Overview
This lecture explains the structure and function of carbohydrates and lipids, emphasizing their biological roles, chemical properties, and key examples relevant to living organisms.
Carbohydrates: Structure and Function
- Carbohydrates contain carbon, hydrogen, and oxygen, and mainly provide energy through respiration.
- Glucose is a monosaccharide serving as the main respiration substrate.
- Some polysaccharides store energy (glycogen in animals, starch in plants), while others are structural (cellulose in plants).
- Monosaccharides are the simplest carbohydrates with the general formula Cn(H2O)n.
- Hexoses (e.g., glucose, fructose, galactose) have six carbons; pentoses (e.g., ribose, deoxyribose) have five.
- Glucose is soluble, stable, essential in photosynthesis, and easily transported.
- Alpha and beta glucose differ at the orientation of the hydroxyl group on carbon 1, leading to different properties and enzyme specificity.
Formation and Types of Carbohydrates
- Disaccharides form when two monosaccharides join via a glycosidic bond, producing water (condensation reaction).
- Examples: maltose (glucose + glucose), lactose (glucose + galactose), sucrose (glucose + fructose).
- Hydrolysis is the reverse, using water to split disaccharides.
- Polysaccharides form by repeated condensation; starch (plants) and glycogen (animals) are storage forms made of alpha glucose.
- Starch consists of amylose (unbranched, coiled) and amylopectin (branched).
- Glycogen is highly branched, compact, and well-suited for rapid glucose release in animals.
- Cellulose is made of beta glucose, forms straight unbranched chains with hydrogen bonds for cell wall strength.
Lipids: Structure and Function
- Lipids are not polymers, generally insoluble in water, but soluble in organic solvents.
- Types include triglycerides, phospholipids, and steroids.
- Triglycerides consist of three fatty acids bonded to glycerol via ester bonds (condensation reaction).
- Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds), affecting melting point and fluidity.
- Triglycerides serve as energy stores, insulation, organ protection, and buoyancy.
- Phospholipids have a phosphate group replacing one fatty acid, creating amphipathic molecules (hydrophilic head, hydrophobic tails).
- Phospholipids are key to membrane structure, forming bilayers and influencing fluidity.
- Cholesterol is a steroid with a fused ring structure; it regulates membrane fluidity and is a precursor for steroid hormones.
Key Terms & Definitions
- Monosaccharide — simplest carbohydrate (single sugar unit).
- Disaccharide — two monosaccharides joined by a glycosidic bond.
- Polysaccharide — large carbohydrate made of many monosaccharides.
- Glycosidic linkage — covalent bond between carbohydrate molecules.
- Condensation reaction — joins molecules, releasing water.
- Hydrolysis — splits molecules by adding water.
- Triglyceride — lipid with three fatty acids and one glycerol.
- Ester bond — bond between fatty acid and glycerol in lipids.
- Phospholipid — lipid with a phosphate group; key for membranes.
- Amphipathic — has both hydrophilic and hydrophobic parts.
- Steroid — lipid with a fused ring structure (e.g., cholesterol).
Action Items / Next Steps
- Review the structures of alpha and beta glucose.
- Practice drawing condensation and hydrolysis reactions for carbohydrates and triglycerides.
- Read textbook sections on cell membranes and lipid functions.