Macromolecules: Carbon and Life

Introduction

Macromolecules are large, complex molecules essential to life, composed primarily of carbon and other elements. Their structure and function are central topics in General Biology, as they form the basis of cellular structure and function.

Stanley Miller’s Experiment (1953)

Origin of Organic Molecules

Stanley Miller’s experiment demonstrated that organic molecules could form under conditions thought to resemble those of early Earth, supporting the hypothesis that life’s building blocks could arise abiotically.

• Experimental Setup: Simulated early Earth’s atmosphere using water vapor, methane, ammonia, and hydrogen, with electrical sparks to mimic lightning.

• Results: Formation of amino acids and other organic molecules, indicating that prebiotic chemistry could produce life’s precursors.

• Significance: Provided experimental evidence for the chemical origins of life.

Functional Groups in Organic Molecules

Key Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. They are critical in the chemistry of life.

• Hydroxyl group (–OH): Found in alcohols; increases solubility in water.

• Carbonyl group (C=O): Found in aldehydes and ketones; important in sugars.

• Carboxyl group (–COOH): Acts as an acid; found in amino acids and fatty acids.

• Amino group (–NH2): Acts as a base; found in amino acids.

• Sulfhydryl group (–SH): Found in thiols; important for protein structure (disulfide bonds).

• Phosphate group (–PO42–): Found in nucleic acids and ATP; involved in energy transfer.

• Methyl group (–CH3): Affects gene expression and molecular shape.

Macromolecules: The Molecules of Life

Definition and Classes

Macromolecules are large molecules composed of thousands of covalently connected atoms. The four major classes are carbohydrates, lipids, proteins, and nucleic acids. Their molecular structure dictates their function.

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Polymers and Monomers

Most macromolecules (except lipids) are polymers, long molecules made of repeating subunits called monomers.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating unit that serves as a building block of a polymer.

• Examples: Carbohydrates (monomer: glucose), Proteins (monomer: amino acids), Nucleic acids (monomer: nucleotides).

Carbohydrates

Monosaccharides and Polysaccharides

Carbohydrates serve as fuel and building material. They include simple sugars (monosaccharides) and complex carbohydrates (polysaccharides).

• Monosaccharide: Simple sugar (e.g., glucose).

• Disaccharide: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose).

• Polysaccharide: Polymers of sugars with storage (e.g., starch, glycogen) or structural (e.g., cellulose, chitin) roles.

Glycosidic Linkage: Covalent bond formed between two monosaccharides by a dehydration reaction.

Storage Polysaccharides

• Starch: Storage form in plants; composed of glucose monomers.

• Glycogen: Storage form in animals; more branched than starch.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls; glucose monomers linked by β (1→4) glycosidic bonds, forming straight chains.

• Chitin: Found in exoskeletons of arthropods and fungal cell walls.

Enzymes that hydrolyze α linkages in starch cannot hydrolyze β linkages in cellulose, making cellulose indigestible for most animals (passes as fiber).

Lipids

Diverse Group of Hydrophobic Molecules

Lipids are the only class of large biological molecules that do not form polymers. They are hydrophobic due to their long hydrocarbon chains.

• Types: Triglycerides (fats), phospholipids, steroids.

• Function: Energy storage, membrane structure, signaling.

Fats (Triglycerides)

• Constructed from glycerol and fatty acids.

• Glycerol: Three-carbon alcohol with a hydroxyl group attached to each carbon.

• Fatty Acid: Carboxyl group attached to a long hydrocarbon chain.

• Saturated Fatty Acids: No double bonds; solid at room temperature; mostly animal fats.

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

Phospholipids

• Two fatty acids and a phosphate group attached to glycerol.

• Hydrophobic tails and hydrophilic head; form bilayers in water, major component of cell membranes.

Steroids

• Lipids with a carbon skeleton of four fused rings.

• Cholesterol: Essential component of animal cell membranes; precursor for steroid hormones.

Proteins

Diversity of Structure and Function

Proteins account for more than 50% of the dry mass of most cells and perform a wide range of functions.

• Functions: Catalysis (enzymes), structural support, storage, transport, cellular communication, movement, defense.

Structure of Proteins

• Polymers of amino acids (20 types), linked by peptide bonds.

• Polypeptide: Unbranched polymer of amino acids.

• Protein: One or more polypeptides folded into a specific 3D structure.

Levels of Protein Structure

• Primary: Unique sequence of amino acids.

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

• Tertiary: 3D shape formed by interactions among side chains (R groups).

• Quaternary: Association of multiple polypeptide chains.

Protein function depends on its specific shape, which is determined by its amino acid sequence. Even a single amino acid change (mutation) can affect function, especially in enzymes.

Protein Functions: Examples

Nucleic Acids

Store, Transmit, and Express Hereditary Information

Nucleic acids are polymers that store and transmit genetic information. The two types are DNA and RNA.

• Gene: A unit of inheritance made of DNA.

• DNA (Deoxyribonucleic acid): Stores genetic information.

• RNA (Ribonucleic acid): Involved in protein synthesis and gene regulation.

• Central Dogma: DNA → RNA → Protein

Structure of Nucleic Acids

• Polynucleotides: Polymers made of nucleotide monomers.

• Nucleotide: Composed of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA Structure: Double helix with two antiparallel strands held together by hydrogen bonds between complementary bases (A-T, G-C).

• RNA Structure: Usually single-stranded; can form complex shapes.

Example Equation:

Additional info: The structure of DNA allows for the precise replication and transmission of genetic information from one generation to the next.