Water and Life Notes

2.1 Life depends on the properties of water

  • Water is essential for life on Earth. Life originated in water and water availability influences where and how species live. Many cells are surrounded by water, whether inside organisms or in habitats (lakes, oceans). Water is the single most abundant molecule inside cells, so it is the medium in which life’s molecules interact.

  • NASA strategy: follow the water when searching for extraterrestrial life, because Earth is distinguished by abundant water and life supported by water. Evidence suggests liquid water once existed on ancient Mars, raising questions about past life there.

  • Water is a simple molecule with profound emergent properties arising from interactions among many water molecules, not present in isolated molecules or in hydrogen/oxygen atoms alone.

  • Emergent property (definition): a property of a system that cannot be predicted simply from the properties of its individual parts; for water, the collective behavior of many molecules yields properties critical to life.

  • Water’s basic composition:

    • Water is made of two hydrogen atoms and one oxygen atom, written as extH2extOext{H}_2 ext{O}.
    • The bonds are polar covalent bonds because oxygen is more electronegative than hydrogen, creating partial charges: δ⁺ on H and δ⁻ on O.

  • Focus of this module: explore the chemistry of water itself and how water interacts with other molecules, including chemical reactions in aqueous environments.

  • Key ideas summarized in the section: water’s polar nature, hydrogen bonding, and emergent properties that support life.

  • Focus on the big ideas (AP exam): SYSTEMS INTERACTIONS – examine how water’s properties influence interactions between water molecules and other substances.

  • Hydrogen bonds (definition and role)

    • Because water’s O–H bonds are polar, water molecules orient to minimize repulsion and maximize favorable attractions: the partially positive hydrogen (Hδ⁺) of one molecule is attracted to the partially negative oxygen (Oδ⁻) of another.
    • A hydrogen bond is a weak interaction between a hydrogen atom covalently bound to an electronegative atom (e.g., O, N) and another electronegative atom in a neighboring molecule. It is commonly depicted as a dotted line.
    • Hydrogen bonds are weaker than covalent and ionic bonds, but a large network of them yields substantial collective strength and many emergent properties.
    • In water, hydrogen bonds continually form and break, giving liquid water its dynamic structure.

  • Cohesion, adhesion, and surface tension

    • Cohesion: water molecules stick to each other due to hydrogen bonding. This drives drops, puddles, streams, rivers, lakes, oceans, and supports movement of water in plants.
    • Cohesion in plants enables a water column to rise through xylem from roots to leaves; under ideal conditions, water can rise up to about 100 meters in tall trees like giant sequoias and coast redwoods.
    • Adhesion: water sticks to other surfaces/materials (e.g., dew droplets sticking to a leaf), aiding movement through plant transport channels.
    • Surface tension: strong binding at the surface due to cohesive forces among surface molecules creates a film on the surface; water has high surface tension, allowing leaves to float and enabling some insects (e.g., water strider) to distribute their weight on the surface.

  • Structure of water across phases

    • Ice (solid): most water molecules form hydrogen bonds to four neighbors, creating an open crystalline lattice. This makes ice less dense than liquid water.
    • Liquid water: some hydrogen bonds are broken, allowing molecules to come closer together; liquid water is denser than ice.
    • Steam (gas): molecules move rapidly, bonds break frequently, no regular hydrogen-bond network.
    • Consequence: liquid water is denser than ice, allowing ice to float. This unusual property helps aquatic life survive winter by insulating deeper waters.

  • Density of ice vs liquid water

    • Unlike most substances, water expands upon freezing, so ice is less dense than liquid water; ice floats.
    • This property creates an insulating ice layer on bodies of water, protecting aquatic life in winter.

  • Specific heat and heat buffering

    • Water has a high capacity to resist temperature change due to extensive hydrogen bonding.
    • The energy required to change water’s temperature is the specific heat; for liquid water, it is c=1.000extcalg−1ext°C−1c = 1.000 ext{ cal g}^{-1} ext{°C}^{-1}.
    • Other liquids have lower specific heats (e.g., c<em>extethanol ≈0.6 c</em>extwaterc<em>{ ext{ethanol}} \, \approx 0.6\, c</em>{ ext{water}}).
    • This high specific heat buffers cellular temperatures, stabilizes environmental temperatures (oceans, climate), and helps cool fires: a lot of energy is consumed to break hydrogen bonds before temperature rises.

