Fundamentals of Human
Energy Transfer
Chapter 5
Copyright © 2006 Lippincott Williams & Wilkins.
Fundamental Definitions
• Bioenergetics studies diverse means
for energy transfer for biologic work
within living organisms.
• Aerobic and anaerobic breakdown of
food nutrients provides energy source
for synthesizing the chemical fuel for all
biologic work.
Copyright © 2006 Lippincott Williams & Wilkins.
Energy: The Capacity for Work
Copyright © 2006 Lippincott Williams & Wilkins.
First Law of Thermodynamics
• Conservation of energy
• Dictates that the body
does not produce,
consume, or use up
energy;
rather, energy
transforms from one
form into another as
physiologic systems
undergo continual
change.
Copyright © 2006 Lippincott Williams & Wilkins.
Definition of Energy
• Potential Energy: stored, inactive; ability to
do work even if it is not doing work at the time.
• Kinetic Energy: energy at work, active.
Copyright © 2006 Lippincott Williams & Wilkins.
Potential Energy as
– Energy of position
(gravitational)
– Mechanical potential
energy (elastic
deformation)
– Bound energy within
internal structure
Releasing potential
energy transforms
into kinetic energy
of motion.
Copyright © 2006 Lippincott Williams & Wilkins.
Energy-Releasing and
Energy-Conserving Processes
• Exergonic reactions
– Chemical processes that release energy to
its surroundings
– Downhill processes
• Endergonic reactions
– Chemical processes that store or absorb
energy
– Uphill processes
Copyright © 2006 Lippincott Williams & Wilkins.
Coupled Reactions
Copyright © 2006 Lippincott Williams & Wilkins.
Second Law of Thermodynamics
• Tendency to degrade potential energy
to kinetic energy with lower capacity for
work (i.e., increase entropy).
• Food and other chemicals represent
excellent sources of potential energy,
yet energy decreases as compounds
decompose via normal oxidation.
• Example: flashlight battery.
Copyright © 2006 Lippincott Williams & Wilkins.
Forms of Energy
• During energy
conversions, a
loss of potential
energy from one
source often
produces increase
in potential
energy of another
source.
Copyright © 2006 Lippincott Williams & Wilkins.
Biologic Work
in Humans
1. Performance of
mechanical work
2. Chemical work in
biosynthesis of
macromolecules
3. Active transport of
molecules and ions
concentrating
various substances
in & out of cells.
Copyright © 2006 Lippincott Williams & Wilkins.
Key Point
• The limits of exercise intensity
ultimately depend on the rate that cells,
extract, conserve, and transfer chemical
energy in the food nutrients to the
contractile filaments of skeletal muscle
Copyright © 2006 Lippincott Williams & Wilkins.
Factors Affecting
Bioenergetics
• Enzymes
– Reaction rates: operation rate of
enzymes
– Enzyme mode of action: how an
enzyme reacts with its specific
substrate
• Coenzymes
Copyright © 2006 Lippincott Williams & Wilkins.
Enzymes
• Are highly specific protein catalysts
• Accelerate the forward and reverse
reactions
• Are neither consumed nor changed in the
reaction
• pH and temperature dramatically affect
enzyme activity
• Named for functions they perform “-ase”
Copyright © 2006 Lippincott Williams & Wilkins.
Enzymes
Six major classes of enzymes.
1. Hydrolase: hydrolysis break chemical bonds by
insertion of water molecule.
2. Isomerase: convert one isomer to another.
3. Ligase: bond two substrate molecules together
4. Lyase: catalyze the breakage of molecule
5. Transferase: transfer a specific group from one
molecule to another.
6. Oxidoreductase: catalyze oxidation – reduction
reactions.
Copyright © 2006 Lippincott Williams & Wilkins.
Reaction Rates
• The rate of exergonic and endergonic reactions
depends on:
– Substrate availability
– Enzyme availability
– Metabolic state of the cell
– Cellular conditions (temperature, pH)
• Energy charge is maintained.
Copyright © 2006 Lippincott Williams & Wilkins.
Coenzymes
• Complex nonprotein organic substances
facilitate enzyme action by binding the
substrate with its specific enzyme
• Coenzymes are smaller molecules than
enzymes
• Many vitamins serve as coenzymes, e.g.
riboflavin (flavin adenine dinucleotide) and
niacin (nicotinamide adenine dinucleotide).
Copyright © 2006 Lippincott Williams & Wilkins.
Phosphate-Bond Energy
Copyright © 2006 Lippincott Williams & Wilkins.
ATP: The Energy Currency
• Adenosine triphosphate provides the required energy for
all cellular functions
• Cell’s “energy currency”
• ATP + H2O ↔ ADP + Pi + ∆ G (free energy) + heat
• ATP hydrolysis yields 7.3 kcal of free energy.
Copyright © 2006 Lippincott Williams & Wilkins.
Structure of Adenosine Triphosphate
(ATP)
Copyright © 2006 Lippincott Williams & Wilkins.
ATP  ADP Cycle is the
fundamental mode of
energy release in
biological systems.
