Download electron transport chain

Metabolism and Bioenergetics
• Nutrients from foods are the substrates for metabolism and
are provided and stored as:
– Carbohydrate
– Fat
– Protein
• Each cell contains chemical pathways that convert these
substrates to usable energy, a process called bioenergetics
• The energy we derive from food is stored in cells in the form
of adenosine triphosphase (ATP)
• ATP serves as the immediate source of energy for most body
functions including muscle contraction
Kilocalorie
• Energy in biological systems is measured in kilocalories
• All energy eventually degrades to heat
• One kilocalorie is the amount of heat energy needed to raise
1 kg of water from 1 °C to 15 °C
Energy Sources
• At rest, the body uses carbohydrate and fat almost equally
for energy
• Protein provides little energy for cellular activity, but serves
as the building blocks for the body’s tissues
• During intense short-duration muscular effort, the body
relies mostly on carbohydrate to generate ATP
• Longer, less intense exercise utilizes carbohydrates and fat
for sustained energy production
Carbohydrate
• All dietary carbohydrate is ultimately converted to glucose
• Glucose is taken up by muscles and liver and converted to
the complex sugar molecule called glycogen
• Glycogen is stored in the cytoplasm of muscle cells, where it
can be quickly used to form ATP
• Glycogen is also stored in the liver, where it is converted
back to glucose as needed and transported by the blood to
the muscles to form ATP
Fat
• Provides substantial energy during prolonged, low-intensity
activity
• Body stores of fat are larger than carbohydrate reserves
• Fat is less readily available for cellular metabolism compared
to carbohydrate
• Fat is stored as triglycerides and must be broken down to
free fatty acids (FFAs) to be used in metabolism
• More energy is derived from breaking down fat (9.4 kcal/g)
compared to carbohydrate (4.1 kcal/g)
Protein
• Protein can be used as a minor source of energy, but it must
be converted to glucose via glucogenesis
• Proteins can generate FFAs during starvation through
lipogenesis
• Protein can supply up to 5-10% of energy during prolonged
exercise
• Proteins must be broken down to their basic units—amino
acids—to be used for energy
The Lock-and-Key Action of Enzymes
in the Catabolism of Compounds
• Enzymes control the rate of
free energy release from
substrates
• Enzyme names end with the
suffix -ase
Energy for Cellular Metabolism
Key Points
• We derive energy from food and store it as the highenergy compound ATP
– Carbohydrate (4.1 kcal/g)
– Fat (9.4 kcal/g)
– Protein (4.1 kcal/g)
• Carbohydrate is stored as glycogen in muscles and the liver
and is more accessible than protein or fat
• Glucose, directly from food or broken down from glycogen,
is the usable form of carbohydrate
• Fat is stored as triglycerides in adipose tissue and broken
down to FFA
ATP Is Generated
Through 3 Energy Systems
1. ATP-PCr system
2. Glycolytic system
3. Oxidative system
ATP Is Generated
Through 3 Energy Systems
ATP-PCr System
• Cells store small amounts of ATP, and phosphocreatine
(PCr), which is broken down to regenerate ATP
• Release of ATP from PCr is facilitated by the enzyme
creatine kinase
• This process does not require oxygen (anaerobic)
• ATP and PCr sustain the muscle’s energy needs for 3-15 sec
during an all-out sprint
• 1 mole of ATP is produced per one mole of PCr
ATP-Phosphocreatine System
ATP-PCr
Although ATP is being used at a very high rate, the energy from PCr
is used to resynthesize ATP, preventing the ATP level from
decreasing. At exhaustion, both ATP and PCr concentrations are low.
Glycogen Breakdown
and Synthesis
Glycolysis is the breakdown of glucose; it may be anaerobic
or aerobic.
Glycogenesis is the process by which glycogen is synthesized
from glucose to be stored in the liver or muscle.
