Class 4 - carbon

Physicochemical Properties of Water and Hydrogen Bonding

• Physical Basis of Water's Biological Significance:

  • Water is the fundamental medium in which all terrestrial biochemical processes occur.

  • Molecular polarity and hydrogen-bonding networks give water anomalous thermodynamic, mechanical, and solvent characteristics critical to cellular life.

• Strict Structural Criteria for Hydrogen Bonding:

  • Partially Positive Hydrogen Donor: A hydrogen atom must carry a partial positive charge ($\delta^+$) resulting from a polar covalent bond to a strongly electronegative element (such as oxygen, $O$, or nitrogen, $N$).

  • Electronegative Acceptor Atom: A terminal, highly electronegative atom must carry a partial negative charge ($\delta^-$) resulting from a covalent bond with a significantly less electronegative element (exemplified by the carbonyl oxygen in $C=O$ or fluorine in $H-F$).

  • Spatial Proximity: Both interacting functional groups must reside within approximately one atomic diameter of one another for an electrostatic hydrogen bond to successfully assemble.

• Core Emergent Properties of Water:

  • Polarity and Extensive Hydrogen Bonding: Unequal sharing of valence electrons between oxygen and hydrogen produces a bent dipole ($104.5^\circ$ bond angle) where oxygen is partially negative ($\delta^-$) and each hydrogen is partially positive ($\delta^+$).

  • Universal/Excellent Solvent: Capable of dissolving high concentrations of polar compounds and ionic lattices by forming hydration cages.

  • Chemical Reactivity: Acts directly as a chemical reactant or product in metabolic dehydration and hydrolysis pathways.

  • High Specific Heat Capacity: Absorbs or dissipates significant thermal energy with minimal fluctuations in temperature, stabilizing cellular and environmental thermal profiles.

  • High Heat of Vaporization: Requires substantial thermal input to break extensive hydrogen bonding during phase transition, enabling evaporative cooling mechanisms.

  • Cohesion and Adhesion: Cohesive intermolecular hydrogen bonding creates high surface tension; adhesive interactions between water and polar surfaces drive capillary transport.

  • Density Anomaly (Ice Floats): Below $4^\circ C$, water molecules expand into an open crystalline hexagonal lattice, rendering solid ice less dense than liquid water and insulating aquatic biomes beneath frozen surface layers.

  • Optical Transparency: Allows photosynthetically active radiation (PAR) to penetrate aquatic depths, driving primary metabolic production.

• Water as a Direct Metabolic Participant:

  • Metabolism consists of organized sequences of enzyme-catalyzed chemical reactions that form and cleave bonds to extract, transfer, and store free energy.

  • The Citric Acid Cycle (Krebs cycle) illustrates water's indispensable role as a direct substrate, reagent, and product:

  • Pyruvate Dehydrogenase: Converts pyruvate to Acetyl-CoA ($\text{CH}_3\text{CO-S-CoA}$) with the participation of $H_2O$, $CoASH$, and $NAD^+$, releasing $CO_2$, $NADH$, and hydronium ions ($H_3O^+$).

  • Citrate Synthetase: Catalyzes the condensation of oxaloacetate and acetyl-CoA, consuming water ($H_2O$) to yield citrate, free $CoASH$, and $H_3O^+$.

  • Aconitase: Converts citrate into cis-aconitate through dehydration (extracting $H_2O$), followed immediately by rehydration (incorporating $H_2O$) to synthesize isocitrate.

  • Isocitrate Dehydrogenase: Oxidizes isocitrate to oxalosuccinate and decarboxylates it to $\alpha$-ketoglutarate, consuming $H_2O$ alongside $NAD^+$ to yield $CO_2$, $NADH$, and $H_3O^+$.

  • $\alpha$-Ketoglutarate Dehydrogenase: Oxidatively decarboxylates $\alpha$-ketoglutarate into succinyl-CoA, releasing $CO_2$, $NADH$, and $H_3O^+$ in a process that consumes $H_2O$, $CoASH$, and $NAD^+$.

  • Succinyl-CoA Synthetase: Cleaves succinyl-CoA into succinate and $CoASH$, driving substrate-level phosphorylation of $GDP + P_i$ into $GTP$.

  • Succinate Dehydrogenase: Oxidizes succinate into fumarate via $FAD \rightarrow FADH_2$.

