Overview
This lecture explains the importance of carbon in biological molecules and details the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids, describing their structures and functions.
The Importance of Carbon
- Carbon atoms form the backbone of biological molecules due to their ability to make four covalent bonds.
- Carbon can bond with itself and other elements, creating chains, branches, and rings, resulting in molecular diversity.
Carbohydrates
- Carbohydrates have a carbon:hydrogen:oxygen ratio of 1:2:1 and serve as energy sources and structural materials.
- Monosaccharides are simple sugars (e.g., glucose) and can exist as rings or chains.
- Disaccharides form when two monosaccharides join via dehydration reactions (e.g., lactose, sucrose).
- Polysaccharides are long chains of monosaccharides (e.g., starch in plants, glycogen in animals, cellulose in plant cell walls, chitin in exoskeletons).
- Cellulose provides structural support in plants and dietary fiber in animals; chitin is found in arthropod exoskeletons.
Lipids
- Lipids are nonpolar, hydrophobic molecules including fats, oils, waxes, phospholipids, and steroids.
- Triglycerides consist of glycerol and three fatty acids, providing long-term energy storage and insulation.
- Saturated fats have only single bonds; unsaturated fats have one or more double bonds, affecting their physical state and health effects.
- Trans-fats are artificially hydrogenated oils linked to health risks.
- Essential fatty acids (omega-3 and omega-6) must be obtained from the diet.
- Phospholipids, key cell membrane components, have hydrophilic heads and hydrophobic tails forming bilayers.
- Steroids (e.g., cholesterol) have four fused rings and serve as hormone precursors; waxes protect plants and animals.
Proteins
- Proteins are polymers of 20 amino acids with diverse functions (enzymes, hormones, structural, transport).
- Amino acids share a basic structure, differing only in their R group.
- Amino acids join by peptide bonds through dehydration reactions, forming polypeptides.
- Protein function depends on shape, determined by four structural levels: primary (sequence), secondary (helices/sheets), tertiary (3D folding), quaternary (subunit assembly).
- Small changes in amino acid sequence can cause diseases (e.g., sickle-cell anemia).
- Denaturation is loss of protein structure and function due to environmental changes.
Nucleic Acids
- Nucleic acids (DNA, RNA) store and transmit genetic information.
- DNA is double-stranded, with a sugar-phosphate backbone and paired nitrogenous bases forming a double helix.
- RNA is mainly involved in protein synthesis and gene regulation.
- Nucleic acids are polymers of nucleotides, each with a nitrogenous base, pentose sugar, and phosphate group.
Key Terms & Definitions
- Macromolecule — large molecule composed of smaller subunits (monomers).
- Monosaccharide — simple sugar molecule; carbohydrate monomer.
- Disaccharide — two monosaccharides joined by a covalent bond.
- Polysaccharide — long chain of monosaccharide units.
- Lipid — hydrophobic molecule including fats and oils.
- Saturated fat — fat molecule without double bonds in fatty acid tails.
- Unsaturated fat — fat with one or more double bonds in fatty acid tails.
- Phospholipid — lipid with two fatty acids and a phosphate group; main component of cell membranes.
- Steroid — lipid with four fused carbon rings.
- Amino acid — protein monomer with variable R group.
- Peptide bond — covalent bond between two amino acids.
- Denaturation — loss of protein structure/function due to environmental factors.
- Nucleotide — monomer of nucleic acids (DNA/RNA): base, sugar, phosphate.
- Double helix — structure of DNA: two intertwined strands.
Action Items / Next Steps
- Review diagrams of the four biological macromolecule types.
- Study the structures of glucose, amino acids, and nucleotides.
- Read about the effects of dietary fats and proteins on health.