Description: Biochemistry studies the molecular basis of living organisms. We begin studying the simplest chemical components (atoms and molecules) and describing how, from a small number of chemical elements and from different reactions, more complex molecules are formed. In addition, we will describe the carbon atom, the covalent bond, the functional groups and the conformation of biomolecules as well as their three dimensional structures and the stereospecificity concept. Finally, it is pointed out that the chemical reactions in living organisms occur mainly in the aqueous phase where non-covalent interactions between molecules are particularly relevant.