Biological Molecules: Structure and Function in Living Organisms

Biological Molecules

Introduction

Biological molecules, also known as biomolecules, are essential compounds produced by living organisms. They play critical roles in cellular structure, function, and regulation. This chapter explores the major classes of biological molecules, their structures, and their functions in living systems.

Why Is Carbon So Important in Biological Molecules?

Properties of Carbon

• Biological molecules are defined as all molecules produced by living things. Nearly all are organic, meaning they contain carbon.

• Organic molecules typically contain carbon, hydrogen, and often oxygen. Many are synthesized by organisms.

• Inorganic molecules generally lack carbon atoms and are simpler than organic molecules.

Bonding Properties of Carbon

• The bonding properties of carbon are key to the complexity of organic molecules.

• Atoms with partially filled outer electron shells tend to react with one another. Carbon has four outer shell electrons and tends to form four covalent bonds, allowing for a variety of complex structures.

• Common elements in biological molecules: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

Functional Groups

• Functional groups are specific groups of atoms attached to the carbon backbone of organic molecules. They are often more reactive than the carbon backbone and determine the chemical properties of the molecule.

• Examples include hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), and methyl (-CH3).

How Are Large Biological Molecules Synthesized?

Monomers and Polymers

• Large biological molecules are often polymers, made by joining smaller subunits called monomers.

• Examples: Proteins (polymers) are made of amino acids (monomers); nucleic acids are made of nucleotides.

Dehydration Synthesis and Hydrolysis

• Dehydration synthesis (condensation reaction): Monomers are joined by removing a molecule of water, forming a covalent bond.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, splitting the covalent bond.

• These reactions are fundamental to the synthesis and breakdown of biological macromolecules.

What Are Carbohydrates?

Structure and Types

• Carbohydrates are composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio ($C_nH_{2n}O_n$).

• Types of carbohydrates:

  • Monosaccharides: Simple sugars (e.g., glucose, fructose, ribose).

  • Disaccharides: Two monosaccharides joined by dehydration synthesis (e.g., sucrose, lactose, maltose).

  • Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose, chitin).

Functions and Examples

• Monosaccharides serve as primary energy sources (e.g., glucose in cellular respiration).

• Disaccharides are used for short-term energy storage and transport.

• Polysaccharides serve as energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plants, chitin in fungi and arthropods).

Table: Examples of Carbohydrates

Type	Example	Function
Monosaccharide	Glucose	Primary energy source
Disaccharide	Sucrose	Transported in plants
Polysaccharide	Starch	Energy storage in plants
Polysaccharide	Cellulose	Structural support in plants

What Are Proteins?

Structure and Diversity

• Proteins are polymers of amino acids linked by peptide bonds.

• Each amino acid has a central carbon, an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group.

• There are 20 different amino acids, each with a unique R group that determines its properties (hydrophilic, hydrophobic, acidic, basic, etc.).

Levels of Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding (alpha-helix, beta-pleated sheet) stabilized by hydrogen bonds.

• Tertiary structure: Three-dimensional shape formed by interactions among R groups (hydrogen bonds, disulfide bridges, hydrophobic interactions).

• Quaternary structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Functions of Proteins

• Structural support (e.g., keratin, collagen)

• Movement (e.g., actin, myosin)

• Defense (e.g., antibodies)

• Storage (e.g., albumin)

• Signaling (e.g., insulin)

• Enzymatic activity (e.g., amylase, lipase)

Table: Functions of Proteins

Function	Example
Structural	Keratin (hair, nails), silk (spider webs)
Movement	Actin, myosin (muscle contraction)
Defense	Antibodies, venoms
Storage	Albumin (egg white)
Signaling	Insulin (regulates blood glucose)
Enzymatic	Amylase (digests starch)

Protein Denaturation

• Denaturation is the loss of a protein's three-dimensional structure, resulting in loss of function. It can be caused by heat, pH changes, or chemicals.

• Denaturation is often irreversible (e.g., cooking an egg).

What Are Nucleotides and Nucleic Acids?

Structure of Nucleotides

• Nucleotides are composed of a five-carbon sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil).

Functions of Nucleotides

• Energy carriers (e.g., ATP)

• Intracellular messengers (e.g., cAMP)

• Building blocks of nucleic acids (DNA and RNA)

Nucleic Acids: DNA and RNA

• DNA (deoxyribonucleic acid): Double-stranded helix, stores genetic information, composed of deoxyribose sugar and bases A, T, C, G.

• RNA (ribonucleic acid): Single-stranded, involved in protein synthesis, composed of ribose sugar and bases A, U, C, G.

What Are Lipids?

General Properties

• Lipids are hydrophobic molecules composed mainly of carbon and hydrogen. They are insoluble in water and not formed by linking monomers into polymers.

• Major functions: energy storage, waterproofing, membrane structure, and hormone production.

Types of Lipids

• Fats and oils (triglycerides): Composed of glycerol and three fatty acids. Fats are solid at room temperature (saturated), oils are liquid (unsaturated).

• Waxes: Similar to fats, highly saturated, solid at outdoor temperatures, water-repellent.

• Phospholipids: Contain a glycerol, two fatty acids, and a phosphate group. They have hydrophilic heads and hydrophobic tails, forming the bilayer of cell membranes.

• Steroids: Composed of four fused carbon rings. Examples include cholesterol, estrogen, and testosterone.

Table: Types and Functions of Lipids

Type	Structure	Function
Triglyceride	Glycerol + 3 fatty acids	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Cell membrane structure
Wax	Long-chain fatty acids + alcohol	Waterproofing
Steroid	Four fused carbon rings	Hormones, membrane component

Summary Table: Four Principal Classes of Biological Molecules

Class	Monomer	Polymer	Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Proteins	Amino acid	Polypeptide	Enzymes, structure, transport, etc.
Nucleic Acids	Nucleotide	DNA/RNA	Genetic information, protein synthesis
Lipids	Fatty acid, glycerol	Varies (not true polymers)	Energy storage, membranes, hormones