Chapter 5: The Structure and Function of Large Biological Molecules

Macromolecules and Polymers

Macromolecules are large, complex molecules essential to life, including carbohydrates, proteins, and nucleic acids. These are polymers, built from repeating units called monomers. Lipids, though large, are not true polymers.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers).

• Monomer: The repeating unit that serves as a building block for polymers.

• Enzymes: Specialized macromolecules that speed up chemical reactions, including those that build or break down polymers.

Synthesis and Breakdown of Polymers

Polymers are synthesized and broken down by specific chemical reactions:

• Dehydration Reaction: Joins two monomers by removing a water molecule.

• Hydrolysis: Breaks a polymer into monomers by adding a water molecule.

Carbohydrates

Monosaccharides and Classification

Carbohydrates serve as fuel and building material. The simplest carbohydrates are monosaccharides (simple sugars), classified by the location of their carbonyl group and the number of carbons in their skeleton.

• Monosaccharide: Basic unit of carbohydrates, e.g., glucose.

• Aldose: Sugar with an aldehyde group.

• Ketose: Sugar with a ketone group.

Linear and Ring Forms of Glucose

Monosaccharides often form ring structures in aqueous solutions, which are energetically favorable.

• Glucose: Most common monosaccharide, can exist in linear or ring form.

Disaccharides and Glycosidic Linkages

Disaccharides are formed by joining two monosaccharides via a dehydration reaction, creating a glycosidic linkage.

• Disaccharide: Two monosaccharides joined by a covalent bond.

• Glycosidic Linkage: Covalent bond formed between two monosaccharides.

Polysaccharides: Storage and Structural Roles

Polysaccharides are polymers of sugars with storage or structural functions. Their properties depend on monomer composition and glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers.

• Glycogen: Storage polysaccharide in animals, stored in liver and muscle cells.

Structural Polysaccharides: Cellulose and Chitin

Cellulose is a major component of plant cell walls, while chitin forms the exoskeleton of arthropods and cell walls of fungi. The glycosidic linkages in cellulose differ from those in starch, resulting in different properties.

• Cellulose: Polymer of glucose with β linkages, forms straight, unbranched chains.

• Chitin: Structural polysaccharide found in arthropod exoskeletons and fungal cell walls.

Lipids

Fats: Structure and Function

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. Fats are constructed from glycerol and fatty acids, joined by ester linkages to form triacylglycerol (triglyceride).

• Saturated Fatty Acid: No double bonds, solid at room temperature.

• Unsaturated Fatty Acid: One or more double bonds, liquid at room temperature.

• Trans Fat: Unsaturated fat with trans double bonds, created by hydrogenation.

• Adipose Tissue: Stores fats, cushions organs, and insulates the body.

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. They form bilayers in water, which are fundamental to cell membranes.

• Hydrophilic Head: Attracted to water.

• Hydrophobic Tails: Repelled by water.

• Phospholipid Bilayer: Forms the boundary of cells.

Steroids

Steroids are lipids with a carbon skeleton of four fused rings. Cholesterol is a key steroid in animal cell membranes and a precursor for other steroids.

• Cholesterol: Essential for membrane structure, but high levels can contribute to cardiovascular disease.

Proteins

Protein Functions

Proteins are diverse in structure and function, accounting for more than half the dry mass of most cells. Functions include catalysis, defense, storage, transport, communication, movement, and structural support.

• Enzymes: Catalyze chemical reactions.

• Antibodies: Defense against disease.

• Hemoglobin: Transport of oxygen.

• Collagen: Structural support.

Amino Acids and Polypeptides

Proteins are polymers of amino acids, linked by peptide bonds. Each amino acid has a unique side chain (R group) that determines its properties.

• Amino Acid: Organic molecule with amino and carboxyl groups.

• Polypeptide: Polymer of amino acids.

• Peptide Bond: Covalent bond joining amino acids.

Protein Structure

Protein function depends on its three-dimensional structure, which is determined by the sequence of amino acids. There are four levels of protein structure:

• Primary Structure: Unique sequence of amino acids.

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

• Tertiary Structure: Overall shape due to interactions among R groups.

• Quaternary Structure: Association of multiple polypeptide chains.

Sickle-Cell Disease

A single amino acid substitution in hemoglobin can cause sickle-cell disease, affecting protein structure and function.

• Sickle-Cell Disease: Inherited disorder caused by abnormal hemoglobin, leading to sickle-shaped red blood cells.

Protein Denaturation and Folding

Protein structure can be affected by environmental conditions. Denaturation is the loss of native structure, rendering the protein inactive. Protein folding is complex and often assisted by cellular mechanisms.

• Denaturation: Loss of protein structure due to changes in pH, temperature, or salt concentration.

• Renaturation: Restoration of native structure, sometimes possible.

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

Nucleic Acids

Roles and Structure of Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. DNA and RNA are the two types, composed of nucleotide monomers.

• Gene: Unit of inheritance, made of DNA.

• DNA: Deoxyribonucleic acid, stores genetic information.

• RNA: Ribonucleic acid, involved in protein synthesis.

• Gene Expression: DNA → RNA → Protein.

Nucleotide Structure

Nucleotides consist of a nitrogenous base, a pentose sugar, and a phosphate group. There are two families of nitrogenous bases: pyrimidines and purines.

• Pyrimidines: Cytosine, thymine, uracil (single ring).

• Purines: Adenine, guanine (double ring).

• Phosphodiester Linkage: Joins nucleotides in a polynucleotide chain.

DNA and RNA Structures

DNA is a double helix with antiparallel strands and complementary base pairing (A-T, G-C). RNA is single-stranded, with A-U pairing.

• Antiparallel: Strands run in opposite directions.

• Complementary Base Pairing: Ensures accurate replication and transcription.

Genomics and Proteomics

Biological Inquiry and Applications

Genomics and proteomics have revolutionized biological research. Sequencing genomes and analyzing protein sets provide insights into evolution, medical science, conservation, and species interactions.

• Genomics: Study of whole genomes.

• Proteomics: Study of protein sets and their functions.

• Bioinformatics: Use of computational tools to analyze biological data.

Molecular Genealogy

Sequences of genes and proteins document evolutionary relationships among organisms. Molecular biology provides new tools for evolutionary studies.

• Molecular Genealogy: Tracing evolutionary relationships using DNA and protein sequences.

Summary Tables

Key Concepts: Carbohydrates, Lipids, Proteins, Nucleic Acids

Summary tables provide concise comparisons of the main classes of biological molecules, their structures, and functions.

• Carbohydrates: Fuel and building material.

• Lipids: Energy storage, membrane structure, signaling.

• Proteins: Diverse functions including catalysis, structure, transport.

• Nucleic Acids: Information storage and transmission.

*Additional info: Tables summarizing these concepts are present in the original materials but not fully reproduced here due to image limitations.*