Study Notes: Chemical Context of Life & Water and Life (Chapters 2 & 3, Campbell Biology)

Chemical Context of Life

Elements and Compounds

The chemical context of life begins with understanding the elements and compounds that make up living organisms. Elements are substances that cannot be broken down into other substances by chemical reactions. Compounds are substances consisting of two or more elements combined in a fixed ratio.

• Major elements in living organisms: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N)

• Trace elements: Required in minute quantities (e.g., iron, iodine)

• Atoms: The smallest unit of an element, composed of protons, neutrons, and electrons

Chemical Bonds

Atoms interact through chemical bonds to form molecules essential for life.

• Covalent bonds: Atoms share electrons; can be polar (unequal sharing) or nonpolar (equal sharing)

• Ionic bonds: Atoms transfer electrons, resulting in charged ions

• Hydrogen bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen)

Polarity and Electronegativity

Electronegativity is the tendency of an atom to attract electrons. Polar covalent bonds occur when electrons are shared unequally, resulting in partial charges within the molecule.

• Example: Water (H2O) is a polar molecule due to the higher electronegativity of oxygen compared to hydrogen.

Water and Life

Properties of Water

Water is essential for life due to its unique chemical and physical properties, which arise from its polarity and ability to form hydrogen bonds.

• Cohesion: Water molecules stick to each other due to hydrogen bonding.

• Adhesion: Water molecules stick to other substances.

• High specific heat: Water can absorb or release large amounts of heat with little temperature change.

• High heat of vaporization: Water requires significant energy to change from liquid to gas.

• Expansion upon freezing: Ice is less dense than liquid water due to hydrogen bond arrangement.

Hydrogen Bonding in Water

Hydrogen bonds are responsible for many of water's life-supporting properties.

• Each water molecule can form up to four hydrogen bonds with neighboring water molecules.

• Hydrogen bonding leads to surface tension, cohesion, and adhesion.

Biological Importance of Water's Properties

Water's properties are crucial for biological processes such as temperature regulation, nutrient transport, and cellular function.

• Cohesion and adhesion: Enable water transport in plants through xylem vessels.

• High specific heat: Stabilizes temperatures in organisms and environments.

• Solvent properties: Water dissolves many substances, facilitating chemical reactions in cells.

Application: Water Transport in Plants

Water moves upward in plants due to cohesion and adhesion, allowing for efficient transport from roots to leaves.

• Transpiration: Evaporation of water from leaves pulls water upward through the plant.

• Capillary action: Adhesion of water to cell walls helps counteract gravity.

Impact of Disrupted Hydrogen Bonding

If hydrogen bonding in water were disrupted, many of its life-supporting properties would be lost.

• Loss of cohesion and adhesion: Water transport in plants would be impaired.

• Reduced specific heat: Organisms would be less able to regulate temperature.

• Decreased solvent ability: Cellular reactions would be less efficient.

Key Equations

• Number of hydrogen bonds per water molecule: Up to 4

• Example calculation: If a water sample contains 1,000 molecules, the maximum number of hydrogen bonds is $4 \times 1,000 = 4,000$

Comparison Table: Water's Properties

Property	Biological Importance
Cohesion	Transport of water in plants
Adhesion	Water movement against gravity
High Specific Heat	Temperature regulation
Solvent Ability	Facilitates biochemical reactions