Water and Carbon: The Chemical Basis of Life – Study Notes

Chapter 2: Water and Carbon – The Chemical Basis of Life

Introduction to Chemical Evolution

Chemical evolution is the leading explanation for the origin of life on Earth. It describes the process by which simple molecules combined to form increasingly complex carbon-containing substances, eventually leading to molecules capable of self-replication. This transition marked the shift from chemical to biological evolution, where natural selection began to drive the development of life.

• Chemical evolution: Formation of complex organic molecules from simpler inorganic compounds.

• Biological evolution: Began when molecules could replicate and became metabolically active within membranes.

• Five characteristics of life: Cellular organization, metabolism, homeostasis, growth, and reproduction.

2.2 Properties of Water and the Early Oceans

Water as the Basis of Life

Water is fundamental to life, making up about 75% of a cell's mass. Its unique properties as a solvent enable the chemical reactions necessary for life to occur efficiently.

• Solvent: The substance in which solutes dissolve to form a solution.

• Most biochemical reactions occur in aqueous solutions.

Structural Properties of Water

Water's structure is key to its unique properties. It is a small, bent molecule with highly polar covalent bonds, resulting in an overall polarity.

• Bent shape: The angle between hydrogen atoms is about 104.5°.

• Polarity: Oxygen is more electronegative than hydrogen, creating partial charges (δ− on O, δ+ on H).

Water as an Efficient Solvent

Water's polarity allows it to dissolve many substances, especially ions and polar molecules. This property is essential for cellular processes.

• Hydrogen bonds: Weak electrical attractions between the partial positive charge on hydrogen and the partial negative charge on oxygen in adjacent water molecules.

• Hydrophilic molecules: "Water-loving"; ions and polar molecules that dissolve readily in water due to interactions with water's partial charges.

• Hydrophobic molecules: "Water-fearing"; uncharged and nonpolar compounds that do not dissolve in water, instead clustering together via hydrophobic interactions and van der Waals forces.

Cohesion, Adhesion, and Surface Tension

Water molecules exhibit cohesion (attraction to each other) and adhesion (attraction to other substances), leading to high surface tension. These properties are crucial for processes such as water transport in plants and the formation of droplets.

• Cohesion: Attraction between like molecules (water to water).

• Adhesion: Attraction between unlike molecules (water to other polar surfaces).

• Surface tension: The elastic "skin" at the water's surface due to cohesive forces, allowing small objects to rest on water without sinking.

Density of Water: Liquid vs. Solid

Unlike most substances, water is denser as a liquid than as a solid. As water freezes, it forms an open crystal structure stabilized by hydrogen bonds, causing ice to float. This property insulates aquatic environments in cold climates.

• Ice: Forms a lattice structure, less dense than liquid water.

• Ecological impact: Floating ice insulates water below, protecting aquatic life.

Water’s Capacity for Absorbing Energy

Water has a high specific heat and heat of vaporization, meaning it can absorb or release large amounts of energy with little temperature change. This moderates Earth's climate and helps organisms maintain stable internal temperatures.

• Specific heat: Amount of energy required to raise the temperature of 1 gram of a substance by 1°C.

• Heat of vaporization: Energy needed to convert 1 gram of liquid into gas.

• Hydrogen bonds must be broken for water to change state, requiring significant energy input.

The Role of Water in Acid–Base Chemical Reactions

Water participates in acid–base reactions, which are vital for biological systems. Acids donate protons (H+), increasing hydronium ion concentration, while bases accept protons, decreasing it.

• Acids: Substances that increase proton concentration in solution.

• Bases: Substances that decrease proton concentration in solution.

The pH Scale and Buffers

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity. Living cells maintain a near-neutral pH (~7) using buffers, which minimize changes in pH and help maintain homeostasis.

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

• Acidic: pH < 7; Basic: pH > 7; Neutral: pH = 7

• Buffers: Compounds that stabilize pH by absorbing or releasing protons as needed.

2.3 Chemical Reactions, Energy, and Chemical Evolution

Origins of Chemical Evolution

Chemical evolution may have begun in Earth's early atmosphere (rich in volcanic gases) or in deep-sea hydrothermal vents, both providing energy and reactive molecules necessary for the synthesis of organic compounds.

• Atmosphere: Water vapor, carbon dioxide, nitrogen, and trace gases.

• Hydrothermal vents: Hot rocks, reactive gases, and minerals with catalytic properties.

Spontaneity of Chemical Reactions

Chemical reactions are spontaneous if they proceed without continuous external energy input. Two main factors determine spontaneity: lower potential energy in products and increased disorder (entropy).

• Spontaneous reaction: Occurs without added energy.

• Entropy (S): Measure of disorder; reactions tend to increase entropy.

• Potential energy: Stored energy; reactions favor products with lower potential energy.

2.4 Investigating Chemical Evolution

Miller’s Spark-Discharge Experiment

In 1953, Stanley Miller demonstrated that complex organic molecules could be synthesized from simple inorganic molecules under conditions simulating early Earth. His experiment produced amino acids, the building blocks of proteins, supporting the plausibility of chemical evolution.

• Experimental setup: Simulated early Earth atmosphere, applied heat and electrical sparks.

• Results: Formation of amino acids and other organic molecules.

• Conclusion: Chemical evolution can occur if simple molecules with high free energy are exposed to kinetic energy.

2.5 Life is Carbon Based

The Importance of Carbon

Carbon is the backbone of most biological molecules (except water). Its four valence electrons allow it to form four covalent bonds, enabling a vast diversity of molecular structures and functions.

• Organic compounds: Molecules containing carbon bonded to other elements (H, N, O, P, S).

• Carbon forms single, double, or triple bonds, allowing for complex molecular shapes.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity. They are critical for the structure and function of biomolecules.

• Amino group (–NH2): Acts as a base, attracts protons.

• Carboxyl group (–COOH): Acts as an acid, donates protons.

• Carbonyl group (–C=O): Sites for linking molecules into larger compounds.

• Hydroxyl group (–OH): Acts as a weak acid.

• Phosphate group (–PO42−): Contributes negative charge, involved in energy transfer.

• Sulfhydryl group (–SH): Forms disulfide bonds, stabilizing protein structure.

Assembly of Macromolecules

Small organic molecules (monomers) can join to form large macromolecules (polymers) through condensation (dehydration) reactions, which release water. The reverse process, hydrolysis, breaks polymers into monomers and is energetically favored in dilute solutions.

• Polymerization: Formation of polymers from monomers via condensation reactions (loss of water).

• Hydrolysis: Addition of water to break bonds between monomers.

• Macromolecules include proteins, nucleic acids, and carbohydrates.

Additional info: Polymerization is favored only at high monomer concentrations; otherwise, equilibrium favors free monomers due to increased entropy.