Cellular Respiration: Harvesting
Chemical Energy
Chapter 9
Life Is Work
• Living cells require energy from outside sources
• Plants  E from ?
• Animals  E from ?
Energy flows into
ecosystem as light
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2 + H2O
Cellular respiration
in mitochondria
Organic
+
molecules
O2
ATP
ATP powers work
ATP powers most cellular work
Heat
energy
Energy leaves as heat
• Photosynthesis
– Organelle = ?
– Generates O2 and organic molecules
• Cellular respiration
– Organelle = ?
– Uses organic molecules to generate ATP
Catabolic Pathway review
• Organic molecules have potential (chemical)
energy
• Exergonic rxns break down organic molecules
 energy (and heat)
Cellular Respiration
• Aerobic respiration
– Uses O2
– ATP produced
Anaerobic respiration
Does not use O2
ATP produced
Cellular respiration
1. Glycolysis
Occurs in cytoplasm
Anaerobic
Glucose + 2NAD+ + 2ATP  2 pyruvate+ 2NADH + 4ATP
• 1 glucose  2 ATP and 2 pyruvate
• Glucose oxidized to pyruvate (loses electron)
• NAD+ reduced to NADH (gains electron)
Glycolysis
Electron donor
Electrons
carried
via NADH
mitochondrion
Glycolysis
Pyruvate
Glucose
Cytosol
ATP
Substrate-level
phosphorylation
CYTOSOL
Glycolysis
1. Energy investment phase uses 2 ATP
2. Energy payoff phase 
– 4 ATP produced
– 2NAD+ reduced to 2NADH
– 1 glucose split to 2 pyruvate
Glucose + 2NAD+ + 2ATP  2 pyruvate+ 2NADH + 4ATP
Energy investment phase
Glucose
2 ADP + 2 P
2 ATP
used
4 ATP
formed
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
10 enzymatic
steps in
glycolysis
2. Citric acid cycle (Krebs cycle)
• mt matrix
– Matrix is enclosed by the inner membrane
What’s in the matrix?
Enzymes (acetyl CoA)
mtDNA
Ribosomes
Citric acid cycle
2Pyruvate + NAD+ + FADH  2ATP + NADH +
FADH2 + CO2 + H2O
Where did the pyruvate come from?
How did it get into the mt matrix?
# ATP generated?
Waste product? Where does it go?
NADH and FADH2 can donate electrons later
What happened to the sugar?
O2?
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
MITCHONDRION
citric acid cycle
1. Convert 2pyruvate to 2acetyl A (before cycle)
Acetyl CoA links
glycolysis to cycle
Pyruvate diffuses into mt matrix and is
converted to acetyl
CoA
CYTOSOL
MITOCHONDRION
NAD+ NADH +
H+
2
1
Pyruvate
3
CO2
Coenzyme A
Acetyl CoA
Transport protein
Cellular Respiration: Bioflix animation
• 2. The citric acid cycle
Pyruvate
CO2
NAD+
8 enzymatic steps
CoA
NADH
+ H+
Acetyl CoA
CoA
ATP:
For each pyruvate?
CoA
Citric
acid
cycle
FADH2
2 CO2
For each glucose?
3 NAD+
3 NADH
FAD
+ 3 H+
ADP + P i
ATP
For each turn of cycle?
Summary of citric acid cycle
• Per molecule glucose =2 pyruvate
– NADH and FADH 2 electron donors
– 2 ATP (1 per turn) per glucose
• CO 2 produced (2 per turn)  out
• mt matrix
BIO 231 TCA cycle animation: Acetyl CoA formation
Text Activity: The Citric Acid Cycle
3. oxidative phosphorylation in mt cristae
Cristae compartmentalize mt inner membrane = more surface area
What happens?
NADH and FADH 2 donate electrons in the series of steps
Oxygen accepts electrons  water
H+ proton gradient
ADP + P  ATP
34 ATP produced
Add up the ATP yield per glucose:
Glycolysis + Citric acid cycle + Ox Phos =
Oxidative phosphorylation:  34 ATP
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
Stepwise Energy Harvest via Electron
Transport Chain
1. Controlled rxns
H2 + 1/2
O2
2H
(from food via
NADH)
2 H+ + 2
e–
Explosive
release of
heat and
light
energy
1/
2
Controlled
release of
energy for
synthesis
of
ATP
O2
1/
2
O2
(a) Uncontrolled
reaction
(b) Cellular respiration
NADH
50
2 e–
• 2. electron transport
is a fall in energy during
each step to control
release of fuel energy
NAD+
FADH2
2 e–
40
FM
N

