6
Pathways that Harvest and
Store Chemical Energy
Chapter 6 Pathways that Harvest and Store Chemical Energy
Key Concepts
• 6.1 ATP, Reduced Coenzymes, and
Chemiosmosis Play Important Roles in
Biological Energy Metabolism
• 6.2 Carbohydrate Catabolism in the Presence
of Oxygen Releases a Large Amount of Energy
• 6.3 Carbohydrate Catabolism in the Absence of
Oxygen Releases a Small Amount of Energy
Chapter 6 Pathways that Harvest and Store Chemical Energy
• 6.4 Catabolic and Anabolic Pathways Are
Integrated
• 6.5 During Photosynthesis, Light Energy Is
Converted to Chemical Energy
• 6.6 Photosynthetic Organisms Use Chemical
Energy to Convert CO2 to Carbohydrates
Chapter 6 Opening Question
Why does fresh air inhibit the formation of
alcohol by yeast cells?
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Energy is stored in chemical bonds and can be
released and transformed by metabolic
pathways.
Chemical energy available to do work is termed
free energy (G).
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Five principles governing metabolic pathways:
1. Chemical transformations occur in a series of
intermediate reactions that form a metabolic
pathway.
2. Each reaction is catalyzed by a specific
enzyme.
3. Most metabolic pathways are similar in all
organisms.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
4. In eukaryotes, many metabolic pathways
occur inside specific organelles.
5. Each metabolic pathway is controlled by
enzymes that can be inhibited or activated.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
In cells, energy-transforming reactions are often
coupled:
An energy-releasing (exergonic) reaction is
coupled to an energy-requiring (endergonic)
reaction.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Adenosine triphosphate (ATP) is a kind of
“energy currency” in cells.
Energy released by exergonic reactions is stored
in the bonds of ATP.
When ATP is hydrolyzed, free energy is released
to drive endergonic reactions.
Figure 6.1 The Concept of Coupling Reactions
Figure 6.2 ATP
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Hydrolysis of ATP is exergonic:
ATP  H2O  ADP  Pi  freeenergy
ΔG is about –7.3 kcal
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Free energy of the bond between phosphate
groups is much higher than the energy of the
O—H bond that forms after hydrolysis.
text art pg 102 here
(1st one, in left-hand column)
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Phosphate groups are negatively charged, so
energy is required to get them near enough to
each other to make the covalent bonds in the
ATP molecule.
ATP can be formed by substrate-level
phosphorylation or oxidative phosphorylation.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Energy can also be transferred by the transfer of
electrons in oxidation–reduction, or redox
reactions.
• Reduction is the gain of one or more
electrons.
• Oxidation is the loss of one or more electrons.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Oxidation and reduction always occur together.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Transfers of hydrogen atoms involve transfers of
electrons (H = H+ + e–).
When a molecule loses a hydrogen atom, it
becomes oxidized.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
The more reduced a molecule is, the more
energy is stored in its bonds.
Energy is transferred in a redox reaction.
Energy in the reducing agent is transferred to the
reduced product.
Figure 6.3 Oxidation, Reduction, and Energy
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Coenzyme NAD+ is a key electron carrier in
redox reactions.
NAD+ (oxidized form)
NADH (reduced form)
Figure 6.4 A NAD+/NADH Is an Electron Carrier in Redox Reactions
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Reduction of NAD+ is highly endergonic:



NAD  H  2e  NADH
Oxidation of NADH is highly exergonic:

