Intro Biology: Chemistry of Life and Macromolecules

Introduction

• Purpose: Authors asked instructors to suggest key topics that introductory biology students should understand.
• Context: This content accompanies Lecture 1 – Introduction/The Chemistry of Life and Chapter Two – Chemistry from Life: The Science of Biology (12th/9th edition variants referenced in slides).
• Resources mentioned:
  • Vocab list and questions
  • PDF lecture documents (Chapter Two – Chemistry)
  • Color lecture slides (Chapter Two – Chemistry)
  • Web pages of learning objectives
  • JOVE videos (for enjoyment and ease; no exam questions will be based solely on these videos)

Water and Chemical Bonds

Water: key interactive task from slides

• Task: Draw multiple water molecules interacting; indicate:
  1) electron distributions in covalent bonds,
  2) partial charges on each atom,
  3) each hydrogen bond.
• Core concepts: water molecules form hydrogen bonds that create a tetrahedral arrangement with four hydrogen-bonding partners.

Hydrogen vs Covalent vs Ionic vs van der Waals interactions

• Hydrogen bonds: attraction between a covalently bonded hydrogen and a highly electronegative atom (e.g., O, N).
• Covalent bonds: sharing of electron pairs.
• Ionic bonds: attraction between opposite charges.
• van der Waals forces: transient interactions between electrons in nonpolar substances.
• Hydrophobic interactions: nonpolar interactions that become favorable in the presence of polar substances (notably water).
• Bond energy (definition):
  • Bond energy is the amount of energy in kcal/mol needed to separate two bonded or interacting atoms under physiological conditions.
  • Typical order of strengths (rough ranges):
  • Covalent bonds: E_{ ext{bond}} ext{ (covalent)}
    ightarrow 50 ext{--}110 ext{ kcal/mol}
  • Hydrogen bonds: roughly $3 ext{--}7 ext{ kcal/mol}$
  • van der Waals: roughly $1 ext{--}2 ext{ kcal/mol}$
• Visual cue: covalent bonds hold atoms together within a molecule; hydrogen bonds and other noncovalent interactions stabilize interactions between molecules (as in water clustering).

Bond types table (conceptual)

• Covalent bond: basis = sharing of electron pairs; structure example includes single/double bonds such as in CH bonds, C–N, C=O, etc.; strong bonds.
• Hydrogen bond: basis = attraction between H attached to an electronegative atom and another electronegative atom; weaker but numerous in biological systems.
• Ionic bond: basis = attraction of opposite charges; common in salts and some biomolecular interactions.
• Hydrophobic interactions: interaction of nonpolar regions in aqueous environments; important in protein folding and membrane formation.
• van der Waals forces: weak, non-specific interactions due to transient dipoles.

Macromolecules and Fundamental Concepts

• Macromolecules: polymers with molecular weights > 1,000 daltons; contain carbon; held together mostly by covalent bonds.
• Function depends on functional groups (chemical groups with specific properties that recur in biology).
• Major macromolecule classes introduced: proteins, carbohydrates, lipids, nucleic acids.
• Macromolecular assemblies have characteristic sizes (e.g., ribosomes around 30 nm, larger assemblies around 200 nm in some contexts).

Biological organization hierarchy (as depicted in Fig. 1.10 style)

• Atoms → Small molecules → Large molecules (macromolecules) → Cells → Tissues → Organs → Organ systems → Organism.
• Representative components: Oxygen, Methane (as example small molecules), Water, Nucleic acids, Proteins, Carbohydrates, Lipids; Organisms range from unicellular to multicellular.

What Kinds of Molecules Characterize Living Things?

• Major classes: proteins, carbohydrates, lipids, nucleic acids.
• Most biomolecules are polymers built from smaller subunits (monomers).
• Mono/Polymer concept:
  • Monomers: building blocks (e.g., sugars, amino acids, nucleotides).
  • Polymers: assembled from monomers; examples include polysaccharides, proteins, nucleic acids.

