The cellular structure that assembles an organic molecule is the ribosome.
Ribosomes are cellular organelles responsible for the synthesis of proteins from amino acids. The process of protein synthesis involves the assembly of amino acids into long chains, which then fold into the three-dimensional structures that define their function. Ribosomes facilitate this process by reading the genetic instructions encoded in mRNA molecules and using that information to assemble the appropriate sequence of amino acids into a polypeptide chain.