NADH  H 
1

2
O2  NAD  H 2O
Figure 6.4 B NAD+/NADH Is an Electron Carrier in Redox Reactions
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
• In cells, energy is released in catabolism by
oxidation and trapped by reduction of
coenzymes such as NADH.
• Energy for anabolic processes is supplied by
ATP.
Oxidative phosphorylation transfers energy
from NADH to ATP.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Oxidative phosphorylation couples oxidation of
NADH:
NADH  NAD  H   2e   energy
with production of ATP:
energy  ADP  Pi  ATP
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
The coupling is chemiosmosis—diffusion of
protons across a membrane, which drives the
synthesis of ATP.
Chemiosmosis converts potential energy of a
proton gradient across a membrane into the
chemical energy in ATP.
Figure 6.5 A Chemiosmosis
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
ATP synthase—membrane protein with two
subunits:
F0 is the H+ channel; potential energy of the
proton gradient drives the H+ through.
F1 has active sites for ATP synthesis.
Figure 6.5 B Chemiosmosis
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Chemiosmosis can be demonstrated
experimentally.
A proton gradient can be introduced artificially in
chloroplasts or mitochondria in a test tube.
ATP is synthesized if ATP synthase, ADP, and
inorganic phosphate are present.
Figure 6.6 An Experiment Demonstrates the Chemiosmotic Mechanism
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Cellular respiration is a major catabolic pathway.
Glucose is oxidized:
carbohydra te  6O2  6CO2  6H 2O  chemical energy
Photosynthesis is a major anabolic pathway.
Light energy is converted to chemical energy:
6CO2  6H 2O  light energy  6O2  carbohydra te
Figure 6.7 ATP, Reduced Coenzymes, and Metabolism
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Cellular Respiration
A lot of energy is released when reduced
molecules with many C—C and C—H bonds
are fully oxidized to CO2.
Oxidation occurs in a series of small steps in
three pathways:
1. glycolysis
2. pyruvate oxidation
3. citric acid cycle
Figure 6.8 Energy Metabolism Occurs in Small Steps
Figure 6.9 Energy-Releasing Metabolic Pathways
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Glycolysis: ten reactions.
Takes place in the cytosol.
Final products:
2 molecules of pyruvate (pyruvic acid)
2 molecules of ATP
2 molecules of NADH
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 2)
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 3)
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Examples of reaction types common in metabolic
pathways:
Step 6: Oxidation–reduction
Step 7: Substrate-level phosphorylation
text art pg 107 here
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Pyruvate Oxidation:
Products: CO2 and acetate; acetate is then
bound to coenzyme A (CoA)
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Citric Acid Cycle: 8 reactions, operates twice
for every glucose molecule that enters
glycolysis.
Starts with Acetyl CoA; acetyl group is oxidized
to two CO2.
Oxaloacetate is regenerated in the last step.
Figure 6.11 The Citric Acid Cycle
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Final reaction of citric acid cycle:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Electron transport/ATP Synthesis:
NADH is reoxidized to NAD+ and O2 is reduced
to H2O in a series of steps.
Respiratory chain—series of redox carrier
proteins embedded in the inner mitochondrial
membrane.
Electron transport—electrons from the
oxidation of NADH and FADH2 pass from one
carrier to the next in the chain.
Figure 6.12 Electron Transport and ATP Synthesis in Mitochondria
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
The oxidation reactions are exergonic; the
energy is used to actively transport H+ ions out
of the mitochondrial matrix, setting up a proton
gradient.
ATP synthase in the membrane uses the H+
gradient to synthesize ATP by chemiosmosis.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
About 32 molecules of ATP are produced for
each fully oxidized glucose.
The role of O2: most of the ATP produced is
formed by oxidative phosphorylation, which is
due to the reoxidation of NADH.
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen
Releases a Small Amount of Energy
Under anaerobic conditions, NADH is reoxidized
by fermentation.
There are many different types of fermentation,
but all operate to regenerate NAD+.
The overall yield of ATP is only two—the ATP
made in glycolysis.
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen
Releases a Small Amount of Energy
Lactic acid fermentation:
End product is lactic acid (lactate).
NADH is used to reduce pyruvate to lactic acid,
thus regenerating NAD+.
Figure 6.13 A Fermentation
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen
Releases a Small Amount of Energy
Alcoholic fermentation:
End product is ethyl alcohol (ethanol).
Pyruvate is converted to acetaldehyde, and CO2
is released. NADH is used to reduce
acetaldehyde to ethanol, regenerating NAD+ for
glycolysis.
Figure 6.13 B Fermentation
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Metabolic pathways are linked.
Carbon skeletons (molecules with covalently
linked carbon atoms) can enter catabolic or
anabolic pathways.
Figure 6.14 Relationships among the Major Metabolic Pathways of the Cell
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Catabolism:
Polysaccharides are hydrolyzed to glucose,
which enter glycolysis.
Lipids break down to fatty acids and glycerol.
Fatty acids can be converted to acetyl CoA.
Proteins are hydrolyzed to amino acids that can
feed into glycolysis or the citric acid cycle.
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Anabolism:
Many catabolic pathways can operate in reverse.
Gluconeogenesis—citric acid cycle and
glycolysis intermediates can be reduced to
form glucose.
Acetyl CoA can be used to form fatty acids.
Some citric acid intermediates can form nucleic
acids.
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Amounts of different molecules are maintained
at fairly constant levels—the metabolic pools.
This is accomplished by regulation of enzymes—
allosteric regulation, feedback inhibition.
Enzymes can also be regulated by altering the
transcription of genes that encode the
enzymes.
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Photosynthesis involves two pathways:
Light reactions convert light energy into
chemical energy (in ATP and the reduced
electron carrier NADPH).
Carbon-fixation reactions use the ATP and
NADPH, along with CO2, to produce
carbohydrates.
Figure 6.15 An Overview of Photosynthesis
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Light is a form of electromagnetic radiation,
which travels as a wave but also behaves as
particles (photons).
Photons can be absorbed by a molecule, adding
energy to the molecule—it moves to an excited
state.
Figure 6.16 The Electromagnetic Spectrum
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Pigments: molecules that absorb wavelengths in
the visible spectrum.
Chlorophyll absorbs blue and red light; the
remaining light is mostly green.
Absorption spectrum—plot of light energy
absorbed against wavelength.
Action spectrum—plot of the biological activity
of an organism against the wavelengths to
which it is exposed
Figure 6.17 Absorption and Action Spectra
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
In plants, two chlorophylls absorb light energy
chlorophyll a and chlorophyll b.
Accessory pigments—absorb wavelengths
between red and blue and transfer some of that
energy to the chlorophylls.
Figure 6.18 The Molecular Structure of Chlorophyll (Part 1)
Figure 6.18 The Molecular Structure of Chlorophyll (Part 2)
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
The pigments are arranged into lightharvesting complexes, or antenna systems.
A photosystem spans the thylakoid membrane
in the chloroplast; it consists of antenna
systems and a reaction center.
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
When chlorophyll (Chl) absorbs light, it enters an
excited state (Chl*), then rapidly returns to
ground state, releasing an excited electron.
Chl* gives the excited electron to an acceptor
and becomes oxidized to Chl+.
The acceptor molecule is reduced.

