envelope

The term "envelope" in cytology can refer to different structures depending on the cell type. In eukaryotic cells, the nuclear envelope is a double membrane that encloses the cell's nucleus, separating it from the cytoplasm. This envelope contains nuclear pores that regulate the passage of molecules between the nucleus and the cytoplasm.12

In bacteria, the cell envelope is the outer covering of the cell. It typically includes the cytoplasmic membrane, which controls the flow of substances into and out of the cell, and a rigid cell wall made of peptidoglycan. This wall determines the cell's shape and provides structural support. Some bacteria also have an outer membrane that acts as a barrier against harmful substances.3