Chapter 07 HW
11/11/18, 1&58 PM
Chapter 07 HW
Due: 11:59pm on Tuesday, November 13, 2018
You will receive no credit for items you complete after the assignment is due. Grading Policy
Cellular Respiration (2 of 5): Glycolysis (BioFlix tutorial)
In glycolysis, the first stage of cellular respiration, one molecule of glucose is oxidized to two molecules of pyruvate, with the production of ATP and NADH. As you
watch the Glycolysis animation, pay attention to the reactions involved in the production of NADH. These reactions, called redox (oxidation-reduction) reactions,
play a key role in cellular respiration. Also pay attention to the mechanism by which ATP is synthesized. Think about how ATP synthesis at this stage differs from
ATP synthesis during oxidative phosphorylation, where most of the ATP in cellular respiration is made.
Part A - Redox (oxidation-reduction) reactions in glycolysis
In glycolysis, as in all the stages of cellular respiration, the transfer of electrons from electron donors to electron acceptors plays a critical role in the overall
conversion of the energy in foods to energy in ATP. These reactions involving electron transfers are known as oxidation-reduction, or redox, reactions.
Drag the words on the left to the appropriate blanks on the right to complete the sentences.
Hint 1. Redox reactions
Under normal circumstances, redox reactions always occur in pairs. In the first of the two reactions, the electron donor is oxidized (it loses electrons). In
the second reaction, the electron acceptor is reduced (it gains the electrons lost by the first compound). A generic redox reaction showing the transfer of
two electrons is illustrated here.
Note that compounds A and B each exist in two forms: One form is reduced (it carries the extra electrons) while the other form is oxidized (it does not
carry the extra electrons). In the reaction shown here, the electron donor is the reduced form of compound A and the electron acceptor is the oxidized
form of compound B.
Hint 2. The oxidation of carbon-containing compounds
In the net reaction of cellular respiration, glucose (C6H12O6) is completely oxidized to CO2.
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In reality, glucose is not oxidized to CO2 in a single step, as suggested by the net reaction. Instead, there is a step-wise removal of 12 pairs of electrons
from the carbon-containing intermediates of glucose catabolism (one pair at a time) until all of the carbons exist in the form of CO2. Two of the 12 pairs
of electrons are removed in the reactions of glycolysis.
Hint 3. What are the electron carriers in glycolysis?
In glycolysis and the other stages of cellular respiration, electrons removed from intermediates in glucose catabolism are not passed directly to O2.
Instead, electron carriers shuttle the electrons from the oxidation of the carbon-containing compounds to the eventual reduction of O2 to water.
In glycolysis, which molecule picks up the electrons released by the oxidation of glucose?
Hint 1. Redox reactions involving NAD+ and NADH
Recall that compounds involved in redox reactions occur in pairs, an oxidized form and a reduced form. The most common pair of electron
carriers in cellular respiration is NAD+ (the oxidized form) and NADH (the reduced form).
ANSWER:
NADH
O2
ATP
NAD+
ANSWER:
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1. When a compound donates (loses) electrons, that compound becomes oxidized . Such a
compound is often referred to as an electron donor.
2. When a compound accepts (gains) electrons, that compound becomes reduced . Such a
compound is often referred to as an electron acceptor.
3. In glycolysis, the carbon-containing compound that functions as the electron donor is glucose .
water
4. Once the electron donor in glycolysis gives up its electrons, it is oxidized to a compound called
oxygen
pyruvate .
5. NAD+
is the compound that functions as the electron acceptor in glycolysis.
6. The reduced form of the electron acceptor in glycolysis is NADH .
Correct
In the net reaction for glycolysis, glucose (the electron donor) is oxidized to pyruvate. The electrons removed from glucose are transferred to the
electron acceptor, NAD+, creating NADH.
Part B - Energy from glycolysis
Among the products of glycolysis, which compounds contain energy that can be used by other biological reactions?
Hint 1. Energy in carbon-containing compounds
In the overall process of cellular respiration, glucose is oxidized to carbon dioxide and much of the energy generated by the breakdown of glucose is
used to make ATP. In glycolysis, however, glucose is only oxidized to pyruvate; no carbon dioxide is produced in this stage of respiration. In addition,
only a small fraction (<10%) of the total ATP produced by cellular respiration is generated in glycolysis. Consider what this means in terms of the energy
content of pyruvate and its oxidation in subsequent stages of cellular respiration.
Hint 2. ATP-- the energy currency of cells
The interconversion of ATP and ADP + Pi is a key step in the transfer of energy between reactions that release energy and those that require an input of
energy.
Most of the ATP produced in our cells comes from the oxidation of foods in cellular respiration. The reverse reaction, the hydrolysis of ATP to ADP and
Pi, powers most of the energy-requiring reactions that take place in cells, such as active transport and muscle contraction.
Hint 3. What is the fate of NADH produced by glycolysis?
Is the NADH produced in glycolysis used in any other reactions of cellular respiration where ATP is produced?
ANSWER:
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Yes, NADH is an input to the citric acid cycle, where more ATP is produced.
Yes, NADH is an input to oxidative phosphorylation, where more ATP is produced.
Yes, NADH is an input to both the citric acid cycle and oxidative phosphorylation, where more ATP is produced.
No, NADH is used in oxidative phosphorylation, but no additional ATP is produced.
No, NADH is not used in any subsequent stages of cellular respiration.
ANSWER:
pyruvate and ATP only
CO2 only
NADH only
O2 only
ATP only
ATP and NADH only
pyruvate, ATP, and NADH
Correct
ATP is the main product of cellular respiration that contains energy that can be used by other cellular processes. Some ATP is made in glycolysis. In
addition, the NADH and pyruvate produced in glycolysis are used in subsequent steps of cellular respiration to make even more ATP.
Part C - ATP synthesis in glycolysis: substrate-level phosphorylation
The ATP that is generated in glycolysis is produced by substrate-level phosphorylation, a very different mechanism than the one used to produce ATP during
oxidative phosphorylation. Phosphorylation reactions involve the addition of a phosphate group to another molecule.
Sort the statements into the appropriate bin depending on whether or not they correctly describe some aspect of substrate-level phosphorylation in
glycolysis.
Hint 1. The reactions of glycolysis are catalyzed by soluble enzymes
All of the reactions of glycolysis are catalyzed by soluble enzymes located in the cytosol of the cell. None of the enzymes is associated with
membranes.