Anabolic: use extracted
chemical energy from
ATP to synthesize new
compounds.
Catabolic: release energy
for biologic work.
Copyright © 2006 Lippincott Williams & Wilkins.
Catabolism-Anabolism Interactions
Copyright © 2006 Lippincott Williams & Wilkins.
Synthesized
end products
ADP
+
Pi
Anabolism
(endergonic)
Building block
precursors
Carbohydrates,
fats, proteins,
and O2
Catabolism
(exergonic)
ATP
Copyright © 2006 Lippincott Williams & Wilkins.
H2O + CO2
How much ATP can the body store?
Why do cells store small quantity?
Copyright © 2006 Lippincott Williams & Wilkins.
Phosphocreatine
• Energy rich phosphate compound closely related to ATP.
• Contains an energy rich phosphoanhydride bond.
Insert Figure 3.7
Copyright © 2006 Lippincott Williams & Wilkins.
• Released energy is coupled with energy
requirement for re-synthesis of ATP.
– For every mole of PCr broken down, 1 mole of ATP
synthesized.
– The coupled reaction is:
Where is
PCr stored?
Copyright © 2006 Lippincott Williams & Wilkins.
Intramuscular High Energy Phosphates
1. How long can high
energy phosphates
sustain all-out
activity?
2. Mobilization of ATP
and PCr important
in determining
anaerobic power.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• Human energy
dynamics involve
transferring energy by
chemical bonds.
• Energy for
phosphorylation
comes from oxidation
of carbohydrate, lipid,
and protein
macronutrients.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• Oxidation reaction: an
element loses electrons (e-);
a compound loses electrons,
often accompanied by
hydrogen ions (H+),
or it gains oxygen.
• Reduction reaction: an
element gains electrons (e-);
a compound gains electrons,
or it loses oxygen.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• Oxidation-reduction
reactions are coupled.
Every oxidation coincides
with a reduction.
• OIL RIG:
Oxidation Involves Loss
Reduction Involves Gain
• LEO the lion says GER:
Lose Electrons Oxidation
Gain Electrons Reduction
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• Oxidation-reduction
reactions are coupled.
Every reduction coincides
with a oxidation (redox).
• Cellular oxidation-reduction
constitutes mechanism for
energy metabolism.
• Carbohydrate, fat, and
protein provide hydrogen
atoms for this process.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• Hydrogens released from
food molecules picked up
by coenzyme NAD+ &
sometimes FAD in cytosol.
• Substrate oxidizes & loses
hydrogens (electrons),
NAD+ gains a hydrogen & 2
electrons and reduces to
NADH, the other H+ in fluid
• FAD also catalyzes dehydrogenations and accepts pairs
of electrons.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• NADH and FADH2 formed
in macronutrient
breakdown in cytosol carry
electrons to cytochromes
in mitochondrial
membrane. Animation:
chemical rxns mitochondrion
• Cytochromes, a series of
iron-protein electron
carriers, pass pairs of
electrons in bucket brigade
fashion on the inner
membranes of
mitochondrion.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
Transport of electrons by specific carriers
constitutes the respiratory chain or electron
transport chain.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
• The final electron acceptor
(oxidizer) in the
respiratory chain is
oxygen which forms
water.
• Without oxygen as final
oxidizer, respiratory chain
cannot proceed and H
remain in cellular cytosol.
Animation: Electron Transport
Copyright © 2006 Lippincott Williams & Wilkins.
Oxidative Phosphorylation
• Oxidative phosphorylation:
refers to the
phosphorylation of ADP
during the electron
transport from NADH and
FADH2 to oxygen.
• Electrochemical energy
generated in the ETC is
harnessed and transferred
to ATP synthase which
phosphorylates ADP to
ATP.
Copyright © 2006 Lippincott Williams & Wilkins.
Cellular Oxidation
Phosphorylation: the energy transfer through the phosphate
bonds of ATP to other compounds to raise them to a higher
activation level
Oxidation: biologic burning of macronutrients in the body for the
energy needed for phosphorylation
Occurs on inner lining of mitochondrial membranes
Involves transferring electrons from NADH and FADH2 to molecular
oxygen, which release and transfer chemical energy to combine
ATP from ADP plus a phosphate ion.
During aerobic ATP resynthesis, oxygen combines with hydrogen to
form water.
More than 90% of ATP synthesis takes place in the respiratory
chain by oxidative reactions coupled with phosphorylation.
Copyright © 2006 Lippincott Williams & Wilkins.
Energy Release from Food
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Energy Release from
Carbohydrate
• What is the primary function of CHO?
• The only macronutrient whose potential
energy generates ATP anaerobically
• During light & moderate intensity activity
CHO supplies about ????? body’s needs.
• A continual breakdown of CHO is
required so lipid can be used for energy.
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Energy Release from
Carbohydrate
• Compare rate of aerobic breakdown of
CHO to lipid.
• Net energy yield per molecule of
glucose is ?? moles of ATP.
