Download Pathways that Harvest and Store Chemical Energy

6
Pathways that Harvest and
Store Chemical Energy
Chapter 6 Pathways that Harvest and Store Chemical Energy
Key Concepts
6.1 ATP and Reduced Coenzymes 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 and Reduced Coenzymes 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 and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Five principles govern 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 and Reduced Coenzymes 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 and Reduced Coenzymes 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.
•  Two coupling molecules are the coenzymes
ATP and NADH.
Concept 6.1 ATP and Reduced Coenzymes 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 and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Hydrolysis of ATP is exergonic:
ATP + H2O → ADP + Pi + free energy
ΔG is about –7.3 kcal/mol
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
The free energy of the bond between phosphate
groups is much higher than the energy of the
O—H bond that forms after hydrolysis.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Energy can also be transferred by the transfer of
electrons in reduction–oxidation, 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 and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Oxidation and reduction always occur together.
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
It is also useful to think of oxidation and
reduction in terms of gain or loss of hydrogen
atoms:
•  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 and Reduced Coenzymes 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 and Reduced Coenzymes 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 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 1)
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Reduction of NAD+ is highly endergonic:
NAD+ + H+ + 2 e– → NADH
Oxidation of NADH is highly exergonic:
NADH + H+ + ½ O2 → NAD+ + H2O
Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 2)
Concept 6.1 ATP and Reduced Coenzymes Play Important Roles
in Biological Energy Metabolism
Energy is released in catabolism by oxidation
and trapped by reduction of coenzymes such
as NADH.
Energy for anabolic processes is supplied by
ATP.
Most energy-releasing reactions produce NADH,
but most energy-consuming reactions require
ATP.
Oxidative phosphorylation transfers energy from
NADH to ATP.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Cellular respiration: the set of metabolic
reactions used by cells to harvest energy from
food
A lot of energy is released when reduced
molecules with many C—C and C—H bonds
are fully oxidized to CO2.
The oxidation occurs in a series of small steps,
allowing the cell to harvest about 34% of the
energy released.
Figure 6.5 Energy Metabolism Occurs in Small Steps
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Catabolism of glucose under aerobic conditions
(in the presence of O2), occurs in three linked
biochemical pathways:
•  Glycolysis—glucose is converted to
pyruvate.
•  Pyruvate oxidation—pyruvate is oxidized
to acetyl CoA and CO2.
• 
Citric acid cycle—acetyl CoA is oxidized to
CO2.
Figure 6.6 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 6.7 Glycolysis Converts Glucose into Pyruvate (Part 1)
Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 2)
Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 3)
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Steps 6 and 7 are examples of reactions that
occur repeatedly in metabolic pathways:
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Oxidation–reduction (step 6): exergonic;
glyceraldehyde 3-phosphate is oxidized and
energy is trapped via reduction of NAD+ to
NADH.
Substrate-level phosphorylation (step 7): also
exergonic; energy released transfers a
phosphate from 1,3-bisphosphoglycerate to
ADP, forming ATP.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Pyruvate Oxidation
•  Occurs in mitochondria in eukaryotes.
•  Products: CO2 and acetate; acetate is then
bound to coenzyme A (CoA) to form acetyl
CoA.
•  NAD+ is reduced to NADH.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Citric Acid Cycle
•  Eight reactions
•  Occurs in mitochondria in eukaryotes
•  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 6.8 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
Cells transfer energy from NADH and FADH2 to
ATP by oxidative phosphorylation:
•  NADH oxidation is used to actively transport
protons (H+) across the inner mitochondrial
membrane, resulting in a proton gradient.
•  Diffusion of protons back across the
membrane then drives the synthesis of ATP.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
When NADH is reoxidized to NAD+, O2 is
reduced to H2O:
NADH + H+ + ½ O2
→ NAD+ + H2O
This occurs in a series of redox electron carriers,
called the respiratory chain, embedded in the
inner membrane of the mitochondrion.
Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Electron transport: electrons from the oxidation
of NADH and FADH2 pass from one carrier to
the next in the chain.
The oxidation reactions are exergonic, energy
released is used to actively transport H+ ions
across the membrane.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
Oxidation is always coupled with reduction.
When NADH is oxidized to NAD+, the reduction
reaction is the formation of water from O2.
2 H+ + 2 e– + ½ O2
→ H 2O
The key role of O2 in cells is to act as an electron
acceptor and become reduced.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
ATP synthase uses the H+ gradient to drive
synthesis of ATP by chemiosmosis:
•  Chemiosmosis: Movement of ions across a
semipermeable barrier from a region of
higher concentration to a region of lower
concentration.
ATP synthase converts the potential energy of
the proton gradient into chemical energy in
ATP.
Figure 6.10 Chemiosmosis
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
ATP synthase is a molecular motor with two
subunits:
•  F0 is a transmembrane domain that
functions as the H+ channel.
•  F1 has six subunits. As protons pass through
F0, it rotates, causing part of the F1 unit to
rotate.