Hint 2. Oxidative phosphorylation and ATP synthesis
In oxidative phosphorylation, the last stage of cellular respiration, energy released from the oxidation of NADH and FADH2 is used to produce ATP from
ADP and free inorganic phosphate (Pi) ions.
ANSWER:
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Reset
correct statements
Help
incorrect statements
A bond must be broken between an
organic molecule and phosphate
before ATP can form.
An enzyme is required in order for
the reaction to occur.
One of the substrates is a molecule
derived from the breakdown of
glucose.
The enzymes involved in ATP
synthesis must be attached to a
membrane to produce ATP.
The phosphate group added to ADP
to make ATP comes from free
inorganic phosphate ions.
Correct
In substrate-level phosphorylation, an enzyme transfers a phosphate group from one molecule (an intermediate in the breakdown of glucose to
pyruvate) to ADP to form ATP. This is very different from the mechanism of ATP synthesis that takes place in oxidative phosphorylation.
Cellular Respiration (3 of 5): Acetyl CoA Formation and the Citric Acid Cycle (BioFlix tutorial)
Before beginning this tutorial, watch the animation showing
the second and third stages of cellular respiration – acetyl
CoA formation and the citric acid cycle (also known as the
Krebs cycle). In these two stages, the pyruvate produced in
glycolysis is completely oxidized to CO2, with the
production of NADH, FADH2, and some ATP. Consider
carefully the unique cyclic sequence of reactions of the
citric acid cycle and the many redox reactions that occur
within it.
Acetyl CoA Formation.
Cytric Acid Cycle.
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Part A - Carbon atoms in acetyl CoA formation and the citric acid cycle
During acetyl CoA formation and the citric acid cycle, all of the carbon atoms that enter cellular respiration in the glucose molecule are released in the form of
CO2. Use this diagram to track the carbon-containing compounds that play a role in these two stages.
Drag the labels from the left (which represent numbers of carbon atoms) onto the diagram to identify the number of carbon atoms in each
intermediate in acetyl CoA formation and the citric acid cycle. Labels may be used more than once.
Hint 1. How many carbon atoms are in pyruvate and acetyl CoA?
In glycolysis, glucose (a six-carbon molecule) is broken down to form two molecules of pyruvate. Then, during acetyl CoA formation, pyruvate is
converted to acetyl CoA through a series of reactions.
Which statement is true about the number of carbon atoms in the compounds involved in acetyl CoA formation?
ANSWER:
Both pyruvate and acetyl CoA are three-carbon molecules.
Both pyruvate and acetyl CoA are two-carbon molecules.
Pyruvate is a six-carbon molecule, while acetyl CoA is a five-carbon molecule. One molecule of CO2 is released.
Both pyruvate and acetyl CoA are six-carbon molecules.
Pyruvate is a three-carbon molecule, while acetyl CoA is a two-carbon molecule. One molecule of CO2 is released.
Hint 2. Carbon atoms in citrate, the first intermediate in the citric acid cycle
Citrate (citric acid), the first intermediate in the citric acid cycle, contains six carbon atoms.
ANSWER:
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Reset
6C
1C
Help
5C
2C
6C
4C
3C
4C
2C
5C
4C
4C
6C
7C
4C
4C
Correct
This diagram of the citric acid cycle shows the carbon skeletons of each intermediate. The net result of this complex series of reactions is the
complete oxidation of the two carbon atoms in the acetyl group of acetyl CoA to two molecules of CO2.
Part B - Net redox reaction in acetyl CoA formation and the citric acid cycle
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In the sequential reactions of acetyl CoA formation and the citric acid cycle, pyruvate (the output from glycolysis) is completely oxidized, and the electrons
produced from this oxidation are passed on to two types of electron acceptors.
Drag the labels on the left to show the net redox reaction in acetyl CoA formation and the citric acid cycle. Note that two types of electron carriers
are involved.
Hint 1. Redox reactions
Under normal circumstances, redox reactions always occur in pairs. In the first of the two reactions, the electron donor is oxidized (it loses electrons). In
the second reaction, the electron acceptor is reduced (it gains the electrons lost by the first compound). A generic redox reaction involving the transfer
of two electrons is illustrated here.
Note that compounds A and B each exist in two forms: One form is reduced (it carries the extra electrons) while the other form is oxidized (it does not
carry the extra electrons). In the reaction shown here, the electron donor is the reduced form of compound A and the electron acceptor is the oxidized
form of compound B.
Hint 2. What are the electron carriers in acetyl CoA formation and the citric acid cycle?
In acetyl CoA formation and the citric acid cycle, as in glycolysis, electron carriers shuttle the electrons removed from carbon-containing intermediates
to a later stage in cellular respiration. The most common pair of electron carriers in cellular respiration is NAD+ (the oxidized form) and NADH (the
reduced form):
In the citric acid cycle, however, another pair of electron carriers shuttles electrons in addition to NAD+/NADH.
What is the second pair of electron carriers in the citric acid cycle?
ANSWER:
H2O (the oxidized form) and O2 (the reduced form)
ATP (the oxidized form) and ADP (the reduced form)
ADP (the oxidized form) and ATP (the reduced form)
FAD (the oxidized form) and FADH2 (the reduced form)
FADH2 (the oxidized form) and FAD (the reduced form)
O2 (the oxidized form) and H2O (the reduced form)
Hint 3. Can you identify the electron donor and acceptor in a reaction?
One of the reactions in the citric acid cycle is the oxidation of the six-carbon molecule isocitrate to the five-carbon molecule alpha-ketoglutarate, with the
release of one molecule of CO2.
isocitrate + NAD+ → alpha-ketoglutarate + NADH + CO2
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In this reaction, the electron donor is __________ and the electron acceptor is __________.
ANSWER:
isocitrate/alpha-ketoglutarate
isocitrate/NAD+
alpha-ketoglutarate/NADH
NADH/NAD+
NAD+/NADH
ANSWER:
Reset
Help
H2 O
FAD
ADP + P i
CO2
NADH
FADH 2
ATP
O2
Correct
As in glycolysis, the electrons removed from carbon-containing intermediates during acetyl CoA formation and the citric acid cycle are passed to the
electron carrier NAD+, reducing it to NADH. The citric acid cycle also uses a second electron carrier, FAD (flavin adenine dinucleotide), the oxidized
form, and FADH2, the reduced form.
Part C - Why is the citric acid cycle a cyclic pathway rather than a linear pathway?
