Comprehensive University Study Notes on Biological Molecules and Biochemistry

Introduction to Biochemistry and the Molecular Foundation of Life

• Biochemistry constitutes the specialized integrative branch of biological and chemical sciences dedicated to elucidating the biochemical basis of life, detailing how living organisms are structurally assembled from and dynamically maintained by chemical entities.

• The foundational importance of biochemistry lies in uncovering the precise chemical processes operating within living organisms, spanning:

  • The construction and structural maintenance of anatomical and cellular architectures.

  • The nucleocytoplasmic flow of genetic information originating from nuclear DNA for the directed synthesis of enzymatic and structural proteins.

  • The global coordination, regulation, and maintenance of integrated cellular metabolic pathways.

  • The biochemical pathology of abnormal cellular mechanisms, which clarifies the molecular etiology of diseases and enables the rational design and synthesis of pharmaceutical interventions to eliminate functional abnormalities.

• Biochemistry resolves fundamental biological and medical questions:

  • Morphogenesis: Mechanisms through which individual cells recognize and adhere to one another to organize into functional tissues and complex organs ($\text{cells} \rightarrow \text{tissues} \rightarrow \text{organs}$).

  • Cellular Regulation: How genomic instructions encoded within cellular DNA tightly govern cell growth, differentiation, and mitotic proliferation.

  • Oncology: Identification of molecular aberrations, mutations, and signaling malfunctions triggering oncogenesis and cancer progression.

  • Neurobiology: Elucidation of the neurochemical and biophysical pathways underpinning learning, memory consolidation, and neural storage.

• Structural Hierarchy of Living Systems:

  • Organisms are systematically constructed from organ systems and tissues.

  • Tissues are assemblies of specialized cells.

  • Cells are composed of organized macromolecular complexes and individual molecules.

  • Molecules are formed via specific chemical bonds between fundamental atomic elements.

• Central Axiom of Cell Theory: The structural architecture and functional capabilities of any living cell are fundamentally governed by and directly dependent upon its underlying chemical composition.

Chemical Organization and Constituents of the Cell

• Cellular Chemical Zones:

  • Protoplasm: The total living substance and aqueous reaction center of the cell, functioning as the primary site for metabolic transformations. Protoplasm encompasses the water and solutes within the cytoplasm, nucleoplasm, mitochondrial matrix, and chloroplast stroma.

  • Cellular Water vs. Dry Weight:

  • Water constitutes $70\%$ to $90\%$ of the total physical mass of an active living cell.

  • The remaining non-aqueous residual matter represents the cellular dry weight, accounting for $10\%$ to $30\%$ of total cellular mass.

  • Biomolecules: Organic compounds synthesized directly by living systems that comprise this dry weight fraction.

• Elemental Composition of Biomolecules:

  • Six major biological elements make up approximately $98\%$ of total biomolecular mass: Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Nitrogen ($N$), Phosphorus ($P$), and Calcium ($Ca$).

  • Minor elements and trace elements (principally essential transition metals and minerals) constitute the remaining $1\%$ to $2\%$ of biological elemental composition.

• Primary Classes of Cellular Biomolecules:

  • Carbohydrates

  • Proteins

  • Lipids

  • Nucleic Acids

  • Conjugated Molecules

• Comparative Quantitative Chemical Composition (Bacterial Cell vs. Mammalian Cell):

  • Water: $70\%$ in bacterial cells; $70\%$ in mammalian cells.

  • Proteins: $15\%$ in bacterial cells; $18\%$ in mammalian cells.

  • Carbohydrates: $3\%$ in bacterial cells; $4\%$ in mammalian cells.

  • Lipids: $2\%$ in bacterial cells; $3\%$ in mammalian cells.

  • DNA: $1\%$ in bacterial cells; $0.25\%$ in mammalian cells.

  • RNA: $6\%$ in bacterial cells; $1.10\%$ in mammalian cells.

  • Other Organic Compounds (enzymatic intermediates, metabolites): Identical relative concentrations in both cellular types.

  • Inorganic Ions and Mineral Salts: Identical relative concentrations in both cellular types.

Synthesis and Degradation of Macromolecules: Condensation and Hydrolysis

• Macromolecular Architecture:

  • Macromolecules (polymers, from poly = many, mer = unit) are large, complex structures formed through the covalent assembly of repeating small chemical subunits termed monomers (micromolecules).

  • Structural Metaphor: A macromolecule functions like a brick wall; the wall is not a monolithic, indivisible entity, but a composite structure constructed from repeating individual building blocks (bricks/monomers).

• Condensation (Dehydration Synthesis):

  • The biochemical polymerization process wherein two monomeric subunits are covalently joined accompanied by the concurrent elimination of a water molecule ($H_2O$).

  • Reaction Mechanism: An active hydroxyl group ($-OH$) is removed from one monomer, and a hydrogen ion ($H^+$) is extracted from the adjacent monomer, forming a free water molecule and yielding a dimer held by a covalent bond.

  • Progressive condensation reactions sequentially assemble dimers into trimers, oligomers, and high-molecular-weight polymers.

  • Thermodynamic and Kinetic Requirements: Condensation is an endergonic synthetic process that strictly requires specific catalytic enzymes and input of chemical energy.

• Hydrolysis (Hydrolytic Cleavage):

  • The catabolic breakdown of complex macromolecules into their constituent monomeric building blocks through the biochemical addition of water (hydro = water, lysis = splitting).

  • Reaction Mechanism: Operates as the exact biochemical reverse of condensation. A water molecule is split into $H^+$ and $OH^-$ ions; the $H^+$ ion attaches to one cleaved fragment, while the $OH^-$ ion attaches to the adjacent cleaved subunit.

  • Quantitative Stoichiometry: The complete hydrolytic cleavage of a polymer consisting of $n$ monomeric subunits requires precisely $n - 1$ molecules of water. For example, cleaving a tetrameric macromolecule (4 subunits) into four isolated monomers consumes exactly 3 water molecules.

  • Energetics: Hydrolysis is an exergonic breakdown process mediated by specific hydrolytic enzymes, releasing stored bond energy that becomes available to power other physiological reactions.

Dynamic Metabolic Pathways in Heterotrophic and Autotrophic Organisms

• Metabolism encompasses the totality of intracellular chemical reactions involving the simultaneous synthesis (anabolism) and degradation (catabolism) of biological molecules during structural remodeling and turnover.

• Heterotrophic Macromolecular Processing:

  • Digestion: Ingested exogenous dietary macromolecules cannot be directly integrated into host tissues; they are enzymatically fragmented via hydrolytic enzymes into absorbable, low-molecular-weight monomers within digestive cavities.

