Substrate BreakdownSubstrate Breakdown

The free Energy of oxidation of a The free Energy of oxidation of a food is the amount of energy food is the amount of energy

liberated by the complete oxidation liberated by the complete oxidation of the food. of the food.

It is expressed by calories per It is expressed by calories per mole. mole.

Example = there are 686,000 Example = there are 686,000 calories/mole of glucose.calories/mole of glucose.

Regulation of Regulation of CarbohydratesCarbohydrates

Human skeletal muscle contains Human skeletal muscle contains 80-100mM of glycogen per 80-100mM of glycogen per

kilogram of wet weight kilogram of wet weight

15-18 g of glycogen15-18 g of glycogen 70 kg male has about 400g of muscle 70 kg male has about 400g of muscle

glycogen glycogen This is the total muscle pool This is the total muscle pool

Glycogen use by a specific skeletal Glycogen use by a specific skeletal muscle during exercise is limited to muscle during exercise is limited to

its own reservesits own reserves

It cannot borrow from other musclesIt cannot borrow from other muscles although glycogen levels in them may although glycogen levels in them may

decrease due to the catabolic influence of decrease due to the catabolic influence of catecholaminescatecholamines

5-6 G of glucose are available in 5-6 G of glucose are available in the blood (100mg/100ml)the blood (100mg/100ml)

During exerciseDuring exercise

the blood concentration of glucose does not the blood concentration of glucose does not dropdrop

even when considering the considerable even when considering the considerable increase in uptakeincrease in uptake

due to glucose release from the liverdue to glucose release from the liver

The liver has 50-90 g of easily The liver has 50-90 g of easily accessible reserve glycogenaccessible reserve glycogen

When glycogenolysis (breakdown When glycogenolysis (breakdown of glycogen) takes placeof glycogen) takes place

glycogen is broken down into glucose and glycogen is broken down into glucose and released from the liver.released from the liver.

Glucose is also produced in the Glucose is also produced in the liver (gluconeogenesis) fromliver (gluconeogenesis) from

LactateLactate PyruvatePyruvate GlycerolGlycerol Alanine precursors when the precursor Alanine precursors when the precursor

concentration is elevated concentration is elevated

The rate of release of glucose The rate of release of glucose depends upon:depends upon:

Blood glucose concentrationBlood glucose concentration Hormonal interactionsHormonal interactions

– InsulinInsulin– glucagon, norepinephrineglucagon, norepinephrine– epinephrineepinephrine

Insulin Insulin

secreted by the beta cells of the pancreas secreted by the beta cells of the pancreas and regulates glucose transport into the and regulates glucose transport into the cellscells

secreted when blood glucose levels are secreted when blood glucose levels are elevated, inhibiting glycogenolysis and elevated, inhibiting glycogenolysis and gluconeogenesisgluconeogenesis

In this way, insulin controls blood glucose In this way, insulin controls blood glucose metabolism of most tissues.metabolism of most tissues.

Glucagon is a hormone secreted by Glucagon is a hormone secreted by the alpha cells of the pancreasthe alpha cells of the pancreas

Helps to maintain blood glucose levels by Helps to maintain blood glucose levels by stimulating glycogenolysis and stimulating glycogenolysis and gluconeogenesis (The formation of new gluconeogenesis (The formation of new glucose) in the liverglucose) in the liver

Secreted in response to a decrease in blood Secreted in response to a decrease in blood glucose levels.glucose levels.

Most of its actions are through a cyclic AMP Most of its actions are through a cyclic AMP dependent protein kinase.dependent protein kinase.

Epinephrine and NorepinephrineEpinephrine and Norepinephrine

CatecholaminesCatecholamines released from the adrenal glands in released from the adrenal glands in

response to low blood glucose concentration response to low blood glucose concentration in response to exercise or its anticipationin response to exercise or its anticipation Most of their actions are regulated through Most of their actions are regulated through

cyclic AMPcyclic AMP

Epi and Norepi stimulateEpi and Norepi stimulate

GlycogenolysisGlycogenolysis LipolysisLipolysis GluconeogenesisGluconeogenesis

They also influence They also influence

Cardiac outputCardiac output RespirationRespiration Blood pressureBlood pressure Neuromuscular transmissionNeuromuscular transmission

The interaction of exercise and The interaction of exercise and hormoneshormones

Higher intensities of exercise and longer Higher intensities of exercise and longer durationsdurations

result in higher circulating levels of result in higher circulating levels of catecholamines and glucagoncatecholamines and glucagon

With decreases in insulin With decreases in insulin

The resulting elevated hepatic The resulting elevated hepatic glucose output mayglucose output may

spare glycogenspare glycogen delay the onset of fatigue delay the onset of fatigue

During prolonged exercise, During prolonged exercise, gluconeogenesis is very gluconeogenesis is very

importantimportant

compensates for the decreasing hepatic compensates for the decreasing hepatic glycogen storesglycogen stores

in turn, attenuates the lowering of blood in turn, attenuates the lowering of blood glucose concentrationsglucose concentrations

With these regulatory mechanisms, With these regulatory mechanisms, blood glucose is maintained at blood glucose is maintained at

physiologically adequate physiologically adequate concentrations except when muscle concentrations except when muscle

and liver glycogen stores are and liver glycogen stores are largely depletedlargely depleted

Carbohydrate CatabolismCarbohydrate Catabolism

Why is carbohydrate metabolism Why is carbohydrate metabolism important?important?

