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

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Water and Carbon: The Chemical Basis of Life

Introduction to Chemical Evolution

Chemical evolution is the leading scientific 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 simple precursors.
Biological evolution: Began when molecules could replicate and evolve by natural selection.
Five characteristics of life: Metabolic activity, membrane acquisition, and multiplication of original molecules.

Properties of Water and the Early Oceans

Water is fundamental to life, making up about 75% of a cell's mass. Its unique properties as a solvent enable chemical reactions essential for biological processes.

Solvent: A liquid that dissolves solutes to form a solution.
Water as a solvent: Substances are more likely to react when dissolved in water.

Water's Structure and Its Unique Properties

The molecular structure of water gives rise to its remarkable properties, which are crucial for life.

Small size: Water molecules are compact.
Bent shape: The angle between hydrogen atoms creates a bent geometry.
Highly polar covalent bonds: Electrons are shared unequally between oxygen and hydrogen.
Overall polarity: Water molecules have partial positive and negative charges.

Covalent and Ionic Bonding

Chemical bonds are essential for molecular formation and stability.

Covalent bonds: Atoms share electrons (e.g., H2 molecule).
Ionic bonds: Atoms transfer electrons, forming ions (e.g., NaCl).

Water Is Polar and Forms Hydrogen Bonds

Water's polarity allows it to form hydrogen bonds, which are weak electrical attractions between the partial positive charge of hydrogen and the partial negative charge of oxygen in adjacent molecules.

Hydrogen bonds: Responsible for many of water's unique properties.

Water as an Efficient Solvent

Water dissolves many substances due to its polarity and ability to form hydrogen bonds.

Hydrophilic molecules: 'Water-loving' ions and polar molecules that dissolve easily in water.
Hydrophobic molecules: 'Water-fearing' nonpolar compounds that do not dissolve in water; they interact via van der Waals forces.

Cohesion, Adhesion, and Surface Tension

Water molecules exhibit cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules), leading to surface tension.

Cohesion: Water molecules stick together due to hydrogen bonds.
Adhesion: Water molecules adhere to polar or charged surfaces.
Surface tension: Cohesive force at the surface of water acts like an elastic membrane.

Density of Water: Liquid vs. Solid

Unlike most substances, water is denser as a liquid than as a solid. This is due to the open crystal structure formed by hydrogen bonds in ice, causing ice to float and insulate aquatic environments.

Ice: Forms a lattice structure, less dense than liquid water.
Liquid water: Molecules are closer together, making it denser.

Water's Capacity for Absorbing Energy

Water has a high specific heat and heat of vaporization, meaning it can absorb large amounts of energy before changing temperature or state.

Specific heat: (energy required to raise temperature of 1 g by 1°C).
Heat of vaporization: Energy required to convert 1 g of liquid to gas.
Biological relevance: Sweating cools organisms due to water's high heat of vaporization.

Acid-Base Chemistry and pH

Water participates in acid-base reactions, affecting the concentration of protons (H+) in solution, which is measured by pH.

Acids: Donate protons, increasing hydronium ion concentration ().
Bases: Accept protons, decreasing hydronium ion concentration.
pH scale: ; acidic (<7), neutral (=7), basic (>7).
Buffers: Minimize changes in pH, maintaining homeostasis.

Chemical Reactions, Energy, and Chemical Evolution

Chemical evolution may have begun in the atmosphere or deep-sea hydrothermal vents, where energy and reactive molecules were abundant.

Spontaneous reactions: Occur without external energy if products have lower potential energy and higher entropy than reactants.
Entropy (): Measure of disorder; increases in spontaneous reactions.

Investigating Chemical Evolution: Miller's Experiment

Stanley Miller's 1953 experiment demonstrated that complex organic molecules, such as amino acids, could be synthesized from simple molecules under conditions simulating early Earth.

Experimental setup: Simulated early atmosphere, used heat and electrical sparks.
Results: Formation of amino acids, supporting chemical evolution theory.

Life Is Carbon-Based

Carbon is the backbone of organic molecules due to its ability to form four covalent bonds, resulting in a limitless array of molecular shapes and structures.

Organic compounds: Molecules containing carbon bonded to other elements.
Structural diversity: Chains, rings, and various bond types.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that determine their chemical behavior.

Amino group (–NH2): Acts as a base, attracts protons.
Carboxyl group (–COOH): Acts as an acid, donates protons.
Carbonyl group (–CO): Links molecules into more complex compounds.
Hydroxyl group (–OH): Acts as a weak acid.
Phosphate group (–PO4): Has two negative charges.
Sulfhydryl group (–SH): Forms disulfide bonds, stabilizing protein structure.

Assembly of Large Organic Molecules

Small organic molecules can join to form macromolecules through polymerization, which occurs via condensation (dehydration) reactions, while hydrolysis breaks polymers apart.

Macromolecules: Large molecules made of monomers (e.g., proteins, nucleic acids).
Polymerization: Monomers join via condensation reactions, releasing water.
Hydrolysis: Water is used to break bonds, increasing entropy.

Table: Comparison of Hydrophilic and Hydrophobic Molecules

Type	Definition	Solubility in Water
Hydrophilic	Charged or polar molecules	High
Hydrophobic	Uncharged, nonpolar molecules	Low

Table: Major Functional Groups in Organic Molecules

Functional Group	Structure	Chemical Behavior
Amino	–NH2	Acts as base
Carboxyl	–COOH	Acts as acid
Carbonyl	–CO	Links molecules
Hydroxyl	–OH	Weak acid
Phosphate	–PO4	Negative charge
Sulfhydryl	–SH	Forms disulfide bonds

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