In the oxidation of pyruvate to acetyl CoA, one carbon atom is released as CO2. However, the oxidation of the remaining two carbon atoms—in acetate—to
CO2 requires a complex, eight-step pathway—the citric acid cycle. Consider four possible explanations for why the last two carbons in acetate are converted to
CO2 in a complex cyclic pathway rather than through a simple, linear reaction.
Use your knowledge of the first three stages of cellular respiration to determine which explanation is correct.
Hint 1. What is the fate of carbon atoms entering the citric acid cycle?
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With each turn of the citric acid cycle, two carbon atoms from the acetyl group of acetyl CoA join with the four-carbon oxaloacetate (the last intermediate
in the citric acid cycle) to form citrate, a six-carbon acid.
In the remaining steps of the citric acid cycle, the six-carbon citrate is converted back to the four-carbon oxaloacetate.
What happens to the other two carbon atoms?
ANSWER:
They are used to convert ADP and Pi to ATP.
They are used to convert NAD+ to NADH.
They are released as two CO2 molecules.
They are converted into pyruvate.
They increase the number of carbon atoms stored in the intermediates of the citric acid cycle.
Hint 2. How many carbon atoms are in the molecules from which CO2 is released in the citric acid cycle?
Two molecules of CO2 are produced for each molecule of acetyl CoA that enters the citric acid cycle. These two molecules of CO2 are removed from
specific carbon intermediates in the cycle. How many carbon atoms are present in the intermediates from which CO2 is removed?
How many carbon atoms are present in the intermediates from which CO2 is removed?
Hint 1. Reactions that release CO2 in the citric acid cycle
This diagram shows the carbon skeletons of the intermediates in the citric acid cycle. Locate the intermediates involved in reactions in which
CO2 is released to determine how many carbon atoms they contain.
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ANSWER:
2
3
4
5
6
2 or 6
4 or 5
5 or 6
ANSWER:
More ATP is produced per CO2 released in cyclic processes than in linear processes.
It is easier to remove electrons and produce CO2 from compounds with three or more carbon atoms than from a two-carbon compound such as
acetyl CoA.
Redox reactions that simultaneously produce CO2 and NADH occur only in cyclic processes.
Cyclic processes, such as the citric acid cycle, require a different mechanism of ATP synthesis than linear processes, such as glycolysis.
Correct
Although it is possible to oxidize the two-carbon acetyl group of acetyl CoA to two molecules of CO2, it is much more difficult than adding the acetyl
group to a four-carbon acid to form a six-carbon acid (citrate). Citrate can then be oxidized sequentially to release two molecules of CO2.
Cellular Respiration (1 of 5): Inputs and Outputs (BioFlix tutorial)
The reactions of cellular respiration can be broken down into four stages:
1.
2.
3.
4.
glycolysis
acetyl coenzyme A (acetyl CoA) formation
citric acid cycle (also known as the Krebs cycle)
oxidative phosphorylation (electron transport and chemiosmotic ATP synthesis)
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In this tutorial, you will identify the inputs and outputs of each stage of cellular respiration, and identify the cellular compartments where these stages occur. Before
beginning this tutorial, watch the Cellular Respiration animation. Pay close attention to the carbon atoms, electron carriers, and ATP formation. You can review
relevant parts of the animation at any point in the tutorial.
Part A - Glycolysis
From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of glycolysis.
Drag each compound to the appropriate bin. If the compound is not involved in glycolysis, drag it to the "not input or output" bin.
Hint 1. Review the Glycolysis animation
Hint 2. Is there a net input or net output of ATP in glycolysis?
If a compound is both consumed (input) and produced (output) in a process, you need to consider whether more of the compound is consumed or
produced. If more of the compound is consumed than produced, there is a net input of the compound in that process. If more of the compound is
produced than consumed, there is a net output of the compound.
Recall that in the first steps of glycolysis, 2 ATP are consumed per glucose molecule. As glycolysis progresses, 4 ATP are produced per glucose
molecule. Which statement correctly describes the net change in ATP during glycolysis?
ANSWER:
There is a net output of ATP.
There is a net input of ATP.
There is no net input or net output of ATP.
Hint 3. What are the electron carriers in glycolysis?
In the overall process of glycolysis, one molecule of glucose is oxidized to produce two molecules of pyruvate. Recall that oxidation means the loss of
electrons, and that the electrons removed from glucose must be passed on to some other molecule, called an electron carrier.
Drag the compounds on the left to identify the electron carriers in glycolysis.
Hint 1. NAD+ and NADH are a common pair of electron carriers in cells
NAD+ and NADH are the most common pair of compounds that carry electrons among the various stages of cellular respiration. Oxidation and
reduction reactions connect these two compounds.
ANSWER:
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O2
ADP + P i
NAD+
H2 O
NADH
ATP
Hint 4. Reactions that produce ATP usually require ADP and Pi as inputs
The reactions of glycolysis provide the energy needed to produce ATP. To produce ATP, however, ADP and Pi are needed as inputs to the process.
ANSWER:
Reset
net input
net output
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not input or output
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Correct
In glycolysis, the six-carbon sugar glucose is converted to two molecules of pyruvate (three carbons each), with the net production of 2 ATP and 2
NADH per glucose molecule. There is no O2 uptake or CO2 release in glycolysis.
Part B - Acetyl CoA Formation
In acetyl CoA formation, the carbon-containing compound from glycolysis is oxidized to produce acetyl CoA. From the following compounds involved in cellular
respiration, choose those that are the net inputs and net outputs of acetyl CoA formation.
Drag each compound to the appropriate bin. If a compound is not involved in acetyl CoA formation, drag it to the "not input or output" bin. (Note
that not all of the inputs and outputs of acetyl CoA formation are included.)
Hint 1. Review the Acetyl CoA Formation animation
Hint 2. What are the electron carriers in acetyl CoA formation?
In the production of acetyl CoA, one molecule of pyruvate is first oxidized to acetate, then coenzyme A (CoA) is added to form acetyl CoA. Recall that
oxidation is the loss of electrons, and that the electrons removed from pyruvate must be passed to an electron carrier.
Drag the compounds on the left to identify the electron carriers in acetyl CoA formation.
ANSWER:
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ADP + P i
ATP
NAD+
O2
NADH
H2 O
Hint 3. The formation of acetyl CoA requires the input of CoA
To convert pyruvate to acetyl CoA, an input of coenzyme A (CoA) is required. Note that CoA is not metabolized in cellular respiration. It enters and
leaves cellular respiration as CoA, functioning as a carrier of some carbon compounds (such as acetate) that are intermediates in cellular respiration.
