Structure and Function of Large Biological Molecules

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Chapter 5: 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 complex and possess unique properties that arise from the specific arrangement of their atoms.

Carbohydrates: Serve as fuel and building material.
Lipids: Diverse group of hydrophobic molecules.
Proteins: Exhibit a wide range of structures and functions.
Nucleic acids: Store, transmit, and help express hereditary information.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Most biological macromolecules are polymers, long molecules consisting of many similar building blocks called monomers. These are linked by covalent bonds.

Three of the four classes (carbohydrates, proteins, nucleic acids) are polymers.
Lipids are not true polymers.

Polymer Synthesis and Breakdown:

Dehydration Reaction (Synthesis): Two monomers bond together through the loss of a water molecule to form a polymer.
Hydrolysis (Breakdown): Polymers are disassembled to monomers by adding water, breaking the bond.
Enzymes mediate both reactions, speeding up chemical processes in cells.

The Diversity of Polymers

The diversity of macromolecules is based on the sequence and length of 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, cell structure, and cell recognition/identity.

Monosaccharides: Simple sugars (e.g., glucose, fructose).
Oligosaccharides: Short chains of monosaccharides.
Polysaccharides: Long chains, such as starch, glycogen, cellulose, chitin, and peptidoglycan.

General Formula:

Monosaccharides vary by:

Location of carbonyl group (aldose or ketose)
Number of carbon atoms (triose, pentose, hexose)
Spatial arrangement of atoms
Linear vs. ring forms (rings in aqueous solutions)

Type	Examples	Formula
Aldoses	Glyceraldehyde, Ribose, Glucose, Galactose	Triose, Pentose, Hexose
Ketoses	Dihydroxyacetone, Ribulose, Fructose	Triose, Pentose, Hexose

Disaccharides are formed by dehydration reactions, joining two monosaccharides via a glycosidic linkage.

Disaccharide	Monomers	Linkage
Maltose	Glucose + Glucose	1-4 glycosidic
Sucrose	Glucose + Fructose	1-2 glycosidic

Polysaccharides have storage and structural roles:

Starch: Storage in plants (glucose monomers)
Glycogen: Storage in animals (liver, muscle)
Cellulose: Structural in plants (cell wall)
Chitin: Exoskeleton of arthropods, cell walls of fungi
Peptidoglycan: Bacterial cell walls

Polysaccharide	Function	Location
Starch	Energy storage	Plants
Glycogen	Energy storage	Animals
Cellulose	Structural support	Plants
Chitin	Structural support	Arthropods, fungi
Peptidoglycan	Structural support	Bacteria

Carbohydrates in Cell Identity: Glycoproteins and glycolipids on cell surfaces are crucial for cell-cell recognition and signaling.

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, phospholipids, and steroids.

Fats (Triglycerides): Composed of glycerol and fatty acids; function in energy storage, insulation, and protection.
Saturated fatty acids: No double bonds, solid at room temperature (e.g., butter).
Unsaturated fatty acids: One or more double bonds, liquid at room temperature (e.g., oils).
Essential fatty acids: Must be obtained from diet (e.g., omega-3 fatty acids).
Phospholipids: Two fatty acids and a phosphate group attached to glycerol; amphipathic, forming cell membranes.
Steroids: Four fused rings; includes cholesterol and hormones (e.g., estrogen, testosterone).

Lipid Type	Structure	Function
Fat	Glycerol + 3 fatty acids	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Cell membrane structure
Steroid	Four fused rings	Hormones, membrane component

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

Proteins are the most diverse macromolecules, central to cell growth and function. Functions include catalysis (enzymes), defense, storage, transport, communication, movement, and structural support.

Proteins are polymers of amino acids (20 types).
Amino acids have a central carbon, amino group, carboxyl group, and variable R group.
Linked by peptide bonds to form polypeptides.

Levels of Protein Structure:

Primary: Sequence of amino acids (coded by DNA).
Secondary: Coils and folds (α-helix, β-pleated sheet) via hydrogen bonds.
Tertiary: 3D shape from R group interactions (hydrogen, ionic, hydrophobic, disulfide bridges).
Quaternary: Multiple polypeptide chains (e.g., hemoglobin, collagen).

Structure Level	Description	Example
Primary	Amino acid sequence	Insulin
Secondary	α-helix, β-sheet	Keratin
Tertiary	3D folding	Enzymes
Quaternary	Multiple chains	Hemoglobin

Denaturation: Loss of protein structure (and function) due to heat, pH, or chemicals.

Protein Misfolding: Can cause diseases (e.g., Alzheimer's, sickle-cell anemia, prion diseases).

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

Nucleic acids are polymers of nucleotides and include DNA and RNA. 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 (purine or pyrimidine), a pentose sugar, and a phosphate group.

Nucleic Acid	Sugar	Bases	Strands
DNA	Deoxyribose	A, T, G, C	Double
RNA	Ribose	A, U, G, C	Single

Gene Expression: DNA → RNA → Protein

Phosphodiester Linkage: Joins nucleotides in a polynucleotide chain.

Complementary Base Pairing: In DNA, A pairs with T, G pairs with C; in RNA, A pairs with U.

Summary Table: Carbohydrates

Large Biological Molecule	Components	Examples	Functions
Carbohydrates	Monosaccharide monomers	Glucose, fructose, lactose, sucrose, starch, glycogen, cellulose, chitin	Fuel, cell structure, cell identity

Additional info:

Enzymes are crucial for both synthesis and breakdown of macromolecules.
Cell identity is mediated by carbohydrate modifications on cell surfaces.