Transcript Presentation

7
Pathways That Harvest
Chemical Energy
7 Pathways That Harvest Chemical Energy
• 7.1 How Does Glucose Oxidation Release Chemical
Energy?
• 7.2 What Are the Aerobic Pathways of Glucose
Metabolism?
• 7.3 How Is Energy Harvested from Glucose in the
Absence of Oxygen?
• 7.4 How Does the Oxidation of Glucose Form ATP?
• 7.5 Why Does Cellular Respiration Yield So Much
More Energy Than Fermentation?
• 7.6 How Are Metabolic Pathways Interrelated and
Controlled?
7.1 How Does Glucose Oxidation Release Chemical Energy?
Fuels: molecules whose stored energy
can be released for use.
The most common fuel in organisms is
glucose. Other molecules are first
converted into glucose or other
intermediate compounds.
7.1 How Does Glucose Oxidation Release Chemical Energy?
Principles governing metabolic pathways:
• Complex chemical transformations occur in
a series of reactions.
• Each reaction is catalyzed by a specific
enzyme.
• Metabolic pathways are similar in all
organisms.
• In eukaryotes, metabolic pathways are
compartmentalized in organelles.
• Each pathway is regulated by key
enzymes.
7.1 How Does Glucose Oxidation Release Chemical Energy?
Burning or metabolism of glucose:
C6 H12O6  6O2  6CO2  6H 2O  free energy
Glucose metabolism pathway traps the
free energy in ATP:
ADP  Pi  free energy  ATP
7.1 How Does Glucose Oxidation Release Chemical Energy?
ΔG from complete combustion of glucose
= –686 kcal/mole
Highly exergonic; drives endergonic
formation of many ATP
Figure 7.1 Energy for Life
Three metabolic pathways involved in harvesting the energy of glucose
7.1 How Does Glucose Oxidation Release Chemical Energy?
If O2 is present, four pathways operate:
• Glycolysis, pyruvate oxidation, citric acid
cycle, and electron transport chain.
If O2 is not present, pyruvate is
metabolized in fermentation.
Figure 7.2 Energy-Producing Metabolic Pathways
7.1 How Does Glucose Oxidation Release Chemical Energy?
Redox reactions: one substance
transfers electrons to another substance
Reduction: gain of one or more electrons
by an atom, ion, or molecule
Oxidation: loss of one or more electrons
Also applies if hydrogen atoms are gained
or lost.
7.1 How Does Glucose Oxidation Release Chemical Energy?
Oxidation and reduction always occur
together.
The reactant that becomes reduced is the
oxidizing agent.
The reactant that becomes oxidized is the
reducing agent.
Figure 7.3 Oxidation and Reduction Are Coupled
7.1 How Does Glucose Oxidation Release Chemical Energy?
In combustion of glucose, glucose is the
reducing agent, O2 is the oxidizing
agent.
Energy is transferred in a redox reaction.
Energy in the reducing agent becomes
associated with the reduced product.
7.1 How Does Glucose Oxidation Release Chemical Energy?
Coenzyme NAD is an electron carrier in
redox reactions.
Two forms:
NAD+ (oxidized)
NADH + H+ (reduced)
Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (A)
7.1 How Does Glucose Oxidation Release Chemical Energy?

NAD  2 H  NADH  H

A hydride ion (H–) is transferred, leaving a
free H+
H– : a proton with two electrons
Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (B)
7.1 How Does Glucose Oxidation Release Chemical Energy?
Oxygen accepts electrons from NADH:


NADH  H  2 O2  NAD  H 2O
1
exergonic—ΔG = –52.4 kcal/mole
Oxidizing agent is molecular oxygen—O2
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
Glycolysis takes place in the cytosol.
Involves 10 enzyme-catalyzed reactions
Results in: 2 molecules of pyruvate
4 molecules ATP
2 molecules NADH
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 1)
probably 4 parts?
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 2)
probably 4 parts?
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 3)
probably 4 parts?
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 4)
probably 4 parts?
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 5)
probably 4 parts?
Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 6)
probably 4 parts?
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
A kinase is an enzyme that catalyzes
transfer of a phosphate group from ATP
to another substrate.
In the first half of glycolysis, the glucose
molecule is split into two 3-carbon
molecules (G3P).
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
Phosphorylation: addition of a phosphate
group
Enzyme-catalyzed transfer of a
phosphate group to ADP is called
substrate-level phosphorylation.
Figure 7.6 Changes in Free Energy During Glycolysis
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
Pyruvate Oxidation:
• Links glycolysis and the citric acid cycle
• Pyruvate is converted to acetyl CoA
• Takes place in the mitochondrial matrix
Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)
1st part – pyruvate oxidation
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
Acetyl CoA is the starting point of the citric
acid cycle:
• Coenzyme A is removed in the first
reaction and can be reused
• The cycle is in steady state: the
concentrations of the intermediates don’t
change
• Outputs: CO2, reduced electron carriers,
and ATP
Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)
citric acid cycle
Figure 7.7 The Citric Acid Cycle Releases Much More Free Energy Than Glycolysis Does
7.2 What Are the Aerobic Pathways of Glucose Metabolism?
The electron carriers that are reduced
during the citric acid cycle must be
reoxidized to take part in the cycle again.
Fermentation—if no O2 is present
Oxidative phosphorylation—O2 is present
7.3 How Is Energy Harvested from Glucose in the Absence of
Oxygen?
Fermentation occurs in the cytosol.
Pyruvate is reduced by NADH + H+ and
NAD+ is regenerated.
7.3 How Is Energy Harvested from Glucose in the Absence of
Oxygen?
Lactic acid fermentation:
• Occurs in microorganisms, some
muscle cells.
• Pyruvate is the electron acceptor.
Figure 7.9 Lactic Acid Fermentation
7.3 How Is Energy Harvested from Glucose in the Absence of
Oxygen?
Alcoholic fermentation:
• Yeasts and some plant cells
• Pyruvate is converted to acetaldehyde,
CO2 is released
• Acetaldehyde is reduced by NADH + H+,
producing NAD+ and ethyl alcohol
Figure 7.10 Alcoholic Fermentation
7.4 How Does the Oxidation of Glucose Form ATP?
Oxidative phosphorylation: ATP is
synthesized as electron carriers are
reoxidized in the presence of O2.
Two stages:
Electron transport chain
Chemiosmosis
7.4 How Does the Oxidation of Glucose Form ATP?
Why does the electron transport chain
have so many steps?
Why not


