Biology 09/02

PAL Sessions and Study Guidance

  • PAL sessions start today. Your PAL leaders will send communications and post materials to Canvas. Check the times on the board and in Canvas, especially if you want direction on how to study.

  • If you have questions about the study guide (chapter 2, differences between hydrophilic and hydrophobic compounds), ask or review now because the slides tie into those questions and they will accumulate as content builds toward the first exam.

  • Stay on top of daily work to stay honest and prepared. The first exam is still a bit away, but more content will be covered before then.

Water: Universal Chemistry for Life

  • Water is not an organic molecule (no carbon), but it is crucial for life and our planetary chemistry.

  • Water can form up to a maximum of four hydrogen bonds per molecule, which drives its interactions with other water molecules and with charged or polar substances.

  • Hydrophilic vs hydrophobic:

    • Hydrophilic: water-loving, interacts with polar or charged species.

    • Hydrophobic: water-fearing, interacts poorly with water; hydrophobic compounds tend to separate from water and aggregate (e.g., oil on water).

  • Amphipathic molecules: have both hydrophilic and hydrophobic regions. These are important as we approach cell membranes (outer membrane considerations).

  • Hydrogen bonding and liquid water vs ice:

    • In liquid water, most molecules form and break hydrogen bonds rapidly; many still form some hydrogen bonds.

    • At colder temperatures, water can form a crystalline structure (ice) with maximum hydrogen bonding (up to four per molecule) and more spacing between molecules.

    • Ice is less dense than liquid water, so it floats. This has ecological and environmental implications, e.g., polar ecosystems and insulation of underwater habitats.

  • Water’s role in temperature regulation:

    • Water exhibits high specific heat, resisting temperature changes and buffering environments and organisms.

    • Specific heat of water: c=1 calg⋅∘Cc = 1\ \frac{\text{cal}}{\text{g} \cdot {}^\circ\text{C}}

    • To raise 1 g of water by 1°C requires 1 calorie of energy, due to breaking hydrogen bonds and increasing molecular motion.

    • Energy transfers (heat) flow from higher to lower thermal energy bodies.

  • Thermal energy vs temperature:

    • Kinetic energy describes motion; thermal energy is the subset related to molecular movement.

    • Temperature measures the average thermal energy of a system.

    • Heat is the transfer of thermal energy between bodies with different thermal energies (hot to cold).

  • Water buffering and biology:

    • Water’s hydrogen bonding helps buffer and absorb heat from metabolic reactions, protecting organisms from overheating.

    • In humid environments (e.g., South Carolina summers), heat is retained in the atmosphere, reducing nighttime cooling relative to drier climates.

pH, Acids, Bases, and Water Ionization

  • Water ionization:

    • Dissociation: H2O⇌H++OH−\mathrm{H_2O \rightleftharpoons H^+ + OH^-}

    • In pure water, [H extsuperscript{+}] = [OH extsuperscript{−}] ≈ 10−7 M10^{-7}\ \text{M}.

    • Ion product of water: Kw=[H+][OH−]=10−14 M2K_w = [H^+][OH^-] = 10^{-14}\ \text{M}^2

  • pH concept:

    • Definition: pH=−log⁡10[H+]\mathrm{pH} = -\log_{10} [H^+]

    • Pure water has a neutral pH of 7; numbers greater than 7 are basic (lower hydrogen ion concentration), numbers less than 7 are acidic (higher hydrogen ion concentration).

    • Each unit change on the pH scale represents a tenfold change in hydrogen ion concentration: e.g., pH 6 has [H+]=10−6 M[H^+] = 10^{-6}\ \text{M}; pH 8 has [H+]=10−8 M[H^+] = 10^{-8}\ \text{M}.

  • Acids and bases in solution:

    • Acid: increases hydrogen ion concentration (lowers pH).

    • Base: reduces free hydrogen ion concentration (increases pH) or accepts hydrogen ions.

  • Quick comparison practice:

    • If comparing ammonia at pH 11 and baking soda at pH 9, baking soda has about 100× more hydrogen ions than ammonia because the pH decrease from 11 to 9 corresponds to a 10^2 increase in [H^+].

iClicker Review: Key Concept Checks

  • True/False: The importance of any element in an organism is proportional to its concentration. Answer: False. Trace elements can be essential even at very low concentrations (e.g., iron makes up <0.01% of body mass but is essential for oxygen transport).

  • Multi-select: Identify false statements about chemical equilibrium:

    • Equilibrium involves a steady ratio of reactants to products, not all reactants converted to products.

    • Equilibrium is affected by environmental conditions (temperature, etc.).

    • Equilibrium does not halt chemical reactions; reactions continue at equal rates in both directions.

Carbon: Versatility and the Basis of Life

  • Carbon’s four covalent bonds:

    • Carbon can form up to four bonds due to its four valence electrons, allowing tetrahedral geometry and a vast diversity of molecules.

    • This versatility underlies the structural diversity of life: long chains, branched structures, rings, and multiple functional groups.

  • Isomers:

    • Isomers have the same chemical formula but different connectivity or structure.

    • Example: two butene isomers differ by the location of a double bond; both are C\