Chapter 3 Notes: Carbon and the Molecular Diversity of Life

Valence and Bonding in Organic Molecules

• Major elements highlighted as most abundant in organisms: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O); also Phosphorus (P) and Sulfur (S) are important for biological molecules.
• Valence basics:
  • Hydrogen: valence 1
  • Oxygen: valence 2
  • Nitrogen: valence 3
  • Carbon: valence 4
• The formation of bonds with carbon relies on carbon’s ability to form four covalent bonds, enabling diverse molecular structures.

Formation of Bonds with Carbon

• Carbon can form single, double, or triple bonds, creating a variety of skeletons.
• Examples illustrating tetra-valence and bond formation tendencies (illustrative):
  • Methane: $ext{CH}_4$
  • Ethane: $ext{C}<em>2 ext{H}</em>6$
  • Simple chain or branched chains, with potential for multiple functional groups.

Four Ways That Carbon Skeletons Can Vary

• Variation types (with representative examples):
  • Length: Ethane, Propane, Butane
  • Branching: 2‑Methylpropane (isobutane)
  • Double bond position: 1‑Butene, 2‑Butene
  • Presence of rings: Cyclohexane, Benzene
• Abbreviated structural formulae use corners as carbon centers; each corner carries attached hydrogens as needed.
• Note: Carbon skeletons influence properties such as stability, reactivity, and solubility.

Extracellular vs Intracellular Hydrophobic/Hydrophilic Characteristics and Lipids

• Phospholipid bilayer basic arrangement:
  • Hydrophobic tails form the interior of the bilayer
  • Hydrophilic heads face aqueous environments on either side of the membrane
  • A phospholipid shows a glycerol backbone with two fatty acid tails and a phosphate-containing head
• Conceptual takeaway: membranes are formed by amphipathic phospholipids, balancing hydrophobic and hydrophilic interactions.

Isomers

• A. Structural isomers
  • Example: Pentane vs 2‑methylbutane (structural isomers differ in connectivity)
  • Structural isomer illustration: H–C–C–H patterns differ between isomers.
• B. Geometric isomers (cis/trans)
  • Example: 2‑butene; cis has identical substituents on the same side, trans on opposite sides.
  • Notation: cis-2-butene vs trans-2-butene shows different physical properties.
• C. Enantiomers (mirror-image isomers)
  • Examples: L- and D- forms of chiral molecules like amino acids (e.g., glycine is achiral; others such as amino acids show chirality).
• D‑enantiomer and L‑enantiomer differences can lead to drastically different biological activities.
• Example in pharmaceuticals: L‑Dopa is effective against Parkinson’s disease, whereas D‑Dopa is biologically inactive due to receptor/enzyme specificity.

The Chemical Groups Most Important to Life (Functional Groups)

• Functional groups are chemical groups that affect molecular function by taking part in chemical reactions; each group reacts in characteristic ways.
• Common functional groups discussed: OH, C=O, -COOH, -NH2, -SH, -OPO3^2-, -CH3.

Functional groups (specific examples)

• Hydroxyl group (-OH): Alcohol
  • Example: Ethanol (CH3–CH2–OH)
• Carbonyl group (>C=O): Ketone or Aldehyde
  • Ketone example: Acetone (CH3–CO–CH3)
  • Aldehyde example: Propanal (CH3–CH2–CHO)
• Carboxyl group (-COOH): Carboxylic acid (organic acid)
  • Example: Acetic acid (CH3–COOH)
• Amino group (-NH2): Amine
  • Example: Glycine (NH2–CH2–COOH)
• Sulfhydryl group (-SH): Thiol
  • Example: Cysteine (HS–CH2–CH2–SH); note cysteine contains a thiol that can form disulfide bridges, stabilizing protein structure
• Phosphate group (-OPO3^2-): Organic phosphate
  • Example: Glycerol phosphate
• Methyl group (-CH3): Methylated compounds
  • Example: 5‑Methylcytosine (a methylated DNA base)

ATP: An Important Source of Energy for Cellular Processes

• ATP structure: Adenosine with three phosphate groups linked by high-energy bonds (O–P–O–P–O–P).
• ATP hydrolysis provides energy for cellular work:
  • Reaction (simplified): ext{ATP} + ext{H}2 ext{O} ightarrow ext{ADP} + ext{P}i + ext{Energy}
• The inorganic phosphate (P_i) and ADP are products; energy release powers cellular processes.

