• Chapter 9
Cellular Respiration:
Harvesting Chemical
Energy
Main Objective is discovering
how cells use the energy stored
in food molecules to make
ATP. Catabolic pathways do
not directly move anything,
they are linked to work by a
chemical drive shaft: ATP.
Cellular Respiration
• Cellular respiration is a cellular process that breaks
down nutrient molecules with the concomitant production
of ATP
• Consumes oxygen and produces carbon dioxide (CO2)
 Cellular respiration is an aerobic process.
• Usually involves the breakdown of glucose to CO2 and
H2O
 Energy is extracted from the glucose molecule:
• Released step-wise
• Allows ATP to be produced efficiently
 Oxidation-reduction enzymes include NAD+ and FAD as
coenzymes
2
Cellular Respiration
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Oxidation
C6H12O6
+
6O2
6CO2
+
6H2O
+ energy
glucose
Reduction


Electrons are removed from substrates and
received by oxygen, which combines with
H+ to become water.
Glucose is oxidized and O2 is reduced
3
ATP
• Remember that ATP is like a loaded spring. It has
three negatively charged Phosphate groups attached
and that the 3 P’s close together are unstable.
• Cells must recycle the inorganic P that comes off
ATP in order to regenerate its supply of ATP
• Muscle cells recycle the ATP at a rate of 10 million
molecules per second
Cellular Respiration
NAD+ (nicotinamide adenine dinucleotide)
 A coenzyme of oxidation-reduction. It can:
• Oxidize a metabolite by accepting electrons
• Reduce a metabolite by giving up electrons
 Each NAD+ molecule is used over and over again
• FAD (flavin adenine dinucleotide)
 Also a coenzyme of oxidation-reduction
 Sometimes used instead of NAD+
 Accepts two electrons and two hydrogen ions (H+) to
become FADH2
6
Why does the decomposition of
Glucose lead to energy?
• Transfer of Electrons!
• Relocation of e- releases energy stored in
food molecules and this energy is used to
synthesize ATP
• Activation energy of Glucose is high, much
higher than body temperature. Enzymes help
to lower activation energy.
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Cellular Respiration
• Cellular respiration includes four phases:
 Glycolysis is the breakdown of glucose into
two molecules of pyruvate
• Occurs in the cytoplasm
• ATP is formed
• Does not utilize oxygen
 Preparatory (prep) reaction
• Both pyruvates are oxidized and enter the
mitochondria
• Electron energy is stored in NADH
• Two carbons are released as CO2 (one from each
pyruvate)
10
Cellular Respiration
 Citric acid cycle
• Occurs in the matrix of the mitochondrion and
produces NADH and FADH2
• In series of reactions, it releases 4 carbons as CO2
• Turns twice per glucose molecule (once for each
pyruvate)
• Produces two immediate ATP molecules per
glucose molecule
 Electron transport chain (ETC)
• Extracts energy from NADH & FADH2
• Passes electrons from higher to lower energy
states
• Produces 32 or 34 molecules of ATP
11
Outside the Mitochondria:
Glycolysis
• Glycolysis occurs in cytoplasm outside mitochondria
• Energy Investment Step:
 Two ATP are used to activate glucose
 Glucose splits into two G3P molecules
• Energy Harvesting Step:
 Oxidation of G3P occurs by removal of electrons and hydrogen
ions
 Two electrons and one hydrogen ion are accepted by NAD+
resulting in two NADH
 Four ATP are produced by substrate-level phosphorylation
 Net gain of two ATP (4 ATP produced - 2 ATP consumed)
 Both G3Ps converted to pyruvates
12
Outside the Mitochondria:
Glycolysis
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Glycolysis
inputs
outputs
6C glucose
2 NAD+
2
2 (3C) pyruvate
2 NADH
2 ADP
ATP
4 ADP + 4 P
2
4
ATP
ATP
total
net gain
13
Glycolysis
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Animation
D:\ImageLibrary1-17\09-CellularRespiration\09-09Glycolysis.mov
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Outside the Mitochondria:
Fermentation
• Pyruvate is a pivotal metabolite in cellular
respiration
• If O2 is not available to the cell,
fermentation, an anaerobic process,
occurs in the cytoplasm.
 During fermentation, glucose is incompletely
metabolized to lactate, or to CO2 and alcohol
(depending on the organism).
• If O2 is available to the cell, pyruvate
enters the mitochondria for aerobic
respiration.
18
Outside the Mitochondria:
Fermentation
•
Fermentation is an anaerobic process that reduces
pyruvate to either lactate or alcohol and CO2
NADH transfers its electrons to pyruvate
Alcoholic fermentation, carried out by yeasts, produces
carbon dioxide and ethyl alcohol
•
•

