Chapter 8
Cellular Respiration
1
8.1 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
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
3
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)
4
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
5
The Four Phases of Complete
Glucose Breakdown
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NADH
e–
e–
NADH
e–
e–
Cytoplasm
e–
NADH and
FADH2
e–
e–
Glycolysis
Citric acid
cycle
Preparatory reaction
glucose
Mitochondrion
pyruvate
Electron transport
chain and
chemiosmosis
2 ATP
2 ATP
4 ADP
4 ATP total
2
ATP
net gain
2 ADP
2
ATP
32 ADP
or 34
32
or 34
ATP
6
8.2 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
7
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
8
Glycolysis
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Glycolysis
Energy-investment Step
glucose
–2
ATP
ATP
ATP
ADP
G3P
glyceraldehyde-3-phosphate
BPG
1,3-bisphosphoglycerate
3PG
3-phosphoglycerate
ADP
Two ATP are used to get started.
Splitting produces two
3-carbon molecules.
G3P
Energy-harvesting Steps
NAD+
G3P
NAD+
E1
NADH
NADH
BPG
ADP
Oxidation of G3P occurs as
NAD+ receives high-energy
electrons.
BPG
ADP
E2
Substrate-level ATP synthesis.
+2
ATP
ATP
ATP
3PG
3PG
E3
H2O
H2O
PEP
ADP
+2
2
ATP
PEP
ADP
E4
ATP
ATP
(net gain)
Oxidation of 3PG occurs by
removal of water.
Substrate-level ATP synthesis.
ATP
pyruvate
pyruvate
Two molecules of pyruvate
are the end products of
glycolysis.
8.3 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.
10
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.
11
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
 Only 2 ATP per glucose are produced, compared to 36 or 38
ATP molecules per glucose produced by cellular respiration.
12
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
13
8.4 Inside the Mitochondria
• The Preparatory (prep) Reaction
 Connects glycolysis to the citric acid cycle
 End product of glycolysis, pyruvate, enters the
mitochondrial matrix
 Pyruvate is converted to a 2-carbon acetyl group
• Attached to Coenzyme A to form acetyl-CoA
• Electrons are picked up (as hydrogen atom) by NAD+
• CO2 is released and transported out of mitochondria into the
cytoplasm
14
Inside the Mitochondria
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2 NAD+
O
OH
C
2
C
O + 2 CoA
CH
3
pyruvate
2 pyruvate + 2 CoA
2 NADH
CoA
2 C
O + 2 CO2
CH 3
carbon
acetyl CoA dioxide
2 acetyl
CoA + 2 carbon
dioxide
15
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
16
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
17
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.
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
19
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
20
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+
21
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
22
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+
Inside the Mitochondria
• Energy yield from glucose metabolism:
 Net yield per glucose:
• From glycolysis – 2 ATP
• From citric acid cycle – 2 ATP
• From electron transport chain – 32 or 34 ATP
 Energy content:
• Reactant (glucose) 686 kcal
• Energy yield (36 ATP) 263 kcal
• Efficiency is 39%
• The rest of the energy from glucose is lost as heat
24
Accounting of Energy Yield per
Glucose Molecule Breakdown
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2
net
glycolysis
ATP
2
NADH
2
NADH
6
NADH
2
FADH2
2 pyruvate
Mitochondrion
2 acetyl CoA
2 CO2
2
ATP
4 or 6
ATP
6
ATP
18
ATP
4
ATP
subtotal
32
or 34
ATP
Electron transport chain
Cytoplasm
glucose
Citric acid
cycle
4 CO2
6 O2
subtotal
4
ATP
36 or 38
total
ATP
6 H2O
25
8.5 Metabolic Pool
• Foods:
 Sources of energy rich molecules
 Carbohydrates, fats, and proteins
• Degradative reactions (Catabolism) break down
molecules
 Tend to be exergonic (release energy)
• Synthetic reactions (anabolism) build molecules
 Tend to be endergonic (consume energy)
26
The Metabolic Pool Concept
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proteins
carbohydrates
amino
acids
glucose
Glycolysis
fats
glycerol
fatty
acids
ATP
pyruvate
acetyl CoA
Citric
acid
cycle
ATP
Electron
transport
chain
ATP
© C Squared Studios/Getty Images.
27
Metabolic Pool
• All metabolic compounds are part of the
metabolic pool
• Intermediates from respiratory pathways can be
used for anabolism
• Anabolism (synthetic reactions of metabolism):
 Carbohydrates
• Start with acetyl-CoA
• Basically reverses glycolysis (but different pathway)
 Fats
• G3P converted to glycerol
• Acetyl groups are connected in pairs to form fatty acids
28
Metabolic Pool
• Anabolism (cont.):
 Proteins:
• Made up of combinations of 20 different amino
acids
• Some amino acids (11) can be synthesized by
adult humans
• However, other amino acids (9) cannot be
synthesized by humans
– Essential amino acids
– Must be present in the diet
29
Photosynthesis vs. Cellular
Respiration
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Photosynthesis
grana
enzymes
membrane
H2O
NADPH
O2
CO2
O2
ATP
NAD+
H2O
NADH
Cellular Respiration
CH2O
membrane
CH2O
enzyme-catalyzed
reactions
ATP production
via chemiosmosis
ADP
NADP+
CO2
cristae
enzymes
30