Carbon Compounds in Cells: Four Major Classes

• Carbohydrates: Fuel and building material
• Lipids: Membranes, energy storage, signaling (amphipathic molecules such as phospholipids)
• Proteins: Catalysis, structure, transport, signaling
• Nucleic Acids: Information storage and transfer (DNA, RNA)

Formation of Macromolecules Within Cells

• Macromolecules are built from monomers:
  • Nucleotides form Nucleic Acids
  • Amino acids form Proteins
  • Simple sugars form Polysaccharides (carbohydrates)
  • Glycerol + fatty acids form Lipids (triglycerides, phospholipids)
• Key elements involved in macromolecule synthesis: Hydrogen (H), Oxygen (O), Nitrogen (N), Carbon (C), Phosphorus (P), and Sulfur (S).
• Conceptual pathway from small molecules to macromolecules includes the formation of monomers, polymerization, and assembly into functional complexes.

Synthesis of Polymers

• Condensation (Dehydration) reactions build polymers by removing a water molecule to form a new covalent bond:
  • Generic form: ext{Monomer} ext{–OH} + ext{Monomer} ext{–H}
    ightarrow ext{Monomer}– ext{Monomer} + H_2O
• Result: Short polymer grows into a longer polymer via successive condensation steps.

Breakdown of Polymers

• Hydrolysis reactions break polymers by adding water, cleaving bonds:
  • Generic form: ext{Polymer} + H_2O
    ightarrow ext{Monomer} + ext{Monomer}
• This process recovers monomers for reuse and is essential for digestion and recycling of cellular materials.

Carbohydrates Serve as Fuel and Building Material

• Carbohydrates include monosaccharides (e.g., glucose), disaccharides (e.g., sucrose), and polysaccharides (e.g., starch, glycogen, cellulose).
• Glucose is a central hexose sugar with formula $C<em>6H</em>{12}O_6$; it exists in linear and ring forms in solution.

Aldoses and Ketoses; Examples

• Aldoses (aldehyde sugars): glyceraldehyde, ribose, glucose, galactose, etc.
• Ketoses (ketone sugars): dihydroxyacetone, ribulose, fructose, etc.
• Trioses: 3-carbon sugars (e.g., glyceraldehyde, dihydroxyacetone)
• Pentoses: 5-carbon sugars (e.g., ribose, ribulose)
• Hexoses: 6-carbon sugars (e.g., glucose, galactose, fructose)
• General note: Monosaccharides can exist in linear or ring forms; equilibrium strongly favors ring forms in aqueous solutions.

Linear and Ring Forms of Glucose

• In solution, glucose exists predominantly as rings due to intramolecular aldol reactions.
• Pathway to ring formation: carbon 1 (C1) bonds to the oxygen of carbon 5 (C5) to form a hemiacetal linkage.
• Abbreviated ring structures show the ring with substituents (OH and H) oriented above or below the plane.
• Important forms:
  • α-D-glucose: OH on C1 points downward in the Haworth projection
  • β-D-glucose: OH on C1 points upward in the Haworth projection
• The ring forms are interconvertible with the linear form in solution; the ring form is greatly favored.

α- and β-Glucose in Polysaccharides

• Amylose (starch component): linear polymer of α-D-glucose units with α-1,4 glycosidic linkages; forms a mostly unbranched helix.
• Amylopectin (starch component): branched polymer with α-1,4 linkages and α-1,6 branch points; results in a branched helical structure.
• Glycogen (animal storage polysaccharide): highly branched α-D-glucose polymer with frequent α-1,6 glycosidic linkages; optimized for rapid glucose release.

Glycosidic Linkages in Polysaccharides

• α-1,4 glycosidic linkage: polymerizes glucose units in the same orientation (as in starch and glycogen).
• α-1,6 glycosidic linkage: creates branch points (as in amylopectin and glycogen).
• β-1,4 glycosidic linkage: found in cellulose; results in a different, linear polymer with different properties.
• Disaccharides such as sucrose are formed by glycosidic linkages between monosaccharide units (e.g., glucose and fructose) with specific linkage types.
• Example: Sucrose is formed by a glucose–fructose glycosidic linkage (glucose C1 to fructose C2): $ext{Glucose}- ext{(1→2)}- ext{Fructose}$, chemical formula $C<em>{12}H</em>{22}O_{11}$.

