Chapter 2: Chemical Components of Cells – Comprehensive Study Notes

Small organic building blocks of the cell

  • The four small organic building blocks in cells: SUGARS, FATTY ACIDS, AMINO ACIDS, and NUCLEOTIDES.
  • Larger organic molecules built from these blocks include: POLYSACCHARIDES, GLYCOGEN, AND STARCH (IN PLANTS); FATS AND MEMBRANE LIPIDS; PROTEINS; and NUCLEIC ACIDS.
  • These building blocks combine to form the macromolecules that carry out cellular structure and function.

Atoms, elements, and basic chemistry in cells

  • Matter is composed of elements; atoms are the smallest unit with chemical properties.
  • Subatomic particles:
    • Protons (+) in the nucleus
    • Neutrons (neutral) in the nucleus
    • Electrons (-) orbiting the nucleus
  • The chemical properties of an atom depend on its electrons, especially the outermost shell.
  • Atoms link to form molecules via chemical bonds.

Atomic number, neutrons, and isotopes

  • Atomic number: determined by the number of protons in an atom.
  • In a electrically neutral atom, electrons = protons.
  • Isotopes: atoms of the same element that differ in the number of neutrons; same atomic number but different mass numbers. Examples:

    • 12<em>6extCextvs14</em>6extC ext(C−12vsC−14)^{12}<em>{6} ext{C} ext{ vs } ^{14}</em>{6} ext{C} \ ext{(C-12 vs C-14)}

How atoms come together to form molecules: covalent bonds

  • A molecule is a collection of atoms held together by covalent bonds.
  • Covalent bonds involve sharing of electrons between atoms.

Electron shells and reactivity

  • Electrons reside in regions around the nucleus called orbitals/shells.
  • First shell can hold up to 2 electrons; second, up to 8; third also up to 8 in many cases.
  • Atoms with a full outer shell are chemically inert; atoms with incomplete outer shells are reactive and form bonds to reach stability.
  • The outermost electrons determine how atoms interact with others.

The octet rule and bonding capacity

  • Octet rule: atoms are most stable when their outer shell is full with 8 electrons (except hydrogen).
  • Hydrogen is stable with 2 electrons in its outer shell.
  • The reactivity (and the number of bonds an atom can form) is driven by unpaired electrons in the valence shell.
  • Bond formation often involves gaining, losing, or sharing electrons to achieve a full outer shell.
  • Bond counts correlate with unpaired electrons: e.g., 2 unpaired electrons allow up to 2 bonds; 3 unpaired electrons allow up to 3 bonds.

Covalent vs noncovalent bonds; types of interactions

  • Covalent bonds: strong chemical bonds where electrons are shared between atoms.
    • Polar covalent bonds: unequal sharing due to differing electronegativities.
    • Nonpolar covalent bonds: equal sharing due to similar electronegativities.
  • Noncovalent bonds: weaker interactions that do not involve full electron transfer or sharing; critical for shape and binding in biology.
    • Ionic (electrostatic) bonds: formed by transfer of electrons creating oppositely charged ions that attract.
    • Hydrogen bonds: a partial positive H atom bonded to an electronegative atom (like O or N) interacts with another electronegative atom.
    • Van der Waals attractions: transient, non-specific interactions when atoms come close.
    • Hydrophobic interactions: nonpolar groups coalesce to minimize disruption to water.
  • Relative strengths: covalent bonds are generally stronger; noncovalent interactions are crucial for dynamic assembly and specificity in cells.

The outermost electrons determine how atoms interact

  • Full outer shell → atoms are less reactive; incomplete outer shells → atoms form bonds.
  • Bond formation involves sharing (covalent) or transfer (ionic) of electrons to achieve stability.
  • Examples illustrating interactions:
    • Water is a polar molecule with partial charges that drive many interactions in biology.
    • NaCl forms ions (Na+, Cl−) that interact with water (solubility via hydration).

