Chapter 5 - Macromolecules

Chapter 5: Macromolecules

Introduction to Macromolecules

• Four main classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

• Macromolecules are large molecules, often consisting of thousands of atoms, with some having a mass greater than 100,000 daltons.

• Biochemists have mapped the structures of macromolecules that exhibit unique emergent properties due to atomic arrangement.

• Some macromolecules are categorized as polymers.

Structure and Function of Polymers

• Polymers: Long molecules made of similar building blocks known as monomers.

  • Monomers are linked by covalent bonds.

  • Common examples of polymers include carbohydrates, proteins, and nucleic acids.

• A dehydration reaction bonds two monomers together, losing a water molecule, while hydrolysis breaks down polymers back into monomers, facilitated by specific enzymes.

Diversity of Macromolecules

• Cells contain thousands of different macromolecules, which vary among organisms.

• Diversity is due to various combinations of 40-50 common monomers and rare monomers.

• Despite such diversity, members of each class share structural and functional similarities.

Carbohydrates

• Carbohydrates consist of sugars and sugar polymers.

  • Monosaccharides: Simple sugars that act as primary energy sources for cells and can serve as building blocks for other molecules.

  • Disaccharides: Formed from two monosaccharides via a glycosidic linkage;

  • Polysaccharides: Long chains of monosaccharides bonded together.

• Monosaccharides typically have formulas that are multiples of CH₂O, with glucose being the most common (C₆H₁₂O₆).

• Classification of Monosaccharides:

  • Based on carbon skeleton size (3-7 carbons) and carbonyl group location (aldose or ketose).

Structural Characteristics of Sugars

• Monosaccharides can differ in spatial arrangements, impacting their function (e.g., glucose vs. galactose).

• In aqueous environments, sugars often form ring structures.

Sugar Functionality

• Monosaccharides like glucose are essential for cellular respiration and serve as precursors for amino acids and fatty acids.

• Disaccharides: Formed via dehydration reactions with examples such as maltose, sucrose, and lactose.

  • Maltose: Two glucose molecules.

  • Sucrose: Composed of glucose and fructose; main transport sugar in plants.

  • Lactose: Glucose and galactose bound together.

• Polysaccharides: Store energy or serve structural roles in cells (例如 starch, glycogen, cellulose).

Properties of Storage and Structural Polysaccharides

• Starch: Storage form in plants made up of glucose; can be branched (amylopectin) or unbranched (amylose).

• Glycogen: Animal storage polysaccharide, highly branched, primarily stored in liver and muscles.

• Cellulose: Major structural polysaccharide in plant cell walls; composed of beta-glucose, enabling it to form strong fibers.

Structural Differences and Functions of Polysaccharides

• Starch (alpha linkages) is helical, while cellulose (beta linkages) is straight and forms microfibrils, essential for plant structure.

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

Proteins and Their Functions

• Proteins account for more than 50% of cell dry mass, serving various functions (enzymes, transport, structural, communication).

• Comprised of 20 different amino acids; arranged into polypeptides through peptide bonds.

  • Functions: Enzymatic activity, immune defense, transport (e.g., hemoglobin), movement, hormonal signaling, and structural support.

Protein Structure Levels

1. Primary Structure: Unique amino acid sequence.

2. Secondary Structure: Coils (alpha helix) and folds (beta sheet) due to hydrogen bonding.

3. Tertiary Structure: Three-dimensional shape from side chain interactions.

4. Quaternary Structure: Multi-polypeptide interactions; e.g., collagen is fibrous and hemoglobin is globular.

Nucleic Acids: DNA and RNA

• Nucleic Acids: DNA and RNA are polymers called polynucleotides, with nucleotides as monomers consisting of a nitrogenous base, a sugar, and phosphate groups.

• DNA: Provides instructions for protein synthesis and enables replication; double helix structure.

  • Comprises two strands running antiparallel; adenine pairs with thymine and guanine with cytosine.

• RNA: Primarily single-stranded; thymine is replaced with uracil. RNA plays roles in protein synthesis by carrying genetic instructions.

Lipids

• Lipids: Diverse group of hydrophobic molecules including fats, phospholipids, and steroids.

• Fats: Made of glycerol and fatty acids; energy-rich with significant diverse configurations affecting state (saturated vs. unsaturated).

• Phospholipids: Key components of cell membranes forming bilayers that separate internal and external environments.

• Steroids: Structurally characterized by four fused carbon rings; include hormones and cholesterol, impacting membranes and signaling.

Summary of Macromolecule Functions

• Carbohydrates: Energy source and structural components.

• Proteins: Catalysts, structural support, transport, hormones.

• Nucleic acids: Store and transmit genetic information.

• Lipids: Energy storage, membrane formation, signaling.