Polysaccharides: Form Fits Function

• Summary of major polysaccharides and roles:
  • Starch: energy storage in plants; composed of amylose (unbranched, helical) and amylopectin (branched)
  • Glycogen: energy storage in animals; highly branched, enabling rapid mobilization of glucose
  • Cellulose: structural polysaccharide in plants; composed of β-D-glucose units linked by β-1,4 glycosidic bonds; provides structural support in cell walls
• Structural differences between α‑ and β‑glucose lead to markedly different properties: digestibility, rigidity, and function.

Cellulose: Structure and Plant Cell Walls

• Cellulose is a polymer of β-D-glucose with β-1,4 glycosidic bonds; every glucose unit is flipped relative to its neighbor (upside-down orientation).
• Parallel cellulose molecules form microfibrils held together by extensive hydrogen bonding.
• Each cellulose molecule is unbranched; hydrogen bonds form between OH groups on C3 and C6 of adjacent glucose units, stabilizing the microfibril structure.
• A typical plant cell wall contains many cellulose microfibrils; ~80 cellulose molecules associate to form a microfibril, the main architectural unit of the wall.

Electronegativity, Polarity, and Practical Implications

• Functional groups influence polarity and solubility:
  • Hydroxyls contribute to hydrogen bonding and water solubility in small molecules but not in highly polymeric polysaccharides like cellulose (due to extensive intermolecular hydrogen bonding and crystallinity).
• Digestibility in humans:
  • α‑glycosidic linkages (starch, glycogen) are digestible by human enzymes (amylases) to release glucose.
  • β‑glycosidic linkages (cellulose) are not digestible by humans because we lack the enzyme β‑glucosidase; thus cellulose functions as dietary fiber.

Learning Objectives and Practice

• Distinguish different carbon skeleton types in cells and understand how they lead to molecular diversity.
• List isomer types and explain their biological significance.
• Write chemical formulas for functional groups and describe properties (polar/nonpolar, acidic/basic).
• Explain dehydration/condensation and hydrolysis reactions in the context of molecule formation and breakdown.
• Draw glucose in linear and ring forms; identify carbons and ring-form interconversion.
• Explain variations among monosaccharides and identify alpha vs beta glucose forms in amylose, amylopectin, cellulose, and glycogen.
• Identify which polysaccharides exhibit branching and determine branch point carbons.
• Provide examples of structural vs storage polysaccharides and their cellular roles.
• Explain why some sugars are digestible while others are not, with examples.

Vocabulary

• functional groups – carbonyl, carboxyl, hydroxyl, methyl, sulfhydryl, amino, phosphate
• isomers – structural, geometric (cis/trans), enantiomers
• monomers, polymers
• Carbohydrate, Lipids, Proteins, Nucleic acids
• aqueous solution
• condensation/dehydration reactions
• hydrolysis reactions
• Alpha glucose, beta glucose
• glycosidic linkages
• Monosaccharides, Disaccharides, Polysaccharides
• Glycogen, Starch, Cellulose
• Amylose, Amylopectin

Learning Resources (Conceptual References)

• Concept 3.1: Carbon atoms can form diverse molecules by bonding to four other atoms
• Concept 3.2: Macromolecules are polymers built from monomers
• Concept 3.3: Carbohydrates serve as fuel and building material
• Suggested videos: Introduction to Biological Molecules; Carbohydrates (external resources listed in the transcript)

Connections to Broader Topics

• The diversity of carbon compounds underpins cellular structure and function across all life.
• Macromolecule architecture (monomer type, linkage type, branching) directly affects stability, digestibility, and biological roles.
• The study of enantiomers emphasizes the importance of stereochemistry in pharmacology and drug design, where only one enantiomer may be biologically active.
• The structure of cellulose illustrates how microscopic organization (microfibrils and hydrogen bonding) translates into macroscopic properties like plant cell wall rigidity and biomass composition.

Quick Reference Formulas and Notations

• Glucose: $C<em>6H</em>{12}O_6$
• Sucrose (glucose-fructose disaccharide): ext{Glucose–Fructose}
  ightarrow ext{Sucrose}, ext{ linkage: } eta ext{- or } ext{α-1,2 (specific to sucrose)}
• Glycosidic linkages: $ext{α-1,4}, ext{α-1,6}, ext{β-1,4}$
• ATP hydrolysis: ext{ATP} + ext{H}2 ext{O} ightarrow ext{ADP} + ext{P}i + ext{Energy}
• Ring form formation: C1 of glucose bonds to O in C5 to form a hemiacetal; ring forms: $ext{α-D-glucose}, ext{β-D-glucose}$
• In cellulose: $\beta ext{-1,4-glycosidic linkage}$ leading to unbranched polymers and strong hydrogen-bonded microfibrils