Subunits, Macromolecules, and Assemblies

• Subunits (monomers) form macromolecules via covalent bonds.
• Noncovalent interactions stabilize macromolecular assemblies (e.g., globular proteins and RNA components like ribosomes).
• Scale notes: macromolecular assemblies can be tens to hundreds of nanometers in size (e.g., 30 nm to 200 nm).

Macromolecules: Polymers and Functional Groups

• Macromolecules are polymers with MW > 1000 Da; carbon-containing; mainly covalent bonds connect monomers.
• Function and behavior depend on functional groups present in polymers.
• Functional groups are groups of atoms with specific chemical properties and consistent behavior; they occur repeatedly in biological molecules.
• Examples: polar groups, acidic groups, etc. A macromolecule can contain multiple different functional groups.

Functional groups and their properties (highlights)

• Hydroxyl group (–OH): polar; enables hydrogen bonding; enables linkage to other molecules via dehydration synthesis.
• Carboxyl group (–COOH): acids that ionize to release a proton; contributes negative charge under physiological pH.
• Carbonyl group (–C=O): participates in hydrogen bonding and polarity.
• Amino group (–NH2): bases; can accept a proton and become positively charged.
• Sulfhydryl group (–SH): can form covalent disulfide bridges that stabilize protein structure.
• Note: Some textbook boxes may be updated; the core concepts remain the same.

Isomers: Structural, Cis/Trans, and Optical

• Isomers: same chemical formula but different structures or spatial arrangement.
• Structural isomers: atoms arranged differently; different properties (e.g., C4H10 is different structural isomers).
• Cis-trans isomers: different orientation around a double bond (C=C) i.e., H–C=C–H vs. different arrangement around the double bond (example: cis- vs trans-Butene).
• Optical isomers (enantiomers): mirror images; typically only one is biologically active (e.g., D-glucose vs L-glucose; amino acids are L-isomers biologically).

Monomers and Polymers; Condensation and Hydrolysis

• Monomers: building blocks of macromolecular polymers.
• Polymers: formed by condensation reactions (dehydration synthesis) where a water molecule is removed per linkage.
• General condensation equation:
  • ext{Monomer}1 + ext{Monomer}2
    ightarrow ext{Dimer} + H_2O
• Hydrolysis: addition of water to break a covalent bond in a polymer, yielding monomers.
  • ext{Polymer} + H2O ightarrow ext{Monomer}1 + ext{Monomer}_2

Major Macromolecules: Overview of Subunit and Polymer Relationships

• Subunits (examples):
  • Sugar → polysaccharide
  • Amino acid → protein
  • Nucleotide → nucleic acid
• Each class is built from its own monomer and assembled via condensation.

Amino Acids and Polypeptides

• Amino acids: both acids and bases; carboxyl group (–COOH) is an acid (proton donor); amino group (–NH2) is a base (proton acceptor).
• Side chains (R groups) determine properties and behavior of amino acids.
• There are 20 amino acids encoded by DNA; all amino acids exist as an optical isomer (L) in biology.
• Polypeptide backbone: repeating unit of –N–Cα–C–; monomers added at the C-terminus; growth direction is from the N terminus to the C terminus (N → C).
• Functions of proteins are diverse, as captured in a representative table of protein functions (see Table 3.1 in the text):
  • Enzymes: catalyze biochemical reactions.
  • Structural proteins: provide stability and movement.
  • Defensive proteins: antibodies; recognize/respond to nonself substances.
  • Signaling proteins: hormones; regulate physiological processes.
  • Receptor proteins: receive/respond to chemical signals.
  • Membrane transporters: regulate passage across membranes.
  • Storage proteins: store amino acids for later use.
  • Transport proteins: bind and carry substances.
  • Gene regulatory proteins: determine gene expression rates.
  • Motor proteins: facilitate movement of cell structures.

