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HOW CELLS RELEASE CHEMICAL ENERGY
When Mitochondria Spin Their Wheels
OVERVIEW OF ENERGY·RELEASING PATHWAYS
Comparison of the Main Types of EnergyReleasing Pathways
Overview of Aerobic Respiration
THE FIRST STAGE: GLYCOLYSIS
SECOND STAGE OF AEROBIC RESPIRATION
Acetyl-eoA Formation
The Krebs Cycle
FERMENTATION PATHWAYS
Alcoholic Fermentation
Lactate Fermentation
ALTERNATIVE ENERGY SOURCES IN THE BODY
The Fate of Glucose at Mealtime and In Between
Meals
Energy From Fats
Energy From Proteins
PERSPECTIVE ON LIFE
THIRD STAGE OF AEROBIC RESPIRATION-THE
BIG ENERGY PAYOFF
Electron Transfer Phosphorylation
Summing Up: The Energy Harvest
Interactive Exercises
When Mitochondria Spin Their Wheels [pp.106-107]
7.1.
0)
OVERVIEW OF ENERGY-RELEASING PATHWAYS [pp.1OS-109]
For additional practice, use the interactive vocabulary exercises linked with your BiologyNow CD·ROM.
Selected Words: Lufl's syndrome [p.106], Friedreich's ataxia [p.106], anaerobic [p.1OS]
Boldfaced Terms
[p.107] free radicals _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.ID8] aeroJ:>ic respiration
[p.108] glycolysis _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.ID8] pyruvate _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
How Cells Release Chemical Energy 81
[p.l09j Krebs cycle _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.l09jNAD+ _ _ _ _ _ _ _ _ _ _ _ _ _~_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.l09jFAD _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.l09j electron transfer phosphorylation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
Short Answer
1. What do Luft's syndrome, Friedreich's ataxia, type 1 diabetes, atherosclerosis, amyotrophlc lateral sclerosis, and Parkinson's, Alzheimer's, and Huntington's diseases have in common? [pp.106-lD7]
Fill in the Blanks
The numbers in this diagram represent information missing in the following narrative, which gives an
overview of the three stages of aerobic respiration. [pp.lDS-lD9]
4
I
3
'~y~~
energy input to
slart reactions
2
'--,------'1--"
\:' e- + H+
"
,-"-
9
I----~J
18r--19
TYPICAL NET ENERGY YIELD:.....!L ATP
The initial stage of aerobic respiration is called (2) _ _ _ _ _ _ [p.lDS], and it occurs in the
(3) _ _ _ _ _ _ [p.lDS]. Here, with no need for oxygen, enzymes cleave and rearrange a
(4) _ _ _ _ _ [p.lDS] molecule into two molecules of (5) _ _ _ _ _ [p.lDS]. During glycolysis, a
net yield of (6) _ _ _ _ _ _ (number) [p.lDS] ATP molecules is produced. Once glycolysis ends, though,
the energy-releasing pathways differ. Only the aerobic pathway continues in a (7) _ _ _ _ _ _ [p.lDS],
where oxygen accepts and removes the electrons that drove the reactions .
•By contrast, anaerobic pathways that produce a low-yield of ATP start and end in the cytoplasm.
82 Chapter Seven
Aerobic respiration makes the most out of a glucose molecule, commonly (S)
(number)
[p.109] ATP molecules, or more, are produced. The second stage consists of a cyclic pathway called the
(9)
[p.109] and a few steps preceding it. Here enzymes break down
pyruvate to (10)
[p.109] and water. Two coenzymes, abbreviated NAD+ and FAD,
~?
serve in glycolysis and the Krebs cycle. When these coenzymes are reduced, we abbreviate them as
(11)
[p.109] and (12)
it produce (13)
[p.109]. Krebs cycle reactions and those just preceding
(number) [p.109] ATP molecules. The big energy harvest comes in the third
stage, after coenzymes give up electrons and hydrogen to electron transfer chains. These chains are the
machinery of (14)
[p.109]. They create H+ concentration
and electric gradients that drive the formation of (15)
(number) [p.109] ATP molecules.
