Glycolysis and Glucose Metabolism

Glycolysis and Glycogenesis

• Glycolysis is the breakdown of glucose to pyruvic acid or lactic acid for the production of energy in the form of ATP.
• Glycogenesis is the process of glycogen formation, while glycogenolysis is the breakdown of glycogen.
• Glucose is converted to glycogen and stored in the liver and skeletal muscle.

Glucose Recovery

• Glucose can be recovered from fat and protein through the process of gluconeogenesis.

Regulation of Blood Glucose Metabolism

• Two major hormones regulate blood glucose metabolism: insulin and glucagon, both produced by the pancreas.
• Insulin is the primary hormone responsible for the entry of glucose into the cell, produced by beta cells in the islets of Langerhans.
• Insulin decreases blood glucose levels by:
  • Transporting glucose to muscle and adipose tissue
  • Increasing glycogenesis, lipogenesis, and glycolysis
  • Inhibiting glycogenolysis
• Glucagon is the primary hormone responsible for increasing blood glucose levels, produced by alpha cells in the islets of Langerhans.
• Other hormones involved in blood glucose regulation include:
  • Epinephrine
  • Glucocorticoids (primarily cortisol)
  • Growth hormone
  • Thyroxine
  • Somatostatin

Hormone Functions

• Insulin:
  • Decreases blood glucose levels
  • Hypoglycemic hormone
• Glucagon:
  • Increases blood glucose levels
  • Hyperglycemic hormone
• Epinephrine:
  • Increases plasma glucose
• Glucocorticoids:
  • Increase plasma glucose
  • Stimulated by ACTH
• Growth hormone:
  • Released from anterior pituitary
  • Stimulated by decreased blood glucose levels
  • Inhibited by increased blood glucose levels
• Somatostatin:
  • Produced by gamma cells in the islets of Langerhans
  • Increases plasma glucose levels by inhibiting insulin

Hypoglycemia

• Hypoglycemia is characterized by blood glucose levels less than normal.
• Symptoms of hypoglycemia usually occur when blood glucose levels fall below 50 mg/dl.
• Warning signs and symptoms of hypoglycemia are related to the central nervous system.
• Classification of hypoglycemia:
  • Reactive (postprandial, or after meals)
  • Fasting (postabsorptive)

Whipple's Triad

• Low blood glucose with classic symptoms of hypoglycemia
• Symptoms improve with glucose administration

Carbohydrates

• Polysaccharides (glycans) differ from each other in:
  • Identity of their recurring monosaccharide units
  • Length of their chains
  • Types of bonds linking the units
  • Degree of branching
• Amylase hydrolyzes starch to disaccharides
• Polysaccharides are of two types:
  • Homopolysaccharides (contain only a single monomeric species)
  • Heteropolysaccharides (contain two or more different kinds of monomeric species)

Chemical Properties of Carbohydrates

• Some carbohydrates are reducing substances, able to reduce or oxidize other compounds.
• Examples of reducing carbohydrates: glucose, maltose, fructose, lactose, and galactose.
• Non-reducing carbohydrates cannot reduce or oxidize compounds.
• Example of non-reducing carbohydrates: sucrose

Functions of Carbohydrates

• Provide energy to fuel cellular reactions
• Are intermediates in the formation of fats and proteins
• Aid in the regulation of nerve tissue and are the energy source of the brain
• Involved in the formation of RNA and DNA
• Involved in the formation of connective tissue

Fate of Glucose

• Ingested carbohydrates are polymers such as starch and glycogen.
• Amylase enzyme is responsible for the digestion of these compounds to disaccharides and dextrin.
• Enzymes sucrase and lactase hydrolyze sucrose to glucose and fructose, and lactose to glucose and galactose.
• Glucose is absorbed from the small intestine, then metabolism of the carbohydrate in the body begins.
• Glucose is transported through the bloodstream to the liver.
• In the liver, glucose is either used or stored.