Amino Acid Structure and Properties

• Side chains (R) vary, leading to differences in:
  • Charge (charged vs uncharged)
  • Polarity (polar vs nonpolar)
  • Hydrophilicity vs hydrophobicity
• Charged side chains are hydrophilic and usually exposed to the aqueous environment; carry positive (NH3+) or negative (COO−) charges.
• Polar but uncharged side chains are hydrophilic and can form hydrogen bonds with water; often exposed to the aqueous environment.
• Nonpolar side chains are hydrophobic; typically buried in the protein core away from water.
• Special properties of certain residues:
  • Proline: rigidifies the polypeptide backbone.
  • Glycine: smallest amino acid; fits into tight spots.
  • Cysteine: forms covalent disulfide bridges (–S–S–) within or between polypeptides, stabilizing structure.
• Disulfide bridges: covalent bonds between cysteine residues; stabilize protein structure in harsh environments.

Protein Structure, Stability, and Folding

• Primary structure: unique amino acid sequence; defines identity of a protein family; changes can dramatically affect function.
• Secondary structure: regular local structures such as
  • α-helix: stabilized by hydrogen bonds between backbone carboxyl and amino groups; side chains project outward.
  • β-pleated sheet: stabilized by hydrogen bonds between different segments of the backbone; can form sheets from multiple polypeptide strands.
• Tertiary structure: overall 3D shape of a single polypeptide; determined by intramolecular hydrogen bonds, ionic interactions, hydrophobic interactions, and covalent disulfide bridges.
• Quaternary structure: arrangement of multiple polypeptide subunits in proteins that have more than one polypeptide chain; stabilized by noncovalent interactions and sometimes disulfide bridges.
• Proteins can change conformation upon binding to other molecules or via covalent modification; structural changes underpin enzyme activity and signaling.

Protein–Ligand Interactions

• Proteins bind noncovalently to specific molecules; specificity arises from:
  • Shape: general fit of 3-D shapes between protein and ligand.
  • Chemistry: interactions such as ionic, hydrophobic, and hydrogen bonds involving surface R groups.
• Binding can be influenced by conformational changes in proteins and covalent modifications (e.g., phosphorylation).

Carbohydrates

• Roles: sources of stored energy, carbon skeletons for other molecules, extracellular structures such as cell walls.
• Monomer to polymer progression:
  • Monosaccharides → disaccharides → oligosaccharides (3–10 units) → polysaccharides (hundreds to thousands of units).
• Monosaccharides: simple sugars; common energy source is glucose.
• Pentoses: five-carbon sugars (e.g., ribose and deoxyribose) with different biological roles (RNA vs DNA).
• Hexoses: six-carbon sugars (e.g., glucose, fructose, mannose); isomeric structural forms with formula $C<em>6H</em>{12}O_6$; aldose vs ketose classification; α vs β anomers in ring form, determined by the orientation of the hemiacetal/hemiketal linkage.
• Disaccharides: formed by condensation reactions creating glycosidic bonds; examples include sucrose and lactose; α- and β- glycosidic linkages contribute different structures.
• Polysaccharides: linear (α-1,4) or branched (α-1,6) linkages; β-1,4 linkages also present; structural and energy storage roles vary by linkage type.

Lipids and Membranes

• Lipids: not always considered macromolecules; essential building blocks in cells; do not form true polymers with repeating monomer units in the same sense as proteins or nucleic acids.
• Triglycerides: synthesized from glycerol and fatty acids via condensation; main energy storage form.
• Phospholipids: building blocks of biological membranes; consist of a glycerol backbone, two fatty acid tails (nonpolar, hydrophobic), and a phosphate-containing head group (polar, hydrophilic).
• Membranes: phospholipids organize spontaneously into a bilayer in aqueous environments; bilayer has a hydrophilic head facing water and a hydrophobic core formed by fatty acid tails.