It is in this final stage of aerobic respiration that so many ATP molecules are produced. As it ends,
(16)
[p.109] inside the mitochondrion accepts the "spent" (17)
[p.109] from
the last component of each transport system. At the same time, oxygen (taken into an organism from the
atmosphere) also picks up (IS)
[p.109] ions and thereby forms (19)
[p.109].
Fill in the Blanks
Using a glucose molecule as the reactant, complete this equation, which summarizes the degradative pathway known as aerobic respiration:
(20)
+
0, - > 6
+6
[p.IOS]
(21) Supply the appropriate information to state the equation in question 20 for aerobic respiration in
words: one molecule of glucose plus six molecules of
enzymes) yields
molecules of carbon dioxide plus
(in the presence of appropriate
molecules of metabolic
water [p.10S]. Now return to Figure 7.3 [p.109] in the textbook and locate the components of this basic
equation.
7.2. THE FIRST STAGE: GLYCOLYSIS [pp.llO-l11]
Selected Words: energy-requiring steps [p.110], energy-releasing steps [p.llO], net yield of glycolysis [p.llO]
Boldfaced Terms
[p.llO] substrate-level phosphorylation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
How Cells Release Chemical Energy 83
i;
'i
,i::
Sequence
Arrange the steps of glycolysis, the first stage of aerobic respiration, in the correct chronological sequence.
Write the letter of step 1 next to 1, the letter of step 2 next to 2, and so on. [pp.llO-lll]
a. Each PGAL gives up two electrons and a hydrogen atom to NAD+,
forming two NADH.
b. Intermediates form; each releases a hydrogen atom and an -OH group.
These combine as water. Two molecules of PEP form by the reactions.
c. First, one ATP molecule transfers a phosphate group to glucose, then
another; atoms are rearranged, because the cell has now invested two ATP
molecules already present.
d. Each PEP transfers a phosphate group to ADP; once again, two ATP
molecules have formed by substrate level phosphorylation.
e. Each PGAL also picks up an inorganic phosphate and transfers a
pnosphate group to ADP, forming ATP (substrate level pho~phorylation);
the original cell investment of two ATP is now paid off.
f. In sum, the net energy yield is two ATP for each molecule of glucose
entering glycolysis; two molecules of pyruvate are the end product.
g. Glucose molecules are present in the cytoplasm.
h. The rearranged and phosphorylated glucose splits into two PGAL
molecules, each with a tluee-carbon backbone.
1.
2.
3.
4.
5.
6.
7.
8.
7.3. SECOND STAGE OF AEROBIC RESPIRATION [pp.ll2-113]
Selected Words: acetyl-CoA [p.1l2], coenzymes [p.1l2], coenzyme A [p.1l2], citric acid cycle [p.1l2]
Identification
Identify each numbered structure or location in the sketch. [p.1l2]
AEROBIC
RESPIRATION
start (glycolysis)
in cytoplasm
1. ___________ membrane
2. ___________ membrane
3. _ _ _ _ _ compartment
4. _ _ _ _ _ compartment
84
Chapter Seven
Fill in the Blanks
The numbers in this diagram represent information missing in the following narrative about aerobic
respiration.
Preparatory
~
5
steps
'>
o
--,..,,~/ 7
.'.
."----..,~.6
11 ;--11
NAO+
1
00-8
---------
------------------
Krebs Cycle
\\
t/V,<i>
GoA
.s.······ •.
Y"""~
~
citrate
Ii
<,
f
NAO+
12' -{l
.~
\i.-- NAO+
:}:A'DH~' ~~
~\",,13
12
;'1
NAD+,
iJ~ ~6
/j~
~
Two (5)
~6
~
<':'<"/"""" ___ ".",-/>'/'/
~~~)
\\---
'0
i"-.
