Carbohydrate Metabolism and Classification

Carbohydrate Metabolism

• Carbohydrates are hydrosoluble molecules with the empirical formula (CH₂O)ₙ. Monosaccharides are the building blocks of larger carbohydrates.
• Carbohydrates serve various functions, including energy storage, structural roles (like cellulose in plant cell walls), and as components of other molecules (like glycoproteins and DNA).

Carbohydrate Classification

• Monosaccharides: Simple sugars, classified based on the carbonyl group (aldehyde or ketone).
  • Aldoses have an aldehyde group.
  • Ketoses have a ketone group.
  • Examples include trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), and hexoses (6 carbons) like glucose, fructose, and galactose.
• Oligosaccharides: Chains of 2-10 monosaccharides linked by glycosidic bonds.
  • Disaccharides, like lactose (galactose + glucose), sucrose (glucose + fructose), and maltose (glucose + glucose), are examples of oligosaccharides.
• Polysaccharides: Chains of more than 10 monosaccharides.
  • Homopolysaccharides are composed of a single type of monosaccharide unit, such as starch (glucose polymer in plants) and glycogen (branched glucose polymer in animals). Cellulose (unbranched glucose polymer in plants) is another example of a storage form.
  • Heteropolysaccharides are composed of multiple types of monosaccharide units. An example is hyaluronic acid which is found in the synovial fluid and vitreous humor of the eye.

Digestion of Carbohydrates

• Polysaccharides and oligosaccharides are broken down into monosaccharides by enzymes in the digestive tract.
• Specific enzymes, like amylase (breaks down 1-4 bonds), lactase, and sucrase, target different bonds.
• The resulting monosaccharides are absorbed into the bloodstream.

Glucose Homeostasis

• Maintaining stable blood glucose levels (homeostasis) is crucial for energy provision to the brain and other tissues.
• After a meal, insulin is released, promoting glucose uptake by tissues and glycogen synthesis.
• During fasting or exercise, glucagon is released, leading to glycogen breakdown and gluconeogenesis (new glucose production).

Glycolysis

• The universal central pathway of glucose catabolism (breaking down).
• Glycolysis converts one molecule of glucose into two molecules of pyruvate in the cytoplasm.
• During glycolysis, some free energy released from glucose is conserved in the form of ATP and NADH.
• The pathway is regulated by key enzymes like hexokinase, phosphofructokinase, and pyruvate kinase.

Gluconeogenesis

• The pathway for synthesizing glucose from non-carbohydrate precursors, like lactate, glycerol, and some amino acids.
• Different enzymes in gluconeogenesis bypass irreversible steps in glycolysis.
• Gluconeogenesis is crucial for maintaining blood glucose levels during fasting.

Cori Cycle

• A cyclical process between muscles and the liver.
• During intense exercise, lactate (product of anaerobic glycolysis) in the muscles is transported to the liver.
• The liver converts lactate back to glucose via gluconeogenesis and then delivers the glucose back to the muscles, maintaining glycogen stores.

Glycogen Metabolism

• Glycogenesis: The synthesis of glycogen from glucose. Insulin stimulates this pathway.
• Glycogenolysis: The breakdown of glycogen into glucose. Epinephrine and glucagon stimulate this pathway.
• Glycogen serves as a storage form of glucose in animals, especially in the liver and muscles.
• Glycogen in muscle is for immediate energy use during contraction.
• Glycogen in the liver maintains blood glucose levels between meals and during fasting.

Regulation of Carbohydrate Metabolism

• Well-fed state: Insulin promotes glycogen synthesis and glucose uptake, and glycolysis.
• Fasting state: Glucagon promotes glycogenolysis and gluconeogenesis, while low blood glucose stimulates hormonal secretions for glucose production.
• Exercise: Epinephrine promotes glycogen breakdown and glycolysis in muscle and liver.