Chapter 5: Biomolecules – Structure and Function of Macromolecules in Biology

Biomolecules

Introduction to Macromolecules

All living organisms are composed of four major classes of large biological molecules, known as macromolecules. These macromolecules are essential for structure, function, and regulation of the body's tissues and organs.

• Lipids

• Carbohydrates

• Proteins

• Nucleic acids

Building Macromolecules

Polymers and Monomers

Macromolecules are often formed by linking smaller units into larger structures. A polymer is a long molecule consisting of many similar or identical building blocks called monomers.

• Only three classes of macromolecules are true polymers: carbohydrates, proteins, and nucleic acids.

• Lipids are not true polymers, as they are not composed of repeating monomer units.

Lipids

General Characteristics

Lipids are a diverse group of hydrophobic molecules, meaning they do not mix well with water. This property is due to their structure, which is dominated by long hydrocarbon chains or rings.

• Major types: fats, phospholipids, and steroids.

• All lipids are hydrophobic and consist mostly of hydrocarbon regions.

Fats (Triglycerides)

Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Glycerol: A three-carbon alcohol with a hydroxyl group (-OH) attached to each carbon.

• Fatty acid: Consists of a carboxyl group (-COOH) attached to a long hydrocarbon skeleton.

• Three fatty acids are joined to glycerol by an ester linkage, forming a triacylglycerol (triglyceride).

• Fatty acids in a fat molecule can be identical or different.

Diagram: The synthesis of a fat involves three dehydration reactions, each forming an ester linkage between glycerol and a fatty acid.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: Have the maximum number of hydrogen atoms possible and no double bonds in the hydrocarbon chain.

• Unsaturated fatty acids: Have one or more double bonds, causing kinks in the chain.

Example: Animal fats (e.g., butter) are typically saturated and solid at room temperature, while plant and fish fats are usually unsaturated and liquid at room temperature.

Functions of Lipids

• Energy storage (fats store large amounts of energy)

• Insulation and protection (adipose tissue cushions organs)

• Structural roles (phospholipids in cell membranes)

• Signaling (steroids act as hormones)

Carbohydrates

Overview and Classification

Carbohydrates are sugars and polymers of sugars. They serve as fuel and building material for cells.

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

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose)

• Polysaccharides: Polymers of many monosaccharides (e.g., starch, glycogen, cellulose, chitin)

Monosaccharides

• General formula: $(CH_2O)_n$

• Classified by the location of the carbonyl group (aldose or ketose) and the number of carbons (triose, pentose, hexose)

• Most common: Glucose ($C_6H_{12}O_6$)

Disaccharides

• Formed by a dehydration reaction that joins two monosaccharides via a glycosidic linkage.

• Examples: Maltose (glucose + glucose), sucrose (glucose + fructose)

Polysaccharides

• Storage polysaccharides: Starch (plants), glycogen (animals)

• Structural polysaccharides: Cellulose (plants), chitin (arthropods and fungi)

• Structure and function depend on sugar monomers and glycosidic linkages.

Comparison Table: Major Polysaccharides

Name	Organism	Function	Structure
Starch	Plants	Energy storage	α-glucose, helical
Glycogen	Animals	Energy storage	α-glucose, branched
Cellulose	Plants	Structural (cell wall)	β-glucose, straight
Chitin	Fungi, Arthropods	Structural (exoskeleton, cell wall)	β-glucose with nitrogen group

Proteins

Functions and Structure

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. They perform a vast array of functions:

• Enzymatic catalysis

• Defense (antibodies)

• Storage (casein, ovalbumin)

• Transport (hemoglobin)

• Cellular communication (hormones, receptors)

• Movement (actin, myosin)

• Structural support (collagen, keratin)

Amino Acids and Polypeptides

• Amino acids: Organic molecules with an amino group, carboxyl group, hydrogen atom, and variable R group (side chain).

• 20 different amino acids are used to build proteins.

• Polypeptide: A polymer of amino acids linked by peptide bonds.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils (α-helix) and folds (β-pleated sheet) formed by hydrogen bonds between backbone atoms.

• Tertiary structure: Overall 3D shape due to interactions among R groups (hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges).

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

Protein Denaturation

• Loss of native structure due to changes in pH, temperature, or salt concentration.

• Denatured proteins are biologically inactive.

Nucleic Acids

DNA and RNA

Nucleic acids store, transmit, and help express hereditary information. The two types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• DNA: Stores genetic information; double-stranded helix; bases are adenine (A), thymine (T), cytosine (C), guanine (G).

• RNA: Various functions in gene expression; single-stranded; bases are adenine (A), uracil (U), cytosine (C), guanine (G).

Nucleotide Structure

• Each nucleotide consists of a nitrogenous base, a pentose sugar (ribose or deoxyribose), and one or more phosphate groups.

• Nucleotides are joined by phosphodiester linkages to form polynucleotide chains.

Base Pairing and Double Helix

• DNA strands are held together by hydrogen bonds between complementary bases: A pairs with T, G pairs with C.

• Strands run antiparallel (5' to 3' and 3' to 5').

Gene Expression

• DNA directs synthesis of messenger RNA (mRNA) in the nucleus (transcription).

• mRNA directs protein synthesis at ribosomes in the cytoplasm (translation).

Summary Table: Macromolecules

Class	Monomer	Polymer	Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, etc.
Nucleic acids	Nucleotide	Polynucleotide	Genetic information
Lipids	Glycerol, fatty acids	Not true polymers	Energy storage, membranes, signaling

Key Terms

• Macromolecule: Large molecule formed by joining smaller molecules.

• Polymer: Long molecule consisting of many similar building blocks (monomers).

• Monomer: Small molecule that serves as a building block of a polymer.

• Dehydration reaction: Chemical reaction that builds polymers by removing water.

• Hydrolysis: Chemical reaction that breaks polymers into monomers by adding water.

Additional info: This guide expands on the provided notes with definitions, examples, and tables for clarity and completeness, suitable for college-level General Biology students.