Biomolecules: Carbon, Isomers, and Functional Groups

Carbon and Bonding: versatility and foundations

• Carbon basics
  • Carbon is tetravalent: forms four bonds, enabling diverse structures.
  • Atomic number: Z=6; neutral atom (no net charge).
  • In the nucleus: 6 protons; balance by 6 electrons.
  • Electron configuration concept: first shell holds 2e^-; with 6 total electrons, the second shell must hold the remaining 4e^-, making the second shell the valence shell; this drives tetravalence and octet-seeking behavior.
  • Octet rule: carbon tends to complete its valence shell with a total of eight electrons around the atom.
  • Carbon’s versatility comes from ability to form chains, double bonds, branching, and rings; 3D shapes are critical in biology, not just flat drawings.
• 3D structures and representations
  • Real biomolecules exist in three dimensions; ball-and-stick and space-filling (covalent bonding and spatial packing) illustrate how carbon skeletons connect to H, O, N, P, etc.
  • Structure dictates function: different carbon frameworks lead to different biological roles.
• Carbon in biological systems
  • Living things assemble carbon skeletons from foods; plants can fix inorganic carbon via photosynthesis, whereas animals cannot convert inorganic carbon (CO₂) directly into organic carbon.
  • Plants assimilate CO₂ through chloroplasts and convert inorganic carbon into carbohydrate skeletons; roots store carbohydrate as starch (e.g., potatoes store starch in roots).
  • Humans ingest carbon-containing molecules (carbohydrates, proteins, lipids, nucleic acids) from plants and animals to meet carbon requirements.
• The carbon cycle and photosynthesis (plants vs humans)
  • Inorganic carbon enters plants as CO₂ from the air; chloroplasts enable carbon fixation into organic molecules.
  • Humans rely on plant or animal sources to obtain carbon-based macromolecules; we cannot synthesize carbon skeletons from inorganic carbon directly.
• Bonding and polarity basics
  • Covalent bonds: sharing electrons between atoms.
  • Nonpolar covalent bonds: bonds between similar atoms with similar electronegativities (e.g., C–C, C–H).
  • Polar covalent bonds: bonds where electrons are not shared equally due to electronegativity differences (e.g., C–O, C–N).
  • electronegativity context
  • Very electronegative elements: O, N, F, Cl; these pull electron density toward themselves.
  • Carbon and hydrogen are relatively less electronegative and similar to each other, so C–H and C–C bonds are largely nonpolar.
• Electronegativity and polarity in biological contexts
  • In watery environments (cytosol and extracellular fluid), polar covalent bonds enable solubility and interactions via hydrogen bonding.
  • Polar vs nonpolar attributes influence dissolution, transport, and reactivity of biomolecules.

