The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Macromolecules are large polymers composed of many similar building blocks called monomers. These include carbohydrates, proteins, and nucleic acids. The synthesis and breakdown of polymers are facilitated by enzymes, which speed up chemical reactions.

• Polymer: A long molecule consisting of many similar or identical monomers linked together.

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

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

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

• Diversity: Cells contain thousands of different macromolecules, which vary among cells, species, and organisms.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates include sugars and polymers of sugars. The simplest carbohydrates are monosaccharides, which serve as major fuel for cells and as raw material for building molecules. Carbohydrate macromolecules are polysaccharides, composed of many sugar monomers.

• Monosaccharides: Simple sugars with molecular formulas usually multiples of CH2O. Glucose is the most common.

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

• Disaccharide: Formed by a dehydration reaction joining two monosaccharides via a glycosidic linkage.

• Polysaccharides: Polymers of sugars with storage (starch in plants, glycogen in animals) and structural roles (cellulose in plants, chitin in arthropods and fungi).

Example: Starch and glycogen are storage polysaccharides; cellulose and chitin are structural polysaccharides.

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Lipids are large biological molecules that do not form true polymers and are hydrophobic. The most important lipids are fats, phospholipids, and steroids.

• Fats: Constructed from glycerol and fatty acids. Three fatty acids joined to glycerol form a triacylglycerol (triglyceride).

• Saturated Fatty Acids: No double bonds, solid at room temperature (animal fats).

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

• Trans Fats: Created by hydrogenation, may contribute to cardiovascular disease.

• Phospholipids: Two fatty acids and a phosphate group attached to glycerol; form bilayers in water, crucial for cell membranes.

• Steroids: Lipids with four fused rings; cholesterol is a key steroid in animal cell membranes.

Concept 5.4: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

Proteins account for more than 50% of the dry mass of most cells and perform a wide range of functions, including catalysis, defense, storage, transport, communication, movement, and structural support.

• Enzymes: Proteins that act as catalysts, speeding up chemical reactions.

• Amino Acids: Organic molecules with amino and carboxyl groups; differ by their side chains (R groups).

• Polypeptides: Unbranched polymers of amino acids linked by peptide bonds.

• Protein Structure: Four levels—primary (sequence of amino acids), secondary (coils and folds, e.g., α-helix and β-pleated sheet), tertiary (overall shape due to R group interactions), quaternary (multiple polypeptide chains).

• Denaturation: Loss of protein structure due to environmental changes; denatured proteins are biologically inactive.

Example: Collagen (fibrous protein) and hemoglobin (globular protein) demonstrate quaternary structure.

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. DNA and RNA are the two types of nucleic acids, essential for storing and transmitting genetic information.

• Gene: Unit of inheritance, made of DNA.

• Nucleotide: Consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Polynucleotide: Nucleotides linked by phosphodiester bonds, forming a sugar-phosphate backbone.

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

• RNA Structure: Single-stranded; complementary pairing can occur within or between RNA molecules (A-U, G-C).

Example: DNA directs synthesis of messenger RNA (mRNA), which controls protein synthesis (gene expression).

Concept 5.6: Genomics and Proteomics Have Transformed Biological Inquiry and Applications

Bioinformatics uses computational tools to analyze large sets of genetic and protein data. Genomics is the study of whole genomes, while proteomics focuses on large sets of proteins and their sequences.

• Genomics: Analysis and comparison of whole genomes.

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

Summary Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structural support
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, signaling
Nucleic Acids	Nucleotide	Polynucleotide	Information storage and transmission
Lipids	Fatty acid (not true polymer)	Varies (e.g., triglyceride)	Energy storage, membranes, signaling

Key Equations and Structures

• Dehydration Reaction: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis: $\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

• Phosphodiester Linkage: $\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Polynucleotide} + \text{H}_2\text{O}$

Additional info: This summary expands on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.