Carbohydrates can be used to generate Carbohydrates can be used to generate ATP anaerobically (ex vigorous exercise ATP anaerobically (ex vigorous exercise requiring rapid energy release)requiring rapid energy release)

They supply half of the energy required for They supply half of the energy required for light and moderate exerciselight and moderate exercise

A constant breakdown of carbohydrates is A constant breakdown of carbohydrates is necessary for the metabolism of fatty acids.necessary for the metabolism of fatty acids.

Fatigue, which we will talk about later, is Fatigue, which we will talk about later, is associated with low levels of stored associated with low levels of stored carbohydrates, glycogen, in muscle and carbohydrates, glycogen, in muscle and liver.liver.

The second most available The second most available method for producing ATP for method for producing ATP for muscle contraction is through muscle contraction is through the anaerobic breakdown of the anaerobic breakdown of

glucoseglucose

Glucose is broken down to carbon dioxide and Glucose is broken down to carbon dioxide and waterwater

6H6H1212O6 + 6 OO6 + 6 O22 →→ 6 CO6 CO22 + 6 H + 6 H2200

Glycolysis: Glycolysis: The catabolic pathway The catabolic pathway responsible for this is through anaerobic responsible for this is through anaerobic

glycolysis.glycolysis.

series of 10 enzymatically controlled series of 10 enzymatically controlled chemical reactions chemical reactions

break one glucose molecule into two break one glucose molecule into two molecules of pyruvic acid, or pyruvate.molecules of pyruvic acid, or pyruvate.

This results in a net production of 2 This results in a net production of 2 ATP and 2 NADH.ATP and 2 NADH.

When this series of reactions starts When this series of reactions starts with stored glycogen, it is called with stored glycogen, it is called

glycogenolysis.glycogenolysis. The breakdown of glycogen for energy is The breakdown of glycogen for energy is

regulated by the enzyme phosphorylase.regulated by the enzyme phosphorylase. Epinephrine greatly influences the activity of Epinephrine greatly influences the activity of

this enzyme.this enzyme. These reactions occur in the cell, outside of These reactions occur in the cell, outside of

the mitochondrion.the mitochondrion.

If the state of the fiber favors If the state of the fiber favors complete oxidation of glucosecomplete oxidation of glucose

pathway for glucose breakdown becomes pathway for glucose breakdown becomes aerobic glycolysis.aerobic glycolysis.

Two molecules of pyruvic acid or Two molecules of pyruvic acid or pyruvate are formed during aerobic pyruvate are formed during aerobic

GlycolysisGlycolysis

rather than lactaterather than lactate Therefore, Glycolysis yields a net of 2 ATPTherefore, Glycolysis yields a net of 2 ATP and either two molecules of lactic acid and either two molecules of lactic acid

(anaerobic Glycolysis) (anaerobic Glycolysis) Two pyruvic acid (aerobic Glycolysis) for Two pyruvic acid (aerobic Glycolysis) for

each glucose molecule catabolizationeach glucose molecule catabolization

Realize that both anaerobic and Realize that both anaerobic and aerobic Glycolysis can occur aerobic Glycolysis can occur

simultaneously within the same cell simultaneously within the same cell or within the same muscle. or within the same muscle.

The metabolic fate of glucose and The metabolic fate of glucose and the ratio of lactate to pyruvate the ratio of lactate to pyruvate formed depends upon several formed depends upon several

factors:factors: Enzyme kineticsEnzyme kinetics Mitochondrial capacity of the cellMitochondrial capacity of the cell Hormonal controlHormonal control Oxygen availabilityOxygen availability Required rate of energy productionRequired rate of energy production

Entry into the cellEntry into the cell

• Glucose enters the cell via facilitated Glucose enters the cell via facilitated diffusiondiffusion

• Glucose binds to a protein carrier Glucose binds to a protein carrier molecule on the lipid matrix of the cell molecule on the lipid matrix of the cell membranemembrane

• Only in this form glucose can diffuse into Only in this form glucose can diffuse into the cellthe cell

Insulin, secreted by the pancreas, Insulin, secreted by the pancreas, regulates the rate of glucose regulates the rate of glucose

transport into the celltransport into the cell

Without insulin, except in the liver and brain Without insulin, except in the liver and brain cellscells

very little glucose enters the cell, not nearly very little glucose enters the cell, not nearly enough to meet the needs of energy enough to meet the needs of energy metabolismmetabolism

Therefore, the rate of carbohydrate Therefore, the rate of carbohydrate storage is controlled by the rate of storage is controlled by the rate of

insulin secretion form the pancreas.insulin secretion form the pancreas.