Chemistry of Life: Structure and Function of Biological Molecules

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Chemistry of Life

Introduction

The chemistry of life focuses on the molecular building blocks and chemical processes that underpin all biological systems. Understanding the structure and function of water, macromolecules, and their subunits is essential for grasping how living organisms grow, reproduce, and maintain homeostasis.

Structure of Water and Hydrogen Bonding

Properties of Water

Polarity: Water is a polar molecule, meaning it has a partial positive charge on one side and a partial negative charge on the other. This results from the unequal sharing of electrons between oxygen and hydrogen atoms.
Hydrogen Bonding: The polarity of water allows it to form hydrogen bonds with other water molecules and with other polar molecules.

Biological Significance of Water's Properties

High Specific Heat Capacity: Water can absorb or release large amounts of heat with little temperature change, helping organisms maintain stable internal temperatures.
High Heat of Vaporization: Water requires significant energy to change from liquid to gas, which is important for cooling mechanisms like sweating and transpiration.
Cohesion and Adhesion: Hydrogen bonds cause water molecules to stick together (cohesion) and to other substances (adhesion), supporting processes like water transport in plants.

Examples and Applications

Water's solvent properties allow it to dissolve a wide range of substances, facilitating biochemical reactions.
Surface tension, a result of cohesion, enables small insects to walk on water.

Elements Essential to Life

Major Elements in Biological Molecules

Carbon (C): Forms the backbone of organic molecules; found in carbohydrates, proteins, lipids, and nucleic acids.
Hydrogen (H) and Oxygen (O): Present in most biological molecules, including water.
Nitrogen (N): Found in proteins and nucleic acids.
Phosphorus (P): Important for nucleic acids and phospholipids.
Sulfur (S): Found in some amino acids and vitamins.

Introduction to Macromolecules

Biological Macromolecules

Carbohydrates, proteins, lipids, and nucleic acids are the four major classes of biological macromolecules.
These macromolecules are polymers, built from smaller subunits called monomers.

Chemical Reactions in Macromolecule Synthesis and Breakdown

Hydrolysis: A reaction that breaks polymers into monomers by adding water. Example: Digestion of starch into glucose.
Dehydration Synthesis (Condensation): A reaction that joins monomers by removing water, forming covalent bonds. Example: Formation of proteins from amino acids.

Carbohydrates

Structure and Function

Monosaccharides: Simple sugars (e.g., glucose, fructose) that serve as the monomers for carbohydrates.
Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose) with storage or structural roles.

Lipids

Structure and Function

Lipids are hydrophobic molecules, including fats, oils, phospholipids, and steroids.
Fats: Composed of glycerol and fatty acids; used for energy storage and insulation.
Saturated Fatty Acids: Contain only single bonds between carbon atoms; typically solid at room temperature.
Unsaturated Fatty Acids: Contain one or more double bonds; typically liquid at room temperature.
Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.
Steroids: Lipids with a characteristic four-ring structure (e.g., cholesterol, hormones).

Nucleic Acids

Structure and Function of DNA and RNA

Nucleic acids are polymers of nucleotides, each consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base.
DNA: Double-stranded helix with complementary base pairing (A-T, G-C); stores genetic information.
RNA: Single-stranded; involved in protein synthesis and gene regulation.

Base Pairing Rules

In DNA: Adenine (A) pairs with Thymine (T), Guanine (G) pairs with Cytosine (C).
In RNA: Adenine (A) pairs with Uracil (U) instead of Thymine.

Directionality

Nucleic acids have directionality, with a 5' end (phosphate group) and a 3' end (hydroxyl group).

Proteins

Structure and Function

Proteins are polymers of amino acids, linked by peptide bonds.
Each amino acid has a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain).

Levels of Protein Structure

Primary Structure: Sequence of amino acids in a polypeptide chain.
Secondary Structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
Tertiary Structure: Three-dimensional shape formed by interactions among R groups, including hydrophobic interactions, ionic bonds, and disulfide bridges.
Quaternary Structure: Association of multiple polypeptide chains to form a functional protein (not present in all proteins).

Function

Protein structure determines its function, including roles as enzymes, structural components, transporters, and signaling molecules.

Summary Table: Major Classes of Biological Macromolecules

Macromolecule	Monomer	Function	Example
Carbohydrates	Monosaccharides	Energy storage, structural support	Starch, cellulose, glycogen
Lipids	Glycerol and fatty acids	Energy storage, membrane structure, signaling	Fats, oils, phospholipids, steroids
Proteins	Amino acids	Catalysis, structure, transport, signaling	Enzymes, hemoglobin, antibodies
Nucleic Acids	Nucleotides	Genetic information storage and transfer	DNA, RNA

Key Equations and Concepts

Dehydration Synthesis:
Hydrolysis:

Enduring Understandings

Living systems are organized in a hierarchy of structural levels that interact.
The organization of living systems requires constant input of energy and exchange of macromolecules.
Heritable information provides continuity of life.