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Chapter 9
Cellular Respiration: Harvesting
Chemical Energy
Light energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
molecules
CO2 + H2O
Cellular respiration
in mitochondria
ATP
powers most cellular work
Heat
+ O2
Ask a Cell Biologist:
What is Cellular Respiration?
 Cellular respiration is the aerobic
harvesting of energy from organic
molecules
 It is a catabolic pathway
 It contains mostly exergonic reactions
that release energy
Summary Equation for Cellular Respiration
C6H12O6 + 6O2
glucose
oxygen
6CO2 + 6H2O + ATP + Heat
Carbon
dioxide
Greater number of bonds = Greater potential
energy
O=C=O
carbon dioxide
glucose
Oxidation-Reduction Reactions
 Oxidation-Reduction (Redox)
reactions
 Transfer electrons from one reactant
to another by oxidation and reduction
Oxidation-Reduction Reactions
 In oxidation
 A substance LOSES electrons, or is
oxidized
 In reduction
 A substance GAINS electrons, or is
reduced
Example of Redox Reaction
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Oxidation of Organic Fuel
 During cellular respiration
 Glucose is oxidized and oxygen is reduced
 It is the movement of hydrogen atoms and
their electrons from glucose that are
important
BECOMES OXIDIZED
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
BECOMES REDUCED
Mitochondrion: Site of Cellular Respiration
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Mitochondrial
DNA
100 µm
Stages of Cellular Respiration
 Glycolysis
 Breaks down glucose into two molecules of
pyruvate (pyruvic acid)
 Citric acid cycle (Krebs Cycle)
 Completes the breakdown of energy originally in
glucose
• Electron transport chain
 Generates lots of ATP
Glycolysis
Glucose
Pyruvate
CYTOSOL
ATP
Pyruvate
oxidation
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
MITOCHONDRION
ATP
ATP
Glycolysis
 Glycolysis
 Means “splitting of sugar”
 Breaks down glucose into pyruvate
 Occurs in the cytoplasm of the cell
 Ancient pathway
 No oxygen required!
Glycolysis: 2 Phases
Glycolysis
ATP
Citric
acid
cycle
Oxidative
phosphorylation
ATP
ATP
Energy investment phase
Glucose
2 ADP + 2
P
2 ATP
used
Energy payoff phase
4 ADP + 4
P
2 NAD+ + 4 e- + 4 H +
4 ATP
formed
2 NADH
+ 2 H+
2 Pyruvate + 2 H2O
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H +
2 Pyruvate + 2 H2O
2 ATP
2 NADH+ 2 H+
Gradual Harvesting of Energy
 Electrons from organic compounds
 Are usually first transferred to NAD+, an electron
shuttle
 NAD+ becomes reduced to NADH as it accepts
electrons and H’s from glucose
 Dehydrogenase is an enzyme which helps move the
electrons by removing 2 H atoms (and their electrons)
from glucose and giving them to NAD+
NAD+ + 2H
NADH + H+
Dehydrogenase
Hydrogen Ions = H+ = Proton
HYDROGEN ATOM
Substrate Level Phosphorylation
• Both glycolysis and the citric acid cycle
• Can generate ATP by substrate-level phosphorylation
ENZYME
ENZYME
ADP
P
SUBSTRATE
+
PRODUCT
ATP
Citric Acid Cycle
 The citric acid cycle completes the
energy-yielding oxidation of organic
molecules
 The citric acid cycle
 Takes place in the matrix of the
mitochondrion
Pre-citric Acid Cycle
 Before the citric acid cycle can begin

Pyruvate must first be converted to acetyl CoA, which
links the cycle to glycolysis
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
CH3
Acetyl CoA
CO2
Coenzyme A
Citric Acid Cycle
Pyruvate
Glycolysis
(from glycolysis,
2 molecules per glucose)
ATP
Citric
acid
cycle
Oxidative
phosphorylation
ATP
ATP
CO2
CoA
NADH
+ 3 H+
Acetyl CoA
CoA
CoA
Oxaloacetate
(4 C)
FADH2
Citrate
(6 C)
Citric
acid
cycle
2 CO2
3 NAD+
3 NADH
FAD+
+ 3 H+
ADP + P i
ATP
Electron Transport Chain
 Mitochondrial inner membrane
proteins (carrier molecules) pass
electrons in a series of steps instead
