Overview
This lecture covers carbohydrates, detailing their structures, functions, classifications, and significance in biology, including energy metabolism, structural roles, and key examples like glycoproteins, glycolipids, and vaccines.
Carbohydrate Basics
- Carbohydrates are molecules made of carbon, hydrogen, and oxygen, serving as a major energy source.
- Plants synthesize carbohydrates (sugars), which animals consume and metabolize for energy.
- Carbohydrates’ empirical formula resembles hydrated carbon (CH2O)n.
Types of Carbohydrates
- Monosaccharides are single sugar units; examples: glucose, fructose, galactose.
- Disaccharides consist of two monosaccharides linked by glycosidic bonds (e.g., sucrose, lactose, maltose).
- Oligosaccharides are short chains (3–9 sugars), often involved in cell signaling.
- Polysaccharides are long chains (10+ sugars), such as starch, glycogen, cellulose, and chitin.
Structural Variations and Isomerism
- Carbohydrates can be aldoses (aldehyde group) or ketoses (ketone group).
- Chirality leads to D- and L- forms, which are mirror-image isomers (enantiomers).
- Arrangement of hydroxyl groups determines sugar identity and properties.
Key Monosaccharides and Their Roles
- Glucose is the primary energy source and crosses the blood-brain barrier.
- Fructose is found in honey, fruits, and vegetables.
- Galactose is found in milk, usually as part of lactose.
Important Disaccharides & Polysaccharides
- Sucrose (glucose + fructose) is table sugar.
- Lactose (glucose + galactose) is milk sugar.
- Maltose (glucose + glucose) is found in germinating grains.
- Starch and glycogen are digestible energy-storage polysaccharides.
- Cellulose is an indigestible plant fiber; chitin forms fungal and arthropod structures.
Biological Functions of Carbohydrates
- Structural support: cellulose in plants, chitin in fungi/arthropods, peptidoglycan in bacteria.
- Energy storage: starch (plants), glycogen (animals).
- Cell signaling: oligosaccharides on glycoproteins and glycolipids serve as cell surface markers.
- Immunity: blood type antigens and bacterial capsules involve glycosylated molecules.
Glycosylation and Its Importance
- Glycosylation attaches sugars to proteins (glycoproteins) or lipids (glycolipids), affecting protein stability, lifespan, and signaling.
- N-glycans are attached to asparagine; O-glycans to serine or threonine residues.
Clinical and Immunological Relevance
- Vaccines targeting polysaccharide capsules (e.g., pneumococcal vaccines) often require conjugation to proteins for strong immune responses.
- HIV infectivity relies on viral glycoproteins binding to host cell receptors.
Key Terms & Definitions
- Monosaccharide — single sugar molecule.
- Disaccharide — two monosaccharides linked by glycosidic bond.
- Polysaccharide — long chain of monosaccharides.
- Glycosidic bond — bond linking sugar units.
- Aldose/Ketose — sugar with aldehyde/ketone group.
- Chirality — property of a molecule with non-superimposable mirror images.
- Glycosylation — addition of carbohydrate to a protein or lipid.
Action Items / Next Steps
- Review carbohydrate structures, especially glucose, fructose, and galactose.
- Study the differences between glycoproteins and glycolipids.
- Prepare for quiz on carbohydrate classification and the functions of major polysaccharides.