\
i
~
·s
~
{ 10
·,°':'12-
ADP+
phosphate
group
[pp.112-113] molecules formed by glycolysis leave the cytoplasm and enter a mito-
chondrion. In this organelle, the second (preparatory steps) and third (electron transfer phosphorylation)
stages of the aerobic pathway are completed. It is during the second stage that glucose is broken down to
carbon dioxide and water. Six carbon atoms, three from each pyruvate, enter these reactions. To start, one
carbon atom from each pyruvate is removed and attached to oxygen molecules, forming (6) _ _ _ _ __
_ _ _ _ _ _ [pp.112-113] (recall that the Krebs cycle turns twice for each glucose molecule entering
glycolysis-all respiration chemistry begins with glycolysis).
Next, the two-carbon fragment that is left over joins with (7)
(8)
[pp.112-113] to make
[pp.112-113]. (8) enters the Krebs cycle and transfers its two carbon
acetyl group to oxaloacetate; this forms the first intermediate compound, citric acid, or citrate (the Krebs
-
=
cycle is also called the citric acid cycle). Two (9)
[pp.112-113] molecules form by
substrate-level phosphorylations in the Krebs cycle, and the intermediate compounds get arranged into
(10)
[pp.112-113]. Cells have only so much (10), which must be regenerated to keep the
reactions going. The two A TP molecules formed in the Krebs cycle don't add much to the small yield of
two ATP from glycolysis. The ten coenzymes (NADH and FADH2) that gain electrons and hydrogen
constitute a big energy payoff for the cell. To summarize these reactions for two turns of the Krebs cycle
How Cells Release Chemical Energy
85
and its preparatory stage: two CO2 form, and two NAD+ are reduced to (11) _ _ _ _ _ _ [p.113] in the
preparatory stage. In the Krebs cycle, four CO, form, two A TP are generated, and six NAD+ are reduced
to (12)
[p.113] and two (13)
[p.113] are reduced to FADH2 • The original
glucose that entered this cycle via glycolysis reactions has lost all of its carbons and can be considered fully
consumed, or oxidized, at this stage.
7.4. THIRD STAGE OF AEROBIC RESPIRATION-THE BIG ENERGY PAYOFF
[pp.114-115]
Selected Words: electron transfer phosphorylation [p.114], electrochemical gradient [p.114], ATP synthases
[p.114]
Fill in the Blanks
The numbers in this diagram represent information missing in the following narrative about electron
transport phosphorylation.
@
4~
glycolysis
86 Chapter Seven
;f
2 NAD+
The aerobic pathway's third stage, electron transfer phosphorylation, generates most of the ATP.
(1)
[p.llS] and (2)
deliver electrons and (3)
[p.llS] that formed during the first and second stages
[p.llS] to electron transfer chains, which are at the membrane
dividing a mitochondrion's interior into two compartments. As the electrons are transferred through the
chains, they attract H+ (unbound hydrogen) from the (4)
are shuttled across the membrane into the (5)
[p.llS] compartment. These-ions
[p.1l5] compartment. Very soon, the H+ con-
centration is greater in the outer compartment than in the inner one. H+ and electric gradients have become
established across the membrane. H+ follows the gradients, through the interior of (6) _ _ _ _ __
_ _ _ _ _ _ [p.1l5]. Energy released by the flow drives formation of (7)
(8)
[p.llS] and unbound (9)
[p.1l5] from
[p.llS]. Hence the name electron transfer
phosphorylation.
