Structure and Function of Large Biological Molecules

Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are large and complex, with unique properties arising from the specific arrangement of their atoms. Understanding their structure is essential to understanding their function in biological systems.

• Carbohydrates: Serve as energy sources and structural materials.

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

• Proteins: Perform a wide range of functions due to their structural diversity.

• Nucleic acids: Store, transmit, and express hereditary information.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

A polymer is a long molecule consisting of many similar or identical building blocks called monomers, linked by covalent bonds. Three of the four classes of biological macromolecules (carbohydrates, proteins, nucleic acids) are polymers.

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

• Dehydration reaction (synthesis): Joins two monomers by removing a water molecule.

• Hydrolysis (breakdown): Adds a water molecule to break a bond between monomers.

Example: During digestion, enzymes hydrolyze proteins into amino acids.

The Diversity of Polymers

The diversity of macromolecules is based on the sequence and length of their monomers. Proteins are made from 20 different amino acids, and DNA is built from just four kinds of nucleotides.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates (sugars) play crucial roles in energy storage, structural support, and cell recognition. They include:

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Oligosaccharides: Short chains of monosaccharides.

• Polysaccharides: Long chains, such as starch, glycogen, cellulose, chitin, and peptidoglycan.

Carbohydrates have the general formula $(CH_2O)_n$ and are hydrophilic due to their carbonyl and hydroxyl groups.

Monosaccharide Variation

• Location of carbonyl group: Aldose (end), Ketose (middle).

• Number of carbons: Triose (3), Pentose (5), Hexose (6).

• Spatial arrangement of atoms (hydroxyl group position).

• Linear vs. ring forms (rings predominate in aqueous solution).

Disaccharides and Glycosidic Linkages

A disaccharide forms when two monosaccharides join via a glycosidic linkage (covalent bond formed by dehydration reaction).

Polysaccharides: Storage and Structural Roles

• Starch: Storage in plants (glucose polymer).

• Glycogen: Storage in animals (mainly liver and muscle).

• Cellulose: Structural, plant cell walls (unbranched glucose polymer).

• Chitin: Structural, exoskeletons of arthropods and fungal cell walls.

• Peptidoglycan: Structural, bacterial cell walls.

Carbohydrates in Cell Identity

• Glycoproteins: Carbohydrates attached to proteins.

• Glycolipids: Carbohydrates attached to lipids.

• Key in cell-cell recognition and signaling.

Type	Component	Example	Function
Monosaccharide	Glucose	Fuel	Energy source
Disaccharide	Lactose, Sucrose	Transport	Energy transport
Polysaccharide	Starch, Glycogen, Cellulose, Chitin	Storage/Structure	Energy storage, cell wall support

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Lipids are not true polymers and are hydrophobic due to their hydrocarbon content. Major types include:

• Fats (triglycerides): Glycerol + 3 fatty acids; energy storage, insulation.

• Phospholipids: Glycerol + 2 fatty acids + phosphate group; form cell membranes (amphipathic).

• Steroids: Four fused carbon rings; hormones (e.g., cholesterol, testosterone).

Saturated fats have no double bonds (solid at room temp), unsaturated fats have one or more double bonds (liquid at room temp).

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

Proteins are polymers of amino acids (polypeptides) and account for more than 50% of cell dry mass. Functions include catalysis (enzymes), defense, storage, transport, communication, movement, and structural support.

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

• Peptide bonds: Link amino acids via dehydration reactions.

Levels of Protein Structure

• Primary: Sequence of amino acids.

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

• Tertiary: 3D shape from R group interactions (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

• Quaternary: Association of multiple polypeptides (e.g., hemoglobin, collagen).

Protein function depends on its structure. Denaturation (by heat, pH, etc.) disrupts function. Misfolded proteins can cause diseases (e.g., Alzheimer's, sickle-cell anemia).

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

Nucleic acids (DNA and RNA) are polymers of nucleotides. They store and transmit genetic information and direct protein synthesis.

• DNA: Deoxyribonucleic acid, double helix, stores genetic information.

• RNA: Ribonucleic acid, single-stranded, involved in protein synthesis.

• Nucleotide: Composed of a nitrogenous base (A, T/U, G, C), a pentose sugar, and a phosphate group.

• Phosphodiester linkage: Joins nucleotides in a polynucleotide chain.

Base pairing: In DNA, A pairs with T, G pairs with C. In RNA, A pairs with U.

Summary Table: Major Biological Macromolecules

Macromolecule	Monomer	Bond Type	Function
Carbohydrate	Monosaccharide	Glycosidic linkage	Energy, structure, recognition
Lipid	Fatty acid, glycerol	Ester linkage	Energy storage, membranes, hormones
Protein	Amino acid	Peptide bond	Catalysis, structure, transport, signaling
Nucleic acid	Nucleotide	Phosphodiester bond	Genetic information, protein synthesis