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. This transition marked the switch from chemical to biological evolution, with natural selection acting on replicating molecules.

• Chemical evolution: Formation of complex molecules from simpler ones, leading to life.

• Biological evolution: Evolution by natural selection acting on replicating molecules.

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

• Example: The first self-replicating molecule multiplied, became metabolically active, and acquired a membrane.

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.

• Solvent: A liquid that dissolves a solute to form a solution.

• Solution: A homogeneous mixture of solute and solvent.

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

Properties Correlated with Water's Structure

Water's unique properties arise from its molecular structure:

• Small size

• Bent shape

• Highly polar covalent bonds

• Overall polarity

Covalent and Ionic Bonding

• Covalent bond: Atoms share electrons (e.g., H2 molecule).

• Ionic bond: Atoms transfer electrons, resulting in charged ions (e.g., NaCl formation).

Water's Polarity and Hydrogen Bonding

Water is a polar molecule, with oxygen carrying a partial negative charge and hydrogens carrying partial positive charges. This allows water molecules to form hydrogen bonds with each other and with other polar molecules.

• Hydrogen bond: Weak electrical attraction between the partial positive charge on hydrogen and partial negative charge on another atom (often oxygen or nitrogen).

• Example: Hydrogen bonds between water molecules give water its unique properties.

Water as an Efficient Solvent

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

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

• Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions interact with water molecules.

Cohesion, Adhesion, and Surface Tension

Definitions and Biological Importance

• Cohesion: Attraction between like molecules (e.g., water molecules sticking together).

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

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

• Example: Water droplets form beads on a surface; insects can walk on water due to surface tension.

Water's Density and Thermal Properties

Density of Water as Solid vs. Liquid

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

• This property allows ice to float, forming an insulating layer on water surfaces.

High Capacity for Absorbing Energy

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

• Heat of vaporization: Energy required to change 1 gram of a substance from liquid to gas.

• Water's high specific heat and heat of vaporization are due to the many hydrogen bonds that must be broken for temperature change or evaporation.

• Example: Sweating cools the body as water evaporates, absorbing heat.

The Role of Water in Acid-Base Chemistry

Acids, Bases, and pH

• Acids: Substances that give up protons (H+), increasing hydronium ion concentration ($\text{H}_3\text{O}^+$).

• Bases: Substances that accept protons, decreasing hydronium ion concentration.

• pH: Logarithmic scale expressing proton concentration in solution. $\text{pH} = -\log_{10}[\text{H}^+]$

• Acidic: pH < 7; Basic: pH > 7; Neutral: pH = 7 (e.g., inside living cells).

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

pH Value	Nature	Example
< 7	Acidic	Lemon juice, stomach acid
7	Neutral	Pure water, human blood
> 7	Basic	Household bleach, milk of magnesia

2.3 Chemical Reactions, Energy, and Chemical Evolution

Origins of Chemical Evolution

• Chemical evolution may have begun in two environments:

  1. The atmosphere: Dominated by volcanic gases (water vapor, CO2, N2, small amounts of H2 and CO).

  2. Deep-sea hydrothermal vents: Hot rocks, gases (CO2, H2), and minerals with reactive metals.

Spontaneity of Chemical Reactions

• Spontaneous reactions: Proceed without continuous external influence or added energy.

• Two factors favor spontaneity:

  1. Products have lower potential energy than reactants.

  2. Products are less ordered (higher entropy) than reactants.

• Entropy (S): A measure of disorder in a system.

2.4 Investigating Chemical Evolution

Stanley Miller's Experiment

• In 1953, Stanley Miller tested whether complex organic compounds could be synthesized from simple molecules present in Earth's early atmosphere.

• He used heat and electrical sparks to simulate early Earth conditions.

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

• It forms four covalent bonds due to its four valence electrons, allowing for a limitless array of molecular shapes.

• Organic compounds contain carbon bonded to other elements, with various combinations of single and double bonds.

Shapes of Carbon-Containing Molecules

• Carbon atoms can be linked in chains or rings, forming diverse structures such as octane (chain) and glucose (ring).

Functional Groups in Organic Molecules

• Amino groups (–NH2): Attract protons, act as bases.

• Carboxyl groups (–COOH): Drop protons, act as acids.

• Carbonyl groups (–CO–): Sites for linking molecules into more complex compounds.

• Hydroxyl groups (–OH): Act as weak acids.

• Phosphate groups (–PO42–): Have two negative charges.

• Sulfhydryl groups (–SH): Link together via disulfide bonds.

Macromolecules and Polymerization

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

• Polymerization: Process of linking monomers via condensation (dehydration) reactions, releasing water.

• Hydrolysis: Reverse reaction; water is used to break bonds between monomers.

• Polymerization is energetically unfavorable unless monomer concentration is high; hydrolysis increases entropy and is energetically favored.

• Example: Proteins, nucleic acids, and carbohydrates are biological macromolecules formed by polymerization.