Short Answer
10. What role does free oxygen play in electron transport phosphorylation? [pp.1l4-ll5]
11. What is the typical yield of ATP from the third stage of aerobic respiration? What is the net ATP
harvest from one molecule of glucose? [p.1l5]
7.5. FERMENTATION PATHWAYS [pp.1l6-ll7]
Selected Words: Saccharomyces cerevisiae [p.1l6], Lactobacillus [p.1l7]
Boldfaced Terms
[p.116] alcoholic fermentation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
[p.117] lactate fermentation _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
How Cells Release Chemical Energy 87
Choice
Choose from the following:
a. alcoholic fermentation
b. lactate fermentation
c. applies to both types of fermentation
1. _ _ Glycolysis is the first stage [p.1l6]
2. _ _ Yeasts are famous for their use of this pathway [p.116]
3. _ _ Humans, rabbits, and many other animals use this for a quick fix of energy [p.1l7]
4. _ _ Pyruvate and NADH form, and the net energy yield is two ATP [p.1l6]
5. _ _ Muscle cells use this pathway but not for long; diverting glucose into this pathway would
waste too much of its energy for too little ATP [p.1l7]
6. _ _ The final steps simply regenerate NAD+, the coenzyme that assists the breakdown reactions
[p.1l6]
7. _ _ Each pyruvate molecule that formed in glycolysis is converted to the intermediate acetaldehyde
[p.1l6]
8. _ _ These reactions do not completely degrade glucose to CO2 and H 20 [p.1l6]
9. _ _ Saccharomyces cerevisiae makes bread dough rise [p.1l6]
10. _ _ Produce no more ATP beyond the rather tiny yield from glycolysis [p.1l6]
11. _ _ Drunken birds such as tipsy wild turkeys and robins [p.1l6]
12. _ _ Lactabaccilus and some other bacteria use only this anaerobic pathway [p.1l7]
13. _ _ NADH transfers electrons and hydrogen to this form and thereby converts it to ethanol [p.1l6]
14. _ _ Yields enough energy to sustain many single-celled anaerobic organisms [p.1l6]
15. _ _ The transfer by NADH converts each pyruvate to a three-carbon compound found in animals
and other organisms that carryon this process [p.1l7]
7.6. ALTERNATIVE ENERGY SOURCES IN THE BODY [pp.1l8-119]
Selected Words: glucose intake [p.1l8], fat reservoir [p.1l8]' fatty acid conversions [p.1l8], fuel of choice
for the brain [p.1l9]
Choice
Choose from the following:
a. fatty acids
f. glucose
c. amino acids
b. triglycerides
g. glycogen
h. acetyl-CoA
d. PGAL
e. glucose-6-phosphate
1. _ _ At mealtime, a rise in blood concentration of this molecule prompts the pancreas to secrete
insulin [p.1l8]
2. _ _ Conversions of these molecules resuit in pyruvate, acetyl-CoA, or one of the intermediates of
the Krebs cycle. [p.1l8]
3. _ _ When you aren't eating, the level of this in blood declines [p.1l8]
4. _ _ A conversion to this compound allows cells to trap incoming glucose [p.1l8]
88 Chapter Seven
5.
~~
A storage polysaccharide created by a glucose-6-phosphate pathway [p.1l8]
6. ___ Muscle and liver cells maintain the largest stores of this compound [p.1l8]
7. ___ Makes up 1 percent or so of the body's total energy reserves [p.1l8]
8.
~~
Enzymes split dietary protein into these units [p.1l8]
.,
9. ___ An excess of these molecules end up as excess fat [p.1l8]
10. ___ Following removal of amino groups, the carbon backbones may be converted to fats or carbohydrates, or they may enter the Krebs cycle [p.1l9]
11.
~~
Yields more ATP than glucose [p.1l8]
12.
~~
Enzymes in the liver convert glycerol to these molecules [p.1l8]
13.
~~
Muscle cells will not give this up [p.1l8]
14.
~~
Accumulate inside the fat cells of adipose tissue at strategic body regions [p.1l8]
15.
~~
Supply about half of the ATP that muscle, liver, and kidney cells require between meals [p.1l8]
16.
~~
Molecules resulting from enzymes cleaving fatly acid backbones; can enter the Krebs cycle
[p.1l8]
7.7. PERSPECTIVE ON LIFE [p.120]
Short Answer
1. Through the evolution of life over a very long period of time, photosynthesis and aerobic respiration
became "linked" on a global scale. In terms of your knowledge of the two chemical processes, explain
what forms this linkage. [p.120]
Self-Quiz
'J
Are you ready for the exam? Test yourself on key concepts by taking the additional tests linked with your
BiologyNow CD-ROM.
