Chapter 5: The Structure and Function of Large Biological Molecules

The Molecules of Life

Introduction to Macromolecules

All living organisms are composed of four major classes of large biological molecules, known as macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules possess unique properties that arise from the specific and orderly arrangement of their atoms.

• Macromolecules are essential for structure, function, and regulation of the body's tissues and organs.

• Three of the four classes (carbohydrates, proteins, nucleic acids) are polymers, composed of repeating units called monomers.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Polymers and Monomers

A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds. The individual subunits are called monomers.

• Carbohydrates: monomer is monosaccharide

• Proteins: monomer is amino acid

• Nucleic acids: monomer is nucleotide

• Lipids are not true polymers

Synthesis and Breakdown of Polymers

• Dehydration reaction: Monomers are joined together by the removal of a water molecule, forming a polymer.

• Hydrolysis reaction: Polymers are broken down into monomers by the addition of water.

• Enzymes catalyze both synthesis and breakdown reactions.

The Diversity of Polymers

Overview

Polymers vary in their monomers and functions. The diversity of macromolecules is key to biological complexity.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates

Carbohydrates are sugars and polymers of sugars. They serve as energy sources and structural materials.

• Monomer: Monosaccharide

• Molecular formula: $CH_2O$

• Classified by:

  • Location of carbonyl group (aldose or ketose)

  • Number of carbons in the skeleton

• Glucose ($C_6H_{12}O_6$) is the most common monosaccharide.

Linear and Ring Forms

Monosaccharides can exist in linear or ring forms. In aqueous solutions, most sugars form rings.

Disaccharides and Polysaccharides

• Disaccharide: Formed when two monosaccharides are joined by a dehydration reaction (e.g., sucrose).

• Polysaccharide: Formed by joining many monosaccharides (e.g., cellulose).

Glycosidic Bonds

The bond between sugars is called a glycosidic linkage. The linkage can be alpha or beta, depending on the position of the hydroxyl group on carbon 1.

Storage Polysaccharides

• Starch: Found in plants, consists entirely of glucose monomers. Stored in plastids.

• Glycogen: Found in animals, stored mainly in liver and muscle cells.

Structural Polysaccharides

• Cellulose: Polymer of glucose with beta glycosidic linkages. Major component of plant cell walls.

• Chitin: Polymer of a variation of glucose, found in arthropod exoskeletons and fungal cell walls.

Connection: Glycosidic Linkage and Digestion

• Enzymes that digest starch (alpha linkages) cannot digest cellulose (beta linkages).

• Cellulose passes through the human digestive tract as insoluble fiber.

• Some microbes can digest cellulose, aiding herbivores.

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Lipids

Lipids are hydrophobic molecules, not true polymers. They are made mostly of hydrocarbons, forming nonpolar covalent bonds.

• Main types: fats, phospholipids, steroids

Fats (Triacylglycerol)

• Function: Energy storage, insulation, cushioning organs

• Constructed from glycerol and fatty acids

• Formed by dehydration reactions, creating ester linkages

Saturated vs. Unsaturated Fats

• Saturated fats: No double bonds, solid at room temperature

• Unsaturated fats: One or more double bonds, liquid at room temperature

• Cis double bonds cause bending in unsaturated fats

Trans Fatty Acids

• Hydrogenation converts unsaturated fats to saturated fats, creating trans fats

• Trans fats may contribute more to cardiovascular disease than saturated fats

Phospholipids

• Composed of two fatty acids and a phosphate group attached to glycerol

• Fatty acid tails are hydrophobic; phosphate head is hydrophilic

• Form phospholipid bilayers in cell membranes

Steroids

• Characterized by a carbon skeleton with four fused rings

• Cholesterol: Component of animal cell membranes, precursor for other steroids

• High cholesterol levels may contribute to cardiovascular disease

Concept 5.4: Proteins

Proteins

Proteins are polymers of amino acids. They perform a wide variety of functions, including catalysis, defense, storage, transport, communication, movement, and structural support.

• Monomer: Amino acid (20 types)

• Structure: Central carbon (alpha carbon) bonded to amino group, carboxyl group, hydrogen, and R group (side chain)

Polypeptides (Amino Acid Polymers)

• Amino acids are linked by peptide bonds via condensation reactions

• Polypeptides have an amino (N-terminus) and carboxyl (C-terminus) end

Amino Acid Classification

Type	Examples
Nonpolar (hydrophobic)	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Proline
Polar (hydrophilic)	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Electrically charged (hydrophilic)	Aspartic acid, Glutamic acid (acidic); Lysine, Arginine, Histidine (basic)

Protein Structure and Function

• The sequence of amino acids determines a protein's three-dimensional structure

• Structure determines function, including the ability to bind other molecules

Protein Functions

Type	Function	Example
Enzymatic	Catalyze chemical reactions	Digestive enzymes
Defensive	Protection against disease	Antibodies
Storage	Store amino acids	Casein, ovalbumin
Transport	Transport substances	Hemoglobin
Hormonal	Coordination of activities	Insulin
Receptor	Response to stimuli	Receptor proteins
Contractile/Motor	Movement	Actin, myosin
Structural	Support	Keratin, collagen

Four Levels of Protein Structure

• Primary structure: Unique sequence of amino acids (peptide bonds)

• Secondary structure: Coils (alpha helix) and folds (beta sheet) stabilized by hydrogen bonds

• Tertiary structure: Determined by interactions among R groups (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds)

• Quaternary structure: Association of multiple polypeptide chains

Sickle-Cell Disease: A Change in Primary Structure

• A single amino acid substitution in hemoglobin leads to sickle-cell disease, affecting protein structure and function

Protein Denaturation

• Physical and chemical conditions (pH, salt, temperature) can cause proteins to lose their native structure (denaturation)

• Denatured proteins are biologically inactive

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Nucleic Acids

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

• Monomer: Nucleotide (joined by phosphodiester bonds)

• Types: DNA (deoxyribonucleic acid), RNA (ribonucleic acid)

• Nucleotide = nitrogenous base + pentose sugar + phosphate group

• Nucleoside = nitrogenous base + sugar

Types of Bases and Sugars

Family	Bases
Pyrimidines	Cytosine, Thymine (DNA), Uracil (RNA)
Purines	Adenine, Guanine

The Roles of Nucleic Acids

• DNA directs synthesis of messenger RNA (mRNA), which controls protein synthesis

• This process is called gene expression

• Genetic information flow: DNA → RNA → Protein

How to Determine Protein Structure

• X-ray crystallography is used to determine protein structure

• NMR spectroscopy and bioinformatics are alternative methods

Additional info: The study notes above expand on the original slides and notes by providing definitions, examples, and tables for classification and comparison, ensuring a comprehensive and self-contained study guide for General Biology students.