Hint 4. Counting carbon atoms in reactions
In cellular respiration, the carbon atoms in glucose are oxidized in stages to produce CO2. In reactions such as acetyl CoA formation, there must be the
same number of carbon atoms as inputs as there are as outputs. Consider whether any carbon atoms have been released during this stage in the form
of CO2. In that case, CO2 should be included as an output.
ANSWER:
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net input
net output
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not input or output
Correct
In acetyl CoA formation, pyruvate (a product of glycolysis) is oxidized to acetyl CoA, with the reduction of NAD+ to NADH and the release of one
molecule of CO2.
Part C - Citric Acid Cycle
In the citric acid cycle (also known as the Krebs cycle), acetyl CoA is completely oxidized. From the following compounds involved in cellular respiration,
choose those that are the net inputs and net outputs of the citric acid cycle.
Drag each compound to the appropriate bin. If a compound is not involved in the citric acid cycle, drag it to the "not input or output" bin. (Note that
not all of the inputs and outputs of the citric acid cycle are included.)
Hint 1. Review the Citric Acid Cycle animation
Hint 2. What are the electron carriers in the citric acid cycle?
In the citric acid cycle, carbon that enters as acetate (acetyl CoA) is completely oxidized to CO2. Recall that oxidation means the loss of electrons, and
that the electrons removed from acetate must be passed on to other molecules, known as electron carriers. One pair of electron carriers that functions
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in the citric acid cyle is FAD (the oxidized form) and FADH2 (the reduced form). However, FAD and FADH2 are not the main electron carriers in the citric
acid cycle.
Drag the compounds on the left to identify the main electron carriers in the citric acid cycle.
ANSWER:
Reset
O2
Help
NAD+
H2 O
NADH
ATP
ADP + P i
Hint 3. Carbon atoms in the citric acid cycle
Carbon enters the citric acid cycle in the form of a two-carbon compound. Although there are many carbon-containing intermediates, think about how
the two carbon atoms leave the cycle. Make sure to include that carbon-containing molecule as an output.
ANSWER:
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net input
net output
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not input or output
Correct
In the citric acid cycle, the two carbons from the acetyl group of acetyl CoA are oxidized to two molecules of CO2, while several molecules of NAD+
are reduced to NADH and one molecule of FAD is reduced to FADH2. In addition, one molecule of ATP is produced.
Part D - Oxidative Phosphorylation
In the last stage of cellular respiration, oxidative phosphorylation, all of the reduced electron carriers produced in the previous stages are oxidized by oxygen
via the electron transport chain. The energy from this oxidation is stored in a form that is used by most other energy-requiring reactions in cells.
From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of oxidative phosphorylation.
Drag each compound to the appropriate bin. If a compound is not involved in oxidative phosphorylation, drag it to the "not input or output" bin.
(Note that not all of the inputs and outputs of oxidative phosphorylation are listed.)
Hint 1. Review the Oxidative Phosphorylation animation
Hint 2. Carbon inputs to oxidative phosphorylation
All six of the carbon atoms that enter glycolysis in glucose are released as molecules of CO2 during the first three stages of cellular respiration. Think
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about whether any carbon compounds play a role in oxidative phosphorylation.
Hint 3. What are the electron carriers in oxidative phosphorylation?
In the first three stages of cellular respiration, the oxidation of carbon-containing molecules (beginning with glucose) was coupled to the reduction of
electron carriers. Think about the fate of these reduced electron carriers in oxidative phosphorylation.
Drag the compounds on the left to identify the main electron carriers in oxidative phosphorylation. (Note that another pair of electron
carriers, FAD and FADH2, is also involved at this stage.)
Hint 1. The role of O2 in cellular respiration
The overall equation for cellular respiration includes two molecules (O2 and H2O) that do not participate in the first three stages of cellular
respiration.
C6H12O6 + 6 O2 → CO2 + 6 H2O
Think about the role that these two molecules play in oxidative phosphorylation.
ANSWER:
Reset
Help
ATP
O2
NADH
NAD+
H2 O
ADP + P i
Hint 4. ATP production in oxidative phosphorylation
During cellular respiration, the energy in sugars such as glucose is converted into energy in the form of ATP. Most of this ATP is produced in oxidative
phosphorylation. Recall that in order to produce ATP as an output, some other compound is required as an input.
ANSWER:
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net input
net output
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not input or output
Correct
In oxidative phosphorylation, the NADH and FADH2 produced by the first three stages of cellular respiration are oxidized in the electron transport
chain, reducing O2 to water and recycling NAD+ and FAD back to the first three stages of cellular respiration. The electron transport reactions supply
the energy to drive most of a cell's ATP production.
Part E - Cellular locations of the four stages of cellular respiration
Each of the four stages of cellular respiration occurs in a specific location inside or outside the mitochondria. These locations permit precise regulation and
partitioning of cellular resources to optimize the utilization of cellular energy.
Match each stage of cellular respiration with the cellular location in which it occurs. Labels may be used once, more than once, or not at all.
Hint 1. Compounds that cross membranes in cellular respiration
In cellular respiration, some of the outputs of one stage must cross both the inner and outer mitochondrial membranes in order to function as inputs to
another stage. For example, both pyruvate and some NADH must cross both membranes. Think about where these products are first produced
(outputs) and where they are consumed (inputs).
ANSWER:
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cytosol
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cytosol
outer mitochondrial
membrane
mitochondrial matrix
mitochondrial matrix
inner mitochondrial
membrane
mitochondrial matrix
intermembrane
space
inner mitochondrial
membrane
Correct
Cellular respiration begins with glycolysis in the cytosol. Pyruvate, the product of glycolysis, then enters the mitochondrial matrix, crossing both the
outer and inner membranes. Both acetyl CoA formation and the citric acid cycle take place in the matrix. The NADH and FADH2 produced during the
first three stages release their electrons to the electron transport chain of oxidative phosphorylation at the inner mitochondrial membrane. The inner
membrane provides the barrier that creates an H+ gradient during electron transport, which is used for ATP synthesis.