Glycogenolysis is the process by which glycogen is broken
down into glucose-1-phosphate to be used for energy
production.
The Glycolytic System
• Requires 10-12 enzymatic reactions to break down
glycogen to lactic acid, producing ATP
• Occurs in the cytoplasm
• Glycolysis does not require oxygen (anaerobic)
• Without oxygen present, pyruvic acid produced by
glycolysis becomes lactic acid
• 1 mole of glycogen produces 3 moles of ATP; 1 mole of
glucose produces 2 moles of ATP because 1 mole is used
to convert glucose to glucose-6-phosphate
• ATP-PCr and glycolysis provide the energy for ~2 min of
all-out activity
Glycolysis
ATP-PCr and Glycolytic Systems
Key Points
• ATP-PCr system
– Pi is separated from PCr by creatine kinase
– Pi is combined with ADP to form ATP
– Energy yield: 1 mole of ATP per 1 mole of PCr
• Glycolytic system
– Glucose or glycogen is broken down to pyruvic
acid
– Without oxygen, pyruvic acid is converted to
lactic acid
– 1 mole of glucose yields 2 moles of ATP
Energy Sources for the Early
Minutes of Intense Exercise
The combined actions of the ATP-PCr and glycolytic
systems allow muscles to generate force in the absence of
oxygen; thus these two energy systems are the major
energy contributors during the early minutes of highintensity exercise.
The Oxidative System
• The oxidative system uses oxygen to generate energy
from metabolic fuels (aerobic)
• Oxidative production of ATP occurs in the mitochondria
• Can yield much more energy (ATP) than anaerobic
systems
• The oxidative system is slow to turn on
• Primary method of energy production during endurance
events
Aerobic Glycolysis and the Electron
Transport Chain
Krebs Cycle
Oxidation of Carbohydrate
1. In the presence of oxygen, pyruvic acid from glycolysis is
converted to acetyl coenzyme A (acetyl CoA)
2. Acetyl CoA enters the Krebs cycle and forms 2 ATP,
carbon dioxide, and hydrogen
3. Hydrogen ion created during glycolysis and through the
Krebs cycle combines with two coenzymes (NAD and
FAD)
4. NAD and FAD carry hydrogen ions to the electron
transport chain
(NAD and FAD → NADH and FADH)
The Electron Transport Chain
1. The electron transport chain splits NADH and FADH,
producing hydrogen ions which are recombined with oxygen
to produce water
2. Electrons produced from the split of NADH and FADH
provide the energy for the phosphorylation of ADP to ATP
3. One molecule of glycogen can generate up to 37-39
molecules of ATP
Oxidative Phosphorylation:
The Electron Transport Chain
Oxidation of Fat
• Lipolysis is the breakdown of triglycerides into glycerol
and free fatty acids (FFAs)
• FFAs travel via blood to muscle fibers and are broken
down by enzymes in the mitochondria into acetic acid,
which is converted to acetyl CoA through β-oxidation
• Acetyl CoA enters the Krebs cycle and the electron
transport chain
• Fat oxidation requires more oxygen compared with
glucose because a FFA molecule contains more carbon
Oxidation of Protein
• Body uses little protein during rest and exercise (5-10%
to sustain prolonged exercise)
• Some amino acids can be converted into glucose or
intermediates of oxidative metabolism
• Energy yield from protein is difficult to determine
• The nitrogen in amino acids is converted into urea,
which requires ATP
Oxidative Capacity of Muscle
• Oxidative capacity of muscle (QO2). is a measure of its maximal
capacity to use oxygen
• Representative enzymes to measure oxidative capacity
– Succinate dehydrogenase
– Citrate synthase
Oxidative Capacity in Muscle
Oxidative Metabolism
Key Points
• The oxidative system involves the breakdown of substrates
in the presence of oxygen
• Oxidation of carbohydrate involves glycolysis, the Krebs
cycle, and the electron transport chain, resulting in the
formation of H2O, CO2, and 38-39 molecules of ATP
• Fat oxidation involves β-oxidation of free fatty acids, the
Krebs cycle, and the electron transport chain to produce
more ATP than carbohydrate
• The maximum rate of ATP formation from fat oxidation is
too low to match the rate of ATP utilization during high
intensity exercise
(continued)
Oxidative Metabolism (continued)
Key Points
• Protein contributes little to energy production, and its
oxidation is complex because amino acids contain nitrogen,
which cannot be oxidized
• The oxidative capacity of muscle fibers depends on their
oxidative enzyme levels, fiber-type composition, and oxygen
availability
Measuring Energy Costs
of Exercise
Direct calorimetry measures the body’s heat production
to estimate energy expenditure
Indirect calorimetry calculates energy expenditure from
the ratio of CO2 produced to O2 consumed
A Direct Calorimeter for
Human Use
Respiratory Exchange Ratio
.
