic for students. Many students perceive
that the processes of cell respiration alone are very complex processes. However, cell
respiration is only on small component of all the metabolic pathways taking place in a
cell. Main Metabolic Pathways for Internet displays a flowchart of the major metabolic
pathways eukaryotic animal cells. The chart can be found at
http://home.wxs.nl/~pvsanten/mmp/mmp.html.
LABORATORY IDEAS
A. Specificity of Glycolysis
Have students perform a simple experiment in which students investigate the ability of
yeast to metabolize different sugars and sweeteners.
a. Tell the class that they will be using yeast as a model for measuring the ability for
various sugars and sweeteners to be used in glycolysis.
b. Students should be able to make a setup that that evaluates metabolic waste
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c.
d.
e.
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production. They can come up with a way to detect carbon dioxide by looking at
bubbles forming in solution or by measuring the drop of pH in the solution due to
the buildup of carbonic acid.
Provide students with the following materials:
i. Fast rising baker’s yeast
ii. Test tube rack
iii. 6 test tubes
iv. pH paper
v. Distilled water (negative control)
vi. 0.5% d-glucose solution (positive control)
vii. 0.5% l-glucose solution
viii. 0.5% fructose solution
ix. 0.5% saccharine solution
x. 0.5% Splenda solution
Instruct the students to design an experiment to test if yeast are able to
metabolism particular sugars or sweeteners.
Tell them that they must first determine which sugar in the samples provided will
be the positive control and how they should compare it to a negative or “no
results” control.
Have the students explain why a particular sugar or sweetener worked or did not
work as a source of material for cellular respiration.
LABORATORY IDEAS
Effect of Temperature on Cellular Respiration
Have students perform a simple experiment in which students investigate the relationship
between temperature and rate of cellular respiration.
a. Tell the class that they will be using yeast as a model investigating the
relationship between temperature and cellular respiration.
b. Students should be able to make a setup that that evaluates carbon dioxide as an
indicator of cellular respiration. They can come up with a way to detect carbon
dioxide by looking at bubbles forming in solution or by measuring the drop of pH
in the solution due to the buildup of carbonic acid.
c. Provide students with the following materials:
i. Fast rising baker’s yeast
ii. 0.5% glucose solution
iii. Test tube rack
iv. Test tubes
v. Thermometer
vi. pH paper
vii. Hot plate
viii. Ice bath
g. Instruct the students to design an experiment to test the relationship between
temperature and cellular respiration.
h. Tell them that they must first determine what would be an appropriate
temperature range to test.
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i. Ask the students to see if their estimates match the literature about the rate of
change of enzymatic reaction in relationship to temperature.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
Students who have successfully mastered the content of Chapter 8 can apply their knowledge for
service learning activities in the following ways:
1. Have students talk to youth sports groups about the risks of high-protein diets.
2. Have students design prepare an electronic presentation for school teachers about the
pathways of cellular respiration.
3. Have students tutor middle school or high school biology students studying cell
metabolism.
4. Have students give demonstrations to boy scouts or girl scouts using yeast a model for
understanding commercial applications of cellular respiration.
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S.
Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and
does not discriminate on the following basis:
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against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political
affiliation or belief; and
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against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on
the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States,
or his or her participation in any WIA Title I-financially assisted program or activity.
This workforce solution was funded by a grant awarded under the President’s CommunityBased Job Training Grants as implemented by the U.S. Department of Labor’s Employment
and Training Administration. The solution was created by the grantee and does not
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respect to such information, including any information on linked sites and including, but not
limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy,
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copyright owner.
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