Cellular Respiration (4 of 5): Oxidative Phosphorylation (BioFlix tutorial)
Oxidative phosphorylation consists of two tightly linked processes - electron transport and ATP synthesis. In electron transport, the NADH and FADH2 produced in
the first three stages of cellular respiration are oxidized by O2 (the oxidative part of this stage). These redox reactions also drive the pumping of protons across the
inner mitochondrial membrane, creating a proton ( H+) gradient. This H+ gradient is used to power the chemiosmotic synthesis of ATP from ADP and Pi (the
phosphorylation part of this stage).
As you watch the Oxidative Phosphorylation animation, pay close attention to how electron transport is coupled to the formation of the H+ gradient and ATP
synthesis.
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Part A - The role of O2 in electron transport
In mitochondrial electron transport, what is the direct role of O2?
Hint 1. The definition of “direct role”
A molecule plays a direct role in a chemical reaction if it participates in the reaction as either a reactant or a product, or if it is part of the catalytic
machinery (enzymes or cofactors) that allows the reaction to occur.
Hint 2. The electron transport chain
The electron transport chain (shown in this diagram) consists of a sequence of electron carriers, most of which are components of the four main protein
complexes (I - IV) embedded in the inner mitochondrial membrane. Two other substances, Q and Cyt c, are mobile carriers that shuttle electrons
between the protein complexes. Note the different locations where electrons from NADH and FADH2 enter the electron transport chain and the role that
O2 plays in this process.
ANSWER:
to oxidize NADH and FADH2 from glycolysis, acetyl CoA formation, and the citric acid cycle
to provide the driving force for the synthesis of ATP from ADP and Pi
to function as the final electron acceptor in the electron transport chain
to provide the driving force for the production of a proton gradient
Correct
The only place that O2 participates in cellular respiration is at the end of the electron transport chain, as the final electron acceptor. Oxygen's high
affinity for electrons ensures its success in this role. Its contributions to driving electron transport, forming a proton gradient, and synthesizing ATP
are all indirect effects of its role as the terminal electron acceptor.
Part B - The effects of anaerobic conditions
How would anaerobic conditions (when no O2 is present) affect the rate of electron transport and ATP production during oxidative phosphorylation? (Note that
you should not consider the effect on ATP synthesis in glycolysis or the citric acid cycle.)
Hint 1. The role of O2 in electron transport
O2 is the final electron acceptor in the electron transport chain. Without O2, there is no place for the electrons from NADH and FADH2 (and ultimately
from glucose) to go.
Hint 2. What is the link between electron transport and ATP synthesis in oxidative phosphorylation?
If electron transport stops because there is no O2 to serve as the final electron acceptor, ATP synthesis associated with oxidative phosphorylation also
stops. Why?
ANSWER:
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ATP synthesis in oxidative phosphorylation is directly driven by electrons moving from NADH and FADH2 to O2.
Without electron transport, the inner mitochondrial membrane becomes leaky to protons and the proton gradient rapidly dissipates.
Without electron transport from NADH and FADH2 to O2, the pumping of protons across the inner mitochondrial membrane ceases.
The synthesis of ATP from ADP and Pi creates a proton gradient across the inner mitochondrial membrane. This gradient is required for
NADH oxidation by O2.
ANSWER:
Neither electron transport nor ATP synthesis would be affected.
Electron transport would stop but ATP synthesis would be unaffected.
Electron transport would be unaffected but ATP synthesis would stop.
Both electron transport and ATP synthesis would stop.
Correct
Oxygen plays an essential role in cellular respiration because it is the final electron acceptor for the entire process. Without O2, mitochondria are
unable to oxidize the NADH and FADH2 produced in the first three steps of cellular respiration, and thus cannot make any ATP via oxidative
phosphorylation. In addition, without O2 the mitochondria cannot oxidize the NADH and FADH2 back to NAD+ and FAD, which are needed as inputs
to the first three stages of cellular respiration.
Part C - Comparing the amount of ATP synthesis from NADH and FADH2
NADH and FADH2 are both electron carriers that donate their electrons to the electron transport chain. The electrons ultimately reduce O2 to water in the final
step of electron transport. However, the amount of ATP made by electrons from an NADH molecule is greater than the amount made by electrons from an
FADH2 molecule.
Which statement best explains why more ATP is made per molecule of NADH than per molecule of FADH2?
Hint 1. How does the number of protons pumped across the membrane per NADH and FADH2 compare?
Once an H+ ion is pumped across the inner mitochondrial membrane and becomes part of the H+ gradient, it has the potential to drive the same amount
of ATP synthesis as any other H+ ion, regardless of where it was pumped across the membrane. Thus, an H+ ion derived from the oxidation of NADH is
equivalent to an H+ ion derived from the oxidation of FADH2.
Because more ATP is made from an NADH molecule than from an FADH2 molecule, what must be true about the number of H+ ions that these
two molecules contribute to the H+ gradient?
ANSWER:
Both NADH and FADH2 contribute the same number of H+ ions .
NADH contributes fewer H+ ions than FADH2.
NADH contributes more H+ ions than FADH2.
Hint 2. NADH and FADH2 donate electrons at different positions along the electron transport chain
Although NADH and FADH2 have very similar functions in cellular respiration (they are both reduced electron carriers), they donate their electrons at
different points along the electron transport chain.
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ANSWER:
FADH2 is made only in the citric acid cycle while NADH is made in glycolysis, acetyl CoA formation, and the citric acid cycle.
The H+ gradient made from electron transport using NADH is located in a different part of the mitochondrion than the H+ gradient made using
FADH2.
Fewer protons are pumped across the inner mitochondrial membrane when FADH2 is the electron donor than when NADH is the electron donor.
It takes more energy to make ATP from ADP and Pi using FADH2 than using NADH.
There is more NADH than FADH2 made for every glucose that enters cellular respiration.
Correct
Electrons derived from the oxidation of FADH2 enter the electron transport chain at Complex II, farther down the chain than electrons from NADH
(which enter at Complex I). This results in fewer H+ ions being pumped across the membrane for FADH2 compared to NADH, as this diagram shows.
Thus, more ATP can be produced per NADH than FADH2.
Part D - The effect of gramicidin on oxidative phosphorylation
When the protein gramicidin is integrated into a membrane, an H+ channel forms and the membrane becomes very permeable to protons (H+ ions). If
gramicidin is added to an actively respiring muscle cell, how would it affect the rates of electron transport, proton pumping, and ATP synthesis in oxidative
phosphorylation? (Assume that gramicidin does not affect the production of NADH and FADH2 during the early stages of cellular respiration.)