  • Assimilation: Absorbed monomers are transported via the bloodstream into individual cells.

  • Reconstitution: Cellular enzymes recombine these monomers via intracellular condensation reactions to synthesize novel, host-specific macromolecules required for cellular architecture and physiological function.

• Autotrophic Macromolecular Processing:

  • Monomer Biosynthesis: Autotrophs (plants) synthesize fundamental organic monomers de novo from environmental inorganic raw materials ($CO_2$, $H_2O$, sulfur oxides, and nitrates).

  • Polymerization in Source Tissues: Synthesized monomers undergo condensation to form storage or structural macromolecules inside metabolic source tissues (tissues where organic nutrients are produced or initially stored, such as photosynthetic leaves).

  • Translocation to Sink Tissues: Stored macromolecules are moved to sink tissues (regions of nutrient utilization or long-term deposit, such as developing roots, fruits, or growing buds).

  • Mobilization: When energy or intermediate building blocks are demanded by distant cells, these stored macromolecules undergo enzymatic hydrolysis back into soluble mobile monomers.

Physicochemical Properties and Biological Significance of Water

• Water as the Universal Medium of Life:

  • Constitutes $70\%$ to $90\%$ of cellular mass and acts as the aqueous reaction matrix for virtually all intracellular metabolic processes.

  • Operates as an active chemical participant in metabolic reactions, serving as an electron/hydrogen donor in photosynthetic carbohydrate synthesis ($CO_2 + H_2O \rightarrow \text{glucose}$) and providing cleavage ions in hydrolytic catabolism.

• Molecular Geometry and Polarity:

  • A single water molecule ($H_2O$) consists of two hydrogen atoms covalently bound to one central oxygen atom.

  • Oxygen exhibits significantly higher electronegativity than hydrogen, pulling the shared bonding electrons strongly toward its own nucleus.

  • This asymmetric electron distribution generates a partial negative charge ($\delta^-$) on the oxygen atom and a corresponding partial positive charge ($\delta^+$) on each hydrogen atom.

  • Molecular Dipole: The spatial separation of these opposing partial electrical charges across the molecule constitutes a permanent electric dipole.

• Hydrogen Bonding:

  • The electrostatic intermolecular attraction established between the partially negative oxygen atom ($\delta^-$) of one water molecule and the partially positive hydrogen atom ($\delta^+$) of a neighboring water molecule.

  • Enables water molecules to dynamically cross-link into extensive transient intermolecular networks (forming cohesive multimolecular clusters).

• Cohesive and Adhesive Forces:

  • Cohesion: The intermolecular attraction between identical molecules. In water, extensive hydrogen bonding causes water molecules to cling tenaciously to one another, forming an unbroken, freely flowing liquid stream within vascular and cellular conduits (such as blood vessels and plant xylem).

  • Adhesion: The electrostatic attraction between dissimilar molecules. Water molecules adhere strongly to charged or polar surfaces, such as the hydrophilic cellulosic and lignified walls of xylem vessels.

  • Cohesion-Tension Theory of Sap Ascent: The combined action of cohesion (holding the vertical water column intact) and adhesion (attaching the column to vascular walls) allows plants to lift water against gravity from subterranean roots to high canopy leaves without column cavitation or rupture.

• High Specific Heat Capacity:

  • Specific heat capacity denotes the amount of thermal energy required to raise the temperature of $1\,\text{g}$ of a substance by $1^\circ\text{C}$.

  • Due to the large amount of energy absorbed in destabilizing hydrogen bonds before kinetic molecular motion can increase, water possesses an exceptionally high specific heat capacity.

  • Biological Role: Acts as a powerful thermal buffer, preventing rapid thermal fluctuations within protoplasm and shielding delicate cellular machinery from environmental temperature extremes.

• High Heat of Vaporization:

  • The quantity of heat energy required to transform a substance from the liquid state into the gaseous (vapor) state.

  • Water requires $574\,\text{kcal/kg}$ ($574\,\text{kilo calories per kilogram}$) of thermal energy to vaporize.

  • Liquid State Stability: High vaporization requirements prevent rapid desiccation of cellular contents under standard physiological conditions.

  • Evaporative Thermoregulation: When water does evaporate, it carries off substantial quantities of latent heat, producing a pronounced cooling effect:

  • Transpiration: Evaporative cooling from plant leaf surfaces maintains optimal internal temperatures under intense sunlight.

  • Perspiration and Panting: Evaporation of sweat in humans or panting in dogs (protruding the moist tongue to maximize airflow across liquid surfaces) dissipates excess metabolic heat.

• Hydrophobic Exclusion:

  • The forced aggregation of non-polar, hydrophobic substances (such as oils and neutral lipids) when introduced into an aqueous environment.

  • Non-polar molecules cannot participate in hydrogen bonding, which disrupts local water lattice stability.

  • Water molecules actively maximize their mutual hydrogen bonds, squeezing non-polar substances out of the aqueous phase and forcing them to coalesce into isolated, compact droplets or distinct compartments.

  • Fundamental for the spontaneous assembly of lipid bilayers and cellular compartmentalization.

• Ionization and Amphoteric/Buffer Nature:

  • Water undergoes slight, reversible self-ionization according to the equilibrium:     $H_2O \rightleftharpoons H^+ + OH^-$

  • Amphoteric Character: Water can function simultaneously as a weak acid (donating $H^+$ ions) and as a weak base (donating $OH^-$ ions).

  • Biological Buffer: Water stabilizes environmental pH across the standard $0-14$ pH scale. When local environments become alkaline ($\text{pH} > 7$), water donates $H^+$ to neutralize alkalinity; when conditions become excessively acidic ($\text{pH} < 7$), water accepts $H^+$ or yields neutralizing $OH^-$.

  • Enzyme Preservation: Preventing acute deviations in cellular pH preserves the three-dimensional catalytic conformations of intracellular enzymes, protecting them from denaturing.

• Anomalous Expansion and Low Density of Ice:

  • Most liquids contract continuously as temperature drops. Water exhibits anomalous expansion when cooled below $4^\circ\text{C}$ down to its freezing point at $0^\circ\text{C}$.

  • Molecular Mechanism: In liquid water, thermal kinetic energy allows molecules to remain densely packed in an irregular arrangement. Below $4^\circ\text{C}$, hydrogen bonding locks the molecules into an open, rigid, hexagonal crystalline lattice where water molecules are held further apart than in the liquid state.

  • Density Inversion: Ice at $0^\circ\text{C}$ is physically less dense than liquid water at $4^\circ\text{C}$, causing it to float.