Chl *  acceptor  Chl  acceptor

Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
The electron acceptor is first in an electron
transport system in the thylakoid membrane.
Final electron acceptor is NADP+, which gets
reduced:
NADP   H   2e   NADPH
ATP is produced chemiosmotically during
electron transport (photophosphorylation).
Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Two photosystems:
• Photosystem I absorbs light energy at 700
nm, passes an excited electron to NADP+,
reducing it to NADPH.
• Photosystem II absorbs light energy at
680 nm, produces ATP, and oxidizes water
molecules.
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Photosystem II
When Chl* gives up an electron, it is unstable
and grabs an electron from another molecule,
H2O, which splits the H—O—H bonds.

2 Chl *  H 2O  2 Chl  H  1 2 O2
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
Photosystem I
When Chl* gives up an electron, it grabs another
electron from the end of the transport system of
Photosystem II. This electron ends up reducing
NADP+ to NADPH.
Concept 6.5 During Photosynthesis, Light Energy
Is Converted to Chemical Energy
ATP is needed for carbon-fixation pathways.
Cyclic electron transport uses only
photosystem I and produces ATP; an electron
is passed from an excited chlorophyll and
recycles back to the same chlorophyll.
Figure 6.20 Cyclic Electron Transport Traps Light Energy as ATP
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
The Calvin cycle: CO2 fixation. It occurs in the
stroma of the chloroplast.
Each reaction is catalyzed by a specific enzyme.
Figure 6.21 The Calvin Cycle
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
1. Fixation of CO2:
CO2 is added to ribulose 1,5-bisphosphate
(RuBP).
Ribulose bisphosphate
carboxylase/oxygenase (rubisco) catalyzes
the reaction.
A 6-carbon molecule results, which quickly
breaks into two 3-carbon molecules: 3phosphoglycerate (3PG).
Figure 6.22 RuBP Is the Carbon Dioxide Acceptor
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
2. 3PG is reduced to form glyceraldehyde 3phosphate (G3P).
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
3. The CO2 acceptor, RuBP, is regenerated from
G3P.
Some of the extra G3P is exported to the cytosol
and is converted to hexoses (glucose and
fructose).
When glucose accumulates, it is linked to form
starch, a storage carbohydrate.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
The C—H bonds generated by the Calvin cycle
provide almost all the energy for life on Earth.
Photosynthetic organisms (autotrophs) use
most of this energy to support their own growth
and reproduction.
Heterotrophs cannot photosynthesize and
depend on autotrophs for chemical energy.
Answer to Opening Question
Pasteur’s findings:
Catabolism of the beet sugar is a cellular
process, so living yeast cells must be present.
With air (O2) yeasts used aerobic metabolism to
fully oxidize glucose to CO2.
Without air, yeasts used alcoholic fermentation,
producing ethanol, less CO2, and less energy
(slower growth).
Figure 6.23 Products of Glucose Metabolism