Overview
This lecture covers the structure, function, and types of carbohydrates, including their monomers and polymers, examples, the role in energy storage and structural components, and their significance in human health.
Carbohydrate Basics
- Carbohydrates are macromolecules composed of carbon, hydrogen, and oxygen with the general formula (CH₂O)n.
- Monomers of carbohydrates are monosaccharides (“one sugar”); polymers are polysaccharides (“many sugars”).
- Disaccharides consist of two monosaccharides bonded together.
Monosaccharides
- All monosaccharides (except one) have equal numbers of carbon and oxygen, and twice as many hydrogens (e.g., C₆H₁₂O₆).
- Common monosaccharides: glucose, galactose, fructose (all C₆H₁₂O₆, structural isomers).
- Ribose (RNA) and deoxyribose (DNA) are five-carbon (pentose) sugars.
- Sugars commonly form ring structures in solution.
- Key functional groups: carbonyl (C=O) and multiple hydroxyl (-OH) groups.
Disaccharides
- Formed via dehydration reaction (removal of water) between two monosaccharides, creating a glycosidic linkage.
- Sucrose = glucose + fructose (plants’ transport sugar).
- Lactose = glucose + galactose (milk sugar).
- Maltose = glucose + glucose (found in wheat).
- Disaccharides must be broken down into monosaccharides to be absorbed by cells.
Polysaccharides
- Storage polysaccharides: starch (plants; amylose and amylopectin) and glycogen (animals; highly branched, stored in liver and muscle).
- Structural polysaccharides: cellulose (plant cell walls, indigestible by animals without symbiotic microbes) and chitin (fungal cell walls, arthropod exoskeletons).
Digestion & Energy Storage
- Starch digestion starts in the mouth (amylase enzyme) and continues in the small intestine.
- Animals store excess glucose as glycogen in the liver (main storage) and skeletal muscle (for quick energy).
- Glucose is critical for ATP production in cellular respiration.
Blood Glucose Regulation & Homeostasis
- Homeostasis maintains constant blood glucose (~90 mg/100 mL).
- Insulin (from pancreas) lowers blood glucose by facilitating glucose uptake and glycogen formation.
- Glucagon (from pancreas) raises blood glucose by stimulating glycogen breakdown.
- Only brain cells can take up glucose without insulin.
Diabetes
- Type 1 diabetes: autoimmune destruction of insulin-producing cells, requiring lifelong insulin.
- Type 2 diabetes: cells become desensitized to insulin (linked to diet and lifestyle; can often be managed with diet/exercise).
- Persistent high blood glucose damages small blood vessels.
Cellulose, Fiber, and Digestion
- Animals cannot hydrolyze cellulose without symbiotic bacteria/protists.
- Cellulose acts as dietary fiber, important for gut and overall health.
Key Terms & Definitions
- Monosaccharide — single sugar molecule, carbohydrate monomer.
- Disaccharide — two monosaccharides covalently bonded.
- Polysaccharide — long chain of monosaccharides (polymer).
- Glycosidic linkage — covalent bond between carbohydrate monomers.
- Dehydration reaction — reaction linking monomers by removing water.
- Hydrolysis — breakdown of polymers by adding water.
- Homeostasis — maintaining constant internal conditions.
- Insulin — hormone lowering blood glucose.
- Glucagon — hormone raising blood glucose.
- Cellulose — structural polysaccharide in plant cell walls.
- Chitin — structural polysaccharide in fungi and arthropods.
Action Items / Next Steps
- Review structures and functions of glucose, galactose, fructose, ribose, and deoxyribose.
- Memorize which monosaccharides combine to form each disaccharide.
- Understand the roles of insulin and glucagon in blood glucose regulation.
- Prepare for exam questions on identifying carbohydrates and their polymers.