  • Ecological Protection: Ice forms an insulating surface sheet over aquatic bodies (lakes, oceans), shielding underlying waters from sub-zero atmospheric temperatures and preserving aquatic life beneath the frozen surface.

Carbohydrates: Structural Chemistry and Functional Classifications

• Definition and Empirical Formula:

  • Literally translates to "hydrated carbons" (water-containing carbon compounds).

  • Composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$), wherein the atomic ratio of hydrogen to oxygen is typically $2:1$, mirroring that of water.

  • General empirical formula: $C_n(H_2O)_n$ or $C_n H_{2n} O_n$, where $n$ represents the total number of carbon atoms.

• IUPAC Definition:

  • Chemically classified as polyhydroxy aldehydes or polyhydroxy ketones, or complex substances that yield polyhydroxy aldehydes or ketones upon hydrolytic cleavage.

  • Structural Characteristics: Molecules possess multiple hydroxyl groups ($-OH$) attached across a carbon backbone along with a reactive carbonyl group ($C=O$).

  • Carbonyl Classifications:

  • Aldehyde group ($-CHO$): Consists of a carbonyl carbon bonded to at least one hydrogen atom, occupying a terminal position on the carbon skeleton (aldoses).

  • Ketone group ($C=O$): Consists of a carbonyl carbon positioned internally between two adjacent carbon atoms (ketoses).

• Biosynthetic Origin and Functions:

  • Synthesized as primary products of photosynthetic carbon fixation ($CO_2 + H_2O \rightarrow \text{glucose}$) within green plants.

  • Serve as metabolic precursors for constructing other cellular organic molecules, including lipids and amino acids.

  • Provide essential energy storage (starch, glycogen) and structural scaffolding (cellulose in plant cell walls and commercial paper; chitin in fungal walls and arthropod exoskeletons).

• Classification by Degree of Polymerization:

  • Monosaccharides (single sugar monomers)

  • Oligosaccharides (short chains of 2 to 10 sugar units)

  • Polysaccharides (complex polymers exceeding 10 to thousands of sugar units)

Monosaccharides: Architecture, Isomerism, and Ring Structures

• Physical and Chemical Properties of Monosaccharides:

  • White, crystalline, sweet-tasting solids that dissolve readily in water.

  • Monomeric units that cannot be further degraded into smaller sugars via hydrolytic cleavage.

  • Named systematically according to their carbon atom count with the characteristic suffix -ose:

  • Trioses ($3C$; general formula $C_3H_6O_3$): Examples include glyceraldehyde (glycerose) and dihydroxyacetone; critical metabolic intermediates generated during glycolysis and cellular respiration.

  • Tetroses ($4C$; general formula $C_4H_8O_4$): Rare in nature; synthesized in limited metabolic pathways within specific bacteria.

  • Pentoses ($5C$; general formula $C_5H_{10}O_5$): Key structural components of nucleic acids, including ribose in RNA and deoxyribose ($C_5H_{10}O_4$) in DNA.

  • Hexoses ($6C$; general formula $C_6H_{12}O_6$): Physiologically dominant monosaccharides, including glucose, fructose, galactose, and mannose.

  • Heptoses ($7C$; general formula $C_7H_{14}O_7$): Seven-carbon metabolic intermediates.

• Natural Distribution of Hexoses:

  • Glucose: Abundant in sweet fruits, ripe sweet corn, and honey; acts as the primary building block of starch, glycogen, and cellulose.

  • Fructose: Highly sweet sugar found naturally in fruit juices and honey, commonly termed "fruit sugar".

• Open-Chain vs. Ring Structures:

  • In aqueous cellular solution, monosaccharides (especially pentoses and hexoses) dynamically convert from open, straight-chain structures into stable cyclic ring forms.

  • Aldohexoses form 6-membered pyranose rings (e.g., glucopyranose), while ketohexoses and aldopentoses form 5-membered furanose rings (e.g., fructofuranose, ribofuranose).

• Hexose Isomerism:

  • Structural isomers share the identical molecular formula ($C_6H_{12}O_6$) but differ in the spatial arrangement of atoms or functional groups.

  • Aldohexoisomers (possessing an aldehyde group): Glucose, Galactose, and Mannose. These differ subtly in the spatial orientation of specific hydroxyl groups ($-OH$) along the carbon backbone.

  • Ketohexoisomers (possessing an internal ketone group): Fructose, Sorbose, and Psicose.

Oligosaccharides, Glycosidic Linkages, and Sugar Transport

• Oligosaccharide Characteristics:

  • Formed through the condensation of 2 to 10 monosaccharide units linked by covalent glycosidic bonds.

  • Taste noticeably less sweet than monosaccharides, show lower aqueous solubility, and yield free monosaccharide units upon enzymatic hydrolysis.

• Disaccharides ($2C$ sugar units):

  • Formed by condensing two monosaccharides with the elimination of one molecule of water.

  • Glycosidic Bond Formation: Formed when a hydroxyl group from one sugar reacts with a hydrogen/hydroxyl group from an adjacent sugar (e.g., between carbon-1 of one hexose and carbon-4 of another, creating a $1\rightarrow 4$ glycosidic linkage).

  • Major Disaccharide Examples:

  • Maltose: Formed by the union of two glucose molecules ($\text{glucose} + \text{glucose}$), commonly yielding an $\alpha(1\rightarrow 4)$ glycosidic bond.

  • Sucrose: Formed by condensing one glucose molecule with one fructose molecule ($\text{glucose} + \text{fructose}$).

  • Lactose: Milk sugar, formed by joining galactose and glucose ($\text{galactose} + \text{glucose}$).

• Reducing vs. Non-Reducing Sugars:

  • Reducing Sugars:

  • Carbohydrates that possess an unbonded, chemically reactive free aldehyde or free ketone carbonyl group.

  • Capable of donating electrons (becoming oxidized themselves) to reduce external chemical reagents without needing prior hydrolysis.

  • Examples: Free monosaccharides (glucose, fructose), and disaccharides like maltose and lactose.

  • Non-Reducing Sugars:

  • Carbohydrates in which the reactive carbonyl groups of both constituent monosaccharides are tied up participating in the glycosidic linkage.

  • Lack free, reactive carbonyl centers; incapable of acting as reducing agents unless first hydrolyzed into free monomers.

  • Examples: Sucrose and the trisaccharide raffinose.

• Physiological Significance of Non-Reducing Sugars in Phloem Transport:

  • Plants translocate photosynthetic assimilates across long distances from autotrophic source tissues (leaves) to heterotrophic sink tissues (roots, developing fruits) exclusively in the form of non-reducing sugars, predominantly sucrose.