NADH  H  2 O2  NAD  H 2O
in one step?
1
7.4 How Does the Oxidation of Glucose Form ATP?
Too much free energy would be released
all at once—it could not be harvested by
the cell.
In a series of reactions, each releases a
small amount of energy that can be
captured by an endergonic reaction.
7.4 How Does the Oxidation of Glucose Form ATP?
The electron transport chain:
• On the inner mitochondrial membrane
• 4 protein complexes: I, II, III, IV
• Cytochrome c
• Ubiquinone (Q)—a lipid
Figure 7.11 The Oxidation of NADH + H+ (Part 1)
Figure 7.11 The Oxidation of NADH + H+ (Part 2)
Figure 7.12 The Complete Electron Transport Chain
7.4 How Does the Oxidation of Glucose Form ATP?
The electron transport chain results in the
active transport of protons (H+) across
the inner mitochondrial membrane.
The transmembrane complexes act as
proton pumps.
Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 1)
Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 2)
7.4 How Does the Oxidation of Glucose Form ATP?
The proton pump results in a proton
concentration gradient and an electric
charge difference across the inner
membrane: potential energy!
Proton-motive force
7.4 How Does the Oxidation of Glucose Form ATP?
The protons must pass through a protein
channel—ATP synthase—to flow back
into the mitochondrial matrix.
Chemiosmosis is the coupling of the
proton-motive force and ATP synthesis.
7.4 How Does the Oxidation of Glucose Form ATP?
ATP synthase allows protons to diffuse
back to the mitochondrial matrix, and
uses the energy of that diffusion to
make ATP from ADP and Pi.
Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 1)
Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 2)
7.4 How Does the Oxidation of Glucose Form ATP?
ATP synthesis can be uncoupled: if a
different H+ diffusion channel is inserted
into the mitochondrial membrane, the
energy of the diffusion is lost as heat.
The protein thermogenin occurs in human
infants and hibernating animals.
7.4 How Does the Oxidation of Glucose Form ATP?
ATP synthase:
• F0 subunit—transmembrane
• F1 subunit—projects into the
mitochondrial matrix, rotates to expose
active sites for ATP synthesis
7.5 Why Does Cellular Respiration Yield So Much More Energy
Than Fermentation?
Energy yields:
Glycolysis and fermentation: 2 ATP
Glycolysis and cellular respiration: 32 ATP
Fermentation by-products have a lot of
energy remaining.
Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)
Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 2)
7.6 How Are Metabolic Pathways Interrelated and Controlled?
Metabolic pathways are interrelated.
Interchange of molecules occurs
between the pathways.
7.6 How Are Metabolic Pathways Interrelated and Controlled?
Catabolic interconversions:
Polysaccharides → hydrolyzed to glucose,
enters glycolysis
Lipids broken down to
glycerol → DAP
fatty acids → acetyl CoA
Proteins → hydrolyzed to amino acids—
feed into glycolysis or the citric acid cycle
at various points
Figure 7.16 Relationships among the Major Metabolic Pathways of the Cell
7.6 How Are Metabolic Pathways Interrelated and Controlled?
Anabolic interconversions:
• Most catabolic reactions are reversible
• Gluconeogenesis: glucose from citric
acid cycle and glycolysis intermediates
Figure 7.17 Coupling Metabolic Pathways
7.6 How Are Metabolic Pathways Interrelated and Controlled?
Metabolic homeostasis: concentrations of
the biochemical molecules remain
constant (e.g., glucose concentration in
blood)
• Allosteric control of enzymes in
catabolic pathways
• Negative and positive feedback controls
Figure 7.18 Regulation by Negative and Positive Feedback
Figure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 1)
Figure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 2)
7.6 How Are Metabolic Pathways Interrelated and Controlled?
The main control point in glycolysis is
phosphofructokinase—allosterically
inhibited by ATP.
The main control point in the citric acid
cycle is isocitrate dehydrogenase—
inhibited by NADH + H+ and ATP.
7.6 How Are Metabolic Pathways Interrelated and Controlled?
Acetyl CoA—accumulation of citrate
diverts acetyl CoA to fatty acid
synthesis.
Cell differentiation: Slow twitch muscle
cells have many mitochondria that
catabolize aerobically, providing a
steady supply of ATP.