Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Overview of Organic Molecules in Living Organisms

Large biological molecules, also known as macromolecules, are essential for life. They include carbohydrates, lipids, proteins, and nucleic acids. Each class has unique structures and functions, and they are built from smaller subunits called monomers.

• Polymers and Macromolecules: Polymers are long molecules made by linking together repeating units called monomers through chemical reactions such as dehydration synthesis (removal of water).

• Enzymatic Reactions: Enzymes catalyze the synthesis and breakdown of macromolecules, making these processes efficient and regulated.

Carbohydrates

Structure and Classification

Carbohydrates are sugars and their polymers. They serve as energy sources and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose). Classified by the number of carbon atoms (triose, pentose, hexose).

• Disaccharides: Formed by joining two monosaccharides via a glycosidic bond (e.g., sucrose, lactose, maltose).

• Polysaccharides: Long chains of monosaccharides. Serve as energy storage (e.g., starch in plants, glycogen in animals) or structural support (e.g., cellulose in plants, chitin in arthropods and fungi).

General Formula:

• Monosaccharides: $C_nH_{2n}O_n$ (e.g., glucose: $C_6H_{12}O_6$)

Comparison Table: Major Polysaccharides

Polysaccharide	Function	Organism
Starch	Energy storage	Plants
Glycogen	Energy storage	Animals
Cellulose	Structural support	Plants
Chitin	Structural support	Fungi, Arthropods

Similarities and Differences: All are polymers of glucose but differ in glycosidic linkages and biological roles.

Lipids

Structure and Types

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers but are grouped together due to their insolubility in water.

• Fats (Triglycerides): Composed of glycerol and three fatty acids. Used for energy storage.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds; solid at room temperature (e.g., butter).

  • Unsaturated: One or more double bonds; liquid at room temperature (e.g., plant oils).

• Phospholipids: Major component of cell membranes. Contain a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails, forming bilayers in water.

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

Why is oil liquid at room temperature, but butter is solid? Oils are rich in unsaturated fatty acids, which prevent tight packing, while butter contains mostly saturated fatty acids, allowing molecules to pack closely and solidify.

Proteins

Structure and Function

Proteins are polymers of amino acids and perform a vast array of functions, including catalysis, structure, transport, and signaling.

• Amino Acids: 20 different types, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable side chain (R group).

• Peptide Bonds: Link amino acids via dehydration synthesis.

• Levels of Protein Structure:

  • Primary: Sequence of amino acids.

  • Secondary: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

  • Tertiary: Overall 3D shape due to interactions among side chains.

  • Quaternary: Association of multiple polypeptide chains.

• Denaturation: Loss of protein structure and function due to changes in pH, temperature, or chemicals.

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits, responsible for oxygen transport in blood.

Nucleic Acids

Structure and Function

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

• Nucleotides: Building blocks of nucleic acids, each consisting of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

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

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

Flow of Genetic Information:

• DNA → RNA → Protein (Central Dogma of Molecular Biology)

Key Differences between DNA and RNA:

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, C, G	A, U, C, G
Function	Genetic storage	Protein synthesis, regulation

Example: During transcription, a DNA sequence is copied into mRNA, which is then translated into a protein.