1-1kpef0oe.pdf
Copyright © 2015 Wolters Kluwer Health | Lippincott Williams & Wilkins
Chapter 6
Energy Transfer in the Body
Copyright © 2015 Wolters Kluwer Health | Lippincott Williams & Wilkins
Adenosine Triphosphate (ATP)
• Food macronutrients
provide major
sources
of potential energy
ut do not transfer
directly to biologic
work
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Adenosine Triphosphate (ATP)
• Cells’ two major
energy-transforming
activities:
▪ Extract potential
energy from food
and conserve it
within the ATP
onds
▪ Extract and transfer
the chemical
energy in
ATP to power
iologic work
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• ATP forms from adenosine linked to three
phosphates
• Adenosine diphosphate (ADP) forms when ATP
joins with water, catalyzed by the enzyme
adenosine triphosphatase (ATPase)
Adenosine Triphosphate (ATP), cont.
ATP + H2O XXXXXXXXXXADP + P - ∆G7.3 kcal/mol
ATPase
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ATP Production
• Free energy
liberated in ATP
hydrolysis powers
all forms of
iologic work
• ATP represents
the cell’s “energy
cu
ency”
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ATP: A Limited Cu
ency
• Cells contain only a small quantity
of ATP so it must continually be
esynthesized
• ATP levels decrease in skeletal
muscle only under extreme
exercise conditions
• The body stores 80 to 100 g of ATP
at any time under normal resting
conditions, enough stored energy
to power 2 to 3 seconds of maximal
exercise
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Phosphocreatine (PCr):
The Energy Reservoi
• Some energy for ATP
esynthesis comes from
anaerobic splitting of a
phosphate from PC
• Cells store
approximately 4 to 6
times more PCr than
ATP
• PCr reaches its
maximum energy yield
in about 10 s
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Biologic Work in Humans
• Three forms of biologic work
1. Chemical: Biosynthesis of cellular molecules
2. Mechanical: Muscle contraction
3. Transport: Transfer of substances among cells
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Factors That Affect Rate of Bioenergetics
• Enzymes
▪ Protein catalysts: accelerate chemical
eactions without being consumed or
changed in the reaction
• Coenzymes
▪ Nonprotein organic substances: facilitate
enzyme action by binding a substrate to
its specific enzyme
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Classifications of Enzymes
• Oxidoreductases (example: lactate dehydrogenase)
• Transferases (example: hexokinase)
• Hydrolases (example: lipase)
• Lyases (example: ca
onic anhydrase)
• Isomerases (example: phosphoglycerate mutase)
• Ligases (example: pyruvate ca
oxylase)
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Turnover Numbe
• Enzymes do not all operate at the same rate
▪ Turnover numbe
- number of moles of substrate that react
to form a product per mole of enzyme per
unit time
- pH and temperature alter enzyme activity
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Lock and Key Mechanism
• Enzyme-substrate
interaction
▪ Enzyme turns on
when its active site
joins in a “perfect fit”
with the substrate’s
active site
▪ Ensures that the
co
ect enzyme
matches with its specific
substrate to perform a
particular function
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Energy Release from Macronutrients
• Three stages lead to release and energy
conservation by cells for biologic work:
▪ Stage 1: Digestion, absorption, and assimilation
of relatively large food macromolecules into
smaller subunits
▪ Stage 2: Degrades amino acids, glucose, and fatty
acid and glycerol units into acetyl coenzyme A
▪ Stage 3: Acetyl-coenzyme A degrades to CO2 and
H2O with considerable ATP production
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Six Macronutrient Fuel Sources
1. Triacylglycerol and glycogen molecules stored
within muscle cells
2. Blood glucose
3. Free fatty acids
4. Intramuscular- and liver-derived ca
on skeletons
of amino acids
5. Anaerobic reactions in the initial phase of glucose
eakdown
6. PCr phosphorylates ADP under enzyme control
(creatine kinase and adenylate kinase)
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Energy Release from Ca
ohydrate
• Ca
ohydrate’s primary function supplies
energy for cellular work
• The complete
eakdown of one mole of
glucose yields 686 kcal of available energy
▪ Bonds within ATP conserve about 263 kcal;
the remaining dissipates as heat
• The complete oxidation of one glucose
molecule in skeletal muscle yields 36 ATPs
C6H12O6 + 6O XXXXXXXXXX6CO2 + 6H2O – ∆G 686 kcal/mol
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Anaerobic Versus Aerobic Glycolysis
• Two forms of ca
ohydrate
eakdown:
1. Anaerobic (rapid) glycolysis results in
pyruvate-to-lactate formation with the
elease of about 5% of energy within the
original glucose molecule
2. Aerobic (slow) glycolysis results in
pyruvate-to-acetyl-CoA-to-citric acid cycle
and electron transport of the remaining
energy within the original glucose
molecule
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Anaerobic Glycolysis: Rapid Glycolysis
• Anaerobic (rapid) glycolysis regulated by:
▪ Glycolytic enzymes hexokinase, pyruvate kinase,
and phosphofructokinase
▪ Fructose 1,6-disphosphate levels
▪ Rapid glycolysis forms lactate with 4 total ATP
produced (2 net ATP – 14.6 kcal/mol)
