Chapter 5 – The Structure and Function of Large Biological Molecules

Introduction to Biomolecules and Macromolecules0:00:00

Dr. D introduces Chapter 5, focusing on large biomolecules. These include proteins, carbohydrates, nucleic acids (DNA/RNA), and lipids. Three of these — carbohydrates, proteins, and nucleic acids — are classified as macromolecules due to their ability to form polymers from repeating monomer units (building blocks). Lipids are biomolecules but not macromolecules because they do not form large polymers. Monomers link together via dehydration reactions (condensation reaction), where water is removed to form covalent bonds. Polymers are broken down into monomers through hydrolysis, a process that uses water to break covalent bonds.

Carbohydrates: Monosaccharides, Disaccharides, and Polysaccharides0:10:03

Carbohydrates include sugars and sugar polymers. Monosaccharides are the simplest sugars and the monomers of carbohydrates, typically following the formula (CH2O)n, such as glucose (C6H12O6). Monosaccharides can exist as linear structures but form ring structures in aqueous solutions. They can also have different isomeric forms, such as aldoses and ketoses. Disaccharides are formed when two monosaccharides link via a glycosidic linkage through a dehydration reaction. Examples include maltose (glucose-glucose), lactose (glucose-galactose), and sucrose (glucose-fructose). Polysaccharides are carbohydrate macromolecules formed from many monosaccharides. They serve roles in energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plant cell walls, chitin in fungi and arthropods). The difference between alpha and beta glucose isomers affects their properties: alpha glucose forms helical storage polysaccharides, while beta glucose forms straight, unbranched structural polysaccharides like cellulose, which is indigestible by humans.

Lipids: Fats, Phospholipids, and Steroids0:34:21

Lipids are biomolecules characterized by their poor mixability with water due to their predominantly hydrocarbon, nonpolar regions. They are not considered macromolecules because they do not form polymers. The three main classes of lipids discussed are fats, phospholipids, and steroids. Fats consist of a glycerol head and three fatty acid tails, linked by ester linkages formed through dehydration reactions. Fats are largely nonpolar and do not mix with water. Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds), which affects their physical properties: saturated fats are solid at room temperature (e.g., animal fats), while unsaturated fats are liquid (e.g., plant oils). Phospholipids are crucial components of cell membranes, composed of a glycerol, two fatty acid tails, and a phosphate group (often with choline). They are amphipathic, with a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails, forming a phospholipid bilayer in membranes. Steroids are lipids characterized by a carbon skeleton of four fused rings, such as cholesterol, which maintains membrane fluidity in animal cells.

Proteins: Amino Acids, Peptide Bonds, and Protein Structure0:59:00

Proteins are vital macromolecules made from 20 naturally occurring amino acid monomers connected by peptide bonds (formed by dehydration reaction). Proteins perform diverse functions: enzymatic catalysis, defense, storage, transport, communication, movement, and structural support. Each amino acid has a common backbone (amino group, carboxyl group, alpha carbon with hydrogen) and a unique side chain (R group). R groups classify amino acids as nonpolar, polar, acidic (negatively charged), or basic (positively charged). Polypeptides have an amino end (N-terminus) and a carboxyl end (C-terminus). Protein structure has four levels: primary (amino acid sequence), secondary (local folding into alpha helices and beta sheets), tertiary (overall 3D shape due to R-group interactions like ionic bonds, hydrophobic interactions, hydrogen bonds, and disulfide bridges), and quaternary (association of multiple polypeptide chains). Proper protein folding is critical for function; misfolding, as seen in sickle cell anemia, can lead to serious diseases. External factors like pH, salt concentration, and temperature can denature proteins, causing them to unfold and lose function, which is usually irreversible.

Nucleic Acids: DNA and RNA Structure and Function1:52:43

Nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are macromolecules that store and transmit genetic information. Genes, composed of DNA, serve as blueprints for proteins. Nucleic acids are polymers of nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose sugar (ribose in RNA, deoxyribose in DNA), and one or more phosphate groups. The difference between ribose and deoxyribose lies in an oxygen atom at the 2' carbon. The nitrogenous bases are adenines (A), guanines (G), cytosines (C), and thymines (T) in DNA, and A, G, C, and uracil (U) in RNA. Pyrimidines (C, T, U) are single-ring bases, while purines (A, G) are double-ring bases. Nucleotides link via phosphodiester bonds, forming a sugar-phosphate backbone with bases projecting outwards. A DNA or RNA strand has a 5' end (phosphate group) and a 3' end (hydroxyl group). DNA typically forms a double helix, with two antiparallel strands held together by hydrogen bonds between complementary base pairs (A with T, G with C). RNA is usually single-stranded but can fold and form internal base pairs (A with U, G with C).