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 mM of glycogen per kilogram of wet weight  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 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  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  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  6H 12 O6 + 6 O 2 →6 CO H 2 0

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:  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 diffusionGlucose enters the cell via facilitated diffusion Glucose binds to a protein carrier molecule on the lipid matrix of the cell membraneGlucose binds to a protein carrier molecule on the lipid matrix of the cell membrane Only in this form glucose can diffuse into the cellOnly 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.