• Of 686 kCal from one mole of glucose,
only 34% (233 kCals) of the energy is
conserved within ATP bonds; the
remainder is dissipated as heat
Copyright © 2006 Lippincott Williams & Wilkins.
Anaerobic vs Aerobic CHO
Anaerobic versus Aerobic CHO Energy
• Stage 1. Anaerobic Glycolysis (rapid):
glucose  2 pyruvate  2 lactate.
No oxygen.
• Stage 2. Aerobic Glycolysis (slow):
glucose  2 pyruvate  acetyl CoA 
citric acid cycle & electron transport.
Oxygen (final electron recipient).
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Glycolysis
Glycolysis: a series of 10
enzymatically controlled
chemical reactions involving
breakdown of glucose to
two molecules of pyruvate.
Anaerobic Glycolysis:
breakdown of glucose to 2
lactates.
Glycogenolysis: same
reactions but begins with
glycogen already within cell.
Net energy from glycogen 3
ATP.
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
• Glycolysis occurs in
cell’s cytoplasm.
• What is energy yield in
glycolysis?
• Energy from glycolysis
is useful performing
what exercise?
• How many pairs of
hydrogen are released
during glycolysis?
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Anaerobic
Glycolysis
Formation of
lactic acid
occurs when
excess H
from NADH
temporarily
combine with
pyruvic acid.
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Body disposes of lactic
acid in two ways:
1. When sufficient O2
available within
muscles, lactate
returns H to pyruvic
acid for aerobic
metabolism.
2. Cori cycle in liver:
gluconeogenesis –
make glucose from
lactate. Animation:
Biochemical Cori Cycle
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Stage Two of energy release from carbohydrate takes place in the
mitochondrion. Two coenzyme A’s pick up the 2 two-carbon acetyl
group and transport into mitochondria, releasing carbon dioxide
molecules. Coenzyme A then releases the acetyl to begin TCA cycle.
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
The citric acid cycle, or
tricarboxylic acid cycle ,
or Kreb’s cycle is a series
of 10 enzymatically
controlled chemical
reactions which begins
with oxaloacetate and
ends with oxaloacetate.
Animation: citric acid cycle
What is the most
important function of the
citric acid cycle?
How many ATP are
formed directly from
substrates?
Copyright © 2006 Lippincott Williams & Wilkins.
Carbohydrate
What is the
net energy
transfer from
the complete
catabolism of
glucose?
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Energy Release from Fat
• Adipocytes
– Site of fat storage and mobilization
– 95% of an adipocyte’s volume is occupied
by triacylglycerol (TG) fat droplets
– Lipolysis splits TG molecules into glycerol
and three water-soluble free fatty acids
(FFA)
– Catalyzed by hormone-sensitive lipase
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Transport and Uptake
of Free Fatty Acids
• After diffusing into the circulation, FFA
are transported within the circulation
bound to albumin
• FFA are then taken up by active skeletal
muscle in proportion to their flow and
concentration
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Breakdown of Glycerol and
Fatty Acids
• Glycerol
– Is converted to 3-phosphoglyceraldehyde,
an intermediate glycolytic metabolite
• FFA
– Are transformed into acetyl–CoA in the
mitochondria during -oxidation
– A process that successively releases 2carbon acetyl fragments split from long
fatty acid chains
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Fuel reserves
from fat
represents
about 60,000
to 100,000
kcals in fat
cells and
30,000 in
intramuscular
triglycerides.
Carbohydrate
reserves is
<2,000 kcal.
Before
energy
release
from fat:
lypolysis.
FA diffuse
from
adipocyte
into
blood,
bind to
albumin &
delivered
to tissues.
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Beta
Oxidation
Copyright © 2006 Lippincott Williams & Wilkins.
Fat
Did You Know?
• As carbohydrate levels decrease, the
availability of oxaloacetate may become
inadequate, which impairs fat
catabolism.
• Fats burn in a carbohydrate flame.
Copyright © 2006 Lippincott Williams & Wilkins.
• Excess
macronutrients
convert to fat.
• Any excess
carbohydrate, lipid,
or protein readily
converted to fatty
acid.
Copyright © 2006 Lippincott Williams & Wilkins.
Energy Release from Protein
• Protein can be
used as energy
substrate during
endurance-type
exercise.
• To provide energy
a.a. must be
converted to
usable form.
Copyright © 2006 Lippincott Williams & Wilkins.
Metabolic Mill
Each
pathway
has ratelimiting
enzyme.
Cellular
[ADP] is
most
important
factor that
affects
enzymes
controlling
energy
metabolism.
Molecules
degraded to
few simple
units, mostly
acetyl CoA.
Notice
reversibility
except
pyruvic acid
to acetyl
CoA.
Copyright © 2006 Lippincott Williams & Wilkins.
References
• Axen & Axen. 2001. Illustrated
Principles of Exercise Physiology.
Prentice Hall.
• Katch, McArdle, Katch. 2011. Essentials
of Exercise Physiology, 4th ed. Image
Bank. Lippincott Williams & Wilkens.
Copyright © 2006 Lippincott Williams & Wilkins.