Atoms and Bonds in Living Systems

Objective: Distinguish atoms, elements, molecules, and compounds.

An atom is a unit of an element whose identity is determined by its number of protons. Electrons influence how atoms interact chemically. A molecule contains two or more atoms joined by chemical bonds; a compound contains atoms of at least two different elements in a fixed chemical relationship. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are especially common in biological molecules. A molecule can contain one element, as O₂ does, so molecule and compound are not interchangeable terms.

Objective: Compare electron sharing, charged particles, and weak attractions.

In a covalent bond, atoms share pairs of electrons; bonds within water and organic molecules are covalent. An ion is an atom or group with a net electric charge after losing or gaining electrons. Oppositely charged ions attract. A hydrogen bond is a relatively weak attraction involving a partially positive hydrogen and an electronegative atom on another part of a molecule or a nearby molecule. These interactions differ in strength and context; a hydrogen bond between water molecules is not the covalent O–H bond within one water molecule.

Objective: Interpret reactants, products, and rearrangement of atoms.

A chemical reaction rearranges atoms as bonds break and form. Reactants are starting substances and products are resulting substances. Matter is conserved in a closed chemical system: atoms are not created or destroyed by ordinary chemical reactions, although they may be reorganized. Reactions may release usable energy or require energy input, but an equation alone does not show the full mechanism or reaction rate. Cells coordinate many linked reactions through pathways and catalysts.