  • Fumarase: Performs a hydration reaction across the double bond of fumarate by adding a water molecule ($H_2O$) to generate L-malate.

  • Malate Dehydrogenase: Catalyzes the oxidation of L-malate back to oxaloacetate using $NAD^+$ and $H_2O$, producing $NADH$ and generating an $H_3O^+$ ion to complete the metabolic cycle.

Solutions, Solubility, and Hydration Mechanics

• Fundamental Terminology of Aqueous Systems:

  • Solution: A completely homogeneous liquid mixture comprising two or more distinct chemical substances.

  • Solvent: The dissolving agent of a solution; present in the greatest volume.

  • Solute: The substance that is dissolved within the solvent.

  • Aqueous Solution: Any solution where water functions specifically as the solvent.

• Ionic Dissolution and Hydration Shell Architecture:

  • When a crystalline ionic compound (e.g., table salt, $NaCl$) is introduced into water, polar water molecules break the electrostatic ionic bonds between the solute ions.

  • Hydration Shell: A concentric sphere of water molecules oriented around an individual dissolved ion:

  • Cations ($Na^+$): Surrounded by the partially negative oxygen atoms ($\delta^-$) of adjacent water molecules.

  • Anions ($Cl^-$): Surrounded by the partially positive hydrogen atoms ($\delta^+$) of water molecules.

• Solvation of Macromolecules and Proteins:

  • Water-soluble proteins maintain their functional native conformation due to thermodynamic distribution of polar and nonpolar amino acid residues:

  • Hydrophilic Exterior: Charged and polar amino acid side chains occupy the outer surface of the protein.

  • Water's partially negative oxygen atoms ($\delta^-$) form hydrogen bonds with partially positive or full positive regions on the protein surface.

  • Water's partially positive hydrogen atoms ($\delta^+$) form hydrogen bonds with partially negative or full negative charges on the protein surface.

  • Hydrophobic Interior: Nonpolar, uncharged amino acid residues are sequestered away from the surrounding aqueous cytoplasm into the core of the protein matrix, driving tertiary folding.

Hydrophobic Interactions and Membrane Assembly

• Physical Principles of Solute-Water Compatibility:

  • Hydrophilic Substances: Compounds possessing ionic bonds or polar covalent bonds have a high electrostatic affinity for water molecules. Because water molecules are polar, they interact readily with other charged or partially charged atoms.

  • Hydrophobic Substances: Compounds dominated by nonpolar covalent bonds (such as hydrocarbons) lack ionic charges and dipole moments. Because water molecules are polar and self-associate via extensive hydrogen bonding, they cannot interact electrostatically with uncharged atoms, causing hydrophobic molecules to cluster together.

• Spontaneous Self-Assembly of Biological Membranes:

  • Phospholipids are amphipathic molecules composed of:

  • Hydrophilic Polar Head: A negatively charged phosphate group ($-OPO_3^{2-}$) linked to a glycerol backbone ($\text{CH}_2-\text{CH}-\text{CH}_2$), which directly contacts extracellular and intracellular aqueous environments.

  • Hydrophobic Fatty Acid Tails: Two nonpolar hydrocarbon chains (one saturated fatty acid and one cis-unsaturated fatty acid with a kinked double bond).

  • In an aqueous environment, phospholipids spontaneously assemble into a continuous phospholipid bilayer:

  • The hydrophilic heads face outward toward the aqueous cytosol and extracellular fluid.

  • The hydrophobic tails face inward toward each other, shielding their nonpolar chains from contact with water.

  • This self-assembly creates a stable hydrophobic barrier essential for cellular compartmentalization, enclosing organelles (nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes) and preventing the free diffusion of polar and charged cellular constituents.

Carbon Chemistry and Molecular Diversity

• The Chemical Basis of Life as Carbon-Based:

  • Organic chemistry encompasses the study of carbon-containing compounds, regardless of their natural or synthetic origin.

  • Carbon functions as the foundational backbone for biological macromolecules due to its unique tetravalent bonding capacity, enabling vast molecular diversity.

• Valence Configurations of the Primary Elements of Life:

  • The chemical reactivity and bonding patterns of atoms are dictated by valence electrons:

  • Hydrogen ($H$): $1$ valence electron; requires $1$ electron to fill its shell; forms $1$ covalent bond (valence = $1$).

  • Oxygen ($O$): $6$ valence electrons; requires $2$ electrons to fill its shell; forms $2$ covalent bonds (valence = $2$).