▪ Rapid glycolysis generate about 5% of the total
ATP during complete glucose
eakdown
▪ Rapid glycolysis occurs without molecular
oxygen involvement
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Glucose-to-Glycogen and
Glycogen-to-Glucose Conversion
• Glycogenesis (glycogen synthesis)
▪ Surplus glucose forms glycogen in low
cellular activity and/or with depleted
glycogen reserves
• Glycogenolysis (glycogen
eakdown)
▪ Glycogen reserves
eak down to produce
glucose in high cellular activity with glucose
depletion
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Regulation of Glycolysis
• Three factors regulate glycolysis:
1. Four key glycolytic enzymes: hexokinase,
phosphorylase, phosphofructokinase,
pyruvate kinase
2. Levels of fructose 1,6-disphosphate
3. Oxygen in abundance inhibits glycolysis
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Lactic Acid Versus Lactate
• Lactic acid forms during anaerobic
glycolysis. In the body, it dissociates
to release a hydrogen ion (H+). The
emaining compound binds with a
positively charged sodium (Na+) ion or
potassium (K+) ion to form the acid salt
lactate
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Slow (Aerobic) Glycolysis: The Citric Acid
Cycle
• Rapid glycolysis releases only about 5% of the
total energy within glucose; the remaining
energy releases when pyruvate converts to
acetyl-CoA and enters the citric acid cycle
(also called the Krebs cycle)
• The citric acid cycle represents the second
stage of ca
ohydrate
eakdown to produce
CO2 and hydrogen atoms within mitochondria
Pyruvate + NAD+ CoA XXXXXXXXXXAcetyl-CoA + CO2 + NADH
+ + H+
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Slow (Aerobic)
Glycolysis
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Citric Acid Cycle (11 Steps)
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Total Energy Transfer from Glucose
Catabolism
• The complete
eakdown of glucose yields
34 ATPs
▪ Because two ATPs initially phosphorylate
glucose, 32 ATP molecules equal the net ATP
yield from glucose catabolism in skeletal muscle
▪ Four ATP molecules form directly from
substrate-level phosphorylation (glycolysis and
citric acid cycle)
▪ 28 ATP molecules regenerate during oxidative
phosphorylation
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Net ATP from
Glucose Catabolism
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Energy Release from Fat
• Three specific energy sources for fat catabolism:
1. Triacylglycerols stored directly in muscle
mitochondria
2. Circulating triacylglycerols in lipoprotein
complexes
3. Circulating free fatty acids mobilized from
triacylglycerols in adipose tissue
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Fat Catabolism
• Complete oxidation of a triacylglycerol
molecule yields about 460 ATP molecules
• Stored fat serves as the most plentiful source
of potential energy
• Fat becomes the primary energy fuel for
exercise and recovery when intense, long-
duration exercise depletes both blood glucose
and muscle glycogen
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Oxidation and Reduction
• Oxidation (always involves electron loss)
▪ Reactions that transfer oxygen, hydrogen atoms,
or electrons
▪ A loss of electrons always occurs with a net gain
in valence
• Reduction (always involves electron gain)
▪ Any process in which atoms in an element gain
electrons, with a co
esponding net decrease
in valence
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Fat Catabolism, cont.
• Fat supplies 30 to 80% of energy for
iologic work depending on nutritional
status, level of training, and intensity and
duration of physical activity
• Total fuel reserves from fat in a young
adult male:
▪ 60,000 to 100,000 kcal stored in adipocytes
▪ 3000 kcal stored in intramuscular
triacylglycerol
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Dynamics of Fat Mobilization
• Hormone-sensitive lipase
stimulates triacylglycerol (TAG)
eakdown into its glycerol and
fatty acid components.
• The blood transports free fatty
acids (FFAs) released from
adipocytes and bound to plasma
albumin.
• Energy releases when TAG stored
within muscle fibers degrades to
glycerol and fatty acids.
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Glycerol and Fatty Acid Catabolism
• Glycerol
▪ Substrate phosphorylation degrades pyruvate
to form ATP
▪ Hydrogen atoms pass to NAD+, and the citric
acid cycle oxidizes pyruvate.
▪ Complete
eakdown of a single glycerol
molecule synthesizes 19 ATP molecules
• Fatty Acids
▪ Transform into acetyl-CoA in mitochondria via
β-oxidation for entry into the citric acid cycle
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Glycerol and Fatty Acid Catabolism, cont.
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Electron Transport
• Electron transport
epresents the final
common pathway where
electrons extracted from
hydrogen pass to oxygen
• Mitochondrial oxygen
levels drive the
espiratory chain by
serving as the final
electron acceptor to
combine with hydrogen
to form wate
Oxidizing hydrogen and
electron transport
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Oxygen’s Role in Energy Metabolism
• Serves as the major oxidizing agent in tissues
• Ensures that energy transfer reactions proceed
at appropriate rate
Aerobic metabolism refers to energy-generating catabolic reactions,
where oxygen serves as the final