Elements of Life

Living things are built almost entirely from a handful of light elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These elements are abundant, form strong and stable covalent bonds, and - critically - carbon and nitrogen can each form multiple bonds, allowing an enormous diversity of molecular shapes and functional groups to be built from a small starting set.

Because these elements are relatively small and have intermediate electronegativities, the covalent bonds they form are strong enough to be stable at biological temperatures yet flexible enough to be broken and remade in metabolism (e.g., via hydrolysis and dehydration synthesis).

Carbon has four valence electrons, allowing it to form up to four covalent bonds simultaneously. This tetravalence lets carbon atoms link into long chains, branched structures, and rings, generating the structural diversity underlying carbohydrates, lipids, proteins, and nucleic acids.

Carbon skeletons can also carry various functional groups - hydroxyl, carboxyl, amino, phosphate, and others - attached at different positions. These functional groups give molecules distinct chemical properties (polarity, acidity, reactivity) even when the underlying carbon skeleton is similar.

Whether two atoms form a nonpolar covalent bond, a polar covalent bond, or an ionic bond depends on the difference in electronegativity between them. Similar electronegativities (e.g., C-H) share electrons roughly evenly, producing nonpolar covalent bonds. A larger difference (e.g., O-H, N-H) produces unequal sharing and polar covalent bonds with partial charges. A very large difference (e.g., Na and Cl) results in electrons being transferred rather than shared, forming an ionic bond.

These bonding patterns explain macroscopic behavior: molecules dominated by nonpolar C-H and C-C bonds (like fats) are hydrophobic, while molecules rich in polar O-H, N-H, or ionic groups are hydrophilic and interact readily with water.

Beyond CHNOPS, organisms require small quantities of other elements for specific functions. Iron is a component of the heme group in hemoglobin and cytochromes involved in the electron transport chain. Calcium ions serve as signaling messengers and are structural components of bone and shell. Magnesium sits at the center of the chlorophyll molecule and is a cofactor for many enzymes. Iodine is required to synthesize thyroid hormones.

Even though trace elements make up a tiny fraction of an organism's mass, their absence can be catastrophic - deficiencies of iron, iodine, or calcium each produce well-documented physiological disorders, illustrating that biological function depends on more than just the six most abundant elements.