Biological macromolecules are large, complex molecules essential to life, formed by smaller subunits through chemical reactions that structure and sustain living organisms.
Chemical Bonds in Macromolecules
Covalent Bonds
Covalent bonds are among the most significant types of chemical bonds in biological molecules. They occur when two atoms share electrons in order to fill their outermost energy levels and achieve stability. Covalent bonds are strong and contribute to the structural backbone of most biological molecules.
There are three primary types of covalent bonds:
Single covalent bond: Involves the sharing of one pair of electrons between two atoms. It is represented in structural formulas by a single line connecting the atoms (e.g., H–H or CH₄).
Double covalent bond: Involves the sharing of two pairs of electrons. These bonds are stronger and shorter than single bonds and are depicted by two lines (e.g., O=O in molecular oxygen).
Triple covalent bond: Involves three pairs of shared electrons, forming very strong and short bonds, as seen in molecules like N≡N (nitrogen gas).
These covalent interactions are what allow for the vast diversity of molecular structures seen in living organisms.
Electronegativity and Bond Polarity
Electronegativity is the tendency of an atom to attract electrons in a chemical bond. When atoms in a covalent bond have similar electronegativities, they share electrons equally, creating a nonpolar covalent bond. This equal sharing results in no significant charge separation across the molecule.
Examples of nonpolar covalent bonds:
H–H (hydrogen gas)
O₂ (oxygen gas)
CH₄ (methane)
When atoms have different electronegativities, electrons are shared unequally, forming polar covalent bonds. This unequal sharing results in partial charges (δ⁺ and δ⁻) on different ends of the molecule, leading to dipoles.
Example:
H₂O (water): The oxygen atom is more electronegative than hydrogen, attracting shared electrons more strongly and creating a molecule with a partial negative charge on the oxygen and partial positive charges on the hydrogens.
This polarity influences the behavior of molecules in water and facilitates the formation of hydrogen bonds, which are critical in biological systems.
Monomers and Polymers
Macromolecules are made by linking monomers into larger structures called polymers through covalent bonds. The process by which they are assembled and disassembled is fundamental to understanding biological chemistry.
Monomers
Monomers are the small, repeating subunits that serve as the building blocks of larger molecules. These subunits are simple in structure but versatile in function and can join in various arrangements to form complex polymers.
Examples:
Glucose (monosaccharide): a monomer of carbohydrates
Amino acids: monomers of proteins
Nucleotides: monomers of nucleic acids
Polymers
Polymers are large molecules made up of chains of monomers covalently bonded together. The arrangement and type of monomers affect the polymer’s shape, function, and properties.
Examples:
Polysaccharides like starch and cellulose are made from glucose units.
Polypeptides (proteins) are chains of amino acids.
DNA and RNA are nucleic acid polymers made of nucleotide chains.
Lipids and Non-polymeric Macromolecules
Although lipids are considered macromolecules due to their large size and biological importance, they are not true polymers. Lipids are typically assembled from two or more types of smaller units such as glycerol and fatty acids, but these units are not repeated in a chain-like fashion.
Types of Chemical Bonds Relevant to Biology
Ionic Bonds
Ionic bonds are formed when one atom donates an electron to another, resulting in the formation of oppositely charged ions. These ions attract each other due to their opposite charges.
For example:
Sodium (Na) loses one electron to become Na⁺.
Chlorine (Cl) gains that electron to become Cl⁻.
Together, they form sodium chloride (NaCl).
In biological systems, ionic bonds are important in maintaining cell membrane potentials, nerve signaling, and muscle contractions.
Image courtesy of Surfguppy.
Metallic Bonds
Metallic bonds occur between metal atoms where electrons are delocalized, meaning they move freely throughout the metallic structure. While not directly involved in forming macromolecules, metallic elements like iron, copper, and zinc play key roles in biological systems, particularly as cofactors in enzymes.
Dehydration Synthesis and Hydrolysis
Biological macromolecules are built and broken through chemical reactions involving water. These reactions are crucial for the maintenance of life processes.
Dehydration Synthesis (Condensation Reaction)
Dehydration synthesis is a chemical reaction in which monomers are joined together to form a polymer, and a molecule of water is removed in the process.
Mechanism:
One monomer contributes a hydroxyl group (–OH).
Another contributes a hydrogen (–H).
Together, they form H₂O, which is removed, and a new covalent bond is formed between the monomers.
This reaction:
Is anabolic (builds complex molecules from simpler ones).
Is endergonic, meaning it requires energy.
Is enzyme-catalyzed, requiring biological catalysts for speed and specificity.
Example: Two glucose molecules combine to form maltose and water.
glucose + glucose → maltose + H₂O
Hydrolysis
Hydrolysis is the reverse process of dehydration synthesis. It involves the addition of water to a polymer, breaking the bond between monomers.