  • Transporting sucrose preserves carbohydrate integrity: because its reactive carbonyl groups are chemically locked within the glycosidic bond, sucrose remains chemically inert during transit.

  • It resists premature oxidative degradation, non-specific enzymatic attacks, or undesirable metabolic reactions en route, ensuring maximum chemical energy is delivered intact to sink tissues.

Polysaccharides: Starch, Glycogen, Cellulose, and Chitin

• High-molecular-weight polymers composed of hundreds to thousands of repeating monosaccharide units joined by glycosidic bonds.

• Characterized by high physical insolubility in water, lack of sweet taste, and non-crystalline properties.

• Primary Polysaccharide Classes:

  • Starch:

  • Primary carbohydrate storage polymer in plants; found in cereals, legumes, storage tubers (potatoes), and fruits.

  • Amylose: Unbranched, straight-chain polymer where glucose residues are connected via $\alpha(1\rightarrow 4)$ glycosidic linkages; soluble in hot water; produces a deep blue color when reacted with iodine reagent.

  • Amylopectin: Highly branched polymer containing both linear $\alpha(1\rightarrow 4)$ linkages and periodic branch-point $\alpha(1\rightarrow 6)$ glycosidic bonds; insoluble in water; produces a characteristic purple to blue-black color reaction with iodine.

  • Glycogen ("Animal Starch"):

  • Major energy reserve polysaccharide found in animal tissues, localized predominantly in skeletal muscle fibers and liver cells; also present in fungi and bacteria.

  • Structurally resembles amylopectin but features significantly more frequent branching along its glucose chains.

  • Produces a distinct reddish-brown color when treated with iodine.

  • Cellulose:

  • The most abundant organic macromolecule on Earth; serves as the primary structural constituent of plant cell walls and raw commercial paper.

  • A linear, completely unbranched polymer composed of repeating glucose units linked via $\beta(1\rightarrow 4)$ glycosidic bonds.

  • Linear cellulose polymers align side-by-side to assemble into rigid microfibrillar bundles and coiled tubular fibers of high tensile strength.

  • Water-insoluble and yields no color change when exposed to iodine.

  • Indigestible within the human gastrointestinal tract due to the absence of endogenous cellulase enzymes capable of hydrolyzing $\beta(1\rightarrow 4)$ glycosidic linkages.

  • Chitin:

  • A modified structural polysaccharide composed of repeating linear units of $N$-acetylglucosamine (an amide derivative of glucose bearing an attached nitrogenous group).

  • Structural and Functional Analogy: Structurally mirrors cellulose by organizing into robust, ordered nanofibers; functionally mirrors structural fibrous proteins like keratin by imparting durable mechanical strength.

  • Nitrogen incorporation reinforces intermolecular hydrogen bonding between parallel chains, increasing overall tensile resistance.

  • Physical States and Natural Occurrences:

    • Pure, Unmodified Chitin: Translucent, pliable, resilient, and structurally tough.

    • Modified Composite Form: Extensively fortified with structural proteins and mineralized calcium carbonate ($CaCO_3$) to construct hard, protective exoskeletons in insects (such as cockroaches) and crustaceans (crabs, lobsters, prawns), as well as outer shells in mollusks.

Stereoisomerism and Medical Applications of Carbohydrates

• Stereoisomers are chemical isomers sharing the identical molecular formula and identical sequence of covalently bonded atoms, but differing distinctly in their three-dimensional spatial orientation.

• Enantiomers (Mirror Images):

  • Carbohydrates exhibit optical stereoisomerism, occurring as non-superimposable mirror-image forms designated as right-handed ($D$-enantiomers) or left-handed ($L$-enantiomers).

  • The structural distinction stems from the relative spatial orientation of specific hydroxyl groups ($-OH$) around asymmetric chiral carbon centers.

• Stereospecificity of Biological Enzymes:

  • Human metabolic enzymes are evolutionarily adapted to recognize and catalyze reactions exclusively with naturally occurring right-handed ($D$-form) sugars.

  • Digestive and cellular glycolytic enzymes possess chiral active sites that physically bind right-handed sugars; they fail to recognize or bind left-handed ($L$-form) sugars.

• Clinical Application in Artificial Sweeteners for Diabetes Management:

  • Left-handed ($L$-) sugars physically interact with sweetness taste receptors on the human tongue, eliciting the identical sensory perception of sweetness as normal sugars.

  • Because human metabolic enzymes lack the capacity to hydrolyze, process, or metabolize $L$-sugars, these molecules transit through the metabolic system intact without undergoing breakdown.

  • Provide zero metabolic calories and trigger zero postprandial elevation in blood glucose levels, serving as non-caloric artificial sweetening agents suitable for diabetic individuals.

Proteins: Amino Acid Architecture and Peptide Bond Mechanics

• Considered the "first rank" of biomolecules due to their structural, catalytic, and regulatory importance across all living cells.

• Elemental Composition: Always composed of Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), and Nitrogen ($N$); frequently contain Sulfur ($S$). The high concentration of nitrogen defines them chemically as nitrogenous organic compounds.

• Natural Occurrences: Construct muscle mass, skin, hair, nails, claws, avian feathers, meat, pulses, and lentils; present as essential structural components even within viral capsids.

• Fundamental Subunits: Amino Acids:

  • Proteins are linear polymers assembled from 20 standard amino acid monomers.

  • Structural Anatomy of an Amino Acid:

  • A central tetrahedral carbon atom termed the alpha-carbon ($\alpha\text{-carbon}$).

  • A basic amino group ($-NH_2$) acting as a base.

  • An acidic carboxylic acid group ($-COOH$) acting as an acid.

  • A single hydrogen atom ($-H$).

  • A variable chemical side-chain designated as the radical or $R$ group.

  • The 20 standard amino acids share this identical molecular backbone; functional individuality and chemical properties are determined entirely by the identity of the variable $R$ group:

  • In Glycine: The $R$ group is a simple hydrogen atom ($-H$).

  • In Alanine: The $R$ group is a methyl group ($-CH_3$).

  • In Serine: The $R$ group is a hydroxymethyl group ($-CH_2OH$).

• Mechanism of Peptide Bond Formation:

  • Monomeric amino acids polymerize via condensation reactions yielding covalent peptide bonds (peptide linkages).

  • Reaction Chemistry: The basic amino group ($-NH_2$) of one amino acid reacts with the acidic carboxylic acid group ($-COOH$) of an adjacent amino acid with the concurrent elimination of water ($H_2O$).

