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.
Energy and Electrons from Glucose
• The sugar glucose (C6H12O6) is the most
common form of energy molecule.
• Cells obtain energy from glucose by the chemical
process of oxidation in a series of metabolic
pathways.
Energy and Electrons from Glucose
• Principles governing metabolic pathways:
 Metabolic pathways are formed by complex
chemical transformations which occur in
separate reactions.
 Each reaction in the pathway is catalyzed by a
specific enzyme.
 Metabolic pathways are similar in all
organisms.
 In eukaryotes, many metabolic pathways are
compartmentalized in organelles.
 The operation of each metabolic pathway can
be regulated by the activities of 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
Energy and Electrons from Glucose
• About half of the energy from glucose is collected
in ATP.
• G for the complete conversion of glucose is
–686 kcal/mol.
• The reaction is therefore highly
exergonic(감소반응), and it drives the
endergonic(증가반응) formation of ATP.
Energy and Electrons from Glucose
• Three metabolic processes are used in the
breakdown of glucose for energy:
 Glycolysis
 Cellular respiration
 Fermentation
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:
• 1) Glycolysis, 2) pyruvate oxidation, 3)
citric acid cycle, and 4) electron
transport chain.
If O2 is not present, pyruvate is
metabolized in 1) fermentation.
Figure 7.2 Energy-Producing Metabolic Pathways
Energy and Electrons from Glucose
• Glycolysis produces some usable energy and
two molecules of a three-carbon sugar called
pyruvate.
• Glycolysis begins glucose metabolism in all cells.
• Glycolysis does not require O2; it is an anaerobic
metabolic process.
Energy and Electrons from Glucose
• Cellular respiration uses O2 and occurs in
aerobic (oxygen-containing) environments.
• Pyruvate is converted to CO2 and H2O.
• The energy stored in covalent bonds of pyruvate
is used to make ATP molecules.
Energy and Electrons from Glucose
• Fermentation does not involve O2. It is an
anaerobic process.
• Pyruvate is converted into lactic acid or ethanol.
• Breakdown of glucose is incomplete; less energy
is released than by cellular respiration.
Energy and Electrons from Glucose
• Redox reactions(산화환원반응) transfer the
energy of electrons.
• A gain of one or more electrons or hydrogen
atoms is called reduction(환원).
• The loss of one or more electrons or hydrogen
atoms is called oxidation(산화).
• Whenever one material is reduced, another is
oxidized.
• Oxidation and reduction always occur together.
Figure 7.3 Oxidation and Reduction Are Coupled
Energy and Electrons from Glucose
• An oxidizing agent(산화제) accepts an electron
or a hydrogen atom.
• A reducing agent(환원제) donates an electron or
a hydrogen atom.
• During the metabolism of glucose, glucose is the
reducing agent (and is oxidized), while oxygen is
the oxidizing agent (and is reduced).
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)
Energy and Electrons from Glucose
• The reduction reaction requires an input of energy:
 NAD+ + 2H  NADH + H+
• The oxidation reaction is exergonic:
 NADH + H+ + ½ O2  NAD+ + H2O
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.
Glycolysis: From Glucose to Pyruvate
• Glycolysis can be divided into two stages:
 Energy-investing reactions that use ATP
 Energy-harvesting reactions that produce ATP
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