Biological Macromolecules: Monomers, Polymers, and Synthesis Reactions

Principles of Monomers, Polymers, and Chemical Reactions

• Overview of Biological Macromolecules:

  • Cells and biological organisms are constructed from exceptionally large molecules called macromolecules or polymers.

  • These large molecules are assembled from smaller repeating molecular building blocks called monomers.

  • Prefix etymology:

  • The prefix "mono-" signifies one.

  • The prefix "poly-" signifies many.

• Polymer Chain Elongation:

  • Synthesis occurs by adding an unlinked individual monomer to an existing short polymer chain.

  • An enzyme catalyzes the formation of a new covalent bond between the short polymer and the incoming monomer.

  • With the addition of each individual monomer, the polymer chain grows in length by one unit.

  • This step-by-step process repeats iteratively to build long polymer chains.

• Dehydration Synthesis (Dehydration Reactions):

  • Chemical mechanism: The primary chemical reaction used to assemble monomers into polymers is a dehydration reaction.

  • Reactant group contributions:

  • One monomer partner contributes a hydrogen atom ($H$).

  • The other monomer partner contributes a hydroxyl group ($OH$).

  • Water byproduct: The removed hydrogen atom ($H$) and hydroxyl group ($OH$) combine and exit as a single molecule of water ($H_2O$).

  • Covalent bonding: An enzyme covalently links the two monomer units together simultaneously with the loss of the water ($H_2O$) molecule.

• Hydrolysis Reactions (Depolymerization):

  • Reversibility of reactions: Biological polymerization reactions are fully reversible.

  • Chemical mechanism: Polymers are disassembled into individual constituent monomers through hydrolysis, which uses water ($H_2O$) to break internal covalent bonds.

  • Functional mechanism:

  • A specific enzyme breaks the covalent bond connecting two monomers within a polymer chain by introducing a water ($H_2O$) molecule.

  • The water ($H_2O$) molecule splits into a hydrogen atom ($H$) and a hydroxyl group ($OH$).

  • The hydrogen atom ($H$) attaches to one monomer, while the hydroxyl group ($OH$) attaches to the adjacent monomer.

  • Biological role: Hydrolysis is the essential mechanism involved in food digestion, breaking down complex dietary polymers into basic monomers for biological utilization.

Class 1: Carbohydrates

• Terminology and Monomers:

  • Carbohydrates constitute the first major class of biological macromolecules.

  • Equivalent terms for sugars include saccharide, carbohydrate, and sugar.

  • The fundamental monomer unit of a carbohydrate is a monosaccharide (simple sugar).

  • Monosaccharide structure: Monosaccharides typically form ring-like chemical structures.

  • Structural chemistry: Monosaccharides possess abundant attached hydrogen ($H$) atoms and hydroxyl ($OH$) groups, making them ideally structured for dehydration synthesis reactions.

  • Primary example: Glucose is the foundational monosaccharide example in biological systems.

• Disaccharide Formation:

  • Definition: A disaccharide consists of two monosaccharides covalently bonded together.

  • Reaction example (Maltose assembly):

  • Two individual glucose monomers undergo a dehydration reaction.

  • A hydroxyl group ($OH$) is removed from one glucose monomer, and a hydrogen atom ($H$) is removed from the other glucose monomer.

  • A water molecule ($H_2O$) is lost as a byproduct.

  • A new covalent bond forms between the two glucose monomers, yielding the disaccharide maltose.

• Polysaccharides (Carbohydrate Polymers):

  • Definition: Polysaccharides are large biological polymers composed of many monosaccharide sugar units linked in long chains via repetitive dehydration reactions.

  • Three key polysaccharide examples:

  • Starch: A glucose polymer synthesized by certain plants as a storage medium for excess calories.

  • Glycogen: A glucose polymer stored within animal and human muscle cells as a storage medium for excess glucose.

  • Cellulose: A structural glucose polymer found in the cell walls of plant cells, conferring extreme mechanical strength to wood and tree trunks.

  • Polysaccharide Digestion:

  • Digestible polysaccharides are broken down via hydrolysis reactions, where enzymes add water ($H_2O$) to cleave covalent bonds and yield free glucose monomers.

Class 2: Lipids

• Key Exception to Polymer Formation:

  • Lipids form the second major biological macromolecule group but represent an explicit exception: lipids do not form true polymers.

  • General chemical composition: Lipids are relatively small molecules consisting predominantly of carbon ($C$) and hydrogen ($H$) atoms.

• Solubility Characteristics:

  • Hydrophobic property: Lipids do not mix well with water ($H_2O$).

  • Physical interaction: When mixed, oil (a type of liquid fat) and water separate into distinct layers.

• Three Subcategories of Lipids:

  • Fats

  • Phospholipids

  • Steroids

Class 3: Proteins

• Biological Importance and Function:

  • Proteins represent the third major class of biological macromolecules.

  • Primary role: Proteins execute the vast majority of cellular functions and biological activities within living bodies.

• Monomers (Amino Acids):

  • The basic monomer building blocks of proteins are amino acids.

  • Key structural functional groups for polymer assembly:

  • Amino acids feature hydrogen ($H$) atoms bound to a nitrogen ($N$) atom.

  • Amino acids feature a hydroxyl group ($OH$) bound to a carbon ($C$) atom.

• Polymerization and Peptide Bond Synthesis:

  • Reaction mechanism: Amino acids are joined into polymers via dehydration synthesis.

  • Cleavage step: A hydroxyl group ($OH$) is removed from the carbon ($C$) atom of one amino acid, while a hydrogen atom ($H$) is removed from the nitrogen ($N$) atom of an adjacent amino acid.

  • Bond formation: The resulting covalent bond formed between the carbon ($C$) of one amino acid and the nitrogen ($N$) of another is called a peptide bond.

  • Polymer nomenclature: A chain of amino acids linked by peptide bonds is termed a polypeptide or a protein.

Class 4: Nucleic Acids

• Biological Role and Major Types:

  • Nucleic acids constitute the fourth major class of biological macromolecules.

  • Primary function: Storage, expression, and transmission of genetic information.

  • Two primary nucleic acids: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).

• Monomers (Nucleotides):

  • The monomer building blocks of nucleic acid polymers are nucleotides.

  • Structural features of a nucleotide monomer:

  1. A central monosaccharide (sugar) ring.

  2. A phosphate group attached to the central sugar ring.

  3. A nitrogenous base attached to the central sugar ring.

• Polymer Assembly:

  • Nucleotide monomers are covalently linked together to form nucleic acid polymers, specifically DNA and RNA.