__ 1. Anaerobic respiration pathways begin and
end.in the
. [p.108]
a. thylakoid of a chloroplast
b. plasma membrane of the cell
c. inner membrane of the mitochondrion
d. cytoplasm
e. outer compartment of the
mitochondrion
_
2. The first energy-releasing step in
glycolysis splits activated glucose into two
molecules of
. [p.108]
a. NAD+
b. PGAL
c. ATP
d. pyruvate
e. PEP
How Cells Release Chemical Energy
89
__ 3. The net energy yield of glycolysis is
_ _ _ _ _ _ ATP molecules.
[pp.108-109j
a. three
b. four
c. thirty-two
d.two
e. eight
__ 4. Pyruvate is regarded as the end product of
_ _ _ _ _ . [p.108]
a. glycolysis
b. acetyl-CoA formation
c. fermentation
d. electron transfer phosphorylation
e. the Krebs cycle
__ 5. During which of the follOWing phases of
aerobic respiration is ATP produced
directly by substrate-level
phosphorylation? [p.llO]
a. glucose formation
b. the Calvin-Benson cycle
c. the Krebs cycle
d. formation of acetyl-CoA
e. electron transfer phosphorylation
__ 6. Select the process by which NADH and
FADH, transfer electrons along a chain of
acceptors to oxygen so as to form water
and set up conditions for producing a
large number of ATP molecules.
[pp.1l4-llS]
a. glycolysis
b. the Krebs cycle
c. acetyl-CoA formation
d. fermentation pathways
e. electron transfer phosphorylation
__ 7. The most efficient means of ATP
production during aerobic respiration
involves
. [pp.ll4-11S]
a. concentration of H + and electric
gradients across a membrane
b. ATP synthases
c. formation of ATP in the inner
mitochondrial compartment
d. NADH and FADH,
e. all of the above
90
Chapter Seven
- - S. The total number of ATP molecules
produced by the complete degradation of
one glucose molecule is often thirty-six.
Which of the following is not a
contribution to that number? [pp.1OS-11S]
a. glycolysis produces two ATP molecules
b. steps preparatory to the Krebs cycle
produce two ATP molecules
c. the Krebs cycle produces two ATP
molecules
d. electron transfer phosphorylation
produces thirty-two ATP molecules
e. none of the above are correct
__ 9. What factor allows fermentation reactions
to continue and produce a lo~ yield of
ATP when a cell has no oxygen? [p.116]
a. the presence of ethanol molecules
b. the presence of lactic acid
c. ATP itself
d. regeneration of the coenzyme NAD+
during glycolysis
e. the Krebs cycle
-
10. Of the follOwing, which one is not
available as an alternative energy source
for the human body? [pp.118-ll9]
a. fats
b. glycogen
c. proteins
d. carbohydrates
e. all of the above are available
-
11. The basic reason why we have an
accumulation of oxygen in our atmosphere
is
. [p.120]
a. that oxygen produces more oxygen
b. cyclic photophosphorylation
c. that oxygen is released in electron
transfer phosphorylation
d. the noncyclic pathway
e. the production of ATP
Chapter Objectives/Review Questions
1. Define the term free radicals and be able to list at least three age-related mitochondrial disorders.
[pp.106-107]
2. Be able to summarize the events that occur during the three stages of aerobic respiration as illustrated
in Figure 7.3. [pp.lOS-109]
3. Be able write the general equation for aerobic respiration; then state it in words. [p.10S]
4. Describe the energy-requiring steps of glycolysis. [p.llO]
5. Describe the energy-releasing steps of glycolysis. [p.llO]
6. What is the role of NAD+ in glycolysis? [p.llO]
7. The cell invests
(number) ATP molecules to jump-start glycolysis. [p.llO]
S.
