Carbohydrate Metabolism and Classification

Carbohydrate Metabolism

• Carbohydrates are hydrosoluble molecules.
• Their empirical formula is (CH₂O)ₙ.
• Monosaccharides are the building blocks of larger carbohydrates.
• Monosaccharides function in energy storage, structural components, and cell identity markers.
• Glucose is a vital monosaccharide for energy generation.

Carbohydrate Classification

• Monosaccharides are simple sugars.
• Oligosaccharides consist of 2-10 monosaccharides bonded together by glycosidic bonds.
• Polysaccharides consist of more than 10 monosaccharides.
• Homopolysaccharides are made up of the same type of monosaccharide.
• Heteropolysaccharides contain more than one type of monosaccharide.

Monosaccharides

• Monosaccharides are classified based on the carbonyl group (aldehyde or ketone).
• Aldoses have an aldehyde group, whereas ketoses have a ketone group.
• Glucose, fructose, and galactose are important hexoses.
• Ribose is an important pentose.

Oligosaccharides

• Oligosaccharides are formed from 2 to 10 monosaccharides linked via glycosidic bonds.
• A common example is maltose.
• Disaccharides such as lactose and sucrose have defined compositions.

Polysaccharides

• Polysaccharides are polymers with more than 10 monosaccharide units.
• Starch and glycogen are storage forms of glucose in plants and animals, respectively.
• Starch is less branched than glycogen.
• Cellulose is a structural polysaccharide found in plant cell walls.
• Hyaluronic acid is a heteropolysaccharide, functioning as a lubricant.

Digestion of Carbohydrates

• Digestion begins in the mouth, using enzymes like α-amylase.
• Pancreatic α-amylase further breaks down starch.
• Oligosaccharides in the small intestine are broken down to monosaccharides by enzymes like lactase and sucrase.
• Monosaccharides are absorbed from the intestines.

Glucose Homeostasis

• Glucose is essential for brain function.
• After a meal, blood glucose rises, triggering insulin release.
• Insulin stimulates glucose uptake by cells and glycogen formation in the liver.
• During fasting, glucagon is released, promoting glycogen breakdown and gluconeogenesis.

Glycolysis

• Glycolysis is a universal central pathway for glucose catabolism.
• Glycolysis occurs in the cytoplasm.
• Glycolysis breaks down glucose into pyruvate, releasing ATP.

Gluconeogenesis

• Gluconeogenesis forms glucose from non-carbohydrate precursors.
• It's a vital pathway during fasting and exercise.
• Gluconeogenesis uses many but not all reversible steps of glycolysis.

Cori Cycle

• The Cori cycle is the metabolic pathway between skeletal muscle and the liver involving lactate, pyruvate, and glucose.
• During vigorous exercise, lactate produced by the muscles is transported to the liver.

Glycogen Metabolism

• Glycogen is the storage form of glucose in animals.
• Glycogenolysis breaks down glycogen to glucose.
• Glycogenesis synthesizes glycogen from glucose.
• Epinephrine and glucagon trigger glycogen breakdown.
• Insulin stimulates glycogen synthesis.

Regulation of Carbohydrate Metabolism

• Hormones like insulin and glucagon regulate carbohydrate metabolism by influencing glycogen synthesis and breakdown, and glucose uptake by cells (in a well-fed or fasting state).
• Exercise causes release of adrenaline, which influences glycogen breakdown and glycolysis.

Aerobic and Anaerobic Respiration

• Aerobic respiration involves oxygen and yields a large amount of ATP.
• Anaerobic respiration produces far less ATP, as in fermentation.