Sort the labels into the correct bin according to the effect that gramicidin would have on each process.
Hint 1. Review the Oxidative Phosphorylation animation
In oxidative phosphorylation, electron transport is coupled to the synthesis of ATP via a proton gradient that forms across the inner mitochondrial
membrane. In the Oxidative Phosphorylation animation, watch how the proton gradient is created by electron transport and how it is used in the
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synthesis of ATP.
Hint 2. How to approach the problem
You know that membranes treated with gramicidin become very leaky to protons. Consider these four questions (in this order) to help you evaluate how
gramicidin alters oxidative phosphorylation.
1. Is a proton gradient across the inner mitochondrial membrane required for ATP synthesis during oxidative phosphorylation?
2. What effect does a membrane that is very leaky to protons have on the ability of the mitochondrion to maintain a proton gradient across
that membrane?
3. What effect does the ability of the mitochondrion to maintain a proton gradient have on the rate of proton pumping?
4. How is the rate of electron transport related to the rate of proton pumping, and are these rates affected by the membrane being leaky to
protons?
Hint 3. How would gramicidin affect electron transport and proton pumping?
Think about how gramicidin would affect proton pumping driven by electron transport.
Which of the following statements is correct?
ANSWER:
Gramicidin would prevent electron transport from pumping protons across the inner mitochondrial membrane.
Electron transport would continue to pump protons across the inner mitochondrial membrane, but gramicidin would allow the protons to leak
back across the membrane.
Hint 4. How would gramicidin affect the proton gradient across the inner mitochondrial membrane?
If gramicidin creates proton channels, think about how this would affect the proton gradient across the inner mitochondrial membrane.
What would happen to the proton gradient in the presence of gramicidin?
ANSWER:
Nothing.
Protons would diffuse back through the channel down their electrochemical gradient and the proton gradient would dissipate.
Protons would be pumped through the channel against their electrochemical gradient and the proton gradient would increase.
ANSWER:
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Reset
remains the same
decreases (or goes to zero)
Help
increases
electron transport rate
size of the proton gradient
proton pumping rate
rate of ATP synthesis
rate of oxygen uptake
Correct
Gramicidin causes membranes to become very leaky to protons, so that a proton gradient cannot be maintained and ATP synthesis stops. However,
the leakiness of the membrane has no effect on the ability of electron transport to pump protons. Thus, the rates of proton pumping, electron
transport, and oxygen uptake remain unchanged.
Cellular Respiration (5 of 5): Summary (BioFlix tutorial)
Before beginning this tutorial, watch the Cellular Respiration animation. You can review relevant parts of the animation at any point in the tutorial.
Part A - The coupled stages of cellular respiration
The four stages of cellular respiration do not function independently. Instead, they are coupled together because one or more outputs from one stage functions
as an input to another stage. The coupling works in both directions, as indicated by the arrows in the diagram below. In this activity, you will identify the
compounds that couple the stages of cellular respiration.
Drag the labels on the left onto the diagram to identify the compounds that couple each stage. Labels may be used once, more than once, or not at
all.
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Hint 1. Can you identify a compound that couples two processes?
Coupling is extremely important in the regulation of metabolic pathways, such as cellular respiration. Two processes are coupled when an output from
one process is an input to the other process. In that way, one process depends on the other to supply the reactant it needs.
What compound couples glycolysis to acetyl CoA formation?
ANSWER:
pyruvate
glucose
ATP
NAD+
NADH
ADP + Pi
Hint 2. Inputs and outputs of the stages of cellular respiration
The following table summarizes the inputs and outputs of the stages of cellular respiration. Note that FAD and FADH2 are not included in this table.
Glycolysis
Acetyl CoA Formation
and
the Citric Acid Cycle
Oxidative
Phosphorylation
Inputs
Outputs
Inputs
Outputs
Inputs
Outputs
glucose
pyruvate
pyruvate
CO2
O2
water
NAD+
NADH
NAD+
NADH
NADH
NAD+
ADP + Pi
ATP
ADP + Pi
ATP
ADP + Pi
ATP
ANSWER:
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Reset
Help
ADP + P i
pyruvate
CO2
NADH
NAD+
NADH
pyruvate
NAD+
NADH
NAD+
ATP
Correct
The main coupling among the stages of cellular respiration is accomplished by NAD+ and NADH. In the first three stages, NAD+ accepts electrons
from the oxidation of glucose, pyruvate, and acetyl CoA. The NADH produced in these redox reactions then gets oxidized during oxidative
phosphorylation, regenerating the NAD+ needed for the earlier stages.
Part B - Anaerobic conditions and acetyl CoA formation
Under anaerobic conditions (a lack of oxygen), the conversion of pyruvate to acetyl CoA stops.
Which of these statements is the correct explanation for this observation?
Hint 1. What compound couples oxidative phosphorylation (the stage directly affected by the lack of oxygen) to acetyl CoA formation?
Acetyl CoA formation is regulated by being coupled to oxidative phosphorylation. A compound produced (an output) in oxidative phosphorylation serves
as a substrate (an input) in a reaction in acetyl CoA formation. If oxidative phosphorylation does not provide that compound, acetyl CoA formation will
stop.
What compound couples oxidative phosphorylation to acetyl CoA formation?
ANSWER:
NADH
ATP
NAD+
ADP + Pi
O2
ANSWER:
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ATP is needed to convert pyruvate to acetyl CoA. Without oxygen, no ATP can be made in oxidative phosphorylation.
Oxygen is an input to acetyl CoA formation.
Oxygen is required to convert glucose to pyruvate in glycolysis. Without oxygen, no pyruvate can be made.
In the absence of oxygen, electron transport stops. NADH is no longer converted to NAD+, which is needed for the first three stages of cellular
respiration.
Correct
NAD+ couples oxidative phosphorylation to acetyl CoA formation. The NAD+ needed to oxidize pyruvate to acetyl CoA is produced during electron
transport. Without O2, electron transport stops, and the oxidation of pyruvate to acetyl CoA also stops because of the lack of NAD+.
Part C - Cellular respiration and a cell's demand for ATP
The rate of cellular respiration is regulated by its major product, ATP, via feedback inhibition. As the diagram shows, high levels of ATP inhibit
phosphofructokinase (PFK), an early enzyme in glycolysis. As a result, the rate of cellular respiration, and thus ATP production, decreases. Feedback inhibition
enables cells to adjust their rate of cellular respiration to match their demand for ATP.
