Chapter 5: The Structure and Function of Large Biological Molecules

Introduction to Macromolecules

Large biological molecules, also known as macromolecules, are essential for life and include carbohydrates, lipids, proteins, and nucleic acids. These molecules are polymers built from smaller subunits called monomers, except for lipids, which are not true polymers.

• Macromolecules: Large molecules composed of thousands of covalently connected atoms.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating units that serve as the building blocks of a polymer.

• Dehydration Reaction: A chemical reaction that builds polymers by removing a water molecule, forming a new bond.

• Hydrolysis: A chemical reaction that breaks polymers into monomers by adding a water molecule.

• Enzyme: Biological catalysts that speed up chemical reactions, including those that build or break polymers.

Types of Macromolecules

There are four major types of macromolecules found in living organisms:

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Carbohydrates

Monosaccharides: Structure and Classification

Carbohydrates serve as fuel and building material. The simplest carbohydrates are monosaccharides, or simple sugars.

• Monosaccharide: The smallest unit of a carbohydrate; typically has a molecular formula that is a multiple of CH2O.

• Glucose (C6H12O6): The most common monosaccharide.

• Classification: Based on the location of the carbonyl group (aldose or ketose) and the number of carbons in the skeleton.

• Aldose: Monosaccharide with the carbonyl group at the end of the carbon chain (e.g., glucose).

• Ketose: Monosaccharide with the carbonyl group within the carbon chain (e.g., fructose).

Disaccharides and Polysaccharides

Disaccharides are formed by joining two monosaccharides via a glycosidic linkage. Polysaccharides are polymers of sugars and serve storage and structural roles.

• Disaccharide: Two monosaccharides joined by a dehydration reaction (e.g., maltose, sucrose).

• Glycosidic Linkage: Covalent bond formed between two monosaccharides.

• Polysaccharide: Macromolecule composed of many monosaccharides.

Storage Polysaccharides

• Starch: Storage polysaccharide in plants; consists of glucose monomers. Stored in chloroplasts and plastids.

• Glycogen: Storage polysaccharide in animals; highly branched and stored mainly in liver and muscle cells.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls; composed of glucose monomers linked by β (1→4) glycosidic bonds.

• Chitin: Found in the exoskeleton of arthropods and cell walls of fungi.

Comparison of Starch and Cellulose

• Starch: α (1→4) glycosidic linkages; forms helical structures.

• Cellulose: β (1→4) glycosidic linkages; forms straight, unbranched chains that allow hydrogen bonding between molecules.

• Digestibility: Enzymes that digest starch cannot hydrolyze β linkages in cellulose; cellulose acts as insoluble fiber in human diets.

Lipids

Characteristics and Types of Lipids

Lipids are hydrophobic molecules that do not form true polymers. They consist mostly of hydrocarbon regions and mix poorly with water.

• Major Types: Fats, phospholipids, steroids.

• Fatty Acid: Carboxyl group attached to a long carbon skeleton.

• Glycerol: Three-carbon alcohol with a hydroxyl group attached to each carbon.

• Triacylglycerol (Triglyceride): Three fatty acids joined to glycerol by ester linkages.

• Function of Fats: Energy storage and cushioning of vital organs.

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acid: No double bonds; maximum number of hydrogen atoms; solid at room temperature.

• Unsaturated Fatty Acid: One or more double bonds; causes kinks in the chain; liquid at room temperature.

• Trans Fats: Unsaturated fats that have been hydrogenated to make them solid; associated with health risks.

Phospholipids

• Structure: Two fatty acids and a phosphate group attached to glycerol.

• Properties: Hydrophobic tails and hydrophilic head; form bilayers in water, which are the basis of cell membranes.

Steroids

• Structure: Four fused carbon rings.

• Cholesterol: Component of animal cell membranes and precursor for other steroids.

Proteins

Structure and Function of Proteins

Proteins are polymers of amino acids and perform a wide range of functions in cells, including catalysis, defense, storage, transport, communication, movement, and structural support.

• Amino Acid: Organic molecule with amino and carboxyl groups and a variable side chain (R group).

• Polypeptide: Polymer of amino acids linked by peptide bonds.

• Peptide Bond: Covalent bond between amino acids in a polypeptide.

• Protein: Biologically functional molecule consisting of one or more polypeptides folded into a specific shape.

Levels of Protein Structure

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Coils and folds due to hydrogen bonding (α helix and β pleated sheet).

• Tertiary Structure: Overall shape due to interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

• Quaternary Structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Protein Denaturation

• Denaturation: Loss of a protein's native structure due to changes in pH, salt concentration, temperature, or other environmental factors; results in loss of function.

• Renaturation: Sometimes possible if the denaturing agent is removed.

Protein Folding and Determination

• X-ray Crystallography: Technique to determine protein structure.

• NMR Spectroscopy: Another method for structure determination.

• Bioinformatics: Computational prediction of protein structure from amino acid sequences.

Nucleic Acids

Structure and Function of Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. The two types are DNA and RNA.

• DNA (Deoxyribonucleic Acid): Stores genetic information; double-stranded helix.

• RNA (Ribonucleic Acid): Involved in protein synthesis; usually single-stranded.

• Gene Expression: DNA directs synthesis of mRNA, which controls protein synthesis.

Components of Nucleic Acids

• Nucleotide: Monomer of nucleic acids; consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: Nitrogenous base + sugar (no phosphate group).

• Pyrimidines: Single six-membered ring (cytosine, thymine, uracil).

• Purines: Six-membered ring fused to a five-membered ring (adenine, guanine).

• Phosphodiester Linkage: Bond that joins nucleotides in a polynucleotide chain.

DNA and RNA Structure

• DNA: Double helix; two antiparallel strands; complementary base pairing (A with T, G with C).

• RNA: Single-stranded; complementary pairing can occur within the molecule or between molecules; uracil replaces thymine.

Genomics and Proteomics

Modern Biological Inquiry

Advances in genomics and proteomics have transformed biological research. Bioinformatics uses computational tools to analyze large sets of genetic and protein data.

• Genomics: Study and comparison of whole genomes.

• Proteomics: Study of large sets of proteins, including their sequences and functions.

• Molecular Genealogy: Use of DNA and protein sequences to study evolutionary relationships.

Key Tables

Comparison of Storage and Structural Polysaccharides

Comparison of DNA and RNA

Key Equations

• General Formula for Monosaccharides:

• Dehydration Reaction (Formation of Maltose):

• Phosphodiester Linkage in Nucleic Acids:

Summary

• Macromolecules are essential for life and include carbohydrates, lipids, proteins, and nucleic acids.

• Each type of macromolecule has unique structures and functions, determined by their monomers and linkages.

• Understanding the structure and function of these molecules is fundamental to biology and the study of life.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard biology curriculum.