  • Water as a solvent (solvent properties of water)

    • Water is the solvent for many biological molecules because it is polar and can form hydrogen bonds.
    • Hydrophilic (water-loving) molecules are polar and dissolve in water by hydrogen bonding (e.g., ammonia, extNH3ext{NH}_3, dissolving via interactions with water’s extHδ+ext{Hδ⁺} and extOδ−ext{Oδ⁻}).
    • Ammonia example: water forms hydrogen bonds with ammonia; water’s Hδ⁺ is attracted to N in ammonia, and the Oδ⁻ is attracted to H in ammonia; dissolves in water when surrounded by water molecules.
    • Hydrophilic molecules generally dissolve because of polar interactions with water; many organic macromolecules (proteins, nucleic acids, carbohydrates) also interact with water via polar/charged groups, though solubility can vary by size and structure.
    • Hydrophobic molecules (oil, nonpolar components) minimize contact with water and tend to separate from aqueous solutions (hydrophobic effect). Mixing oil with water requires agitation; otherwise, they separate.
    • Hydrophobic effects are important for protein folding and cell membrane formation.
    • AP Exam tip on solubility: understanding hydrophilic vs hydrophobic properties is crucial for function of major biological macromolecules.

  • Water dissociation and pH

    • Water can dissociate: extH2extO<br/>ightleftharpoonsextH++extOH−ext{H}_2 ext{O} <br /> ightleftharpoons ext{H}^+ + ext{OH}^-
    • In aqueous solutions, small amounts of extH+ext{H}^+ and extOH−ext{OH}^- are present; these ions influence chemistry and biology. Organisms regulate their concentrations tightly.
    • Acids donate extH+ext{H}^+ ions; bases accept extH+ext{H}^+ ions.
    • pH is a measure of how acidic/basic a solution is; defined by extpH=− log⁡[H+]ext{pH} = -\,\log[\text{H}^+] where [H⁺] is the hydrogen ion concentration.
    • pH scale ranges from 0 to 14. 7 is neutral;
    • Examples: lemon juice (~pH 2) and stomach fluid (~pH 1.5–2.0) are acidic; seawater (~pH 8) and human blood (~pH 7.4) are slightly basic; municipal water at ~7.2 is slightly basic.
    • The pH scale is logarithmic: each unit change corresponds to a tenfold change in [H+][H^+].
    • Practical notes:
    • Small pH changes imply large changes in hydrogen ion concentration.
    • The body maintains pH around 7 for most cells; different body fluids have different pH values to support specific functions.
    • AP Exam Tip on pH: You are not expected to perform pH calculations, but you should understand the logarithmic nature of the scale and how a change of one pH unit changes [H+][H^+] by a factor of 10.

  • Concept checks (quick self-test prompts)

    • 1) Describe how polarity enables hydrogen bonding in water.
    • 2) Identify three properties of water that result from hydrogen bonding.
    • 3) Describe the process by which ammonia dissolves in water.
    • 4) Identify the atoms of water and ammonia that form hydrogen bonds with one another.
    • 5) Explain why municipal water with pH ~7.2 is slightly basic.
    • 6) Calculate how many times more [H+][H^+] is present in a solution with pH 3 than in a solution with pH 4.

  • 2.3 Dehydration synthesis reactions build molecules, and hydrolysis reactions break them down

  • Overview

    • Water participates directly in building (synthesis) and breaking down (hydrolysis) large biological molecules.
    • Polymers are made by linking smaller subunits via covalent bonds; dehydration synthesis releases a water molecule when the bond forms.
    • Hydrolysis breaks covalent bonds by adding water across the bond, producing smaller units.

  • Dehydration synthesis reactions

    • Definition: A reaction that builds larger molecules from smaller units with the removal of a water molecule (hence the name “dehydration”).
    • General form (illustrative): subunitA + subunitB → larger molecule + extH2extOext{H}_2 ext{O}
    • Example in biology: joining amino acids to form proteins; joining monosaccharides to build carbohydrates; joining glycerol and fatty acids to form lipids (e.g., a monoglyceride).
    • Figure 2.8 example (monoglyceride): glycerol + fatty acid → monoglyceride + water; glycerol loses an H, fatty acid loses an OH, water (H₂O) is produced.
    • Biological significance: builds polymers like proteins (polypeptides), polysaccharides, and some lipids; essential for constructing cellular structures and functions.

  • Hydrolysis reactions

    • Definition: Hydrolysis breaks covalent bonds by adding water across the bond; water is split into H and OH that become parts of the products.
    • General form (illustrative): larger molecule + extH2extOext{H}_2 ext{O} → subunitA + subunitB + …
    • Example: hydrolysis of a disaccharide (two-unit sugar) into two monosaccharides (one-unit sugars) by water addition (reverse of dehydration).
    • In biology, hydrolysis breaks down polymers for energy extraction and to obtain subunits for rebuilding biological macromolecules.

