Chapter 2: The Chemical Context of Life – Study Notes

Overview: The Importance of Chemistry to Life

Matter, Elements, and Compounds

Definitions and Properties

• Matter: Anything that takes up space and has mass.

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements in a fixed ratio, exhibiting emergent properties distinct from its constituent elements.

The Elements of Life

Essential and Trace Elements

• About 20–25% of the 92 natural elements are essential for life.

• Essential elements: Required for an organism to live a healthy life and reproduce.

• Trace elements: Required in minute quantities (e.g., iodine for thyroid function in vertebrates).

Atomic Structure and Subatomic Particles

Atoms and Their Components

• Atom = smallest unit of an element

• Made of:

  • protons (+)

  • neutrons (0)

  • electrons (−)

• Chemical reactions involve electron rearrangement that breaks and forms chemical bonds between atoms

• Atoms react by gaining, losing, or sharing electrons to become stable

• Atoms form bonds to achieve stability (full outer shell)

Electron Distribution and Chemical Properties

Valence Electrons and Reactivity

• Valence shell = outermost electron shell

• Valence electrons = outer shell electrons

• Determine bonding and reactivity

  • C = 4

  • H = 1

  • O = 6

  • N = 5

• Atoms want a full outer shell for stability

Chemical Bonds and Molecular Formation

Types of Chemical Bonds

• Covalent bonds: sharing electrons

  • Nonpolar = equal sharing (O₂)

  • Polar = unequal sharing (H₂O)

  • Electronegativity = how strongly atoms attract electrons

• Ionic bonds: transfer electrons, form ions (positive + negative)

  • Example: NaCl

  • Formed when one atom transfers an electron to another, creating ions (cation: positive, anion: negative).

  • Ionic compounds (salts) often form crystalline structures.

• Hydrogen bonds: attraction between H (bonded to O/N/F) and another electronegative atom

Bonds	Description
Ionic	transfer of electrons
Covalent	sharing of electrons
Hydrogen	attraction between molecules

Weak Chemical Interactions

• Weak bonds (hydrogen bonds, van der Waals interactions) are crucial for the structure and function of large biological molecules.

Water and Life

Hydrogen Bonding in Water

• Water is a polar molecule, with a partial negative charge on oxygen and partial positive charges on hydrogen.

• Hydrogen bonds between water molecules give rise to unique properties essential for life.

• Water supports life processes, regulates temperature, and dissolves substances

• Hydrogen bonding is the reason for water’s unique properties

Emergent Properties of Water

• Cohesion: Water molecules stick together due to hydrogen bonding.

• Adhesion: Water molecules stick to other substances.

• Surface tension: The difficulty of breaking the surface of a liquid due to cohesion.

• High specific heat: Water resists temperature changes, stabilizing environments.

• Expansion upon freezing: Ice is less dense than liquid water, allowing it to float.

• Versatility as a solvent: Water dissolves many substances due to its polarity.

Examples

• Cohesion → water droplets

• Adhesion → water climbing glass/plant walls

• Surface tension → water striders

• Capillary action → plants pulling water up

• Solvent → salt dissolving in water

Water as a Solvent

Solutions, Solvents, and Solutes

• Solution: Homogeneous mixture of substances.

• Solvent: Dissolving agent (water in aqueous solutions).

• Solute: Substance dissolved in the solvent.

• Water forms hydration shells around ions and can dissolve polar molecules.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water (e.g., salts, sugars).

• Hydrophobic: Substances that repel water (e.g., oils).

Acids, Bases, and pH

Water Dissociation and pH Scale

• Water can dissociate into H+ (hydronium ion) and OH- (hydroxide ion).

• Acids: Increase H+ concentration.

• Bases: Reduce H+ concentration.

• pH scale: Measures H+ concentration; pH = -log[H+].

• pH 7 is neutral; lower values are acidic, higher values are basic.

• pH affects enzyme shape, reaction rates, and protein function

  • low pH = high H⁺

  • high pH = low H⁺

  • pH affects:

    • enzyme shape

    • reaction rates

    • protein function

Buffers

• Buffers minimize changes in pH by accepting or donating H+ as needed.

• Example: Carbonic acid acts as a buffer in human blood.

• Buffers prevent sudden pH changes in organisms