Water and Carbon: The Chemical Basis of Life (Chapter 2 Study Notes)

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 formed increasingly complex carbon-containing substances, eventually leading to molecules capable of self-replication and the transition from chemical to biological evolution.

• Chemical evolution: Formation of complex molecules from simpler ones, setting the stage for life.

• Biological evolution: Began when molecules could replicate, become metabolically active, and acquire membranes, fulfilling the five characteristics of life.

2.2 Properties of Water and the Early Oceans

Water as the Basis of Life

Water is essential for life, making up about 75% of the cell's mass. Its unique properties as a solvent facilitate the chemical reactions necessary for life.

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

• Substances are more likely to react when dissolved in water.

Structural Properties of Water

Water's structure is responsible for its unique chemical and physical properties.

• Small molecular size

• Bent shape

• Highly polar covalent bonds

• Overall polarity

Covalent and Ionic Bonds

• Covalent bonds: Atoms share electrons (e.g., H2 molecule shares two electrons).

• Ionic bonds: Formed when electrons are transferred from one atom to another, creating ions (e.g., NaCl forms from Na+ and Cl-).

Polarity and Hydrogen Bonding

• Water is polar: Oxygen has a partial negative charge, hydrogen has a partial positive charge.

• Hydrogen bonds form between the partial charges of water molecules, contributing to water's unique properties.

Water as an Efficient Solvent

• Hydrophilic molecules: "Water-loving"; ions and polar molecules that dissolve easily in water due to hydrogen bonding.

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

Cohesion, Adhesion, and Surface Tension

• Cohesion: Attraction between like molecules (e.g., water molecules stick together via hydrogen bonds).

• Adhesion: Attraction between unlike molecules (e.g., water and glass).

• Surface tension: Cohesive force at the surface of a liquid, making water behave like an elastic membrane.

Density of Water

• Water is denser as a liquid than as a solid due to the open crystal structure of ice, which is why ice floats.

• Ice forms an insulating layer on water surfaces, protecting aquatic life in cold environments.

Water's Capacity for Absorbing Energy

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

• Water has a high specific heat due to hydrogen bonding.

• Heat of vaporization: Energy required to change 1 gram of a substance from liquid to gas. Water's high heat of vaporization explains why sweating cools the body.

The Role of Water in Acid-Base Chemical Reactions

Acids, Bases, and pH

• Acids: Substances that give up protons (H+) and increase hydronium ion concentration ($\mathrm{H_3O^+}$).

• Bases: Substances that accept protons and decrease hydronium ion concentration.

• pH: Logarithmic scale expressing proton concentration in solution.

• Acids have pH < 7; bases have pH > 7; neutral solutions have pH ≈ 7 (e.g., inside living cells).

• Buffers: Compounds that minimize changes in pH, helping maintain homeostasis.

Equation:

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

2.3 Chemical Reactions, Energy, and Chemical Evolution

Origins of Chemical Evolution

• Atmosphere hypothesis: Early Earth's atmosphere contained water vapor, CO2, N2, and small amounts of H2 and CO.

• Hydrothermal vent hypothesis: Deep-sea vents provided heat, gases, and reactive minerals for chemical evolution.

Spontaneity of Chemical Reactions

• Reactions are spontaneous if they proceed without continuous external energy input.

• Spontaneity is favored when products have lower potential energy and greater disorder (higher entropy) than reactants.

• Entropy: A measure of disorder in a system.

2.4 Investigating Chemical Evolution

Stanley Miller's Experiment

• Tested whether complex organic compounds could form from simple molecules under early Earth conditions.

• Used heat and electrical sparks to simulate lightning in a closed system containing water, methane, ammonia, and hydrogen.

• Result: Formation of amino acids, the building blocks of proteins.

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

2.5 Life is Carbon Based

Importance of Carbon

• Carbon forms the backbone of almost all biological molecules (except water).

• Can form four covalent bonds, allowing for a variety of molecular shapes and complexity.

• Organic compounds contain carbon bonded to other elements, with diverse structures due to single and double bonds.

Shapes of Carbon-Containing Molecules

• Carbon atoms can be linked in chains or rings, forming molecules like octane (C8H18) and glucose (C6H12O6).

Functional Groups in Organic Molecules

• Amino groups: Attract protons, act as bases.

• Carboxyl groups: Donate protons, act as acids.

• Carbonyl groups: Sites for linking molecules into more complex compounds.

• Hydroxyl groups: Act as weak acids.

• Phosphate groups: Carry two negative charges.

• Sulfhydryl groups: Form disulfide bonds, stabilizing protein structure.

Macromolecules and Polymerization

• Macromolecules: Large molecules made of smaller subunits (monomers) joined by polymerization.

• Condensation (dehydration) reactions: Link monomers, releasing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

• Polymerization is favored at high monomer concentrations; hydrolysis increases entropy and is energetically favorable.

Example: Proteins, nucleic acids, and carbohydrates are biological macromolecules formed by polymerization of amino acids, nucleotides, and sugars, respectively.