Isomerism: three categories and how to recognize them

• Core idea
  • Isomers are compounds with the same number and kind of atoms but different arrangements or spatial orientation.
  • Three main types to recognize: Structural (constitutional) isomers, Geometric (cis/trans) isomers, and Enantiomers.
• Structural isomers (constitutional isomers)
  • Definition: same molecular formula (same number and types of atoms) but different bonding arrangements/connectivity.
  • Key point: they have the same atoms but different connections; may lead to different properties and functions.
  • Example from the transcript: a pair with formula ext{C}3 ext{H}8 ext{O} can differ by where the oxygen is placed; one is an end/terminal oxygen (oxygen on a terminal carbon) vs. oxygen on a middle carbon.
  • Terminology example: isopropanol vs propanol differ in the placement of the hydroxyl group.
• Geometric (cis/trans) isomers
  • Also called geometric isomers.
  • Definition: compounds with the same connectivity but different spatial arrangement around a double bond (alkenes).
  • Rotation around a carbon–carbon double bond is restricted; cannot freely rotate to convert cis to trans without breaking the bond.
  • How to tell them apart:
  • If substituents attached to the double-bond carbons are on the same side, it's cis (Z configuration).
  • If they are on opposite sides, it's trans (E configuration).
  • Notation and examples:
  • Example naming: cis-2-butene vs. trans-2-butene.
  • Health relevance: trans fats arise from trans isomers and are associated with negative health effects; many foods label “no trans fats” for health reasons.
• Enantiomers (chiral isomers)
  • Definition: mirror-image isomers that are non-superimposable; they are chiral due to the presence of at least one chiral center.
  • Chiral center: a carbon atom bonded to four different groups (substituents).
  • Distinctions from other isomers:
  • They are not structural or geometric isomers; they are stereoisomers with handedness.
  • Nomenclature: R/S and D/L (Latin-based) designations.
  • Biological significance: receptors and enzymes are highly stereospecific; only one enantiomer may be biologically active or safe.
  • Examples and case studies from the transcript:
  • Aspirin: tablets often contain both enantiomers, but only the left-handed (one enantiomer) is pharmacologically active in blocking pain signals; the other may be inactive.
  • Thalidomide tragedy: a chiral drug with two enantiomers; one caused teratogenic birth defects when taken during pregnancy; later, it was discovered that one enantiomer can convert to the other inside the body, complicating safety and approvals.
  • Other examples: methamphetamine demonstrates activity differences between enantiomers; S- vs R- forms can have dramatically different effects.
  • Biological receptor recognition: cell surface receptors act like a lock-and-key; only the matching enantiomer fits and triggers a response.
  • Practical exam implication: given two structures, determine whether they are structural, geometric, or enantiomers by checking connectivity, presence of double bonds, and chirality.
• Summary decision rules for the three isomer types (to apply on exams)
  • If same atoms but different bonding arrangement and no chiral center: structural isomer.
  • If a double bond is present in the carbon skeleton and rotation is restricted, creating cis/trans orientation: geometric (cis/trans) isomer.
  • If a carbon is bonded to four different groups (a chiral center) and the pair are mirror images that are non-superimposable: enantiomers.
• Chirality in biology and ethics
  • Enantiomeric specificity is crucial for drug design and safety.
  • Receptors and enzymes distinguish handedness; misalignment can cause loss of efficacy or harmful effects.
  • Historical examples underscore ethical and regulatory implications of racemic drugs and enantioselective pharmacology.

Functional Groups: seven key groups and the four introduced here

• What is a functional group?
  • A functional group is a small, recurring arrangement of atoms within larger molecules that confers characteristic chemical properties and reactivity to that molecule.
  • In larger biomolecules, these groups impart polarity, acidity/basicity, reactivity, and interactions with water.
• The seven functional groups (as introduced in the material)
  • Hydroxyl group: -OH
  • Carbonyl group: =O (as part of aldehydes or ketones)
  • Carboxyl group: -COOH
  • Amino group: -NH2
  • Phosphate group: -PO4H2 (mentioned; to be covered later)
  • Methyl group: -CH3 (mentioned; to be covered later)
  • Sulfhydryl group: -SH (mentioned; to be covered later)
• Hydroxyl group (-OH)
  • What it is:
  • An oxygen atom bonded to hydrogen and to a carbon framework.
  • How to recognize on a molecule:
  • Look for -OH groups protruding from a carbon skeleton.
  • Why it matters:
  • Polar; enables hydrogen bonding; increases solubility in water; contributes to cohesion (e.g., water droplets).
  • Practical examples:
  • Ethanol (C₂H₅OH), isopropyl alcohol (isopropanol, C₃H₇OH).
• Carbonyl group (C=O) and its two main forms
  • Definition: carbon double-bonded to oxygen.
  • Aldehydes (terminal carbonyl): carbonyl group on the terminal carbon of a carbon chain.
  • Ketones (internal carbonyl): carbonyl group on a middle carbon.
  • Relationship to carbohydrates
  • Aldehydes correspond to aldose sugars (terminal carbonyl).
  • Ketones correspond to ketose sugars (internal carbonyl).
  • Polarity and reactivity:
  • Carbonyls contribute polarity and can engage in further reactions depending on position.
  • Examples of naming implications:
  • Sugars end with -ose; aldoses vs ketoses depend on terminal vs middle carbonyl placement.
• Carboxyl group (-COOH)
  • What it is:
  • A carbonyl (C=O) and a hydroxyl (−OH) on the same carbon.
  • Ionization and acidity:
  • The carboxyl hydrogen can dissociate (ionize) to form -COO⁻ and H⁺, increasing acidity of the solution.
  • Consequences for solubility and pH:
  • Carboxyl groups enhance water solubility due to polarity and potential ionization.
  • When ionized, they increase hydrogen ion concentration, lowering pH (more acidic).
  • Example behavior in buffers:
  • In the body, buffering systems help maintain pH near 7.35–7.45 (see pH discussion below).
• Amino group (-NH2)
  • What it is:
  • Nitrogen bonded to two hydrogens and to the rest of the molecule.
  • Basic behavior:
  • Can accept a proton (H⁺) from solution, reducing free hydrogen ion concentration and raising pH (alkaline effect).
  • Contrast with carboxyl group:
  • Carboxyl acts as an acid (donates H⁺); amino acts as a base (accepts H⁺).
• Practical notes on functional groups for exams
  • For a given large biomolecule, you should be able to identify functional groups, circle them on diagrams, name the group, and state a primary function or property.
  • You will see practice slides that place functional groups on carbohydrates, proteins, lipids, and nucleic acids; the task is to locate seven groups and annotate their function.
• The bigger picture: functional groups and macromolecules
  • Four major classes of biomolecules (proteins, lipids, carbohydrates, nucleic acids) all contain these functional groups, which explain their distinct chemical properties and biological roles.
  • The presence and arrangement of functional groups influence solubility, polarity, acid-base behavior, cross-linking, and interactions with receptors or enzymes.