Suppose that a cell’s demand for ATP suddenly exceeds its supply of ATP from cellular respiration.
Which statement correctly describes how this increased demand would lead to an increased rate of ATP production?
Hint 1. How to think about this problem
These steps will help you think this problem through in a logical manner.
1. First, make sure you understand how ATP levels regulate PFK and the rate of cellular respiration. (See Hint 2 if you need more help.)
2. Next, consider how the balance between ATP synthesis (supply) and ATP use (demand) determines the ATP level in the cell. If the
demand changes relative to the supply, how would the level of ATP in the cell change initially? (See Hint 3 if you need more help.)
3. Finally, use your prediction of how ATP levels would change to determine whether the rate of cellular respiration would then increase or
decrease.
Hint 2. Regulation of enzyme activity by ATP
In the case of phosphofructokinase (PFK), ATP acts as an allosteric regulator. Increasing ATP levels inhibit the activity of the enzyme, slowing down
glycolysis and cellular respiration. Decreasing ATP levels reverse the inhibition imposed by high ATP levels, speeding up glycolysis and cellular
respiration.
Hint 3. How do supply and demand initially affect cellular ATP levels?
You are resting comfortably listening to music when an angry dog suddenly enters your room.
What happens to the amount of ATP in your muscle cells in that first second, as you leap out of the room, slamming the door behind you?
ANSWER:
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The cellular ATP level increases.
The cellular ATP level remains about the same.
The cellular ATP level decreases.
ANSWER:
ATP levels would rise at first, decreasing the inhibition of PFK and increasing the rate of ATP production.
ATP levels would rise at first, increasing the inhibition of PFK and increasing the rate of ATP production.
ATP levels would fall at first, decreasing the inhibition of PFK and increasing the rate of ATP production.
ATP levels would fall at first, increasing the inhibition of PFK and increasing the rate of ATP production.
Correct
An increased demand for ATP by a cell will cause an initial decrease in the level of cellular ATP. Lower ATP decreases the inhibition of the PFK
enzyme, thus increasing the rate of glycolysis, cellular respiration, and ATP production. It is the initial decrease in ATP levels that leads to an
increase in ATP production.
Part D - Fermentation - ATP production in the absence of oxygen
Under anaerobic conditions (a lack of oxygen), glycolysis continues in most cells despite the fact that oxidative phosphorylation stops, and its production of
NAD+ (which is needed as an input to glycolysis) also stops. The diagram illustrates the process of fermentation, which is used by many cells in the absence of
oxygen. In fermentation, the NADH produced by glycolysis is used to reduce the pyruvate produced by glycolysis to either lactate or ethanol. Fermentation
results in a net production of 2 ATP per glucose molecule.
During strenuous exercise, anaerobic conditions can result if the cardiovascular system cannot supply oxygen fast enough to meet the demands of muscle
cells. Assume that a muscle cell’s demand for ATP under anaerobic conditions remains the same as it was under aerobic conditions.
What would happen to the cell’s rate of glucose utilization?
Hint 1. How to think about this problem
In Part C of this tutorial, you learned how supply and demand for cellular ATP regulates cellular respiration via a key enzyme in glycolysis. Using this
knowledge, follow these steps to think this problem through in a logical manner.
1. First, remember that the cell’s demand for ATP under anaerobic conditions remains the same as it was under aerobic conditions. Consider
what happens to the supply of ATP under anaerobic conditions.
2. Next, consider how an imbalance between the demand and supply of ATP will affect the ATP level in the cell.
3. Then, think about how the change in ATP level will affect phosphofructokinase (PFK) activity and the rate of glycolysis.
4. Next, determine how the rate of glycolysis will affect the cell’s utilization of glucose.
5. Finally, think about how much ATP is made under anaerobic conditions (fermentation only) compared to aerobic conditions (all of cellular
respiration). See Hint 2 if you need more help.
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Hint 2. How much ATP is made during fermentation compared to during aerobic respiration?
Under aerobic conditions, cellular respiration produces about 30 molecules of ATP per glucose molecule.
How does the amount of ATP produced by fermentation compare to the amount produced by aerobic respiration?
ANSWER:
Fermentation produces about the same amount of ATP as aerobic respiration.
Fermentation produces about 90% of the amount of ATP produced by aerobic respiration.
Fermentation produces about 50% of the amount of ATP produced by aerobic respiration.
Fermentation produces less than 10% of the amount of ATP produced by aerobic respiration.
ANSWER:
Glucose utilization would increase a lot.
Glucose utilization would increase a little.
Glucose utilization would remain the same.
Glucose utilization would decrease a little.
Glucose utilization would decrease a lot.
Correct
ATP made during fermentation comes from glycolysis, which produces a net of only 2 ATP per glucose molecule. In contrast, aerobic cellular
respiration produces about 30 ATP per glucose molecule. To meet the same ATP demand under anaerobic conditions as under aerobic conditions, a
cell’s rate of glycolysis and glucose utilization must increase about 15-fold.
Pathways for Pyruvate
In most organisms, the end product of glycolysis is pyruvate. Pyruvate still contains a substantial amount of energy, which can be further extracted. Whether the
organisms are operating under aerobic or anaerobic conditions determines the metabolic pathway that pyruvate undergoes to produce more ATP. In this tutorial,
you will identify the end products of these metabolic pathways.
Part A - Products of pyruvate metabolism
Match each product of pyruvate metabolism with the condition under which it is produced.
Drag each item to the appropriate bin.
Hint 1. Is the formation of acetyl CoA oxidation or reduction?
When pyruvate is _____ to acetyl CoA, NAD+ is reduced to NADH.
ANSWER:
oxidized
reduced
Hint 2. An example of fermentation
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Recall that wineries use fermentation to turn grape sugar into alcohol. This may help you remember that alcohols are a typical product of fermentation.
Hint 3. Fermentation in muscle cells
When animal muscles metabolize glucose faster than they can be supplied with oxygen, fermentation takes place, producing lactate rather than acetyl
CoA. Organisms that normally produce energy using oxygen as an electron acceptor can use fermentation instead to generate energy when oxygen is
absent.
ANSWER:
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Reset
lactate
ethanol
fermentation in human muscle
fermentation in yeast and bacteria
Help
acetyl CoA
aerobic oxidation
Correct
In the presence of oxygen, human cells carry out aerobic respiration, which yields acetyl CoA. In the absence of oxygen, human cells can carry out
lactic acid fermentation, which yields lactate. Yeasts and many bacteria carry out alcohol fermentation, which takes place under anaerobic conditions,
and produces ethanol.