Nucleotides and Nucleic Acids

• Nucleotides: building blocks of nucleic acids; each nucleotide is composed of a base, a ribose sugar, and a phosphate group.
• Bases: two classes – purines (adenine A, guanine G) and pyrimidines (cytosine C, thymine T in DNA; uracil U in RNA).
• DNA vs RNA sugars: deoxyribose in DNA; ribose in RNA.
• Polynucleotide synthesis: occurs via condensation at the sugar–phosphate backbone; nucleotides are added to the 3' end of a growing chain; elongation proceeds in the 5' to 3' direction.
  • ext{Elongation direction: } 5'
    ightarrow 3'
• Strands orientation: nucleic acids typically form anti-parallel strands in double helices (two strands run in opposite 5' to 3' directions).
• Base pairing in DNA: Adenine pairs with thymine (A–T) via hydrogen bonding; Guanine pairs with cytosine (G–C). In RNA, thymine is replaced by uracil (A–U).
• Double helix geometry: sugar–phosphate backbone on the outside; bases on the inside; complementary base pairing stabilizes the structure.

RNA structure specifics

• RNA is typically single-stranded but can fold back on itself to form regions of complementary base pairing, creating complex secondary structures.
• Examples of RNA sequences and folded structures illustrate how base pairing shapes RNA conformation.

Distinguishing DNA from RNA (Table-style summary)

• Nucleic acid: DNA vs RNA.
• Sugar:
  • DNA: deoxyribose
  • RNA: ribose
• Bases:
  • DNA: A, C, G, T
  • RNA: A, C, G, U
• Strands:
  • DNA: double-stranded
  • RNA: typically single-stranded

Functional Groups: Recap and Significance

• Functional groups confer chemical properties and behaviors to macromolecules (e.g., polarity, acidity, basicity).
• Hydrogen bonding, ionic interactions, and covalent bonds are guided by these functional groups and contribute to macromolecule folding, stability, and interactions.
• Representative functional group highlights:
  • Carboxyl (–COOH): acid—ionizes and contributes negative charge.
  • Amino (–NH2): base—accepts proton and can become positively charged.
  • Hydroxyl (–OH): polarity; participates in hydrogen bonding.
  • Sulfhydryl (–SH): can form disulfide bridges (–S–S–).

Notes on Isomeric Forms and Biological Relevance

• Structural isomers differ in the arrangement of atoms; properties can vary significantly.
• Cis-trans isomerism around C=C influences physical properties and biological recognition.
• Optical isomers (enantiomers) are mirror images; biological systems often utilize a single enantiomer (e.g., L-amino acids, D vs L in sugars).

Takeaway Points

• R groups (side chains) determine protein structure and thus function.
• Macromolecular structure is hierarchical:
  • Primary (sequence) → Secondary (local folding: α-helix, β-pleated sheet) → Tertiary (overall 3D shape) → Quaternary (multi-subunit assemblies).
• Noncovalent interactions drive specific binding and dynamic conformational changes, while covalent bonds provide stability and structural integrity (e.g., disulfide bridges).
• Condensation reactions build polymers; hydrolysis reactions break them down; both are fundamental to metabolism and biosynthesis.
• The chemistry of life is organized around a small set of functional groups and a few recurring motifs that govern macromolecular behavior, interactions, and cellular function.

Quick Reference: Key Formulas and Notations

• Condensation (dehydration) reaction (monomer + monomer → dimer + water):
  ext{Monomer}1 + ext{Monomer}2
  ightarrow ext{Dimer} + H_2O
• Growth direction of polypeptides (N-terminus to C-terminus):
  ext{N-terminus}
  ightarrow ext{C-terminus}
• Nucleic acid synthesis direction (elongation):
  5'
  ightarrow 3'
• Base pairing in DNA: $A ext{ pairs with } T ext{ (or } U ext{ in RNA)}; ext{ } G ext{ pairs with } C$
• Protein backbone growth: $-N- ext{C}_ ext{α}- ext{C}-$ (repeating unit)
• Secondary structure examples:
  • α-helix stabilized by backbone hydrogen bonds between the carboxyl and amino groups within the same polypeptide.
  • β-pleated sheet stabilized by hydrogen bonds between backbones of adjacent polypeptide chains.
• Core organization principle: functional groups drive chemistry; structure drives function; interactions drive life.