Ch06 lecture pathways that harvest and store chemical energy

Pathways that Harvest and
Store Chemical Energy
6

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).

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.

4. In eukaryotes, many metabolic pathways
occur inside specific organelles.
5. Each metabolic pathway is controlled by
enzymes that can be inhibited or activated.

In cells, energy-transforming reactions are often
coupled:
An energy-releasing (exergonic) reaction is
coupled to an energy-requiring (endergonic)
reaction.

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

Hydrolysis of ATP is exergonic:
ΔG is about –7.3 kcal
energyfreePADPOHATP i ++→+ 2

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)

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.

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.

Oxidation and reduction always occur together.

Transfers of hydrogen atoms involve transfers of
electrons (H = H+
+ e–
).
When a molecule loses a hydrogen atom, it
becomes oxidized.

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

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

Reduction of NAD+
is highly endergonic:
Oxidation of NADH is highly exergonic:
NADHeHNAD →++ −++
2
OHNADOHNADH 222
1 +→++ ++

Figure 6.4 B NAD+
/NADH Is an Electron Carrier in Redox Reactions

• 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.

Oxidative phosphorylation couples oxidation of
NADH:
with production of ATP:
energyeHNADNADH +++→ −++
2
ATPPADPenergy i →++

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.

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.

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

Cellular respiration is a major catabolic pathway.
Glucose is oxidized:
Photosynthesis is a major anabolic pathway.
Light energy is converted to chemical energy:
tecarbohydraOenergylightOHCO +→++ 222 666
energychemicalOHCOOtecarbohydra ++→+ 222 666

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

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)

Examples of reaction types common in
metabolic pathways:
Step 6: Oxidation–reduction
Step 7: Substrate-level phosphorylation
text art pg 107 here

Pyruvate Oxidation:
Products: CO2 and acetate; acetate is then
bound to coenzyme A (CoA)

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

Final reaction of citric acid cycle:

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

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.

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.

Lactic acid fermentation:
End product is lactic acid (lactate).
NADH is used to reduce pyruvate to lactic acid,
thus regenerating NAD+
.

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.

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

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.

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.

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

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

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

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)

The pigments are arranged into light-harvesting
complexes, or antenna systems.
A photosystem spans the thylakoid membrane
in the chloroplast; it consists of antenna
systems and a reaction center.

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.
−+
+→+ acceptorChlacceptorChl *

The electron acceptor is first in an electron
transport system in the thylakoid membrane.
Final electron acceptor is NADP+
, which gets
reduced:
ATP is produced chemiosmotically during
electron transport (photophosphorylation).
NADPHeHNADP →++ −++
2

Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems

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.

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.
22
1
2 2*2 OHChlOHChl ++→+ +

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.

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.

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: 3-
phosphoglycerate (3PG).

Figure 6.22 RuBP Is the Carbon Dioxide Acceptor

2. 3PG is reduced to form glyceraldehyde 3-
phosphate (G3P).

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.

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

Ch06 lecture pathways that harvest and store chemical energy

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