• The ratio between
CO2 released (VCO2) and oxygen
.
consumed
. (VO2) .
• RER = VCO2 / VO2
• The RER value at rest is usually 0.78 to 0.80
RER: Determining
Substrate Utilization
Carbohydrate
6 O2 + C6H12O6 → 6 CO2 + 6 H2O + 38 ATP
.
.
RER = VCO
/
VO
2
2 = 6 CO2 / 6 O2 = 1.0
Fat
C16H32O2 +. 23 O2 →
16
CO
+
16
H
O
+
129
ATP
2
2
.
RER = VCO2 / VO2 = 16 CO2 / 23 O2 = 0.70
Calorimetry
Key Points
• Direct calorimetry involves using a large chamber to directly
measure heat production by the body
• Indirect calorimetry involves measuring O2 consumption and
CO2 production
• RER at rest = 0.78 to 0.80
• RER oxidation of fat = 0.70
• RER oxidation of carbohydrate = 1.0
• Isotopes can be used to determine metabolic rate over long
periods of time
Resting Metabolic Rate (RMR)
RMR is the minimum amount of energy required by
the body to sustain basic cellular function
– Fat-free mass
– Body surface area
– Ranges from 1,100 to 2,500 kcal/day
– When activity is added, daily caloric expenditure is
1,700 to 3,100 kcal/day
Factors That Affect RMR
• Age: RMR gradually decreases with age, generally because of
a decrease in fat-free mass
• Body temperature: RMR increases with increasing
temperature
• Psychological stress: Stress increases activity of the
sympathetic nervous system
• Hormones: Thyroxine from the thyroid gland and epinephrine
from the adrenal medulla both increase RMR
Metabolic Rate During
Submaximal Exercise
• Metabolism increases in direct proportion to the increase in
exercise intensity
• During exercise at a constant power output (work rate) VO2
increases
from its resting value to a steady-state value within
.
1-2 minutes
• There is a linear increase in the VO2 with
increases in power
.
output (work rate)
Increase in Oxygen Uptake with
Increasing Power Output
Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in
humans,” Exercise and Sport Sciences Reviews 24: 36.
Increase in Oxygen Uptake with
Increasing Power Output
Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in
humans,” Exercise and Sport Sciences Reviews 24: 36.
Maximal Oxygen Uptake
.
