What are the flattened membranous sacs within a cell called?

Flattened membranous sacs within a cell are called cisternae. Cisternae are typically associated with the endoplasmic reticulum and the Golgi apparatus, where they play crucial roles in the processing, folding, and transport of proteins and lipids. The structure of cisternae facilitates the compartmentalization of cellular functions, allowing for efficient processing and modifications to occur separately from the cytosol.

In the context of cellular organization, vesicles are small, membrane-bound sacs that transport substances within the cell but are not flat or membranous in the same manner as cisternae. Filaments refer to thread-like structures, such as cytoskeletal components, and do not describe membranous sacs. The term structures is too vague and does not specifically refer to any recognized cellular component. Thus, cisternae are the most accurate term for describing these flattened membranous sacs.