The Molecules of Cells: Structure and Function of Biological Molecules

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Introduction

The Importance of Biological Molecules

Biological molecules are essential for the daily functions of living organisms. For example, the enzyme lactase is required to digest dairy products. Individuals who lack this enzyme are lactose intolerant, illustrating the critical role of specific molecules in metabolism and health.

Enzymes are proteins that catalyze biochemical reactions.
Deficiencies in enzymes can lead to metabolic disorders.
Lactose intolerance is a common example of such a disorder.
Application: Understanding enzyme function helps explain dietary restrictions and informs medical treatments.

Introduction to Organic Compounds

Life’s Molecular Diversity and the Role of Carbon

Organic compounds are the foundation of life’s molecular diversity. The unique properties of carbon allow it to form a wide variety of stable and complex molecules.

Carbon’s bonding ability: Carbon can form four covalent bonds, enabling the construction of large, diverse molecules.
Organic compounds are molecules containing carbon atoms bonded to other elements, typically hydrogen, oxygen, and nitrogen.
Carbon chains form the backbone of most organic molecules, which can be straight, branched, or arranged in rings.
Isomers are compounds with the same molecular formula but different structures, resulting in different properties.
Hydrocarbons are composed only of carbon and hydrogen atoms.
Example: Methamphetamine exists as two isomers with different effects—one is a drug, the other a medication—demonstrating how structure determines function.

Structural Variations in Carbon Skeletons

Carbon skeletons can vary in length, branching, and ring formation.
Double bonds can occur at different positions, affecting molecular properties.
Examples: Propane (straight chain), isobutane (branched), cyclohexane (ring), benzene (aromatic ring).

Functional Groups and Chemical Properties

The properties of organic molecules depend on their carbon skeletons and the functional groups attached to them.

Functional groups are specific groups of atoms that confer particular chemical properties to molecules.
Common functional groups include hydroxyl, carbonyl, carboxyl, amino, phosphate, and methyl groups.
Functional groups are often hydrophilic, increasing solubility in water.
Example: The difference between the sex hormones testosterone and estradiol is due to their functional groups.

Macromolecules: Polymers and Monomers

Formation and Breakdown of Polymers

Cells build large molecules, called macromolecules, from smaller units known as monomers. These macromolecules are often polymers, long chains of repeating monomers.

Dehydration reaction: Monomers are joined to form polymers by removing a molecule of water.
Hydrolysis: Polymers are broken down into monomers by adding water.
Enzymes catalyze both dehydration and hydrolysis reactions.
Example: The digestion of proteins into amino acids involves hydrolysis.

Carbohydrates

Monosaccharides: Simple Sugars

Carbohydrates are organic molecules ranging from simple sugars to large polysaccharides. The simplest carbohydrates are monosaccharides.

Monosaccharides have the general formula and contain hydroxyl and carbonyl groups.
Examples: Glucose and fructose are common monosaccharides.

Disaccharides: Double Sugars

Two monosaccharides can be joined by a dehydration reaction to form a disaccharide.

Example: Maltose is formed from two glucose molecules.
Equation:

Polysaccharides: Complex Carbohydrates

Polysaccharides are long chains of sugar units and serve various functions.

Starch and glycogen are storage polysaccharides in plants and animals, respectively.
Cellulose is a structural polysaccharide found in plant cell walls.
Chitin is found in the exoskeletons of insects and the cell walls of fungi.

Lipids

Fats: Energy Storage Molecules

Lipids are hydrophobic molecules primarily composed of carbon and hydrogen. Fats (triglycerides) are a major type of lipid used for energy storage.

Fats consist of glycerol linked to three fatty acids.
Saturated fatty acids have no double bonds and are solid at room temperature (common in animal fats).
Unsaturated fatty acids have one or more double bonds and are usually liquid (common in plant oils).
Trans fats are artificially created by hydrogenating unsaturated fats and are associated with health risks.

Phospholipids and Steroids

Phospholipids are major components of cell membranes, with hydrophilic heads and hydrophobic tails.
Steroids include cholesterol and hormones such as testosterone and estradiol.
Cholesterol is a precursor for other steroids and is found in animal cell membranes.

Proteins

Functions and Structure of Proteins

Proteins are diverse macromolecules that perform a wide range of functions in the body.

Functions include catalysis (enzymes), transport, defense (antibodies), signaling (hormones), reception, movement (muscle contraction), structure (collagen), and storage.
Proteins are polymers of amino acids, of which there are 20 types.
The R group of each amino acid determines its properties.
Denaturation is the loss of a protein’s shape and function due to environmental changes.

Peptide Bonds and Protein Structure

Amino acids are linked by peptide bonds formed through dehydration reactions.
A chain of amino acids is called a polypeptide.
Proteins have four levels of structure:
- Primary structure: Sequence of amino acids.
- Secondary structure: Coiling or folding stabilized by hydrogen bonds (e.g., alpha helix, beta sheet).
- Tertiary structure: Overall 3D shape due to interactions among R groups.
- Quaternary structure: Association of multiple polypeptide chains.
Even a small change in primary structure can affect protein function.

Nucleic Acids

DNA and RNA: Information Polymers

Nucleic acids are polymers of nucleotides, which store and transmit genetic information.

Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
DNA (deoxyribonucleic acid) is a double helix and serves as the molecule of inheritance.
RNA (ribonucleic acid) is usually single-stranded and is involved in protein synthesis.
DNA and RNA direct the synthesis of proteins, controlling cell function.

Genetic Variation and Evolution

Mutations in DNA can lead to traits such as lactose tolerance in populations with a history of dairy farming.
Genetic adaptations can be traced in different human groups, illustrating evolution in response to dietary practices.