  • The resulting covalent bond links the carbonyl carbon to the amide nitrogen ($-CO-NH-$).

  • The resulting dipeptide possesses an intact, chemically reactive free amino terminus ($N\text{-terminus}$) at one end and a free carboxylic acid terminus ($C\text{-terminus}$) at the other.

  • These free reactive ends sequentially condense with additional incoming amino acids, yielding tripeptides, tetrapeptides, and extensive polypeptide chains.

• Catabolic Hydrolytic Breakdown of Proteins:

  • Polypeptides undergo enzymatic hydrolytic cleavage through the progressive re-addition of water molecules:     $\text{Proteins/Polypeptides} \rightarrow \text{Peptones (fragments } > 10 \text{ amino acids)} \rightarrow \text{Peptides} \rightarrow \text{Free Amino Acids}$

Amino Acid Sequencing, Hemoglobin Organization, and Sickle Cell Pathology

• Significance of Primary Amino Acid Sequence:

  • The specific linear sequence of amino acids dictates the precise folding, three-dimensional conformation, and biological functionality of a protein.

  • Sequence data is encoded by instructions stored within cellular DNA, transcribed into messenger RNA, and translated by ribosomes.

  • Even minor deviations or substitutions in the sequence can destabilize the protein, destroying its biological function.

• Frederick Sanger's Landmark Sequencing of Insulin:

  • Frederick Sanger was the first scientist to determine the complete, precise amino acid sequence of a functional protein, analyzing bovine insulin.

  • Insulin comprises a total of 51 amino acids distributed across two distinct polypeptide chains:

  • Alpha chain: Contains 21 amino acids.

  • Beta chain: Contains 30 amino acids.

  • Inter-chain Linkage: The two chains are covalently cross-linked and held together by two inter-chain disulfide bonds ($-S-S-$).

• Quaternary Structure of Hemoglobin:

  • Hemoglobin is a dynamic tetrameric transport protein composed of 4 coordinated polypeptide subunits:

  • Two identical alpha chains (containing 141 amino acids each; noted in the lecture as 151).

  • Two identical beta chains (containing 146 amino acids each).

  • Complete molecule contains an aggregate total of 574 amino acids.

• Sickle Cell Anemia as a Model of Sequence Variation:

  • A genetic disease resulting from the substitution of a single amino acid residue within the hemoglobin tetramer.

  • Molecular Defect: At position 6 of the beta globin chain, normal hydrophilic glutamic acid is erroneously replaced by hydrophobic valine.

  • Pathological Consequences: Under low oxygen tension, abnormal hemoglobin molecules aggregate into rigid, insoluble crystalline fibers. These distort normal flexible, biconcave red blood cells into stiff, fragile, sickle-shaped crescents.

  • Clinical Manifestations: Sickled erythrocytes fail to transit narrow capillaries, causing microvascular occlusions, severe localized tissue hypoxia, vascular crises, and death.

Structural and Functional Classification of Proteins

• Structural Classification (Based on Overall Molecular Shape):

  • Fibrous Proteins:

  • Organized as extended, elongated filamentous strands or sheets.

  • Typically rely on secondary protein structures (spiral $\alpha\text{-helices}$ or intertwined sheets).

  • Physically insoluble in water, non-crystalline, and exhibit high tensile elasticity and mechanical toughness.

  • Function primarily as structural frameworks across tissues.

  • Examples: Keratin (hair, nails, epidermal skin), Collagen (bones, teeth, tendons, cartilage), Elastin (elastic arteries and connective tissues), Myosin and Actin (contractile muscle myofibrils), Silk and Spider Web Fibers (fibroin), Fibrin (insoluble structural mesh of blood clots).

  • Globular Proteins:

  • Folded into compact, spherical, or ovoid three-dimensional conformations.

  • Rely on complex tertiary and quaternary protein structures (super-coiled polypeptide arrangements).

  • Soluble in aqueous solutions, including solutions of mineral salts, mild acids, bases, or ethanol; can be stabilized and crystallized.

  • Function in mobile, highly catalytic, and dynamic metabolic roles.

  • Examples: Metabolic Enzymes, Plasma Antibodies (immunoglobulins), Protein Hormones, Hemoglobin, Myoglobin, Serum Albumin.

• Specific Structural Proteins and Architectural Roles:

  • Actin and Myosin: Filamentous proteins localized in muscle sarcomeres responsible for mechanical contractile work.

  • Collagen: Imparts tensile strength and structural elasticity to the skin, cartilage, tendons, teeth, and skeletal bones.

  • Elastin: Imparts flexible distensibility to elastic arterial walls, lungs, and mobile tissues.

  • Tubulin: Polymerizes into cylindrical microtubules responsible for constructing the mitotic spindle apparatus during cell division and organizing the cytoskeleton.

  • Titin: Provides elastic resistance and structural centering for thick myosin and thin actin filaments in muscle sarcomeres.

  • Extracellular Adhesives: Specialized sticky proteins produced by certain aquatic insects to anchor protective cases or webs securely to submersed rocks.

  • Keratin: Durable, sulfur-rich fibrous protein constructing animal hair, fingernails, hooves, and outer epidermal layers.

• Functional Classification of Dynamic Proteins:

  • Digestive Enzymes: Catalytic proteins that accelerate hydrolytic digestion of dietary nutrients:

  • Amylase: Hydrolyzes complex starch into oligosaccharides and maltose.

  • Lipase: Cleaves ester bonds within lipids and fats.

  • Pepsin and Trypsin: Endopeptidases that hydrolyze peptide bonds within complex proteins.

  • Transport Proteins:

  • Hemoglobin: Binds and transports molecular oxygen ($O_2$) and carbon dioxide ($CO_2$) within circulating blood.

  • Serum Albumin: Regulates colloid osmotic (oncotic) pressure in blood plasma, preventing fluid leakage into interstitial tissue spaces and lymph conduits (which would cause severe systemic edema and tissue inflammation); also binds and transports hydrophobic vitamins, fatty acids, and hormones.

  • Hormonal Regulators:

  • Insulin: Pancreatic hormone governing glucose uptake and glycemic balance.

  • Thyroid-associated Hormones: Govern basal metabolic rates and cellular respiration.

  • Immune Defense Proteins:

  • Immunoglobulins (Antibodies): Bind to and neutralize foreign viral and bacterial antigens.

  • Interferons: Cytokines released by virus-infected cells to induce systemic anti-viral resistance in adjacent uninfected cells.

  • Nutrient and Embryonic Storage Proteins:

  • Ovalbumin (Albumen): The nutrient-rich white fluid surrounding avian egg yolks, providing amino acids, water, and protective buffering to the developing avian embryo.