of in one explosive reaction
 Uses the energy from the electron
transfer to form ATP
Electron Transport Chain
 At the end of the chain
 This makes oxygen the
final electron acceptor
NADH
FADH2
Free energy (G) relative to O2 (kcl/mol)
 Electrons are passed to
oxygen, forming water
50
40
FMN
I
FE•S
CARRIER
MOLECULES
FAD
FE•S
II
O
III
Cyt b
30
FE•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
oxygen
0
2 H + + 12
O2
H2 O
Chemiosmosis
Chemiosmosis and the electron transport chain
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane
ATP
ATP
H+
H+
H+
Intermembrane
space
Protein complex
of electron
carners
Q
I
Inner
mitochondrial
membrane
IV
III
ATP
synthase
II
FADH2
FAD+
NAD+
NADH
Mitochondrial
matrix
H+
Cyt c
2 H+ + 1/2 O2
H2O
ADP +
(Carrying electrons
from, food)
ATP
Pi
H+
Chemiosmosis
Electron transport chain
+
ATP
synthesis
powered by the flow
Electron transport and pumping of protons (H ),
+
+
which create an H gradient across the membrane Of H back across the membrane
Oxidative phosphorylation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chemiosmosis
 At certain steps along the electron
transport chain
 Electron transfer causes protein complexes
to pump H+ from the mitochondrial matrix to
the intermembrane space
 The resulting H+ gradient
 Stores energy
 Drives chemiosmosis in ATP synthase
But what’s Chemiosmosis really?
 Chemiosmosis
 Is an energy-coupling mechanism that
uses energy in the form of a H+
gradient across a membrane to drive
cellular work
 Is referred to as a proton-motive force
Chemiosmosis
 ATP synthase
INTERMEMBRANE
SPACE
H
+
H+
 Is the enzyme that actually
makes ATP
H+
H+
H+
H+
H+
H+
ADP
+
Pi
MITOCHONDRIAL
MATRIX
ATP
Following the Electrons (Energy)
 During respiration, most energy flows in this
sequence
 Glucose to NADH to electron transport chain to
chemiosmosis to ATP
 About 40% of the energy in a glucose molecule
 Is transferred to ATP during cellular respiration, making
approximately 32 ATP
Review of Stages of Cellular Respiration
Electron shuttles
span membrane
2 NADH
Glycolysis
2 Pyruvate
Glucose
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
Pyruvate oxidation
2 Acetyl CoA
 2 ATP
Maximum per glucose:
CYTOSOL
6 NADH
2 FADH2
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
 2 ATP
 about 26 or 28 ATP
About
30 or 32 ATP
So what happens when there is little or no
O2 ?
•Without O2, the ETC will cease to operate
•BUT glycolysis couples with fermentation
to still produce 2 ATP’s
•Without the ETC however, fermentation
needs an alternate way to generate NAD+
Fermentation
• Two common types of fermentation
are:
• Alcohol fermentation
• Lactic acid fermentation
Alcohol Fermentation
Pyruvate is converted to ethanol in two steps
1. Releases CO2 from pyruvate making acetaldehyde
2. Acetaldehyde reduced by NADH to ethanol which
regenerates NAD+
• Many bacteria undergo alcohol fermentation in
anaerobic conditions
• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
Lactic Acid Fermentation
• Pyruvate directly reduced by NADH to form
lactate (ionized form of lactic acid)
•No CO2 released
• Fungi and bacteria undergo lactic acid
fermentation
•Used to make cheese and yogurt
• Human muscle cells will undergo lactic acid
fermentation when O2 scarce during heavy
exercise
Summary of Types of Fermentation
2 ADP  2 P
Glucose
2 ADP  2 P
2 ATP
i
Glycolysis
Glucose
i
2 ATP
Glycolysis
2 Pyruvate
2 NAD 
2 Ethanol
(a) Alcohol fermentation
2 NADH
 2 H
2 NAD 
2 CO2
2 Acetaldehyde
2 NADH
 2 H
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
Fermentation Overview
Glucose
Glycolysis
CYTOSOL
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol,
lactate, or
other products
Pyruvate as a key juncture
in catabolism
Acetyl CoA
Citric
acid
cycle