We observed the functions of organelles under a microscope.
Synonym; 細胞小器官 is a slightly shorter form with the same meaning. Both are used in biology, but 細胞内小器官 explicitly includes 内 (inside) for clarity.
Compound of 細胞 (cell) + 内 (inside) + 小 (small) + 器官 (organ). The term is a direct translation of the English 'organelle'.