  • Legume Storage Proteins: Dense nutrient reserves stored inside cotyledons of leguminous seeds (such as peas), fueling embryonic germination prior to photosynthetic autonomy.

Lipids: Acylglycerols, Fatty Acid Chemistry, and Energy Storage

• Definition and General Characteristics:

  • A chemically heterogeneous class of biological molecules unified by their physical insolubility in water (hydrophobic nature) and solubility in non-polar organic solvents (diethyl ether, ethanol, chloroform, benzene).

  • Coined by Bloor in 1943 to define water-insoluble, organically soluble biological compounds.

• Elemental Composition and Caloric Yield:

  • Composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$).

  • Differ from carbohydrates by possessing an exceptionally low proportion of oxygen relative to carbon and hydrogen.

  • Model Molecular Formula: Stearin (a representative neutral fat) possesses the empirical formula $C_{57}H_{110}O_6$, illustrating this severe oxygen deficit relative to carbon.

  • Caloric Density: Because of their high density of reduced carbon-hydrogen bonds ($C-H$), lipids yield more than twice the chemical energy per gram ($\approx 9\,\text{kcal/g}$) upon oxidation compared to carbohydrates ($\approx 4\,\text{kcal/g}$).

• Acylglycerols (Fats and Oils):

  • Neutral esters formed between the trihydroxy alcohol glycerol and unbranched fatty acid chains.

  • Glycerol: A three-carbon alcohol containing three separate hydroxyl groups ($\text{trihydroxy alcohol}$), with one $-OH$ group bound to each carbon atom.

  • Fatty Acids: Long aliphatic hydrocarbon chains terminating in a carboxylic acid group ($-COOH$).

  • Esterification (Condensation):

  • Occurs via dehydration synthesis: each of the three hydroxyl groups on glycerol condenses with the carboxylic acid group of a fatty acid, eliminating 3 water molecules.

  • The resulting covalent linkages linking the chains are ester bonds ($-COO-$).

  • The resulting molecule is a neutral triglyceride (triacylglycerol).

  • The compound is chemically neutral because the polar, charged functional groups of both glycerol and fatty acids are completely eliminated during ester bond formation.

• Saturated vs. Unsaturated Acylglycerols:

  • Saturated Acylglycerols (Fats):

  • Fatty acid chains lack double bonds between carbon atoms ($C-C$ single bonds only); carbon valencies are fully saturated with hydrogen atoms.

  • Chains pack tightly together, maintaining semi-solid or solid states at room temperature.

  • Predominantly of animal origin (e.g., beef tallow, stearin).

  • Unsaturated Acylglycerols (Oils):

  • Fatty acid chains contain one or more double bonds ($C=C$) along the hydrocarbon backbone.

  • Double bonds introduce physical kinks into the chains, preventing close molecular packing; remain liquid at standard room temperatures.

  • Predominantly of plant origin, extracted from seeds and fruits (e.g., linolenic acid / linolein).

• Physiological Energy Storage:

  • Lipids serve as the primary long-term metabolic energy depot in mammals.

  • An average human body stores approximately $16\,\text{kg}$ of adipose fat tissue.

  • Complete oxidation of this $16\,\text{kg}$ adipose reserve yields approximately $1.44 \times 10^5\,\text{kcal}$ of metabolic energy ($01:44 \times 10^3\,\text{kilo calories}$ in lecture recording), providing a substantial energy reserve.

Phospholipids, Biological Membranes, and Waxes

• Phospholipids (Phosphatids):

  • Structural Derivatives of Phosphatidic Acid: Consist of one glycerol molecule, two fatty acid hydrocarbon chains, a phosphoric acid group, and an attached nitrogenous base (typically choline).

  • Molecular Architecture:

  • Formed like an acylglycerol, except the third hydroxyl position on glycerol is esterified to a phosphate group rather than a third fatty acid.

  • Choline (a positively charged nitrogenous compound) attaches directly to the phosphate group.

  • Amphipathic Character:

  • Polar Hydrophilic Head: Formed by the ionized phosphate and choline groups; exhibits high affinity for water ("water-loving").

  • Non-Polar Hydrophobic Tails: Formed by the two insoluble, non-polar fatty acid chains; repelled by water ("water-fearing").

  • Bilayer Organization in Biological Membranes:

  • In aqueous cellular environments, phospholipids spontaneously self-assemble into a continuous double-layered sheet (the lipid bilayer).

  • The polar hydrophilic heads point outward toward the intra- and extracellular aqueous fluids.

  • The hydrophobic fatty acid tails sequester inward, facing each other to create a non-polar central permeability barrier.

  • Present across all biological membranes: plasma membranes, the nuclear envelope, and inner/outer membranes of mitochondria and chloroplasts.

  • Regulation of Membrane Permeability: The length and degree of chemical saturation within the fatty acid tails govern membrane fluidity and selective permeability, controlling the transmembrane transport of ions and polar metabolites.

• Waxes:

  • Chemical Composition: Natural esters formed by the condensation of a long-chain fatty acid with a high-molecular-weight, long-chain monohydroxy alcohol (e.g., triacontanol).

  • Physical Characteristics: Completely non-polar, hydrophobic, chemically inert, non-reactive, and resistant to atmospheric oxidation.

  • Biological Distribution and Functions:

  • Plants: Secreted over aerial stems, flower stalks, fruit skins, and leaf surfaces as a protective, water-repellent layer termed the cuticle (composed of cutin).

  • Prevents excessive transpirational water loss, mechanical desiccation, and fungal or microbial invasion.

  • Animals: Secreted as protective, water-repellent coatings on mammalian skin, animal fur, and avian feathers (preen gland waxes); constructed by insects (such as beeswax from honeybees).

  • Synthetic and Commercial Waxes:

  • Industrial synthetic waxes are manufactured from refined petroleum distillates or polyethylene polymers.

  • Utilized as stable machine lubricants.

  • Historical Marine Source: Spermaceti wax harvested from the cranial spermaceti organ of the sperm whale.

Terpenoids: Terpenes, Steroids, Carotenoids, and Prostaglandins

• Terpenoids constitute a major, structurally diverse class of biological lipids built from repeating five-carbon isoprene (isoprenoid) units.

• Isoprene Building Block: Consists of a five-carbon hydrocarbon with eight hydrogen atoms ($C_5H_8$).

• Major Terpenoid Subclasses:

  • Terpenes:

  • Volatile, low-molecular-weight aromatic lipids formed from small numbers of condensed isoprenoid units (e.g., diterpenes, triterpenes).

