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6
Pathways that Harvest and
Store Chemical Energy
Introduction to Metabolism: Energetics
• 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).
• 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
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
Figure 6.1 The Concept of Coupling Reactions
ATP – The “Power Molecule”
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.
ATP  H 2O  ADP  Pi  freeenergy
Hydrolysis of ATP is
exergonic
Hydrolysis of ATP is
exergonic
ΔG is about –7.3 kcal
• Free energy of the bond
between phosphate groups
is much higher than the
energy of the
O—H bond that forms after
hydrolysis.
• 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
Redox Reactions
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.
Redox Reactions
Oxidation and reduction always occur together.
Redox Reactions
• 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 in the reducing agent is transferred to
the reduced product.
Redox Reactions
Coenzymes – electron carriers
Coenzyme NAD+ is a key electron carrier in
redox reactions.
NAD+ (oxidized form- lost electrons)
NADH (reduced form – gained electrons)
Figure 6.4 A NAD+/NADH Is an Electron Carrier in Redox Reactions
Coenzymes
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
Oxidative Phosphorylation
• 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.
REMEMBER:
Catabolism – breaking down large to small
Anabolism – making small to large
Oxidative phosphorylation
Oxidative phosphorylation couples oxidation of
NADH:
NADH  NAD  H   2e   energy
with production of ATP:
energy  ADP  Pi  ATP
Chemiosmosis
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.
*** Animated Tutorial 6.1 – BIOPORTAL***
Cellular Respiration
Quick Overview – Cellular Respiration and Photosynthesis
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.
• Six-carbon glucose is converted to two threecarbon pyruvate
• Final products (NET GAIN):
2 molecules of pyruvate (pyruvic acid)
2 molecules of ATP (“use 2 to get 2”)
2 molecules of NADH
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)
Endergonic
reactions,
requiring energy
from ATP
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 2)
6-carbon
molecule is
cleaved into
two 3-carbon
molecules
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 3)
Exergonic
reactions,
resulting in
ATP and
NADH
being
produced
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
Step 6
text art pg 107 here
Step 7
Pyruvate Oxidation – aka Pyruvate Fixation
Pyruvate Oxidation:
Allows for the Citric Acid Cycle to occur in the
mitochondrion
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.