Life depends on four main types of biological macromolecules, each built from smaller subunits and serving distinct roles. Carbohydrates provide quick energy and structural support, lipids store energy over the long term and form insulating layers, proteins perform structural work, act as enzymes, and serve as signaling molecules, and nucleic acids carry the genetic information that guides every cellular process. Together, these molecules make up the bulk of living matter and are responsible for nearly every structure and function inside cells.
Water is the most important substance for life on Earth. Its properties make it a universal solvent, capable of dissolving a wide range of substances, which is why it is so effective at transporting materials within and between cells. Water also helps regulate temperature, participates in hydrolysis reactions that break down macromolecules, and supports cell shape through cohesion and adhesion. Without water, the chemistry of life would grind to a halt.
The reactions that sustain life are accelerated by enzymes, which are mostly proteins that act as biological catalysts. Enzymes speed up reactions by lowering activation energy without being consumed, allowing processes such as digestion and DNA replication to occur fast enough to support life. These enzymes are highly sensitive to pH, a measure of hydrogen ion concentration on a scale from 0 to 14. Most biological processes function best at a neutral pH around 7, because extremes of acidity or alkalinity can denature enzymes and render them inactive.