Mechanism:
Water (H₂O) is split into H⁺ and OH⁻.
The H⁺ is added to one monomer, and the OH⁻ to the other, separating them.
This reaction:
Is catabolic (breaks complex molecules into simpler ones).
Is exergonic, meaning it releases energy.
Also requires enzymes for proper function.
Example: Sucrose is broken into glucose and fructose using water.
sucrose + H₂O → glucose + fructose
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The Four Classes of Biological Macromolecules
Each type of macromolecule has a unique set of monomers, structure, and biological function.
Carbohydrates
Monomer: Monosaccharides (e.g., glucose, fructose)
Elements: Carbon, hydrogen, and oxygen (typically in a 1:2:1 ratio)
Function:
Provide short-term energy (e.g., glucose metabolism)
Structural support in plants (e.g., cellulose)
Energy storage in animals (e.g., glycogen)
Types:
Monosaccharides: single sugar molecules
Disaccharides: two sugar molecules (e.g., sucrose = glucose + fructose)
Polysaccharides: long chains (e.g., starch, cellulose, glycogen)
Proteins
Monomer: Amino acids
Elements: Carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur
Function:
Enzymatic catalysis (e.g., amylase)
Structural support (e.g., keratin, collagen)
Transportation (e.g., hemoglobin)
Signaling and immune response (e.g., hormones, antibodies)
Structure:
Each amino acid has a central carbon attached to:
An amino group (–NH₂)
A carboxyl group (–COOH)
A hydrogen atom
An R-group (variable side chain)
Polypeptides form through peptide bonds via dehydration synthesis.
Nucleic Acids
Monomer: Nucleotides
Each consists of:
A phosphate group
A five-carbon sugar (ribose or deoxyribose)
A nitrogenous base (A, T, G, C, or U)
Elements: CHONP (carbon, hydrogen, oxygen, nitrogen, phosphorus)
Function:
DNA stores genetic information.
RNA is involved in protein synthesis.
Bonds:
Nucleotides are joined via phosphodiester bonds.
Base pairing in DNA is stabilized by hydrogen bonds.
Lipids
Not true polymers
Subunits: Fatty acids and glycerol
Elements: Mostly carbon and hydrogen, with some oxygen
Function:
Long-term energy storage
Major component of cell membranes (phospholipids)
Hormone production (e.g., steroids)
Types:
Triglycerides: Glycerol + 3 fatty acids
Phospholipids: Glycerol + 2 fatty acids + phosphate group
Steroids: Four fused carbon rings (e.g., cholesterol)
Intramolecular and Intermolecular Bonds
Understanding the differences between internal and external bonds helps explain how macromolecules maintain their structure and interact.
Intramolecular Bonds
Intramolecular bonds occur within a molecule. These are the covalent bonds that connect atoms and define the molecule’s basic structure.
Examples:
Peptide bonds between amino acids in proteins
Glycosidic linkages in carbohydrates
Phosphodiester bonds in DNA and RNA
These bonds determine the primary structure of macromolecules.
Intermolecular Bonds
Intermolecular bonds are interactions between molecules, important for higher-order structure and molecular recognition.
Hydrogen Bonds
Hydrogen bonds are weak interactions that occur when a hydrogen atom, covalently bonded to an electronegative atom (typically oxygen or nitrogen), is attracted to another nearby electronegative atom.
Not covalent, but stronger than van der Waals forces
Play a critical role in:
DNA base pairing (A-T, G-C)
Protein folding and secondary structure (alpha helices and beta sheets)
Cohesion and surface tension in water
Molecular Examples and Key Elements
Understanding macromolecules also involves recognizing the specific compounds and elements involved in their formation.
Methane (CH₄): Nonpolar covalent molecule
Carbon Monoxide (CO): Polar covalent molecule
Sodium Chloride (NaCl): Ionic compound
Lithium Fluoride (LiF): Another ionic compound
Iodine Monobromide (IBr): Polar covalent compound
Biologically important elements include:
Iron (Fe): Part of hemoglobin
Zinc (Zn): Enzyme function
Copper (Cu): Electron transport
Cobalt (Co): Component of Vitamin B12
Silver (Ag): Antimicrobial
Gold (Au) and Platinum (Pt): Used in research and medicine
Each plays a unique role in maintaining the structure and function of living systems through their chemical interactions.
FAQ
Carbon is uniquely suited to form the backbone of biological macromolecules due to its versatile bonding properties. It has four valence electrons, which means it can form up to four covalent bonds with other atoms, allowing it to build complex, stable molecules in various shapes and lengths. This tetravalency enables the creation of chains, branched molecules, and ring structures that serve as the foundation for macromolecules like carbohydrates, proteins, lipids, and nucleic acids.
