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Cellular Respiration
How Cells Harvest Energy
Chapter 6
ATP Is Universal
Energy Source

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Photosynthetic organisms get energy
from the sun
The light energy is converted to the
chemical bond energy of ATP
Main Types of
Energy-Releasing Pathways
Anaerobic pathways Aerobic pathways

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Evolved first
Don’t require oxygen
Start with glycolysis in
cytoplasm
Completed in cytoplasm
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Evolved later
Require oxygen
Start with glycolysis in
cytoplasm
Completed in
mitochondria
Redox reactions release energy when
electrons “fall” from a hydrogen carrier to
oxygen

NADH delivers electrons to a series of protein
complexes in an electron transport chain

As electrons move from carrier to carrier, their
energy is released in small quantities
In cellular respiration, electrons “fall” down an
energy staircase and finally reduce O2
NADH
NAD+
+
ATP
2e
Controlled
release of
energy for
synthesis
of ATP
H+
+
2 H
2e
1
2
H2O
O2
Main Pathways Start
with Glycolysis

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Glycolysis occurs in cytoplasm
Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
Glucose
Pyruvate
Net Energy Yield
from Glycolysis
Energy investment phase:
2 ATP invested
Energy releasing phase:
2 NADH formed
4 ATP formed
Net yield: 2 ATP and 2 NADH
Pyruvate is chemically groomed
for the Krebs cycle
• Each molecule of pyruvate is broken down to
form CO2 and acetyl co-A, which enters the
Krebs cycle
Acetyl CoA
(acetyl
coenzyme A)
Pyruvic
acid
CO2
The Krebs cycle completes the oxidation
of glucose, generating many NADH and
FADH2 molecules
• The Krebs cycle is a
series of redox
reactions in which
enzymes strip away
electrons and H+
Acetyl CoA
KREBS
CYCLE
2
2 CO2
Oxidative phosphorylation
powers most ATP production

The electrons from NADH and FADH2
travel down the electron transport chain
to oxygen
Chemiosmosis powers most
ATP production
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Energy released by the electrons is used to
pump H+ into the space between the
mitochondrial membranes (intermembrane
space) by active transport
In chemiosmosis, the H+ ions diffuse back
through the inner membrane through ATP
synthase , which capture the energy to
synthesize ATP
Overview:
Oxidative Phosphorylation
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Occurs in the inner mitochondrial
membrane
Coenzymes (NADH, FADH2) deliver
electrons to electron transport systems
Electron transport sets up H+ ion
gradients
Flow of H+ down concentration
gradients powers ATP formation
Overview:
Electron Transport System
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Electron transport systems are embedded in
inner mitochondrial compartment
NADH and FADH2 give up electrons that they
picked up in earlier stages to electron
transport system
Electrons are transported through the system
The final electron acceptor is oxygen
Summary of Aerobic Cellular
Respiration

Glycolysis
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Krebs cycle
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2 ATP formed by substrate-level phosphorylation
ETC & Chemiosmosis (oxidative
phosphorylation)
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2 ATP formed by substrate-level phosphorylation
32 - 34 ATP formed
Total ATP molecules formed: 36 - 38
Anaerobic Pathways
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Do not use oxygen
Produce less ATP than aerobic pathways
Two types
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Lactic acid fermentation
Alcoholic fermentation
Lactic Acid Fermentation
2
NAD+
2
2
NADH
NADH
2
NAD+
GLYCOLYSIS
2 ADP + 2
P
2
ATP
2 Pyruvate
Glucose
2 Lactate
Alcoholic Fermentation
2
NAD+
2
2
NADH
NADH
2
NAD+
GLYCOLYSIS
2 ADP + 2
Glucose
P
2
2
ATP
2 Pyruvate
CO2
released
2 Ethanol