Large Biological Molecules

Introduction to Macromolecules

Large biological molecules, also known as macromolecules, are essential for life and are composed of thousands of atoms. Living organisms are primarily made up of four major classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each class has unique properties that arise from the specific arrangement of their atoms.

• Carbohydrates: Serve as fuel and building material.

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

• Proteins: Account for more than 50% of the dry mass of cells, with diverse functions including catalysis, defense, transport, and structure.

• Nucleic Acids: Store and transmit genetic information.

Macromolecules as Polymers

Polymers and Monomers

Most macromolecules are polymers, which are long chains made by linking together smaller units called monomers. The process of building polymers from monomers is fundamental to biological structure and function.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) connected in a series.

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

• Three of the four classes of macromolecules are polymers: carbohydrates, proteins, and nucleic acids.

• Lipids are not true polymers, but are built from sets of building blocks (fatty acids and glycerol).

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) that are the monomers of carbohydrates.

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage (covalent bond).

• Polysaccharides: Large polymers of sugars, such as starch, glycogen, and cellulose.

Carbohydrates can be classified by the number of carbon atoms and the location of the carbonyl group (aldose or ketose). In aqueous solutions, most sugars form ring structures.

Polysaccharides: Storage and Structure

• Starch: Storage polysaccharide in plants, composed of α-glucose monomers.

• Glycogen: Storage polysaccharide in animals, highly branched and composed of α-glucose.

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose; most animals cannot digest cellulose.

Lipids

Structure and Types

Lipids are a diverse group of hydrophobic molecules, not true polymers. They are composed mainly of carbon, hydrogen, and oxygen, but with a much lower proportion of oxygen than carbohydrates.

• Fats (Triglycerides): Composed of three fatty acids joined to glycerol by ester linkages. Function in energy storage, insulation, and protection.

• Phospholipids: Composed of two fatty acids, a phosphate group, and glycerol. Major component of cell membranes, forming bilayers.

• Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol, hormones).

Saturated fats have no double bonds and are solid at room temperature; unsaturated fats have one or more double bonds and are liquid at room temperature.

Proteins

Structure and Function

Proteins are polymers made from amino acid monomers. They perform a wide variety of functions in cells, including catalysis (enzymes), defense, transport, communication, movement, and structural support.

• Amino Acids: Organic molecules with amino and carboxyl groups, differing in their side chains (R groups).

• Polypeptide: A polymer of amino acids linked by peptide bonds.

• Protein: One or more polypeptides folded into a specific three-dimensional structure.

Levels of Protein Structure

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Coils and folds (α-helix, β-pleated sheet) formed by hydrogen bonds between backbone atoms.

• Tertiary Structure: Overall three-dimensional shape formed by interactions among side chains (R groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Quaternary Structure: Association of multiple polypeptide chains.

Protein function depends on its ability to recognize and bind to other molecules. Denaturation (loss of structure) renders proteins inactive.

Nucleic Acids

Structure and Function

Nucleic acids are polymers called polynucleotides, made from nucleotide monomers. They store and transmit genetic information.

• DNA (Deoxyribonucleic Acid): Double-stranded helix, stores genetic information.

• RNA (Ribonucleic Acid): Single-stranded, involved in protein synthesis and gene regulation.

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

DNA strands are antiparallel and held together by complementary base pairing (A-T, C-G). RNA uses uracil (U) instead of thymine (T).

Central Dogma of Molecular Biology

The flow of genetic information in cells follows the central dogma:

• DNA → RNA → Protein

This process involves transcription (DNA to RNA) and translation (RNA to protein).

Summary Table: Macromolecules

Key Equations and Concepts

• Dehydration Synthesis: Formation of polymers by removal of water.

• Hydrolysis: Breakdown of polymers by addition of water.

Examples and Applications

• Starch in plants: Energy storage, broken down by amylase.

• Glycogen in animals: Energy storage, especially in liver and muscle.

• Cellulose in plants: Structural support, indigestible by most animals.

• Phospholipid bilayer: Forms the basis of cell membranes.

• Enzymes: Proteins that catalyze biochemical reactions.

• DNA and RNA: Store and transmit genetic information, direct protein synthesis.

Additional info: These notes expand on the original slides by providing definitions, examples, and tables for comparison, ensuring a self-contained study guide suitable for exam preparation in General Biology.