Chapter 2 second half

Chemistry of Life

The Nature of Water

Water is a fundamental molecule for life, with unique chemical and physical properties that make it essential for biological systems. Its structure and interactions underlie many processes in living organisms.

• Composition: A water molecule (H2O) consists of one oxygen atom and two hydrogen atoms.

• Polarity: Water is a polar molecule due to a difference in electronegativity between oxygen and hydrogen atoms. Oxygen is more electronegative, pulling shared electrons closer and creating a partial negative charge on the oxygen and partial positive charges on the hydrogens.

• Covalent Bonds: The bonds between hydrogen and oxygen within a single water molecule are covalent bonds, where electrons are shared.

• Hydrogen Bonds: Water molecules interact with each other through hydrogen bonds, which are weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.

Properties of Water

Water's unique properties arise from its polarity and ability to form hydrogen bonds. These properties are critical for life and influence biological processes at all levels.

• High Specific Heat: Water has a high specific heat, meaning it can absorb or release a large amount of heat with only a slight change in its own temperature. This property helps stabilize temperatures in organisms and environments.

• Heat Absorption and Release:

  • Heat absorbed by water breaks hydrogen bonds.

  • Heat released by water forms hydrogen bonds.

  • During evaporation, the highest energy ("hottest") molecules leave, cooling the remaining liquid (evaporative cooling).

• Cohesion and Adhesion:

  • Cohesion: The attraction between molecules of the same substance (e.g., water molecules sticking together via hydrogen bonds).

  • Adhesion: The attraction between molecules of different substances (e.g., water molecules sticking to plant cell walls).

• Surface Tension: Water has a high surface tension due to hydrogen bonding, making it difficult to break the surface of a liquid. This allows small insects, like water striders, to walk on water.

• Capillary Action: The combined effect of cohesion and adhesion enables water to move upward through narrow tubes, such as the xylem in plants, helping transport water from roots to leaves.

• Universal Solvent: Water's polarity allows it to dissolve many substances, especially other polar molecules and ionic compounds. This property is summarized by the phrase "like dissolves like." Water facilitates chemical reactions in cells by dissolving reactants and products.

• Density of Ice vs. Liquid Water: Water is less dense as a solid (ice) than as a liquid. In ice, hydrogen bonds form a stable lattice, keeping molecules further apart. In liquid water, hydrogen bonds constantly break and reform, allowing molecules to be closer together. This property allows ice to float on water, insulating aquatic life in cold environments.

Acids, Bases, and pH

Water can dissociate into ions, influencing the acidity or basicity of solutions. The pH scale measures the concentration of hydrogen ions in a solution.

• Dissociation of Water: Water molecules can break apart (dissociate) into hydrogen ions (H+) and hydroxide ions (OH-).

• Acids: Substances that increase the concentration of H+ ions in a solution.

• Bases: Substances that increase the concentration of OH- ions or decrease H+ concentration.

• pH Scale: The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

  • pH is calculated as: $pH = -\log_{10}[H^+]$

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions as needed.

Acidification and Environmental Impact

Changes in the pH of natural waters, such as ocean acidification, can have significant effects on living organisms and ecosystems.

• Ocean Acidification: The process by which increased atmospheric CO2 dissolves in seawater, forming carbonic acid and lowering ocean pH.

• Root Cause: The primary cause is the absorption of excess CO2 from the atmosphere due to human activities such as burning fossil fuels.

• Impact on Organisms: Acidification can harm marine organisms, especially those with calcium carbonate shells or skeletons (e.g., corals, mollusks), by making it harder for them to form and maintain their structures.

• Solutions: Solutions include reducing CO2 emissions, protecting and restoring marine ecosystems, and developing technologies to remove CO2 from the atmosphere.

Table: Comparison of Water's Properties

Property	Description	Biological Importance
High Specific Heat	Absorbs/releases heat with little temperature change	Stabilizes organism and environmental temperatures
Cohesion	Water molecules stick together	Enables transport in plants, surface tension
Adhesion	Water molecules stick to other substances	Helps water move up plant vessels
Surface Tension	Difficulty in breaking water's surface	Allows small organisms to move on water
Universal Solvent	Dissolves many substances	Facilitates biochemical reactions
Lower Density as Solid	Ice floats on liquid water	Insulates aquatic life in cold climates

Example: Capillary action in plants allows water to move from roots to leaves, supporting photosynthesis and nutrient transport.

Additional info: Some explanations and context were expanded for clarity and completeness based on standard biology curriculum.