Substrate Breakdown
The free Energy of oxidation of a
food is the amount of energy
liberated by the complete oxidation
of the food.
It is expressed by calories per
mole.
Example = there are 686,000
calories/mole of glucose.
Regulation of
Carbohydrates
Human skeletal muscle contains
80-100mM of glycogen per
kilogram of wet weight
 15-18 g of glycogen
 70 kg male has about 400g of muscle
glycogen
 This is the total muscle pool
Glycogen use by a specific skeletal
muscle during exercise is limited to
its own reserves
 It cannot borrow from other muscles
 although glycogen levels in them may
decrease due to the catabolic influence of
catecholamines
5-6 G of glucose are available in
the blood (100mg/100ml)
During exercise
 the blood concentration of glucose does not
drop
 even when considering the considerable
increase in uptake
 due to glucose release from the liver
The liver has 50-90 g of easily
accessible reserve glycogen
When glycogenolysis (breakdown
of glycogen) takes place
 glycogen is broken down into glucose and
released from the liver.
Glucose is also produced in the
liver (gluconeogenesis) from
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Lactate
Pyruvate
Glycerol
Alanine precursors when the precursor
concentration is elevated
The rate of release of glucose
depends upon:
 Blood glucose concentration
 Hormonal interactions
– Insulin
– glucagon, norepinephrine
– epinephrine
Insulin
 secreted by the beta cells of the pancreas
and regulates glucose transport into the
cells
 secreted when blood glucose levels are
elevated, inhibiting glycogenolysis and
gluconeogenesis
 In this way, insulin controls blood glucose
metabolism of most tissues.
Glucagon is a hormone secreted by
the alpha cells of the pancreas
 Helps to maintain blood glucose levels by
stimulating glycogenolysis and
gluconeogenesis (The formation of new
glucose) in the liver
 Secreted in response to a decrease in blood
glucose levels.
 Most of its actions are through a cyclic AMP
dependent protein kinase.
Epinephrine and Norepinephrine
 Catecholamines
 released from the adrenal glands in
response to low blood glucose concentration
 in response to exercise or its anticipation
 Most of their actions are regulated through
cyclic AMP
Epi and Norepi stimulate
 Glycogenolysis
 Lipolysis
 Gluconeogenesis
They also influence
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Cardiac output
Respiration
Blood pressure
Neuromuscular transmission
The interaction of exercise and
hormones
 Higher intensities of exercise and longer
durations
 result in higher circulating levels of
catecholamines and glucagon
 With decreases in insulin
The resulting elevated hepatic
glucose output may
 spare glycogen
 delay the onset of fatigue
During prolonged exercise,
gluconeogenesis is very
important
 compensates for the decreasing hepatic
glycogen stores
 in turn, attenuates the lowering of blood
glucose concentrations
With these regulatory mechanisms,
blood glucose is maintained at
physiologically adequate
concentrations except when
muscle and liver glycogen stores
are largely depleted
Carbohydrate Catabolism
Why is carbohydrate metabolism
important?
 Carbohydrates can be used to generate ATP
anaerobically (ex vigorous exercise
requiring rapid energy release)
 They supply half of the energy required for
light and moderate exercise
 A constant breakdown of carbohydrates is
necessary for the metabolism of fatty acids.
 Fatigue, which we will talk about later, is
associated with low levels of stored
carbohydrates, glycogen, in muscle and
liver.
The second most available
method for producing ATP for
muscle contraction is through
the anaerobic breakdown of
glucose
 Glucose is broken down to carbon dioxide
and water
 6H12O6 + 6 O2 → 6 CO2 + 6 H20
Glycolysis: The catabolic pathway
responsible for this is through anaerobic
glycolysis.
 series of 10 enzymatically controlled
chemical reactions
 break one glucose molecule into two
molecules of pyruvic acid, or pyruvate.
This results in a net production of 2
ATP and 2 NADH.
When this series of reactions starts
with stored glycogen, it is called
glycogenolysis.
 The breakdown of glycogen for energy is
regulated by the enzyme phosphorylase.
 Epinephrine greatly influences the activity of
this enzyme.
 These reactions occur in the cell, outside of
the mitochondrion.
If the state of the fiber favors
complete oxidation of glucose
 pathway for glucose breakdown becomes
aerobic glycolysis.
Two molecules of pyruvic acid or
pyruvate are formed during aerobic
Glycolysis
 rather than lactate
 Therefore, Glycolysis yields a net of 2 ATP
 and either two molecules of lactic acid
(anaerobic Glycolysis)
 Two pyruvic acid (aerobic Glycolysis) for
each glucose molecule catabolization
Realize that both anaerobic and
aerobic Glycolysis can occur
simultaneously within the same cell
or within the same muscle.
The metabolic fate of glucose and
the ratio of lactate to pyruvate
formed depends upon several
factors:
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Enzyme kinetics
Mitochondrial capacity of the cell
Hormonal control
Oxygen availability
Required rate of energy production
Entry into the cell
• Glucose enters the cell via facilitated
diffusion
• Glucose binds to a protein carrier
molecule on the lipid matrix of the cell
membrane
• Only in this form glucose can diffuse into
the cell
Insulin, secreted by the pancreas,
regulates the rate of glucose
transport into the cell
 Without insulin, except in the liver and brain
cells
 very little glucose enters the cell, not nearly
enough to meet the needs of energy
metabolism
Therefore, the rate of carbohydrate
storage is controlled by the rate of
insulin secretion form the pancreas.