Chapter 5: The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for life. They include carbohydrates, lipids, proteins, and nucleic acids. Each class of macromolecule has unique structures and functions that are fundamental to biological processes.

Types of Macromolecules

Overview of the Four Major Macromolecules

• Carbohydrates: Serve as fuel and building material.

• Lipids: Diverse group of hydrophobic molecules, important for energy storage, membranes, and signaling.

• Proteins: Perform a vast array of functions including catalysis, defense, storage, transport, cellular communication, movement, and structural support.

• Nucleic Acids: Store, transmit, and help express hereditary information.

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, often called simple sugars. They have molecular formulas that are usually multiples of CH2O. The most common monosaccharide is glucose (C6H12O6).

• Smallest unit: Monosaccharide (e.g., glucose, fructose, galactose).

• Classification: By the location of the carbonyl group (aldose or ketose) and the number of carbons in the skeleton (triose, pentose, hexose, etc.).

• Aldose vs. Ketose: Aldoses have an aldehyde group; ketoses have a ketone group.

Disaccharides and Glycosidic Linkages

Disaccharides are formed when two monosaccharides are joined by a glycosidic linkage through a dehydration reaction.

• Examples: Maltose (glucose + glucose), Sucrose (glucose + fructose).

Polysaccharides

Polysaccharides are macromolecules composed of many monosaccharides joined by glycosidic linkages. Their function is determined by their sugar monomers and the positions of glycosidic linkages.

• Storage polysaccharides: Starch (plants), Glycogen (animals).

• Structural polysaccharides: Cellulose (plant cell walls), Chitin (exoskeletons of arthropods, fungal cell walls).

Comparison of Storage Polysaccharides

Comparison of Structural Polysaccharides

• Difference between cellulose and starch: Starch has α (alpha) glycosidic linkages (helical), while cellulose has β (beta) linkages (straight, forms microfibrils).

• Enzymatic digestion: Most animals cannot digest cellulose due to the β linkages; some herbivores rely on symbiotic microbes.

Lipids

Characteristics and Types

Lipids are hydrophobic molecules that do not form true polymers. They are important for energy storage, membrane structure, and signaling.

• Distinguishing characteristics: Hydrophobic, mostly hydrocarbons, not polymers.

• Three major types: Fats (triacylglycerols), Phospholipids, Steroids.

Fats (Triacylglycerols)

• Structure: Glycerol (three-carbon alcohol) + three fatty acids (carboxyl group attached to a long hydrocarbon chain) joined by ester linkages.

• Function: Energy storage, insulation, cushioning of organs.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds, straight chains, solid at room temperature (most animal fats).

  • Unsaturated: One or more double bonds, kinked chains, liquid at room temperature (plant and fish fats).

Phospholipids

• Structure: Glycerol + two fatty acids + phosphate group (with additional polar group).

• Amphipathic: Hydrophobic tails, hydrophilic head.

• Function: Major component of cell membranes, form bilayers in aqueous environments.

Steroids

• Structure: Four fused carbon rings.

• Example: Cholesterol (component of animal cell membranes, precursor for steroid hormones).

Proteins

Structure and Function

Proteins are polymers of amino acids and perform a wide variety of functions in the cell.

• Building blocks: Amino acids (20 types, each with a unique side chain or R group).

• Peptide bond: Covalent bond linking amino acids in a polypeptide chain.

• Functions: Enzymatic, defensive, storage, transport, hormonal, receptor, contractile, structural.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

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

• Tertiary structure: Overall 3D 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 native structure due to changes in pH, salt concentration, temperature, or other environmental factors; usually results in loss of function.

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

Protein Folding and Determination

• Folding: Determined by primary structure and environmental conditions.

• Determination methods: X-ray crystallography, NMR spectroscopy, bioinformatics.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit hereditary information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Monomers: Nucleotides (composed of a nitrogenous base, a pentose sugar, and one or more phosphate groups).

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

• Types of nitrogenous bases:

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA); single six-membered ring.

  • Purines: Adenine (A), Guanine (G); six-membered ring fused to a five-membered ring.

• Sugar: Deoxyribose in DNA, ribose in RNA.

DNA and RNA Structure

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

• RNA: Usually single-stranded, can form secondary structures by base pairing within the molecule; uracil replaces thymine.

Gene Expression

• Central Dogma: DNA → RNA → Protein

• Process: DNA is transcribed to mRNA, which is translated into protein.

Genomics and Proteomics

Modern Biological Inquiry

• Genomics: Study and comparison of whole genomes, enabled by advances in DNA sequencing and computational analysis.

• Proteomics: Large-scale study of proteins, including their sequences and functions.

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

Key Equations and Concepts

• Dehydration Reaction (Polymerization): Formation of a covalent bond with the removal of a water molecule.

• Hydrolysis: Breaking of a covalent bond by the addition of a water molecule.

General formula for a monosaccharide:

Central Dogma of Molecular Biology:

Example: The difference between starch and cellulose is the type of glycosidic linkage: starch has α (1→4) linkages, while cellulose has β (1→4) linkages, resulting in different three-dimensional structures and biological properties.

Additional info: Some details, such as the specific chemical structures of nucleotides and the process of gene expression, have been expanded for clarity and completeness.