  • Nitrogen ($N$): $5$ valence electrons; requires $3$ electrons to fill its shell; forms $3$ covalent bonds (valence = $3$).

  • Carbon ($C$): $4$ valence electrons; requires $4$ electrons to fill its shell; forms $4$ covalent bonds (valence = $4$).

• Three-Dimensional Geometry of Carbon Bonds:

  • Tetrahedral Geometry (Single Bonds): When a carbon atom forms single covalent bonds with four other atoms, hybrid orbitals adopt a tetrahedral geometry with bond angles of approximately $109.5^\circ$:

  • Methane ($CH_4$): A single carbon atom bound to four hydrogens, forming a standard regular tetrahedron.

  • Ethane ($C_2H_6$): Two tetrahedrons conjoined via a central single carbon-carbon ($C-C$) bond.

  • Planar Geometry (Double Bonds): When two carbon atoms are joined by a double covalent bond, all atoms bonded to the carbons lie within the same flat geometric plane:

  • Ethene / Ethylene ($C_2H_4$): Contains a $C=C$ double bond, creating an inflexible planar structure.

  • Carbon Dioxide ($CO_2$): Carbon forms two covalent double bonds with two oxygen atoms ($O=C=O$), producing a completely linear, symmetrical molecule.

• Modes of Variation in Carbon Skeletons:

  • Length: Hydrocarbon chains can vary from short chains (ethane, $C_2H_6$) to medium chains (propane, $C_3H_8$) to long extended chains.

  • Branching: Skeletons can be completely unbranched (butane, $C_4H_{10}$) or branched (2-methylpropane / isobutane, $C_4H_{10}$).

  • Double Bond Position: The location of double bonds along the chain can shift (1-butene vs. 2-butene), drastically altering chemical reactivity and mechanical properties.

  • Presence of Rings: Carbon chains can close into ring systems, either fully saturated (cyclohexane, $C_6H_{12}$) or containing alternating resonance double bonds (benzene, $C_6H_6$).

• Hydrocarbons and Cellular Energy Storage:

  • Hydrocarbons: Organic molecules composed strictly of carbon and hydrogen atoms.

  • The covalent bonds linking carbon and hydrogen ($C-H$) are nonpolar because carbon ($2.55$) and hydrogen ($2.20$) possess very similar electronegativities.

  • Hydrocarbons are hydrophobic and insoluble in water.

  • Oxidation of hydrocarbon chains breaks high-energy nonpolar bonds, releasing large amounts of free energy.

  • In mammals, specialized adipose cells pack large fat droplets filled with triacylglycerols (fat molecules with three hydrocarbon fatty acid chains attached to a glycerol backbone), providing dense energy reserves within a $10\,\mu m$ cellular scale.

Prebiotic Chemistry and the Miller Experiment

• Abiotic Synthesis of Organic Compounds:

  • In 1953, Stanley Miller conducted a classic experiment to test whether organic molecules could synthesize spontaneously under conditions simulating the primitive Earth.

  • Apparatus and Methodology:

  • "Sea" Flask: Liquid water ($H_2O$) was heated gently with a Bunsen burner to produce water vapor that circulated through the closed glass system.

  • "Atmosphere" Flask: Contained a simulated prebiotic reducing atmosphere comprising methane ($CH_4$), ammonia ($NH_3$), and hydrogen gas ($H_2$), mixed with incoming water vapor.

  • Electrodes: Introduced continuous electric sparks into the atmosphere flask to simulate atmospheric lightning.

  • Condenser: Circulated continuous cold water around the vapors, causing the gas to cool and condense into artificial "rain".

  • Collection Trap: Liquid carrying newly synthesized compounds collected at the base of the loop for subsequent chemical analysis.

  • Significance and Findings:

  • Miller identified diverse organic molecules, including amino acids (the monomers of proteins) and complex hydrocarbons, demonstrating that the primary chemical foundations of living systems could assemble abiotically.

  • These findings suggest that organic compound synthesis, possibly occurring near volcanoes or hydrothermal vents, was a crucial early phase in the origin of life.

  • Chemical Uniformity Across Life:

  • Across all living species, the fundamental elemental proportions of life—Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Nitrogen ($N$), Sulfur ($S$), and Phosphorus ($P$)—remain remarkably uniform.

