_____ separates cell from its external environment.

The cell membrane, also known as Plasma Membrane is the outermost covering of the cell and separates the cell from its external environment. The cell membrane is a selectively permeable membrane that controls the movement of substances in and out of the cell. It is composed of lipids and proteins and is crucial in maintaining the cell's shape and integrity.

 The cell wall is found in plant cells and provides structural support, but it is located outside the cell membrane. The nucleus contains the cell's genetic material but is located within the cell's interior. The cytoplasm is the gel-like substance that fills the cell but does not separate it from the external environment.