(number) NADH and
(number) ATP molecules form during glycolysis, but the net energy yield from the process is
(number) ATP molecules. [p.llO]
9. For every glucose molecule entering glycolyis, two
molecules are produced as an end
product. [p.llO]
10. Describe exactly where in the mitochondria the second and third stages of aerobic respiration occur.
[p.112]
11. For one glucose molecule, how many molecules of carbon dioxide and acetyl-CoA are produced in the
preparatory steps? [pp.1l2-113]
12. For one pyruvate molecule during two turns of the Krebs cycle,
(number) molecules of
NADH,
(number) molecules of FADH"
(number) of ATP molecules,
and
(number) molecules of CO, are formed. [p.1l3]
13. During electron transport phosphorylation, electron transfer chains and ATP synthases are involved to
produce
(number) molecules of ATP. [p.llS]
14. The total net harvest of ATP, involving all stages of aerobic respiration, is _ _ _ _ __
=--:-::___ (number) molecules of ATP. [p.llS]
15. Be able to summarize the biochemistry of alcoholic fermentation and lactate fermentation and tell what
types of organisms these processes occur in. [pp.1l6-117]
16. What keeps both types of fermentation operational is the regeneration of
; this allows
even a low yield of
energy production to provide a quick fix of energy for cells lacking
oxygen. [p.1l6]
17. Summarize the metabolic processing of glucose at and between meals. [p.llS]
18. Explain how the body metabolizes fats and proteins when glucose is not available. [pp.llS-1l9]
19. Briefly explain how the pathways of photosynthesis and respiration are biochemically linked on a
global scale. [p.l20]
.-
Media Menu Review Questions
Questions 1-3 are drawn from the following Infotrac College Edition article: "My Personal Challenge."
Jackleen Armstrong. The Exceptional Parent, August 1995.
1. Jackleen Armstrong suffers from a disease known as
process is characterized by dysfunction in skeletal _ _ _ _ __
2. List th~ characteristics of her disease, as told by teenager Jackleen,Armstrong.
. This disease
3. What is the name of the medicine used by Jackleen that does give her some relief?
How Cells Release Chemical Energy 91
1 :1
Questions 4-10 are drawn from the following Infotrac College Edition article: "Mitochondria: Cellular
Energy Co.: Researchers Strive to Keep the Energy Pipeline Open in the Face of Damaging Cellular
Insults." Amy Adams. The Scientist, June 24, 2002.
4.
S.
6.
7.
8.
that impair mitochondrial proteins, encoded by both
and mitochondrial genes, cause widespread disturbances in muscle, nerve, kidney, and other high-energy cells.
Several cellular insults, including mutations, toxins, and lack of
, can induce mitochondrial failure, but the end result is the same: too little
to meet the cell's demands.
Neurons are particularly susceptible to damage from low levels of ATP, owing to their high-energy
needs and almost exclusive reliance on
for energy.
With milder mutations that impair mitochondrial proteins, the diseases proceed as a _ _ _ _ __
process. As long as there is minimal ATP energy, the person may be impaired but not ill. In these cases.
the disease may not appear until later in life, as in
and
diseases.
Researchers discovered the link between
and Parkinson disease in 1982 when symptoms very much like Parkinson disease developed in drug addicts. The heroin they had injected contained a compound called
which is metabolized into a strong inhibitOl' of the first
protein complex in the electron transport chain. This led researchers to track down _ _ _ _ __
_ _ _ _ _ _ mutations in patients with Parkinson disease.
A known complex I inhibitor called
was administered to rats. The rats exhibited symptoms similar to those of Parkinson disease. They also had nerve cell
in the substantia
nigra, the same brain region affected in patients with Parkinson disease. Protein
were
also found that Were similar to those found in people who died from Parkinson disease.
It is not known why cells in the substantia nigra of the brain are so affected by complex I mutations.
One possibility is that the synthesis and degradation of the
used by these cells prodUCE
H 20, as a by-product. It is suspected that many cellular defects associated with Parkinson disease
result from the damage to critical proteins and the harmful mutations caused by long-term exposure
to
. If the cause is enigmatic, the effect is clear: the cell is unable to function and
o·
9.
10.
,
Integrating and Applying Key Concepts
1. Evaluate this statement: Every atom in your body has first passed through the cell(s) of a photosyn-
thetic organism.
92
Chapter Seven