Part B - Reactants and products of lactic acid fermentation
Sort the following items according to whether they are reactants or products in the anaerobic reduction of pyruvate during lactic acid fermentation.
Drag each item to the appropriate bin.
Hint 1. How to approach the problem
During a reduction-oxidation reaction such as the anaerobic reduction of pyruvate in lactic acid fermentation, there are always two entities involved. One
of them gets reduced (gains electrons), and the other one gets oxidized (loses electrons). In the case of the reduction of pyruvate (the reducing agent),
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you need to determine which form of nicotinamide adenine dinucleotide gets oxidized.
Hint 2. What is the reduced form of nicotinamide adenine dinucleotide?
Which of the following molecules is the reduced form of nicotinamide adenine dinucleotide?
ANSWER:
NAD+
NADH
NADH2
FADH2
ANSWER:
Reset
reactants
pyruvate
Help
products
NADH
lactate
NAD+
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Correct
When an animal engages in strenuous usage of its muscles, anaerobic conditions ensue, and pyruvate is reduced to lactate. In the process, NADH is
oxidized to NAD+. This NAD+ can further oxidize glyceraldehyde-3-phosphate to produce more ATP.
Scientific Skills Exercise: Making a Bar Graph and Evaluating a Hypothesis
Does thyroid hormone level affect oxygen consumption in cells?
Some animals maintain a relatively constant body temperature above that of their environment, using heat produced as a byproduct of metabolism. When the core
temperature of these animals drops below an internal set point, their cells are triggered to reduce the efficiency of ATP production by the electron transport chain in
mitochondria. More body heat is generated when extra fuel must be consumed to generate the same number of ATPs. Researchers hypothesized that thyroid
hormone might trigger this cellular response.
In this exercise, you will use a bar graph to visualize data from an experiment that compared the metabolic rate (by measuring oxygen consumption) in the
mitochondria of cells from animals with different levels of thyroid hormone.
Liver cells were isolated from sibling rats that had either low, normal, or elevated thyroid hormone levels. The oxygen consumption rate due to activity of the
mitochondrial electron transport chain of each type of cell was measured under controlled conditions. The table shows the data for each cell type.
Thyroid Hormone Level
Oxygen Consumption Rate
(nmol O2/min ⋅ mg cells)
Low
4.3
Normal
4.8
Elevated
8.7
Data from M. E. Harper and M. D. Brand, The quantitative
contributions of mitochondrial proton leak and ATP turnover reations
to the changed respiration rates of hepatocytes from rats of different
thyroid status, Journal of Biological Chemistry 268: 14850-14860
(1993).
Part A - Making a graph with the data
To see patterns in the data from an experiment like this, it is helpful to graph the data. A bar graph is used instead of a line graph because each type of liver
cell was independent of the others. But first, you must determine which variable should go on each axis of the graph.
What variable did the researchers intentionally vary in the experiment, and what are the units for this variable?
ANSWER:
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oxygen consumption rate in liver cells, in nmol O2/min • mg cells
genetic background of the rats chosen for the experiment, in type of breed
concentration of liver cells, in mg/mL
thyroid hormone level of the rats chosen for the experiment, in relative units
Correct
Thyroid hormone level is the independent variable, which goes on the x-axis.
Part B
What variable responded to thyroid hormone level (the independent variable), and what are the units for this variable?
ANSWER:
oxygen consumption rate of liver cells, in nmol O2/min • mg cells
ATP production, in nmol ATP/min • mg cells
temperature of rats, in degrees Celsius
thyroid hormone level of the rats, in relative units
Correct
Oxygen consumption rate is the dependent variable, which goes on the y-axis.
Part C
Now that you have determined which variable goes on each axis, the graph can be constructed.
Assuming that the x-axis tick marks will be used to identify the thyroid hormone level of each type of rat, what bars should appear on the x-axis?
ANSWER:
Low, Medium, High
4.3, 4.8, 8.7
Low, Normal, Elevated
Cold, Normal, Hot
Correct
Part D
Assuming that the y-axis tick marks will be separated by 1.0 (0.0, 1.0, 2.0, and so on), what is the largest value that should appear on the y-axis?
ANSWER:
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0
5.0
9.0
25.0
Correct
Part E
Which of the following graphs correctly represents the data from the experiment?
ANSWER:
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Correct
Part F - Interpreting the graph
Select Figure 2 from the drop-down menu above the table to
see a graphical version of the data.
Which cell type(s) had the highest rate of oxygen
consumption?
ANSWER:
cells from rats with normal thyroid hormone
cells from rats with low or normal thyroid hormone
cells from rats with elevated thyroid hormone
cells from rats with low thyroid hormone
Correct
Part G
Which cell type(s) had the lowest rate of oxygen consumption?
ANSWER:
cells from rats with low thyroid hormone
cells from rats with normal thyroid hormone
cells from rats with elevated thyroid hormone
cells from rats with low or normal thyroid hormone
Correct
Part H
Do the results in the graph support the researchers’ hypothesis?
ANSWER:
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Yes; cells that were exposed to elevated thyroid hormone levels showed increased oxygen consumption, indicating that the efficiency of the
electron transport chain was reduced.
Yes; cells that were exposed to elevated thyroid hormone levels were warmer than normal, indicating that the efficiency of the electron transport
chain was reduced.
No; only one value was significantly different from normal so the experiment didn’t work.
No; they did not measure cell temperature so there is no way to know if extra heat was generated in response to elevated thyroid hormone levels
and therefore if the efficiency of the electron transport chain was reduced.
Correct
Part I - Making a prediction
Based on what you know about mitochondrial electron transport and heat production, predict which rats had the highest body temperature, and
which had the lowest body temperature.
ANSWER:
Rats with normal thyroid hormone had the highest body temperature; rats with low thyroid hormone had the lowest.
Rats with low thyroid hormone had the highest body temperature; rats with elevated thyroid hormone had the lowest.
Rats with elevated thyroid hormone had the highest body temperature; rats with normal thyroid hormone had the lowest.
Rats with elevated thyroid hormone had the highest body temperature; rats with low thyroid hormone had the lowest.
Correct
Score Summary:
Your score on this assignment is 102%.
You received 7.15 out of a possible total of 7 points.
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