Ionic bonds and salts in biology

  • Ionic bonds form by transfer of electrons between atoms.
    • Example: Na+ and Cl− attract each other via electrostatic forces.
  • Salts are compounds with ions held together by ionic bonds.
  • Ions are attracted to polar molecules like water; hence salts are soluble in water due to hydration shells around ions.

Hydrogen bonding and its biological importance

  • Hydrogen bond: a partial positive charge on hydrogen, bonded to electronegative atom (O, N) interacts with another electronegative atom.
  • Elements involved include O, N; H bonds can occur between molecules containing O or N.
  • Although hydrogen bonds are not chemical bonds in the sense of covalent bonds, they collectively stabilize structures of large molecules (e.g., in DNA, proteins, and water networks).
  • Key concept: individual hydrogen bonds are relatively weak, but together they provide major stabilization.

Hydrophilic vs hydrophobic molecules

  • Hydrophilic (water-loving) substances dissolve readily in water: ions and polar molecules form interactions with water.
  • Hydrophobic (water-fearing) substances are largely nonpolar; water does not readily solvate them, leading to phase separation.
  • Water's polarity drives hydration of ions and polar solutes; nonpolar hydrocarbons (e.g., many C–H compounds) are typically insoluble in water.
  • Examples:
    • Hydrophilic: table salt (NaCl), urea, many sugars.
    • Hydrophobic: hydrocarbons; fatty acid tails are hydrophobic.
  • Rough electronegativity references (for context): O: 3.44, C: 2.55, H: 2.2, N: 3.04.

Noncovalent bonds help bring molecules together in cells

  • Noncovalent interactions help assemble macromolecular complexes and stabilize conformations:
    • Ionic (electrostatic)
    • Hydrogen
    • Van der Waals
    • Hydrophobic forces
  • Van der Waals interactions are transient and nonspecific, arising when atoms come into close contact.
  • Hydrophobic forces drive clustering of nonpolar groups to minimize disruption to the water hydrogen-bond network.

pH, acids, and bases in biology

  • Acids (pH < 7): release a proton (H+) into water, increasing H3O+ concentration.
  • Bases (pH > 7): accept a proton, increasing OH− concentration.
  • Water autoionization provides H+ and OH−; pH is a measure of H+ concentration.
  • pH scale ranges from 0 to 14; pH 7 is neutral where [H+] = [OH−].
  • Common examples:
    • Strong acids: HCl; strong bases: NaOH.
    • Weak acids/bases include H2CO3 and NH3/NH2- systems.

Carbon’s bonding behavior and organic chemistry basics

  • Carbon atoms can form up to four covalent bonds, enabling diverse and complex organic structures.
  • Organic compounds contain carbon bound to hydrogen and other elements (often N, O, S, P).
  • Carbon skeletons can be:
    • Chains (linear)
    • Branched trees
    • Rings
  • These skeletons serve as backbones for more complex molecules; the specific arrangement determines chemical properties and biological function.

Carbon skeletons: examples of structures

  • Chain: C–C–C–C–…
  • Branched tree: C with multiple C branches
  • Ring: cyclic carbon frameworks (e.g., benzene-like rings in larger biomolecules)

Chemical groups (functional groups) and their roles

  • Functional groups are atom clusters covalently bonded to carbon backbones that impart specific properties.
  • Common groups and notes:
    • Amino group (-NH2): polar; part of amino acids; participates in peptide bonds; basic.
    • Carboxyl group (-COOH): acidic; in fatty acids and amino acids; deprotonates to -COO− in water; involved in peptide bonds.
    • Carbonyl groups: (-C=O) found in ketones and aldehydes; polar; reactive.
    • Aldehyde (-CHO) and Ketone (-C=O): differ in attachment; orientation matters in sugars.
    • Hydroxyl group (-OH): polar; forms hydrogen bonds; present in alcohols, sugars, steroids, proteins.
    • Phosphate groups (-PO4 or -O-PO3): polar; often linked as esters; essential in energy transfer (phosphoanhydride bonds) and as part of nucleotides and phospholipids.
    • Phosphate esters: formed between phosphate and a free hydroxyl; common in proteins and nucleotides.
    • Sulfhydryl group (-SH): forms disulfide bridges in proteins; important in protein folding.
    • Sulfate (-SO4^2−) and Phosphate esters: polar and often negatively charged at physiological pH; found in nucleic acids and phospholipids.
    • Methyl group (-CH3): nonpolar; often attached to DNA, proteins, carbohydrates; influences hydrophobicity.
  • Application notes:
    • Glycosidic bonds form when monosaccharides join to form disaccharides/polysaccharides.
    • Ester and amide linkages connect lipids and proteins to other groups.

