1: Chemical basis of life

Section 1: Core Principles - 1: Chemical Basis of Life

Life, at its core, is an intricate symphony of chemical processes governed by fundamental physical principles. All living organisms are composed of matter organized into complex structures, with specific elements forming the essential building blocks. The four most abundant elements in biological molecules are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) – often abbreviated as CHON. Trace elements like phosphorus (P), sulfur (S), calcium (Ca), and iron (Fe) also play critical roles in structures and functions.

Carbon's Central Role: Carbon's unique chemical properties are foundational. Its ability to form four stable covalent bonds allows for immense molecular diversity. Carbon atoms readily bond with other carbon atoms, creating chains, branched structures, and rings. This versatility enables the formation of complex, stable organic molecules – the macromolecules essential for life (carbohydrates, lipids, proteins, nucleic acids). The specific arrangement of carbon atoms and their attached functional groups ($\ce{-OH}$, $\ce{-COOH}$, $\ce{-NH2}$, $\ce{-SH}$, $\ce{-PO4}$) determines a molecule's chemical behavior and biological function.

The Importance of Water: Water ($\ce{H2O}$) is the universal solvent of life, constituting the majority of cellular mass. Its polarity (uneven electron distribution creating a partial positive charge near hydrogens and a partial negative charge near oxygen) and capacity for hydrogen bonding are paramount. Hydrogen bonds form between water molecules and between water and other polar/charged substances. This polarity makes water an excellent solvent for ions (salts) and polar molecules (like sugars, amino acids), facilitating chemical reactions within the aqueous cellular environment. Water's hydrogen bonding also contributes to high specific heat (temperature stabilization), cohesion/adhesion, and surface tension.

Chemical Bonds Define Structure and Function: Biological molecules interact and maintain structure through distinct chemical bonds:

• Covalent Bonds: Strongest bonds; involve sharing electron pairs between atoms. Form the stable backbone of organic molecules (e.g., $\ce{C-C}$, $\ce{C-H}$, $\ce{C-O}$ bonds). Single, double, or triple bonds affect molecular geometry and reactivity.
• Ionic Bonds: Result from the electrostatic attraction between oppositely charged ions (e.g., $\ce{Na+}$ and $\ce{Cl-}$ forming $\ce{NaCl}$). Important for mineral structure and electrolyte function.
• Hydrogen Bonds: Weaker attractions between a hydrogen atom (covalently bonded to $\ce{O}$, $\ce{N}$, or $\ce{F}$) and another electronegative atom. Crucial for stabilizing the 3D structures of proteins and nucleic acids (DNA double helix), enabling enzyme-substrate binding, and the properties of water.
• Hydrophobic Interactions: Not true bonds, but a driving force where nonpolar molecules or regions aggregate in water to minimize disruptive contact with polar water molecules. Critical for membrane formation and protein folding. Van der Waals forces (weak, transient attractions between nonpolar molecules) also contribute.

Understanding these chemical fundamentals – the essential elements, the centrality of carbon, the unique properties of water, and the nature of chemical bonds – provides the essential framework for comprehending the structure, function, and interactions of all biological molecules and processes.