Carbon can bond with itself and many other elements (H, O, N, P, S).
Its ability to form single, double, and triple bonds increases molecular diversity.
Organic molecules built from carbon provide structural and functional roles in all living systems.
Carbon-based molecules can be isomers (same formula, different structures), allowing further variety.
Functional groups are specific clusters of atoms within molecules that determine their chemical behavior and interactions. These groups contribute to the molecule's polarity, acidity/basicity, solubility, and reactivity, directly affecting the structure and function of macromolecules in biological systems.
Hydroxyl (–OH) groups make molecules polar and increase solubility (e.g., sugars).
Carboxyl (–COOH) acts as an acid by donating H⁺, common in amino acids and fatty acids.
Amino (–NH₂) groups act as bases, picking up H⁺ ions and are found in amino acids.
Phosphate (–PO₄) groups are charged and important in nucleic acids and energy molecules like ATP.
Sulfhydryl (–SH) groups form disulfide bridges in protein structure.
Functional groups allow for hydrogen bonding, interaction with water, and participation in chemical reactions.
Enzymes are biological catalysts that significantly speed up the chemical reactions of dehydration synthesis and hydrolysis, which would otherwise occur too slowly to support life. Each enzyme is specific to a reaction or type of bond, ensuring precise control in biological systems.
During dehydration synthesis, enzymes position monomers correctly and lower the activation energy needed to form a covalent bond while water is removed.
In hydrolysis, enzymes help break covalent bonds by facilitating the addition of water, splitting the molecule into its monomer components.
Enzymes remain unchanged by the reaction and can be reused.
Their activity can be regulated by factors such as pH, temperature, and the presence of inhibitors or activators.
Examples include DNA polymerase (in nucleic acid synthesis) and amylase (in carbohydrate breakdown).
Each class of biological macromolecule exhibits unique structural characteristics, leading to varying degrees of complexity and functional diversity. This variability allows cells to build structures, store information, and carry out specialized functions.
Carbohydrates: Can be linear or branched; complex polysaccharides have structural (cellulose) or storage (glycogen) roles.
Proteins: Have four levels of structure (primary to quaternary); shape is highly specific to function due to interactions like hydrogen bonding, ionic interactions, and disulfide bridges.
Nucleic acids: DNA forms a double helix; RNA is usually single-stranded and can fold into complex structures for functions like catalysis (ribozymes).
Lipids: Though not true polymers, they form varied structures such as triglycerides, phospholipid bilayers, and ringed steroids.
Structural complexity arises from different monomer sequences, bond types, and interactions between parts of the molecule.
Macromolecules are distinguished from smaller biological molecules by their large size, polymeric structure (except lipids), and their central role in maintaining life’s processes. Unlike small molecules such as water, salts, or vitamins, macromolecules form the framework of cells and are involved in nearly all functions of life.
Macromolecules typically contain thousands of atoms and are made from repeating units (monomers).
They require specific enzymatic processes for synthesis and breakdown.
Their size allows for specialized 3D structures, which enables precise biological roles.
Information storage (DNA/RNA), catalysis (enzymes), energy storage (starch, glycogen), and membrane formation (lipids) are all functions that depend on macromolecular architecture.
The complexity and functionality of macromolecules are directly related to their ability to interact with other molecules and structures in the cell.
Practice Questions
Describe the processes of dehydration synthesis and hydrolysis in the formation and breakdown of biological macromolecules. Include the role of water in each reaction.
Dehydration synthesis is the process by which monomers are joined to form polymers through the removal of a water molecule. One monomer donates a hydroxyl group (–OH) while the other donates a hydrogen (–H), forming water and a covalent bond between the monomers. This reaction is anabolic and requires energy. In contrast, hydrolysis is the process of breaking down polymers into monomers by adding water. The water molecule is split, with –H and –OH attaching to separate monomers, breaking the covalent bond. Hydrolysis is catabolic and releases energy. Both reactions require enzymes to occur efficiently in biological systems.
Explain the relationship between monomers and polymers in biological macromolecules and provide one example for each of the four major macromolecule types.
Biological macromolecules are formed when monomers, which are small molecular subunits, join together through covalent bonds to create larger molecules called polymers. This relationship allows for complexity and diversity in structure and function. For carbohydrates, glucose is the monomer and starch is the polymer. In proteins, amino acids link to form polypeptides. For nucleic acids, nucleotides are the monomers that form DNA or RNA polymers. Although lipids are not true polymers, they are formed from smaller units like glycerol and fatty acids. These structural relationships enable macromolecules to perform vital roles in cell structure, metabolism, and information storage.