Sugars and carbohydrates (Carbohydrates overview)

  • Carbohydrates have the general formula (CH<em>2O)</em>n(CH<em>2O)</em>n where n = 3–6 for common monosaccharides.
  • Functions:
    • Energy sources (fuel)
    • Structural materials (cell walls, exoskeletons in some organisms)
    • Signaling and recognition (cell identity)
  • Monosaccharides:
    • General formula: (CH<em>2O)</em>n,extwithn=3,4,5,6(CH<em>2O)</em>n, ext{ with } n = 3,4,5,6
    • Aldoses vs. Ketoses based on presence of aldehyde or ketone groups.
    • Examples: glyceraldehyde (triose), ribose (pentose), glucose (hexose), dihydroxyacetone (triose), ribulose (pentose), fructose (ketose).
  • Ring formation in solution:
    • Aldehyde or ketone reacts with a hydroxyl to form a ring (hemiacetal or acetal formation).
    • This ring formation is a key step in stabilizing sugars in solution.
  • Disaccharides:
    • Formed by condensation (glycosidic) bonds between two monosaccharides, releasing water.
    • Examples: sucrose, lactose.
    • Hydrolysis is the reverse process, breaking the glycosidic bond and consuming water.
  • Polysaccharides:
    • Large polymers of monosaccharide subunits.
    • Glycogen (animals) and starch (plants) for energy storage.
    • Cellulose (plants) and chitin (animals/fungi) as structural materials.
    • Branch points exist (e.g., glycogen has many branches).
  • Important structural theme: sugars can exist in linear or ring forms; ring formation is common in solution and affects reactivity and recognition.

Lipids: structure, properties, and roles

  • Lipids are hydrophobic or amphipathic molecules with hydrocarbon chains or rings.
  • Major roles:
    • Concentrated energy stores
    • Structural components of membranes (phospholipids)
    • Cell signaling and hormones (steroids and glycolipids)
  • Fatty acids:
    • Have a carboxyl group at one end (head) and a long hydrocarbon tail (hydrophobic).
    • Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds, introducing kinks that influence packing and fluidity.
    • Examples: palmitic acid (C16:0, saturated); oleic acid (C18:1, monounsaturated).
  • Triacylglycerols (triglycerides):
    • Three fatty acids esterified to glycerol.
    • Main energy reserve stored in fat droplets in adipocytes.
    • Fatty acids can be mobilized and oxidized for energy when needed.
  • Phospholipids:
    • Amphipathic: hydrophilic (polar) head group and hydrophobic (nonpolar) tails.
    • Structure: glycerol backbone, two fatty acid tails, and a phosphate-containing head group (often linked to choline or other polar groups).
    • Major constituent of cell membranes; form phospholipid bilayer.
  • Steroids:
    • Characteristic multi-ring structure.
    • Examples: cholesterol (membrane component); testosterone (steroid hormone).
  • Glycolipids:
    • Lipids with hydrophobic tails and a polar sugar-containing headgroup, but no phosphate.
    • Important in membranes for cell recognition and signaling.
  • Phospholipid bilayer concept:
    • Polar head groups face aqueous environments; tails face inward, forming a hydrophobic interior that acts as a barrier.