• VO2max: The maximal capacity for oxygen consumption by
the body during maximal exertion
• Single best measurement of cardiorespiratory endurance
and aerobic fitness
• Increases with physical training
• Generally expressed relative to body weight
(ml · kg-1
· min-1)
• Normally active untrained college-aged students = 38-42
-1
-1
ml
. · kg · min
• VO2max declines in active people after age 25-30 by ~ 1% per
year
Relationship Between Exercise Intensity and
Oxygen Uptake in Trained and Untrained Man
Estimating Anaerobic Effort
• O2 consumption requires several minutes to reach the
required steady state level at which the aerobic processes
are fully functional
• Oxygen deficit is calculated as the difference between the
oxygen required for a given exercise intensity and the actual
oxygen consumption
• Anaerobic effort can be estimated by examining excess
postexercise oxygen consumption (EPOC)—the mismatch
between O2 consumption and energy requirements
Oxygen Requirement During
Exercise and Recovery
Factors Responsible for EPOC
• Rebuilding depleted ATP and PCr supplies
• Clearing lactate produced by anaerobic metabolism
• Replenishing O2 supplies borrowed from hemoglobin and
myoglobin
• Removing CO2 that has accumulated in body tissues
• Increased metabolic and respiratory rates due to increased
body temperature
Lactate Threshold
• It is the point at which blood lactate begins to accumulate
substantially above resting concentrations during exercise of
increasing intensity
• The rate at which lactate production exceeds lactate
clearance
• Usually expressed as a percentage of maximal oxygen uptake
• A high lactate threshold can indicate potential for better
endurance performance
• Lactate accumulation contributes to fatigue
Relationship Between Exercise Intensity and
Blood Lactate Concentration
Lactate Threshold and
Endurance Performance
Lactate threshold. (LT), when expressed as a percentage of
VO2max, is one of the best determinants of an athlete’s pace
in endurance events such as running and cycling. While
untrained people typically have LT around. 50% to 60% of
their VO2max, elite athletes may not reach LT until around
70%
to 80% VO2max.
.
Economy of Effort
Fatigue and its Causes
• Energy delivery (ATP-PCr, anaerobic glycolysis, and oxidation)
• Accumulation of metabolic by-products, such as lactate and
H+
• Failure of the muscle fiber’s contractile mechanism
• Alteration in the nervous system
Energy Systems and Fatigue
• PCr depletion
• Glycogen depletion (“hitting the wall”)
– Pattern of glycogen depletion from Type I and II
fibers depends on the duration and intensity of
the activity
– Glycogen depletion is selective to the muscle
groups involved in the activity
– Depletion of liver glycogen to increase blood
glucose increases muscle glycogen utilization
Decline in Muscle Glycogen
Adapted, by permission, from D.L. Costill, 1986, Inside running: Basics of sports physiology (Indianapolis: Benchmark Press).
Copyright 1986 Cooper Publishing Group, Carmel, IN.
High Muscle Temperature Impairs Skeletal Muscle
Function and Metabolism
Adapted, by permission, from S.D.R. Galloway and R.J. Maughan, 1997, "Effects of ambient temperature on the capacity to
perform prolonged cycle exercise in man," Medicine and Science in Sports and Exercise 29: 1240-1249.
Metabolic By-Products and Fatigue
• Short-duration activities depend on anaerobic glycolysis and
produce lactate and H+
• Cells buffer H+ with bicarbonate (HCO3–) to keep cell pH
between 6.4 (at exhaustion) and 7.1
• Intercellular pH lower than 6.9, however, slows glycolysis
and ATP production
• When pH reaches 6.4, H+ levels inhibit glycolysis and result
in exhaustion
Changes in Muscle pH During
Sprint Exercise and Recovery
Neuromuscular Fatigue
Fatigue may involve:
1. Decreased release or synthesis of acetylcholine
2. Hyperactive acetylcholinesterase
3. Hypoactive acetylcholinesterase
4. Increased threshold for stimulation of the muscle fiber
5. Competition with ACh for the receptors on the muscle
membrane
6. Potassium may leave the intracellular space, decreasing the
membrane potential below resting values
7. Central nervous system fatigue
Causes of Fatigue
Key Points
• Fatigue may result from depletion of PCr or glycogen,
which impairs ATP production
• The H+ generated by lactic acid leads to fatigue by
decreasing muscle pH, which impairs the cellular
processes of energy production and muscle contraction
• Failure of neural transmission may cause some fatigue
• The central nervous system may also limit exercise
performance as a protective mechanism