  • Interplay and significance

    • Water’s involvement in both dehydration synthesis and hydrolysis makes it central to metabolism, growth, and recycling of cellular components.
    • Life’s dependence on water’s unique properties is reinforced by the fact that under Earth-like conditions, water is especially suitable for these kinds of reactions.

  • TABLE 2.1: Special properties of water (summary)

    • Polar: Regions of partial positive and negative charges on the molecule drive interactions and solvent behavior.
    • Cohesive: Hydrogen bonding between water molecules leads to cohesion.
    • Adhesive: Hydrogen bonding between water and other molecules facilitates interactions with surfaces and materials.
    • High surface tension: Creates a surface film on water, enabling phenomena like surface-floating objects.
    • Less dense as a solid than as a liquid: Ice floats on liquid water; ice forms an open crystal lattice due to hydrogen bonding.
    • High specific heat: Hydrogen bonding gives water unusually high specific heat, making it resistant to temperature changes.
    • Good solvent: Water dissolves many polar and charged substances; water forms hydrogen bonds with solutes and solvent properties influence macromolecule behavior.

  • Emergent properties in context

    • The combination of polarity, hydrogen bonding, cohesion, adhesion, high surface tension, and high specific heat collectively underpin water’s role as the medium of life and its ability to facilitate chemical reactions.

  • Concept Check (further questions)

    • 7) Describe a dehydration synthesis reaction.
    • 8) Describe a hydrolysis reaction.
    • 9) Identify the functions of dehydration synthesis reactions and hydrolysis reactions.

  • Analytical note: Scientific notation and measurement concepts (as they relate to the content above)

    • Scientific notation basics (as reviewed in the module): a number is written as aimes10na imes 10^{n} where 1 ≤ a < 10 and n is an integer.
    • Addition and subtraction in scientific notation require matching exponents; when exponents do not match, terms must be converted so exponents match before combining coefficients.
    • Example: (3.1×107)+(4.6×107)=7.7×107(3.1 \times 10^{7}) + (4.6 \times 10^{7}) = 7.7 \times 10^{7}
    • Significant figures in addition/subtraction are determined by the term with the fewest decimal places.
    • Example: (8.3×103)−(5.22×103)=3.1×103(8.3 \times 10^{3}) - (5.22 \times 10^{3}) = 3.1 \times 10^{3}
    • Multiplication/division in scientific notation: multiply/divide coefficients and add/subtract exponents.
    • Example: (1.1×106)×(7.2×102)=(1.1×7.2)×106+2=7.92×108=7.9×108(1.1 \times 10^{6}) \times (7.2 \times 10^{2}) = (1.1 \times 7.2) \times 10^{6+2} = 7.92 \times 10^{8} = 7.9 \times 10^{8}
    • Example (division): 6.5×10−53.25×10−2=6.53.25×10−5−(−2)=2.0×10−3\frac{6.5 \times 10^{-5}}{3.25 \times 10^{-2}} = \frac{6.5}{3.25} \times 10^{-5 - (-2)} = 2.0 \times 10^{-3}
    • Significant figures rules for multiplication/division: the final answer has as many significant figures as the term with the fewest significant figures.

  • Worked practice and examples from the text

    • Example 1: Aluminum has a specific heat of 0.215 extcal g−1 °C−10.215\ ext{cal}\, \text{g}^{-1}\,\text{°C}^{-1}. In scientific notation: 2.15×10−1 cal g−1 °C−12.15\times 10^{-1}\ \text{cal}\,\text{g}^{-1}\,\text{°C}^{-1} with three significant figures.
    • Example 2: Gold has a specific heat of 0.0301 cal g−1 °C−10.0301\ \text{cal}\,\text{g}^{-1}\,\text{°C}^{-1}. In scientific notation: 3.01×10−2 cal g−1 °C−13.01\times 10^{-2}\ \text{cal}\,\text{g}^{-1}\,\text{°C}^{-1} with three significant figures.
    • Your Turn (beekeeping example): Two hives with 12,500 bees and 4.1×10⁴ bees. Sum: 12,500 + 41,000 = 53,500 = 5.35×10⁴. Because the second quantity has two significant figures, the result should be reported with two significant figures: 5.4×1045.4\times 10^{4}.

  • Key takeaways

    • Water’s polarity and hydrogen bonding generate a suite of properties that support life: solvent capability, cohesion/adhesion, surface tension, high heat capacity, and the unique density behavior of ice.
    • Water’s role as the medium of life is inseparable from its chemical reactivity (dehydration synthesis and hydrolysis) and its interaction with acids/bases (pH).
    • The ability of water to dissolve polar/charged substances underpins many biochemical processes and macromolecule behavior.