Overview
This lecture introduces carbohydrates (sugars/saccharides), discussing their structure, classification, properties, and key biological functions, with emphasis on types relevant for exams.
Structure and Classification of Carbohydrates
- Carbohydrates are organic molecules with a carbon chain backbone, and contain hydrogen and oxygen.
- Each carbon atom typically has a hydroxyl group; one carbon has either an aldehyde (aldose) or ketone (ketose) group.
- Common monosaccharides: glucose (aldose), galactose (isomer of glucose), and fructose (ketose).
- In biological systems, sugars are typically found in cyclic (ring) forms due to aqueous environments.
- Carbohydrates serve nutritional (energy/reserve material) and structural (building material) roles.
Types of Carbohydrates
- Classified as monosaccharides (simple sugars), disaccharides (two monosaccharide units), and polysaccharides (many monosaccharide units).
- Oligosaccharides are chains of several simple sugars but are not covered here.
Monosaccharides
- Properties: highly water-soluble, sweet, osmotically active (cause osmosis), and easily cross cell membranes.
- Not stored as reserve material to prevent cellular swelling from osmosis.
- Classified by carbon atoms: trioses (3C), pentoses (5C, e.g., ribose, deoxyribose), and hexoses (6C, e.g., glucose, fructose, galactose).
- Ribose/deoxyribose are components of nucleic acids.
- Glucose is the main energy source for most cells, especially the brain; abnormal glucose levels cause neurological symptoms.
- Fructose is found in fruits and honey and is sweeter than glucose.
- Galactose is a component of lactose.
Disaccharides
- Sucrose: glucose + fructose; main sugar in plants (transport form).
- Lactose: glucose + galactose; found in milk.
- Maltose: glucose + glucose; forms during starch breakdown.
Polysaccharides
- Polysaccharides consist only of glucose units (either alpha or beta forms).
- Storage polysaccharides: starch (plants, alpha-glucose, amylose and amylopectin) and glycogen (animals, alpha-glucose, highly branched).
- Structural polysaccharides: cellulose (plants, beta-glucose, unbranched, forms fibers) and chitin (fungi/arthropods, beta-glucosamine, forms exoskeletons/cell walls).
- Only starch and glycogen are digestible by humans; beta-glycosidic bonds in cellulose/chitin can't be broken down by most animals.
- Highly branched structure of glycogen allows rapid glucose release for energy in animals.
- Polysaccharides are not osmotically active, so can be safely stored in cells.
Detection of Polysaccharides
- Starch can be detected in materials using Lugol’s iodine, which turns navy blue or red when reacting with amylose in starch.
Key Terms & Definitions
- Carbohydrate — organic molecule with carbon, hydrogen, and oxygen; includes sugars and polysaccharides.
- Monosaccharide — simple sugar, basic carbohydrate unit (e.g., glucose).
- Disaccharide — carbohydrate formed from two monosaccharides (e.g., sucrose).
- Polysaccharide — large carbohydrate composed of many monosaccharide units (e.g., starch, cellulose).
- Aldose — sugar with an aldehyde group.
- Ketose — sugar with a ketone group.
- Osmotically active — able to cause water movement via osmosis.
- Glycosidic bond — linkage between sugar molecules.
- Amylose/Amylopectin — two types of starch molecules (unbranched/branched).
- Beta-glucosamine — modified glucose in chitin.
Action Items / Next Steps
- Practice detecting starch in foods using Lugol’s iodine solution.
- Review the section on membrane transport for more on osmosis.
- Prepare for the next lecture on nucleic acids.