This document provides an introduction to biochemistry and the key building blocks that make up living organisms. It states that water makes up 70% of a cell's weight, and carbon is the basis for nearly all cellular molecules except water. The four most abundant elements in organisms are carbon, hydrogen, nitrogen, and oxygen, which make up 96.5% of an organism's weight. Carbon has unique properties that allow it to form long chains and rings that are the basis for important biomolecules like proteins, nucleic acids, carbohydrates, and lipids. These macromolecules are assembled through enzyme-controlled reactions and their function depends on the sequence of their monomer subunits.