BackWater and Carbon: The Chemical Basis of Life – 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 formation of increasingly complex carbon-containing substances, which eventually led to molecules capable of self-replication. This transition marked the switch from chemical to biological evolution, where evolution by natural selection began to operate on these molecules, fulfilling the five characteristics of life.
Chemical evolution: Formation of complex molecules from simpler ones.
Biological evolution: Natural selection acting on replicating molecules.
Key milestone: Acquisition of metabolic activity and a membrane.
Five characteristics of life: Cellular organization, metabolism, homeostasis, growth, and reproduction.
Properties of Water and the Early Oceans
Water is fundamental to life, making up about 75% of a cell's mass. Its unique chemical structure and properties make it an excellent solvent, facilitating chemical reactions necessary for life.
Solvent: A substance that dissolves solutes to form a solution.
Water's role: Increases likelihood of chemical reactions by dissolving reactants.
Water’s Structure and Its Unique Properties
Water’s molecular structure is characterized by its small size, bent shape, and highly polar covalent bonds, resulting in overall polarity. This structure underlies many of water’s unique properties.
Bent shape: Causes partial charges on oxygen and hydrogen atoms.
Polarity: Leads to hydrogen bonding between water molecules.
Water as an Efficient Solvent
Water’s polarity allows it to dissolve a wide range of substances, especially ions and polar molecules. Hydrogen bonds form between water molecules and solutes, making water an efficient solvent for hydrophilic substances.
Hydrophilic: "Water-loving"; ions and polar molecules dissolve readily.
Hydrophobic: "Water-fearing"; nonpolar molecules do not dissolve and cluster together via hydrophobic interactions and van der Waals forces.
Hydrogen bonds: Weak electrical attractions between partial charges.
Example: Table salt (NaCl) dissolves in water as water molecules surround and stabilize Na+ and Cl- ions.
Cohesion, Adhesion, and Surface Tension
Water exhibits both cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules). These properties contribute to surface tension, allowing water to resist forces that increase its surface area.
Cohesion: Water molecules stick together via hydrogen bonds.
Adhesion: Water molecules adhere to polar or charged surfaces.
Surface tension: Cohesive force at the surface acts like an elastic membrane.
Example: Water forms a meniscus in a test tube and supports small objects due to high surface tension.
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 due to hydrogen bonding, causing ice to float and act as an insulating layer.
Crystal structure: Hydrogen bonds create open lattice in ice.
Biological significance: Floating ice insulates aquatic environments.
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. This property is crucial for temperature regulation in organisms and environments.
Specific heat: Energy needed to raise temperature of 1 gram by 1°C.
Heat of vaporization: Energy required to convert 1 gram from liquid to gas.
Biological application: Sweating cools organisms by evaporating water.
Acid–Base Chemistry and pH
Acids and bases affect the concentration of protons (H+) in solution, which is measured by pH. Buffers help maintain stable pH in biological systems, ensuring homeostasis.
Acids: Donate protons, increase hydronium ion concentration.
Bases: Accept protons, decrease hydronium ion concentration.
pH scale: Logarithmic scale; pH < 7 is acidic, pH > 7 is basic, pH = 7 is neutral.
Buffers: Minimize changes in pH.
Equation: pH is calculated as
Chemical Reactions, Energy, and Chemical Evolution
Chemical evolution may have begun in environments such as the atmosphere (rich in volcanic gases) or deep-sea hydrothermal vents (hot rocks and reactive minerals). Spontaneous chemical reactions occur when products have lower potential energy and higher entropy than reactants.
Spontaneous reactions: Proceed without external energy input.
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. This provided evidence for chemical evolution.
Experimental setup: Simulated early Earth atmosphere with heat and electrical sparks.
Result: Formation of amino acids, precursors to proteins.
Life is Carbon-Based
Carbon is the backbone of most biological molecules, forming four covalent bonds and enabling a vast array of molecular shapes and functions. Organic compounds contain carbon bonded to other elements.
Valence electrons: Carbon has four, allowing diverse bonding.
Organic compounds: Molecules with carbon bonded to H, N, O, P, S, etc.
Functional Groups in Organic Molecules
Functional groups are specific clusters of atoms within molecules that determine their chemical behavior. Common functional groups include amino, carboxyl, carbonyl, hydroxyl, phosphate, and sulfhydryl.
Amino group: Acts as a base, attracts protons.
Carboxyl group: Acts as an acid, donates protons.
Carbonyl group: Links molecules into more complex compounds.
Hydroxyl group: Acts as a weak acid.
Phosphate group: Carries two negative charges.
Sulfhydryl group: Forms disulfide bonds.
Assembly of Small Organic Molecules into Macromolecules
Macromolecules are large molecules formed by joining smaller subunits (monomers) through polymerization. Condensation (dehydration) reactions link monomers, releasing water, while hydrolysis breaks polymers apart, increasing entropy.
Macromolecules: Proteins, nucleic acids, carbohydrates, etc.
Polymerization: Formation of polymers from monomers.
Condensation reaction: Monomers join, water is lost.
Hydrolysis: Water added, polymers broken into monomers.
Equation: General condensation reaction:
Example: Formation of proteins from amino acids via condensation reactions.