ADP and Pi bind to active sites that become
exposed on the F1 unit as it rotates, and ATP is
made.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
ATP synthase structure is similar in all
organisms.
•  In prokaryotes, the proton gradient is set up
across the cell membrane.
•  In eukaryotes, chemiosmosis occurs in
mitochondria and chloroplasts.
The mechanism of chemiosmosis is similar in
almost all forms of life.
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
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 6.11 An Experiment Demonstrates the Chemiosmotic Mechanism
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 is formed by
oxidative phosphorylation, which is due to the
reoxidation of NADH.
Some bacteria and archaea use other electron
acceptors.
•  Geobacter metallireducens can use iron
(Fe3+) or uranium, making it potentially
useful in environmental cleanup.
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, regenerating NAD+.
Occurs in many microorganisms and complex
organisms, including vertebrate muscle during
exercise when O2 can not be delivered to the
muscle fast enough.
Figure 6.12 Fermentation (Part 1)
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+.
Occurs in certain yeasts and some plant cells
under anaerobic conditions.
Figure 6.12 Fermentation (Part 2)
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Metabolic pathways are linked. There is an
interchange of molecules into and out of the
pathways for synthesis and breakdown.
Carbon skeletons (molecules with covalently
linked carbon atoms) can enter catabolic or
anabolic pathways.
These relationships comprise a metabolic
system.
Figure 6.13 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 enters 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 cycle intermediates can
form nucleic acids.
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
Amounts of different molecules are maintained at
fairly constant levels in the metabolic pool.
This is accomplished by regulation of enzymes—
allosteric regulation and feedback inhibition.
Enzymes can also be regulated by altering the
transcription of genes that encode the
enzymes. This is slower than feedback
inhibition.
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
ATP and reduced coenzymes link catabolism,
anabolism, and photosynthesis.
Cellular respiration and photosynthesis are
linked by their reactants and products and by
the energy “currency” of ATP and reduced
coenzymes.
Concept 6.4 Catabolic and Anabolic Pathways Are Integrated
In cellular respiration glucose is oxidized:
glucose + 6 O2 → 6 CO2 + 6 H2O + chemical energy
In photosynthesis, light energy is converted to
chemical energy:
CO2 + H2O + light energy → carbohydrates + O2
Figure 6.14 ATP, Reduced Coenzymes, and Metabolism
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
Photosynthesis (anabolic) 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 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; it is
propagated as a wave but also behaves as
particles (photons).
The amount of energy in the radiation is
inversely proportional to its wavelength.
Figure 6.16 The Electromagnetic Spectrum
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
Photons can be absorbed by specific receptor
molecules, which are raised to an excited state
(higher energy).
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 wavelength.
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
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
The pigments are arranged into light-harvesting
complexes, or antenna systems.
A photosystem spans the thylakoid membrane
in the chloroplast
•  It consists of multiple antenna systems
surrounding a reaction center.
Figure 6.19 Photosystem Organization
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 –
The reaction center has converted light energy
into chemical energy.
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
The electron acceptor is the first carrier in an
electron transport system in the thylakoid
membrane.
The final acceptor is NADP+, which gets reduced:
NADP+ + H+ + 2 e–
→
NADPH
ATP is produced chemiosmotically during electron
transport (photophosphorylation).
Figure 6.20 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 and passes an excited electron to
NADP+, reducing it to NADPH.
•  Photosystem II absorbs light energy at
680 nm, oxidizes water, and initiates ATP
production.
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 H2O, which splits
the H—O—H bonds.
2 Chl* + H2O → 2 Chl + 2 H+ + ½ O2
Concept 6.5 During Photosynthesis, Light Energy Is Converted to
Chemical Energy
The excited (energetic) electron is passed
through a series of thylakoid membrane-bound
carriers to a final acceptor at a lower energy
level.
A proton gradient is generated and used by ATP
synthase to make ATP.
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 last carrier in 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. The
noncyclic light reactions would not provide
enough ATP.
Cyclic electron transport uses only
photosystem I and produces only ATP.
An electron is passed from an excited
chlorophyll, through the electron transport
chain, and recycles back to the same
chlorophyll.
Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP
Concept 6.6 Photosynthetic Organisms Use Chemical Energy
to Convert CO2 to Carbohydrates
Calvin cycle: the energy in ATP and NADPH is
used to “fix” CO2 in reduced form in
carbohydrates
•  Occurs in the stroma of the chloroplast.
•  Each reaction is catalyzed by a specific
enzyme.
The cycle is composed of three distinct
processes.
Figure 6.22 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
splits into two 3-carbon molecules:
3-phosphoglycerate (3PG)
Figure 6.23 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.
In air (O2), yeasts use aerobic metabolism to
fully oxidize glucose to CO2.
Without air, yeasts use alcoholic fermentation,
producing ethanol, less CO2, and less energy
(slower growth).
Figure 6.24 Products of Glucose Metabolism