Amino acids, proteins, and peptide bonds

  • Amino acids:
    • Central (α) carbon attached to:
    • Amino group (–NH2, protonated to –NH3+ at physiological pH)
    • Carboxyl group (–COOH, deprotonated to –COO− at physiological pH)
    • Hydrogen atom
    • Side chain (R group) that distinguishes amino acids
    • At pH ~7, amino acids exist as zwitterions: extH3extN+−extC(extR)−extCOO−ext{H}_3 ext{N}^+- ext{C}( ext{R})- ext{COO}^-
    • 20 standard amino acids with three-letter and one-letter codes (e.g., Ala/A, Phe/F, Lys/K, Glu/E, etc.).
  • Amino acid families by side chain properties:
    • Nonpolar (hydrophobic): e.g., Ala (A), Val (V), Leu (L), Ile (I), Met (M), Phe (F), Pro (P), Gly (G), Trp (W).
    • Polar uncharged: e.g., Ser (S), Thr (T), Asn (N), Gln (Q), Cys (C), Tyr (Y).
    • Acidic (negatively charged at neutral pH): Asp (D), Glu (E).
    • Basic (positively charged at neutral pH): Lys (K), Arg (R), His (H).
  • Peptide bonds:
    • Amino acids are linked by a peptide bond (amide linkage) formed between the carboxyl group of one amino acid and the amino group of the next.
    • General reaction (condensation):
      ext{AminoAcid}1- ext{COOH} + ext{AminoAcid}2- ext{NH}2 ightarrow ext{AminoAcid}1- ext{NH-CO-AminoAcid}2 + ext{H}2 ext{O}
    • The resulting polymer is a polypeptide; proteins are long polypeptides and are directed from the N-terminus to the C-terminus.
    • The peptide bond is planar with limited rotation around the C–N bond, giving directionality: N-terminus on the left, C-terminus on the right.
  • Protein structure and folding:
    • The sequence of amino acids (primary structure) determines the conformational possibilities.
    • Noncovalent interactions (H-bonds, ionic interactions, van der Waals, hydrophobic effects) drive folding into higher-level structures (secondary, tertiary, quaternary) and define function.

Nucleotides, nucleic acids, and energy currency

  • Nucleotides are the subunits of DNA and RNA.
  • Constituents of a nucleotide:
    1) Phosphate group
    2) Five-carbon sugar (ring) (deoxyribose in DNA; ribose in RNA)
    3) Nitrogen-containing base (A, T/U, C, G)
  • Nucleotides polymerize via phosphodiester bonds between the 5' carbon of one sugar and the 3' carbon of the next sugar, forming a sugar-phosphate backbone with directional polarity (5' to 3').
  • DNA:
    • Contains four bases: A, T, C, G; sugar is deoxyribose.
    • Forms a double helix with base pairing: A with T, C with G.
  • RNA:
    • Four bases: A, U, C, G; sugar is ribose; usually single-stranded.
    • Roles include carrying genetic information and participating in protein synthesis.
  • Nucleotides also serve as:
    • Energy carriers (e.g., ATP)
    • Building blocks for nucleic acids (DNA, RNA)
    • Coenzymes (e.g., NAD, FAD, CoA)
    • Chemical messengers (e.g., cyclic AMP)
  • ATP as energy currency:
    • ATP contains three phosphoanhydride bonds; energy released upon hydrolysis drives cellular work:
      ext{ATP} + ext{H}2 ext{O} ightarrow ext{ADP} + ext{P}i + ext{energy}
    • This energy can be used directly for cellular processes or to drive endergonic reactions.
  • Nucleic acids and derivatives:
    • Nucleotides can be modified to form derivatives like cyclic AMP (cAMP) used in signaling, or coenzyme A (CoA) for metabolic reactions.

Macromolecules and their assembly

  • Macromolecules include:
    • Nucleic acids (DNA, RNA)
    • Proteins
    • Polysaccharides
    • Lipids (membrane lipids and energy storage lipids)
  • Macromolecules are assembled from subunits via covalent bonds (e.g., peptide bonds, phosphodiester bonds) and are stabilized by noncovalent interactions.
  • The specific sequence of monomer units (e.g., amino acids in a protein, nucleotides in DNA) is essential for function and is not random.
  • Noncovalent bonds define the precise shape and binding specificity of macromolecules; they allow dynamic interactions with other molecules while maintaining overall structural integrity.
  • Both covalent and noncovalent bonds are required for proper macromolecule assembly and function (e.g., ribosome-mediated protein synthesis uses noncovalent interactions to correctly fold and assemble proteins; covalent bonds link subunits into polymers).

