Stereochemistry is the study of the three‐dimensional structure of molecules. The cis and trans isomers are forms of stereoisomers, differing structurally only in the location of the atoms of the molecule in three‐dimensional space. Such stereoisomers can have different physical and chemical properties. For example, the cis and trans isomers of butenedioic acid show vast differences in their physical and chemical properties.
Stereochemistry is of particular interest to biochemists because the reactivity and toxicity of molecules change with their stereochemistry. Most body reactions are stereospecific, meaning that receptor sites on cells accept only molecules with specific spatial arrangements of their atoms. Other configurations of the same chemical either will not react or may be toxic to the living being.
There are other types of stereoisomers in addition to the cis and trans arrangements of atoms. To understand these other isomers, you must first understand the three‐dimensional structures of the molecules.