Question

some materials can flow freely across a cell membrane, such as oxygen and water, but other materials have to be actively pumped in or out of a cell. which characteristic of the cell membrane allows it to control what enters and leaves the cell? it contains charged ions and water molecules. it is a porous layer made of cellulose. it is a semipermeable lipid bilayer. it is encased by a smooth protein shell.

Explanation:

The cell membrane is a semi - permeable lipid bilayer. This structure allows small, non - polar molecules like oxygen and water to pass through freely, while controlling the movement of other substances through active transport or other mechanisms. The presence of charged ions and water molecules within the membrane doesn't explain its selective permeability. Cellulose is found in plant cell walls, not in cell membranes. And the cell membrane is not encased by a smooth protein shell.

Answer:

It is a semipermeable lipid bilayer.