Chemical Functional Groups

• Functional Groups in Biological Molecules:

  • Functional groups are specific configurations of atoms commonly attached to the carbon skeletons of organic molecules that participate directly in chemical reactions.

  • Each functional group confers distinct physical and chemical characteristics, determining the biological activity of the overall molecule.

• Properties of the Seven Primary Functional Groups:

  • Hydroxyl Group ($-OH$ or $HO-$):

  • Compound Name: Alcohol (e.g., Ethanol).

  • Properties: Highly polar due to the electronegativity of oxygen; readily forms hydrogen bonds with water molecules, significantly aiding the solubility of organic compounds.

  • Carbonyl Group ($>C=O$):

  • Compound Name: Ketone (when the carbonyl is located within a carbon skeleton, e.g., Acetone) or Aldehyde (when the carbonyl is located at the terminus of a carbon skeleton, e.g., Propanal).

  • Properties: Creates a localized dipole; ketones and aldehydes give rise to major structural isomer variants found in sugars (ketoses and aldoses).

  • Carboxyl Group ($-COOH$):

  • Compound Name: Carboxylic acid, or organic acid (e.g., Acetic acid).

  • Properties: Acts as an acid; can donate a proton ($H^+$) because the covalent bond between oxygen and hydrogen is so polar, yielding an ionized carboxylate ion ($-COO^-$).

  • Amino Group ($-NH_2$):

  • Compound Name: Amine (e.g., Glycine).

  • Properties: Acts as a base; can pick up an ambient $H^+$ from surrounding solution, yielding the positively charged ionized form ($-NH_3^+$).

  • Sulfhydryl Group ($-SH$ or $HS-$):

  • Compound Name: Thiol (e.g., Cysteine).

  • Properties: Two sulfhydryl groups can react covalently, cross-linking and forming a stable disulfide bond ($-S-S-$) that stabilizes protein tertiary and quaternary structures.

  • Phosphate Group ($-OPO_3^{2-}$):

  • Compound Name: Organic phosphate (e.g., Glycerol phosphate).

  • Properties: Contributes a negative charge to the molecule (carrying a $2-$ charge when positioned at the terminus of a chain, or a $1-$ charge when located internally within a chain); possesses high potential energy when bonded in series (as in adenosine triphosphate, ATP).

  • Methyl Group ($-CH_3$):

  • Compound Name: Methylated compound (e.g., 5-Methylcytosine).

  • Properties: Nonpolar and hydrophobic; serves as an epigenetic tag on DNA altering gene expression, and alters the shape and function of sex hormones.

• Functional Group Specificity in Steroid Sex Hormones:

  • Estradiol ($C_{18}H_{24}O_2$) and Testosterone ($C_{19}H_{28}O_2$) are vertebrate steroid sex hormones that share an identical fused four-ring carbon skeleton (three six-carbon rings fused to one five-carbon ring).

  • Biological specificity and anatomical differentiation arise entirely from small differences in attached functional groups:

  • Estradiol: Contains an aromatic A-ring bearing a terminal hydroxyl group ($-OH$) and lacks a methyl group at the ring junction.

  • Testosterone: Contains a non-aromatic ring bearing a carbonyl/ketone group ($=O$) and retains an additional methyl group ($-CH_3$) at carbon-$10$.

Questions & Discussion

• Chemical Bonds in Oils:

  • Question: What kind of bonds are present in oil?

  • A. Polar

  • B. Non-polar

  • Answer: B. Non-polar.

  • Explanation: Oils consist almost exclusively of long hydrocarbon chains. Because carbon and hydrogen share electrons almost equally due to negligible differences in electronegativity, oil molecules are nonpolar and cannot establish stabilizing electrostatic interactions with polar water molecules.

• Identifying the Most Hydrophobic Biological Molecule:

  • Question: Given four biological molecules—(1) a dipeptide carrying a charged carboxylate side group ($-COO^-$), (2) a four-carbon polyol sugar containing multiple hydroxyl groups ($-OH$), (3) an eight-carbon straight-chain aliphatic molecule terminating in an aldehyde, and (4) a purine base (guanine derivative) containing multiple ring nitrogens and amine groups—which molecule is the most hydrophobic, and why?

  • Answer: Molecule 3 is the most hydrophobic.