  • Responsible for characteristic plant scents and essential oils; widely extracted for commercial perfumes and flavorings.

  • Characteristic Examples:

    • Myrcene: Extracted from bay oil.

    • Geraniol: Extracted from rose oil.

    • Limonene: Extracted from citrus and lemon oils.

    • Menthol: Volatile terpene extracted from peppermint oil.

  • Metabolic Intermediates: Serve as precursors for synthesizing fat-soluble Vitamin A, phytol chains of chlorophyll, commercial rubber, and natural plant latex.

  • Steroids:

  • High-molecular-weight lipids characterized by a condensed, four-ring steroid nucleus consisting of 17 carbon atoms arranged as three hexagonal (six-sided) rings and one pentagonal (five-sided) ring.

  • Biological individuality is determined by the identity, placement, and oxidation state of functional side groups attached to this 17-carbon core.

  • Cholesterol: The central animal steroid, intercalating into cell membranes to regulate physical fluidity; serves as the metabolic precursor for synthesizing all other steroid hormones.

  • Endocrine Steroid Hormones: Synthesized directly from cholesterol:

    • Testosterone: Primary male androgenic steroid hormone.

    • Progesterone and Estrogen: Female reproductive and gestational steroid hormones.

  • Carotenoids:

  • Polyterpenoid pigments formed from extended conjugated chains of isoprenoid units, terminating in cyclical hydrocarbon rings at one or both ends.

  • Function as light-harvesting accessory pigments and photoprotective agents in photosynthetic tissues, imparting vibrant yellow, orange, red, and brown colors.

  • Examples: Carotenes, xanthophylls, and structural complexes associated with chlorophyll, cytochromes, and phytochromes.

  • Prostaglandins:

  • Biologically active, hormone-like lipid derivatives produced locally within mammalian tissues in response to mechanical trauma, tissue injury, or infection.

  • Autocrine and Paracrine Functions:

    • Amplify and sustain localized inflammatory cascades.

    • Sensitize peripheral nociceptors, amplifying the perception of pain.

    • Mediate local vasodilation, increasing microvascular blood flow to damaged sites.

    • Promote platelet aggregation and activate blood coagulation cascades.

    • Stimulate uterine smooth muscle contractions during female labor and childbirth.

  • Pharmacological Action of Aspirin:

  • Aspirin (acetylsalicylic acid) acts as an inhibitor of the enzymatic synthesis of prostaglandins.

  • By blocking prostaglandin production, aspirin lowers systemic fever (antipyretic action), alleviates pain (analgesic action), and diminishes localized swelling and inflammation.

Nucleic Acids: Structural Nucleotides, High-Energy Carriers, and Coenzymes

• Historical Discovery:

  • First isolated in 1869 by the Swiss physician Friedrich Miescher from the nuclei of white blood cells present in discarded surgical pus.

  • Named "nuclein" due to its exclusive localization within cellular nuclei.

  • Later renamed "nucleic acid" upon demonstrating its strongly acidic properties, which stem from its ionized phosphate groups.

• Biological Distribution and Primary Classes:

  • Present universally within all living organisms, from complex eukaryotes to bacteria, and packaged inside viral capsids.

  • Comprises two primary classes of unbranched linear polynucleotides:

  • Deoxyribonucleic Acid (DNA): Built from repeating deoxyribonucleotide monomers.

  • Ribonucleic Acid (RNA): Built from repeating ribonucleotide monomers.

• Anatomy of a Nucleotide:

  • A nucleotide is composed of three covalently linked chemical components:

  1. A five-carbon pentose sugar (ribose in RNA; deoxyribose in DNA).

  2. A heterocyclic nitrogenous base bonded to carbon-1 ($C-1$) of the pentose sugar.

  3. An inorganic phosphate group esterified to carbon-5 ($C-5$) of the pentose sugar.

  • Nucleoside vs. Nucleotide:

  • Nucleoside: A two-part molecular complex composed exclusively of a pentose sugar bonded to a nitrogenous base, lacking an attached phosphate group.

  • Nucleotide: Formed when a nucleoside is phosphorylated via an ester bond at the $C-5$ position of its pentose sugar ($\text{Nucleotide} = \text{Nucleoside} + \text{Phosphate Group}$.

• Heterocyclic Nitrogenous Bases:

  • Purines: Larger nitrogenous bases featuring a bicyclic (double-ring) structure consisting of Adenine ($A$) and Guanine ($G$).

  • Pyrimidines: Smaller nitrogenous bases featuring a monocyclic (single-ring) structure consisting of Cytosine ($C$), Thymine ($T$), and Uracil ($U$).

  • Base Complement of DNA vs. RNA:

  • DNA contains: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Thymine ($T$).

  • RNA contains: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Uracil ($U$) (Thymine is absent in RNA and replaced by Uracil).

• Mononucleotides as Intracellular Energy Carriers (ATP and ADP):

  • Nucleotides also operate as isolated mononucleotides that mediate cellular energy transactions.

  • Adenosine Triphosphate (ATP):

  • Composed of the purine base adenine linked to a ribose sugar (forming the nucleoside adenosine) and three sequentially attached phosphate groups.

  • Adenosine Diphosphate (ADP) carries two phosphate groups; Adenosine Monophosphate (AMP) carries one.

  • Hydrolytic cleavage of the terminal phosphoanhydride bond converting ATP into ADP releases $7.3\,\text{kcal/mol}$ ($31.81\,\text{kJ/mol}$) of free energy:       $\text{ATP} + H_2O \rightarrow \text{ADP} + P_i + 7.3\,\text{kcal/mol}$

  • This liberated energy directly powers endergonic cellular work, including protein, carbohydrate, and lipid biosynthesis; active transport of ions against concentration gradients; muscle contraction; cytoplasmic streaming (cyclosis); flagellar and ciliary beating; and mitotic chromosome movement.

• Dinucleotides as Oxidation-Reduction Coenzymes (NAD and FAD):

  • Formed by two complete nucleotide units joined through their phosphate groups via covalent bonds.

  • Nicotinamide Adenine Dinucleotide (NAD):

  • Incorporates the vitamin nicotinamide linked to an adenosine nucleotide core.

  • Functions as an essential electron-carrying coenzyme for dehydrogenase enzymes in cellular respiration.

  • During redox reactions, it accepts two electrons and one proton to become reduced to NADH:       $\text{NAD}^+ + 2e^- + 2H^+ \rightarrow \text{NADH} + H^+$

  • Flavin Adenine Dinucleotide (FAD / FADH2) functions analogously as an essential electron-shuttling redox dinucleotide.