Connections to broader biology and practical implications

  • The chemical components of cells beneath the macromolecules explain:
    • How energy is stored and released (glycogen, starch as storage polysaccharides; ATP in energy transfer).
    • How information is stored and transmitted (DNA/RNA and sequence-specific information).
    • How membranes are built and maintained (phospholipids and cholesterol in membranes).
    • How structure and function co-evolve (amphipathic lipids form bilayers; proteins fold based on amino acid sequence; carbohydrates provide recognition signals).
  • Ethical, philosophical, and practical implications include:
    • Understanding metabolism guides medical and environmental interventions.
    • Insight into protein misfolding and disease can drive therapeutics.
    • Knowledge of macromolecular assembly informs biotechnology and synthetic biology.

Quick reference equations and key formulas

  • General carbohydrate formula: (CH<em>2O)</em>n(CH<em>2O)</em>n
  • Monosaccharide formula examples: extGlucose=C<em>6H</em>12O<em>6ext{Glucose} = C<em>6H</em>{12}O<em>6, extRibose=C</em>5H<em>10O</em>5ext{Ribose} = C</em>5H<em>{10}O</em>5
  • Ring formation concept (hydroxyl reacts with carbonyl): aldehyde/ketone + alcohol → cyclic hemiacetal/acetal (in aqueous solution).
  • Condensation (dehydration) reaction for disaccharide formation:
    ext{Monosaccharide}1 + ext{Monosaccharide}2
    ightarrow ext{Disaccharide} + H_2O
  • Hydrolysis (glycosidic bond cleavage):
    ext{Disaccharide} + H2O ightarrow ext{Monosaccharide}1 + ext{Monosaccharide}_2
  • Peptide bond formation in proteins (condensation):
    ext{AminoAcid}1 - ext{COOH} + ext{AminoAcid}2 - ext{NH}2 ightarrow ext{AminoAcid}1- ext{NH-CO-AminoAcid}2 + H2O
  • Phosphodiester bond linking nucleotides (5' to 3' direction):
    ext{Nucleotide}n - ext{phosphate} - ext{Sugar} - ext{OH} ightarrow ext{Nucleotide}n- ext{O}- ext{P}- ext{O}- ext{Sugar}( ext{Next}) {where the phosphate links the 5' carbon of one sugar to the 3' carbon of the next}
  • ATP hydrolysis (energy release):
    ext{ATP} + ext{H}2 ext{O} ightarrow ext{ADP} + ext{P}i + ext{energy} $$

Note

  • The table summarizing the chemical composition of bacterial cells lists broad classes of cellular constituents (water, ions, sugars, amino acids, nucleotides, fatty acids, phospholipids, and macromolecules) and their roles/types; the exact numerical values in that table are presented in the source material and illustrate the relative abundance and diversity of cellular components (e.g., water is a major component; macromolecules constitute most of the organic mass).
  • The four main families of small organic molecules synthesized in cells are reiterated: SUGARS, FATTY ACIDS, AMINO ACIDS, and NUCLEOTIDES, which combine to form polysaccharides, glycogen, starch, lipids, proteins, and nucleic acids.
  • The material emphasizes that chemistry governs biology: bond types, functional groups, and noncovalent interactions shape structure, recognition, and function across all biomolecules.

Sources for further study

  • Review the slide deck on Chapter 2 (Chemical Components of Cells) for visualizations of orbital diagrams, ring formations, bond types, and the specifics of each functional group.
  • Revisit the sections on nucleotides, ATP, and nucleotide derivatives (cAMP, CoA) for signaling and metabolic roles.
  • Practice drawing covalent vs noncovalent interactions and predicting solubility (hydrophilic vs hydrophobic) of simple molecules based on polarity and hydrogen-bonding capacity.