  • Explanation: Molecule 3 is dominated by an extended nonpolar hydrocarbon chain ($-CH_2-CH_2-CH_2-$) composed strictly of nonpolar $C-C$ and $C-H$ bonds. Molecules 1, 2, and 4 possess abundant polar hydroxyl ($-OH$), amino ($-NH_2$), carbonyl ($C=O$), or charged carboxylate groups that interact readily with water dipoles via hydrogen bonding and electrostatic attractions.

• Comparative Polarity and Hydrophobicity of Methanol and Ethane:

  • Question 1: Which molecule(s) are polar among methanol ($CH_3OH$), ethane ($C_2H_6$), and water ($H_2O$)?

  • Answer: Methanol and water.

  • Question 2: Which molecule(s) are nonpolar?

  • Answer: Ethane.

  • Question 3: Which molecule is "water-fearing" or hydrophobic?

  • Answer: Ethane.

  • Question 4: Which molecule is "water-loving" or hydrophilic?

  • Answer: Methanol.

  • Question 5: What is the fundamental relationship between molecular polarity and hydrophobicity?

  • Answer: In general, nonpolar molecules are hydrophobic because they lack the ability to form hydrogen bonds or electrostatic dipole-dipole attractions with water; polar and charged molecules are hydrophilic because their partial charges interact favorably with water dipoles.

• Hydrophilic vs. Hydrophobic Classification of Core Biomolecules:

  • Molecular Evaluations:

  • Compound A (Ribose/Deoxyribose Pentose Sugar Ring): Hydrophilic. Dominated by multiple polar hydroxyl groups ($-OH$) and a polar ring oxygen atom, which form hydrogen bonds with water.

  • Compound B (Serine Amino Acid): Hydrophilic. Contains polar and charged functional groups, including an amino group ($-NH_2$), a carboxyl group ($-COOH$), and a polar hydroxyl side chain ($-CH_2OH$).

  • Compound C (Cytidine Diphosphate Nucleotide): Hydrophilic. Possesses a ribose sugar, a nitrogenous cytosine ring, and a negatively charged diphosphate moiety ($-O-PO_2^--O-PO_3^{2-}$) that forms extensive hydration shells.

  • Compound D (Palmitic Acid / Saturated Fatty Acid): Hydrophobic. Dominated by a $16$-carbon unbranched aliphatic hydrocarbon chain ($C_{15}H_{31}-$); the hydrophobic effect of the nonpolar chain outweighs the hydrophilic contribution of the single terminal carboxyl group.

• Structural Analysis of Organic Carbon Skeletons:

  • Question 1: How many carbon atoms are present in an unlabelled polygonal/skeletal line diagram representing an organic chain with 12 vertices?

  • Answer: Exactly 12 carbons. In skeletal organic line drawings, every vertex and every unlabelled terminus represents a carbon atom saturated with hydrogens.

  • Question 2: What kinds of bonds predominate in this long hydrocarbon chain?

  • Answer: E. Mostly nonpolar $C-H$ bonds.

  • Question 3: Where could such a molecule form hydrogen bonds with water?

  • Answer: Only at specific terminal electronegative functional sites bearing polar atoms (such as oxygen or nitrogen atoms carrying partial charges), whereas the interior hydrocarbon skeleton remains non-interacting.

• Comparing Relative Hydrophilicity of Carbohydrates and Hydrocarbons:

  • Question: Between Molecule 1 (a six-carbon ringed hexose sugar / glucose) and Molecule 2 (a hydrocarbon chain):

  • A. Molecule 1, because of its many polar covalent bonds

  • B. Molecule 2, because of its many polar covalent bonds

  • C. Molecule 1, because of its many nonpolar covalent bonds

  • D. Molecule 2, because of its many nonpolar covalent bonds

  • Answer: A. Molecule 1, because of its many polar covalent bonds.

  • Explanation: Hexose sugars have five hydroxyl ($-OH$) groups attached along the carbon ring, each containing polar covalent bonds that interact favorably with water via hydrogen bonding.

• Chemical Bonding of Functional Groups to Carbon Skeletons:

  • Question: What type of chemical bond joins a functional group to the carbon skeleton of a large biological molecule?

  • A. Covalent bond

  • B. Hydrogen bond

  • C. Ionic bond

  • D. Double bond

  • E. Disulfide bond

  • Answer: A. Covalent bond.

  • Explanation: Functional groups are attached directly to the carbon backbone through strong, stable covalent bonds involving shared pairs of outer valence electrons, ensuring structural integrity in cellular systems.