The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction to Macromolecules

Large biological molecules, also known as macromolecules, are essential for the structure and function of all living organisms. The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Each class has unique properties and functions that contribute to cellular processes and life itself.

• Carbohydrates: Serve as fuel and building material.

• Lipids: Diverse group of hydrophobic molecules, important for energy storage and membrane structure.

• Proteins: Perform a wide range of functions, including catalysis, defense, transport, and structural support.

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

Macromolecules: Polymers and Monomers

Most macromolecules are polymers, long chains made by linking smaller units called monomers. The process of forming and breaking polymers involves specific chemical reactions:

• Dehydration Reaction: Synthesizes polymers by removing a water molecule, forming a new bond between monomers.

• Hydrolysis: Breaks down polymers by adding a water molecule, breaking the bond between monomers.

Enzymes are biological catalysts that facilitate these reactions.

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as the building blocks for more complex sugars. They typically have molecular formulas that are multiples of CH2O.

• Smallest unit: Monosaccharide (e.g., glucose, C6H12O6).

• Most common monosaccharide: Glucose.

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

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

• Ketose: Carbonyl group within the carbon chain (e.g., fructose).

Disaccharides

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

• Maltose: Glucose + Glucose

• Sucrose: Glucose + Fructose

Polysaccharides

Polysaccharides are large polymers of sugars and serve as storage or structural materials.

• Storage polysaccharides: Starch (plants) and glycogen (animals).

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

The function and structure of polysaccharides depend on their sugar monomers and the positions of glycosidic linkages.

Comparison of Storage Polysaccharides

Polysaccharide	Organism	Structure	Function
Starch	Plants	Glucose monomers, amylose (unbranched), amylopectin (branched)	Energy storage
Glycogen	Animals	Highly branched glucose polymer	Energy storage

Comparison of Structural Polysaccharides

Polysaccharide	Organism	Structure	Function
Cellulose	Plants	Unbranched, β(1→4) glycosidic linkages	Cell wall structure
Chitin	Arthropods, fungi	Modified glucose monomers	Exoskeleton, cell wall

Starch vs. Cellulose

• Similarity: Both are polymers of glucose.

• Difference: Starch has α(1→4) linkages (helical), cellulose has β(1→4) linkages (straight, forms sheets).

• Digestibility: Humans can digest starch but not cellulose due to enzyme specificity.

Lipids

Characteristics and Types

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.

Fats

Fats are constructed from glycerol and fatty acids. Three fatty acids are joined to glycerol by ester linkages, forming a triacylglycerol (triglyceride).

• Function: Energy storage and cushioning of organs.

• 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).

Comparison of Saturated and Unsaturated Fats

Type	Structure	Physical State	Source
Saturated	No double bonds, straight chains	Solid	Animal fats
Unsaturated	One or more double bonds, kinked chains	Liquid	Plant/fish fats

Phospholipids

Phospholipids have two fatty acids and a phosphate group attached to glycerol. The fatty acid tails are hydrophobic, while the phosphate head is hydrophilic. In water, phospholipids self-assemble into bilayers, forming the basis of cell membranes.

Steroids

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

Proteins

Functions and Types

Proteins are the most diverse macromolecules, performing a wide range of functions:

• Enzymatic proteins: Catalyze chemical reactions.

• Defensive proteins: Protect against disease (e.g., antibodies).

• Storage proteins: Store amino acids.

• Transport proteins: Move substances (e.g., hemoglobin).

• Hormonal proteins: Coordinate activities (e.g., insulin).

• Receptor proteins: Respond to chemical stimuli.

• Contractile and motor proteins: Movement (e.g., actin, myosin).

• Structural proteins: Support (e.g., collagen).

Amino Acids and Peptide Bonds

Proteins are polymers of amino acids, linked by peptide bonds. Each amino acid has a central carbon, an amino group, a carboxyl group, and a variable side chain (R group).

• 20 different amino acids: Differ by their R groups.

• Peptide bond: Covalent bond joining amino acids.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

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

• Tertiary structure: Overall 3D shape, determined by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

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

Factors Affecting Protein Structure

• Primary structure (amino acid sequence)

• Environmental conditions: pH, salt concentration, temperature

• Denaturation: Loss of native structure, often irreversible

Nucleic Acids

DNA and RNA

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

• DNA: Double-stranded helix, stores genetic information.

• RNA: Single-stranded, involved in protein synthesis.

Nucleotides

Nucleic acids are polymers of nucleotides. Each nucleotide consists of:

• Nitrogenous base: Pyrimidines (cytosine, thymine, uracil) or purines (adenine, guanine)

• Pentose sugar: Deoxyribose (DNA) or ribose (RNA)

• Phosphate group

A nucleoside is a nitrogenous base plus a sugar, without the phosphate group.

Comparison of Pyrimidines and Purines

Type	Structure	Bases
Pyrimidines	Single six-membered ring	Cytosine, Thymine, Uracil
Purines	Six-membered ring fused to five-membered ring	Adenine, Guanine

Structure of DNA and RNA

• DNA: Two antiparallel strands forming a double helix. Complementary base pairing: Adenine (A) pairs with Thymine (T), Guanine (G) pairs with Cytosine (C).

• RNA: Usually single-stranded; uracil (U) replaces thymine.

Gene Expression and Information Flow

DNA directs its own replication and the synthesis of messenger RNA (mRNA), which controls protein synthesis. The flow of genetic information is summarized as:

• DNA → RNA → Protein

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. Comparative genomics and proteomics help understand evolutionary relationships and molecular genealogy.

Key Equations and Concepts

• General formula for monosaccharides: $C_nH_{2n}O_n$

• Dehydration reaction (formation of a polymer): $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis (breakdown of a polymer): $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

• Phosphodiester linkage in nucleic acids: $\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$

Example: The difference between starch and cellulose is due to the type of glycosidic linkage: α(1→4) in starch (digestible by humans) and β(1→4) in cellulose (not digestible by humans).

Additional info: Some context and definitions have been expanded for clarity and completeness, including the explanation of bioinformatics and the flow of genetic information.