FAD
Multiprotein
complexes
FAD
Fe•S
Fe•S

Q

Cyt b
30
Fe•S
Cyt c1
I
V
Cyt c
Cyt a
20
10
0
Cyt a3
2 e–
(from NADH
or FADH2)
2 H+ + 1/2 O2
H2O
Electron Transport Chain
powered by redox reactions
Overview: Wiley
Electron Transport: Wiley
Watch the electrons
 BIO 231 Electron transport animation
Watch the electrons
• In addition to electron transfer…….
• 3. H+ ions pumped out
H+ gradient, a proton force
• ET chain e- pumps H+ across mt membrane
• H+ gradient drives ATP production
• Interactive concepts
• Watch the H+ ions
• Mcgraw hill electron transport
• Watch the H+, no audio
Chemiosmosis couples energy of
electron transport to ATP synthesis
INTERMEMBRANE SPACE
H
+
Stator
Rotor
• ATP synthase
– H+ ion enters for one turn
– ADP + P  ATP
Virtual Cell: Electron
Transport Chain animation
Internal
rod
Catalytic
knob
ADP
+
P
i
ATP
MITOCHONDRIAL MATRIX
H+
H+
H+
H+
Cyt c
Protein complex
of electron
carriers
V
Q



FADH2
NADH
(carrying electrons
from food)
ATP
synthase
FAD
2 H+ + 1/2O2
NAD
H2O
ADP + P i
+
ATP
H+
1 Electron transport chain
Oxidative phosphorylation
2 Chemiosmosis
An Accounting of ATP Production by
Cellular Respiration
• Most energy:
glucose  NADH  electron transport chain 
proton-motive force  ATP
= ~38 ATP total
Electron shuttles
span membrane
CYTOSOL
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
Glycolysis
Glucose
2
Pyruvate
+2 ATP
2 NADH
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Citric
acid
cycle
2
Acetyl
CoA
+2ATP
Maximum per glucose:
Glycolysis
Cytosol
2 FADH2
6 NADH
About
36 or 38 ATP
+ about 32 or 34
ATP
Citric Acid Cycle
mt
Ox. Phos.
mt
Anaerobic respiration (no O2)
Anaerobic respiration (cytoplasm)
Prokaryotes
Eukaryotes
Generate ATP without O2
1. Glycolysis
2. Fermentation
Fermentation
No electron transport chain
NAD+ reused in glycolysis (way to keep generating
ATP without O2)
Alcohol fermentation
• Pyruvate + NADH  ethanol + NAD+ + CO2
• Bacteria
• Yeast by humans for:
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 Ethanol
(a) Alcohol fermentation
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
Lactic acid fermentation
Pyruvate + NADH  lactate + NAD+
• Bacteria, fungi in cheese making
• Human muscle cells use lactic acid fermentation to
generate Pyruvate + NADH  lactate + NAD+
• ATP when O2 is low.
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 NAD+
2 Lactate
(b) Lactic acid fermentation
2 NADH
+ 2 H+
2 Pyruvate
Fermentation (no O2) vs. Aerobic
Respiration
• Both use glycolysis to oxidize glucose (and other
organic fuels ) to pyruvate
• ATP
– Cellular respiration  38 ATP per glucose
– Fermentation  2 ATP per glucose
• Obligate anaerobes
– fermentation
– cannot survive in the presence of O2
– Ex. clostridium botulinum
• Facultative anaerobes
– Yeast and many bacteria
– can survive using either fermentation or cellular
respiration (pyruvate can be used either way)
– Ex. E. coli, Streptococcus
Facultative anaerobe
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol
or
lactate
Acetyl CoA
Citric
acid
cycle
The Evolutionary Significance of
Glycolysis
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient
prokaryotes before O2 on planet
Glycolysis and the citric acid cycle
connect to other metabolic pathways
The Versatility of Catabolism
• Glycolysis and fuel
– Carbohydrates – many accepted
– Proteins  amino acids;  glycolysis or the citric
acid cycle
– Fats  glycerol  glycolysis
– Fatty acids  acetyl CoA
– An oxidized gram of fat produces >2X ATP as
oxidized gram of carbohydrate
Proteins
Carbohydrates
Amino
acids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde-3-
NH3
P
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Fatty
acids
Fermentation and anaerobic
respiration enable cells to produce ATP
without the use of oxygen
• Most cellular respiration requires O2 to produce
ATP
• Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
Regulation of Cellular Respiration via
Feedback Mechanisms
• Feedback inhibition is the most common
mechanism for control
• If ATP concentration begins to drop, respiration
speeds up; when there is plenty of ATP, respiration
slows down
• Control of catabolism is based mainly on regulating
the activity of enzymes at strategic points in the
catabolic pathway
Biosynthesis (Anabolic Pathways)
• The body uses small molecules to build other
substances
• These small molecules may come directly from
food, from glycolysis, or from the citric acid cycle