•
Lactic acid fermentation, carried out by certain bacteria
and fungi, produces lactic acid (lactate)

•
Used in the production of alcoholic spirits and breads.
Used commercially in the production of cheese, yogurt, and
sauerkraut.
Other bacteria produce chemicals anaerobically,
including isopropanol, butyric acid, proprionic acid, and
acetic acid.
19
Fermentation
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glucose
–2
ATP
2
ATP
E1
2ADP
G3P
2NAD;
E2
2 NADH
BPG
4ADP
E3
+4
ATP
4
ATP
pyruvate
E4
or
2
ATP
2CO 2
(net gain)
2 lactate or 2 alcohol
Animals
Plants
20
Fermentation Helps Produce
Numerous Food Products
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© The McGraw Hill Companies, Inc./Bruce M. Johnson, photographer
21
Fermentation Helps Produce
Numerous Food Products
22
Fermentation Helps Produce
Numerous Food Products
23
Outside the Mitochondria:
Fermentation
• Advantages
 Provides a quick burst of ATP energy for muscular activity.
• Disadvantages
 Lactate and alcohol are toxic to cells.
 Lactate changes pH and causes muscles to fatigue.
• Oxygen debt
 Yeast die from the alcohol they produce by fermentation
• Efficiency of Fermentation
 Two ATP produced per glucose of molecule during fermentation
is equivalent to 14.6 kcal.
 Complete oxidation of glucose can yield 686 kcal.
 Only 2 ATP per glucose are produced, compared to 36 or 38
ATP molecules per glucose produced by cellular respiration.
24
Outside the Mitochondria:
Fermentation
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Fermentation
inputs
outputs
glucose
2 ADP + 2
2 lactate or
2 alcohol and 2 CO2
P
2
ATP
net gain
25
½ Step between Glycolysis and
Kreb’s
1. Carboxyl Group is broken off and expelled as CO2
2. A two Carbon fragment is left and oxidized into
acetate. An enzyme transfers the extracted e- to
NAD+ reducing it to NADH
3. CoEnzyme A will attach to acetate and make Acetyl
CoA
4. This only happens if Oxygen is present
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs
cycle
Mitochondrion Structure and Function
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Cristae: location
of the electron
transportchain
(ETC)
Matrix: location
of the prep
reaction and the
citric acid cycle
outer
membrane
inner
membrane
cristae
intermembrane
space
matrix
45,0003X
© Dr. Donald Fawcett and Dr. Porter/Visuals Unlimited
28
Inside the Mitochondria
•
Citric Acid Cycle

Also called the Krebs cycle

Occurs in the matrix of mitochondria

Begins with the addition of a two-carbon acetyl
group (from acetyl-CoA) to a four-carbon molecule
(oxaloacetate), forming a six-carbon molecule (citric
acid)

NADH and FADH2 capture energy rich electrons

ATP is formed by substrate-level phosphorylation

Turns twice for one glucose molecule (once for
each pyruvate)

