Structure and Function of Large Biological Molecules

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Chapter 5: Structure and Function of Large Biological Molecules

Molecules of Life

Living organisms are composed of four major classes of large biological molecules, each with unique properties and functions essential for life. These molecules are often referred to as macromolecules due to their large size and complex structure.

Carbohydrates: Serve as energy sources and structural materials.
Lipids: Function in energy storage, membrane structure, and signaling.
Proteins: Perform a vast array of functions including catalysis, structure, transport, and regulation.
Nucleic Acids: Store and transmit genetic information.

Macromolecules are polymers built from smaller subunits called monomers. The arrangement and type of monomers determine the properties and functions of each macromolecule.

Polymer Formation and Breakdown

Most biological macromolecules are polymers, long chains of repeating monomers joined by covalent bonds. The processes of building and breaking down polymers are fundamental to cellular function.

Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.
Monomer: The repeating subunit that serves as the building block of a polymer.

Dehydration Reaction (Synthesis)

Polymers are synthesized by dehydration reactions, where two monomers are covalently bonded with the loss of a water molecule.

One monomer provides a hydroxyl group (–OH), the other provides a hydrogen (–H).
This reaction is repeated to add more monomers, forming a polymer.

Equation:

Hydrolysis (Breakdown)

Polymers are disassembled by hydrolysis, the reverse of dehydration synthesis. A water molecule is added, breaking the bond between monomers.

Hydrogen (H) and hydroxyl (OH) groups from water are added to the separated monomers.
Example: Digestion of food polymers into absorbable monomers.

Equation:

Carbohydrates

Carbohydrates are sugars and polymers of sugars, serving as fuel and building material for cells. They are classified based on the number and arrangement of their monomers.

Monosaccharides: Simple sugars (e.g., glucose, fructose).
Disaccharides: Double sugars formed by joining two monosaccharides via a glycosidic linkage (e.g., sucrose).
Polysaccharides: Large polymers of monosaccharides (e.g., starch, glycogen, cellulose).

Monosaccharides

Monosaccharides are the simplest carbohydrates, typically having the formula (e.g., glucose: ). They contain a carbonyl group and multiple hydroxyl groups.

Classified by the location of the carbonyl group: Aldose (aldehyde sugar) or Ketose (ketone sugar).
Number of carbon atoms: trioses (3C), pentoses (5C), hexoses (6C).
Examples: Glucose (aldose), Fructose (ketose).

Disaccharides

Disaccharides are formed by joining two monosaccharides via a dehydration reaction, creating a glycosidic bond.

Example: Sucrose (glucose + fructose).

Polysaccharides

Polysaccharides are large polymers consisting of hundreds to thousands of monosaccharide units. They serve as energy storage or structural components.

Starch: Storage polysaccharide in plants, composed of glucose monomers.
Glycogen: Storage polysaccharide in animals, found in liver and muscle cells.
Cellulose: Structural polysaccharide in plant cell walls; most abundant organic compound on Earth.
Chitin: Structural polysaccharide in the exoskeletons of arthropods and cell walls of fungi.

Comparison of Polysaccharides

Polysaccharide	Function	Monomer	Linkage Type
Starch	Energy storage (plants)	Glucose	α (alpha) glycosidic
Glycogen	Energy storage (animals)	Glucose	α (alpha) glycosidic
Cellulose	Structural (plants)	Glucose	β (beta) glycosidic
Chitin	Structural (arthropods, fungi)	Modified glucose	β (beta) glycosidic

Note: Animals can digest α linkages (starch, glycogen) but not β linkages (cellulose).

Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They are primarily composed of hydrocarbons and are insoluble in water due to their nonpolar nature.

Fats: Energy storage molecules formed from glycerol and fatty acids.
Phospholipids: Major components of cell membranes.
Steroids: Include hormones and cholesterol.

Fats (Triglycerides)

Fats are constructed from two types of smaller molecules: glycerol (an alcohol) and fatty acids (hydrocarbon chains with a carboxyl group).

Three fatty acids join to one glycerol by ester linkages, forming a triacylglycerol (triglyceride).

Saturated vs. Unsaturated Fats

Type	Structure	Properties
Saturated Fat	No double bonds in fatty acid chains	Solid at room temperature; found in animal fats
Unsaturated Fat	One or more cis double bonds in fatty acid chains	Liquid at room temperature; found in plant oils

Example: Palmitic acid is a saturated fatty acid; oleic acid is an unsaturated fatty acid.

Proteins

Proteins are polymers of amino acids and perform a wide variety of functions in cells, including catalysis (enzymes), structure, transport, and regulation.

Amino acids: Monomers of proteins, each with a central carbon, amino group, carboxyl group, and variable side chain (R group).
Polypeptide: Polymer of amino acids linked by peptide bonds.
Enzymes: Specialized proteins that speed up chemical reactions.

Example: Alcohol dehydrogenase is an enzyme that catalyzes the breakdown of alcohols in the body.

Nucleic Acids

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

DNA (Deoxyribonucleic Acid): Stores genetic information; double-stranded helix.
RNA (Ribonucleic Acid): Involved in protein synthesis; single-stranded.
Nucleotide: Monomer consisting of a sugar, phosphate group, and nitrogenous base.

Summary Table: Major Biological Macromolecules

Macromolecule	Monomer	Bond Type	Main Function
Carbohydrates	Monosaccharide	Glycosidic linkage	Energy, structure
Lipids	Fatty acid, glycerol	Ester linkage	Energy storage, membranes
Proteins	Amino acid	Peptide bond	Catalysis, structure, transport
Nucleic Acids	Nucleotide	Phosphodiester bond	Genetic information

Additional info: The notes and images provided are consistent with introductory college-level General Biology, focusing on the structure and function of macromolecules. Some details (e.g., specific chemical structures, enzyme examples) were inferred and expanded for completeness.