Biological context: receptors, chirality, and real-world relevance

• Receptors and chirality
  • Cell surface receptors act like antennas on cells; they bind specifically to certain enantiomers of molecules.
  • The fit between a receptor and a molecule is highly stereospecific, akin to a key fitting a lock.
  • Enantiomeric selectivity explains why one enantiomer may be therapeutic while the other is inactive or harmful.
• Real-world drug examples and ethics
  • Aspirin: the enantiomer-specific effect in pain relief; the therapeutic activity may reside largely in one enantiomer.
  • Thalidomide tragedy: two enantiomers with vastly different developmental effects; one caused birth defects when used during pregnancy; the body can interconvert enantiomers, complicating safety.
  • The thalidomide case spurred stricter drug testing and awareness of enantiomeric risk in pharmacology.
  • Other examples discussed: methamphetamine enantiomers differ in psychoactive properties; some enantiomers act as vasodilators or other functions.
• Practical implications for drug development and labeling
  • Isolating and using a single enantiomer can improve efficacy and safety, but in some cases, interconversion means a racemic or mixed formulation remains problematic.
  • Regulatory and ethical considerations emphasize evaluating each enantiomer’s activity, safety, and potential side effects.

Carbohydrates, lipids, and energy flow: linking structure to function

• Carbohydrate classification and naming hints
  • Carbonyl position in sugars determines aldose vs ketose classification.
  • Sugars often end with -ose (e.g., glucose, ribose, fructose).
• Energy flow and storage implications
  • Carbohydrates serve as quick energy stores (e.g., starch in plants, glycogen in animals).
  • Lipids act as long-term energy storage and structural components; their cis/trans configurations influence membrane properties and metabolic processing.
• Practical exam connection: identifying functional groups on macromolecules
  • Expect to locate and name hydroxyls, carbonyls, carboxyls, and amino groups on proteins, lipids, carbohydrates, or nucleic acids in drawn molecules.
  • For each identified group, state its typical function in that molecule (e.g., hydroxyls promote polarity and hydrogen bonding; carboxyls contribute acidity).

Quick recap: key takeaways for exam preparation

• Carbon’s tetravalence and flexible bonding underpin the diversity of biological macromolecules.
• Three major isomer categories to recognize:
  • Structural isomers: same formula, different connectivity.
  • Geometric (cis/trans) isomers: double bonds restrict rotation and define spatial arrangement around the double bond.
  • Enantiomers: chiral, mirror-image, non-superimposable; R/S and D/L naming; biological relevance via receptors and enzymes.
• Functional groups are the small, recurring motifs that dictate behavior of larger molecules; seven groups exist, with hydroxyl, carbonyl, carboxyl, and amino being the four introduced here.
• The interplay between polarity, solubility, and ionization is central to biomolecule behavior in aqueous environments and to physiological pH control (buffers).
• Real-world relevance:
  • Enantiomer-specific drug activity and safety concerns (e.g., thalidomide, albuterol, aspirin).
  • Lipids and sugars exhibit cis/trans configurations with health and metabolic implications.
• Exam strategy for this unit:
  • Be able to circle functional groups on macromolecule diagrams, name the group, and state a primary function.
  • Distinguish between structural, geometric, and enantiomeric relationships using connectivity, double bonds, and chirality.
• Foundational links to broader chemistry and biology: structureDictates function; the carbon cycle and photosynthesis provide context for how organisms obtain and transform carbon; receptor-based specificity explains how molecular handedness affects biology.