Produces 4 CO2, 2 ATP, 6 NADH and 2 FADH2 per
glucose molecule
D:\ImageLibrary1-17\09CellularRespiration\09-12-KrebsCycle.mov
29
Citric Acid Cycle
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NADH
e–
NADH
e–
e–
e–
e–
e–
NADHand
FADH2
e–
Glycolysis
Preparatory reaction
glucose
Electron transport
chain and
chemiosmosis
Citric acid
cycle
pyruvate
Matrix
2 ATP
2 ADP
4 ADP
2
4 ATP total
ATP
net
2ADP
2
ATP
32ADP
or 34
32
or 34
ATP
CoA
1. The cycle begins when a
C2 acetyl group carried by
CoA combines with a C4
molecule to form citrate.
acetyl CoA
C2
citrate
C6
NAD
NADH
oxaloacetate
C2
NADH
5. Once again a substrate
is oxidized, and NAD+
is reduced to NADH.
2. Twice over, substrates
are oxidized as NAD+ is
Reduced to NADH,
and CO2 is released.
Citric acid
cycle
NAD+
CO2
fumarate
C4
ketoglutarate
C5
NAD+
succinate
C4
FAD
4. Again a substrate is
oxidized, but this time
FAD is reduced to FADH2
CO2
NADH
FADH
ATP
3. ATP is produced as an
energized phosphate is
transferred from a
substrate to ADP.
Figure 9.11 A closer look at the Krebs cycle (Layer 4)
h
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33
Inside the Mitochondria
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Citric acid cycle
inputs
outputs
2 (2c) acetyl groups
4 CO2
6 NAD+
6
NADH
2
FADH2
2 FAD
2 ADP +2
P
2
ATP
34
Inside the Mitochondria
• Electron Transport Chain (ETC)
• Location:
 Eukaryotes: cristae of the mitochondria
 Aerobic prokaryotes: plasma membrane
• Series of carrier molecules:
 Pass energy-rich electrons successively from one to another
 Complex arrays of protein and cytochrome
• Cytochromes are proteins with heme groups with central iron atoms
• The electron transport chain
 Receives electrons from NADH & FADH2
 Produces ATP by oxidative phosphorylation
• Oxygen serves as the final electron acceptor
 Oxygen combines with hydrogen ions to form water
35
Inside the Mitochondria
• The fate of the hydrogens:
 Hydrogens from NADH deliver enough energy to
make 3 ATPs
 Those from FADH2 have only enough for 2 ATPs
 “Spent” hydrogens combine with oxygen
• Recycling of coenzymes increases efficiency
 Once NADH delivers hydrogens, it returns (as NAD+)
to pick up more hydrogens
 However, hydrogens must be combined with oxygen
to make water
 If O2 is not present, NADH cannot release H+
 No longer recycled back to NAD+
36
Inside the Mitochondria
• The electron transport chain complexes pump H+ from the
matrix into the intermembrane space of the mitochondrion
• H+ therefore becomes more concentrated in the
intermembrane space, creating an electrochemical
gradient.
• ATP synthase allows H+ to flow down its gradient.
• Flow of H+ drives the synthesis of ATP from ADP and
inorganic phosphate by ATP synthase.
• This process is called chemiosmosis
 ATP production is linked to the establishment of the H+ gradient
• ATP moves out of mitochondria and is used for cellular
work
 It can be broken down to ADP and inorganic phosphate
 These molecules are returned to the mitochondria for more ATP
production
D:\ImageLibrary1-17\09CellularRespiration\09-15ElectronTransport.mov
37
Organization and Function of the
Electron Transport Chain
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e-
NADH
NADH
e-
e-
e-
NADH and
FADH2
ee-
eGlycolysis
glucose
Electron transport
chain and
chemiosmosis
Citric acid
cycle
Preparatory reaction
pyruvate
2ATP
2ADP
4 ADP
4 ATP total
2
ATP
net
2
H+
ATP
Electron transport chain
NADH-Q
reductase
H+
32 or
34
32 ADP
or 34
ADP
H+
H+
H+
H+
cytochrome
reductase
H+
H+
H+
cytochrome c
coenzyme Q
H+
cytochrome
oxidase
H+
H+
e:
H+
FADH2
high energy
electron
H+
NADH
NAD+
FAD
+
2 H+
low energy
electron
e:
H+
H+
2 H+
H+
H+
H+
ATP
H2O
ADP + P
1O
2 2
H+
H+
H+
Matrix
H+
Intermembrane
space
H+
H+
H+
ATP
synthase
+
complex H
H+
H+
ATP
channel
protein
ATP
H+
Chemiosmosis
H+
H+
H+
H+
Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis
Figure 9.14 ATP synthase, a molecular mill
Review: Cellular Respiration
•
Glycolysis:
2 ATP (substrate-level
phosphorylation)
• Kreb’s Cycle:
2 ATP (substrate-level
phosphorylation)
• Electron transport & oxidative
phosphorylation:
2 NADH (glycolysis) = 6ATP
2 NADH (acetyl CoA) = 6ATP
6 NADH (Kreb’s) = 18 ATP
2 FADH2 (Kreb’s) = 4 ATP
• 38 TOTAL ATP/glucose
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Animation
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