• Phosphodiester Backbone of Polynucleotides:

  • Linear polynucleotide chains are assembled by linking adjacent nucleotides via covalent phosphodiester bonds.

  • Bond Formation: The phosphate group attached to carbon-5 ($C-5$) of one pentose sugar condenses with the free hydroxyl group attached to carbon-3 ($C-3$) of the preceding pentose sugar, eliminating a water molecule.

  • Assembles a continuous, alternating sugar-phosphate backbone that confers an intrinsic directional polarity ($5'\rightarrow 3'$ orientation) along the nucleic acid strand.

DNA Architecture, Genetic Information Flow, and RNA Diversity

• Watson-Crick Double Helical Model of DNA:

  • Elucidated by James Watson and Francis Crick, revealing DNA as a right-handed, double-helical structure composed of two polynucleotide strands.

  • Antiparallel Directionality: The two strands run in opposite physical directions; one strand is oriented in the $5'\rightarrow 3'$ direction, while its complementary partner runs in the $3'\rightarrow 5'$ direction.

  • Spatial Architecture:

  • The structural outer uprights ("rails") are formed by the alternating hydrophilic deoxyribose-phosphate backbone.

  • The internal horizontal steps ("rungs") are formed by paired nitrogenous bases oriented perpendicular to the helical axis.

  • Complementary Base Pairing:

  • Nitrogenous bases pair across strands via specific hydrogen bonds.

  • Adenine always pairs with Thymine ($A=T$) via precisely 2 hydrogen bonds.

  • Guanine always pairs with Cytosine ($G\equiv C$) via precisely 3 hydrogen bonds.

  • Hydrogen bonds can be reversibly denatured and separated (strand unzipping) and reannealed without damaging the covalent sugar-phosphate backbones.

  • Physical Dimensions of the Helix:

  • The uniform cross-sectional diameter of the double helix measures $20\,\text{Å}$ ($20\,\text{Angstroms}$).

  • One complete $360^\circ$ helical turn occurs every 10 base pairs.

• The Gene Concept and the Central Dogma of Molecular Biology:

  • A gene is a specific functional locus and nucleotide sequence within chromosomal DNA encoding instructions for synthesizing a single polypeptide chain or functional RNA.

  • Genes govern phenotypic biological characters (such as eye color, hair texture, body height, and facial features) by encoding functional enzymatic and structural proteins that direct development.

  • Two-Step Flow of Genetic Information:

  • Transcription: Genetic sequence data encoded within nuclear DNA is copied into a mobile, single-stranded messenger RNA (mRNA) intermediate.

  • Translation: The mRNA translocates out of the nucleus into the cytoplasm, binding to ribosomes ("the protein factories"). Transfer RNA (tRNA) and ribosomal RNA (rRNA) decode the mRNA sequence, linking amino acids into a functional protein.

• Classification, Structure, and Functions of RNA:

  • General Features: A single-stranded polynucleotide composed of ribose sugars, a phosphate-sugar backbone, and the nitrogenous bases Adenine, Guanine, Cytosine, and Uracil.

  • Three Major RNA Species:

  • Messenger RNA (mRNA):

    • Linear, single-stranded transcript whose length varies with the size of the structural gene transcribed.

    • Represents $3\%$ to $4\%$ of total cellular RNA.

    • Carries the genetic message from nuclear DNA organized as a sequence of three-base genetic units termed codons.

  • Transfer RNA (tRNA):

    • The smallest RNA species, consisting of only 70 to 90 nucleotides.

    • Represents $10\%$ to $20\%$ of total cellular RNA.

    • Adopts a folded cloverleaf secondary structure maintained by localized, internal complementary base pairing.

    • Carries a specific three-base anticodon loop that base-pairs with the complementary codon on mRNA during translation.

    • Transports specific activated amino acids from the cytosol to the ribosomal assembly site.

    • Approximately 60 distinct tRNA species exist globally; the human genome encodes 45 distinct functional tRNA varieties.

  • Ribosomal RNA (rRNA):

    • The largest, structurally predominant RNA class, constituting up to $80\%$ of total cellular RNA.

    • Transcribed in the nucleolus and complexes with ribosomal proteins to assemble the small and large ribosomal subunits.

    • Catalyzes peptide bond formation (peptidyl transferase activity) between adjacent amino acids during protein synthesis.

Conjugated Molecules and Biological Functions

• Conjugated molecules are composite macromolecules formed by the stable chemical union of two distinct biomolecular classes, acting together as an integrated functional unit.

• Glycolipids (Cerebrosides):

  • Covalent complexes of carbohydrates (oligosaccharide chains) with lipids.

  • Cerebrosides are heavily concentrated in the white matter of the brain and throughout the central nervous system.

  • Major structural components of the protective myelin sheath insulating neuronal axons, accelerating saltatory nerve impulse conduction.

  • Present within outer leaflets of cellular plasma membranes and photosynthetic chloroplast lamellae.

• Glycoproteins (Mucoids):

  • Covalent conjugates formed when oligosaccharide or polysaccharide chains link to structural proteins.

  • Synthesized extensively as mucoids within viscous mucus secretions lining the respiratory tract, oral cavity, and gastrointestinal epithelium, protecting surfaces from mechanical shear and chemical degradation.

  • Integral Membrane Roles: Project outward from plasma membranes as cell-surface receptors, cellular adhesion molecules, transport complexes, and major histocompatibility/blood group antigens (determining ABO blood group specificity on human erythrocytes).

  • Found dissolved in the nutrient-dense albumen of avian egg whites and functioning as systemic peptide hormones (e.g., gonadotropins, thyroid-stimulating hormone).

• Lipoproteins:

  • Biochemical assemblies composed of lipids complexed with carrier apoproteins.

  • Function as circulating lipid transport vehicles, solubilizing and ferrying hydrophobic triacylglycerols, phospholipids, and cholesterol through the aqueous bloodstream.

  • Serve as structural components in the construction of biological membranes, including mitochondrial inner membranes, endoplasmic reticulum, the nuclear envelope, and chloroplast thylakoid membranes; also concentrated within embryonic egg yolk.

• Nucleoproteins:

  • High-molecular-weight complexes formed by the association of nucleic acids (DNA or RNA) with basic structural proteins (histones or protamines).

  • Chromatin and Chromosomes: Eukaryotic chromosomal material consists of double-stranded genomic DNA wrapped around core histone protein octamers.

  • Ribosomes: Intracellular ribonucleoprotein complexes composed of ribosomal RNA (rRNA) structurally interwoven with ribosomal proteins to constitute functional translation machinery.