No matter what the cell type, prokaryotic or eukaryotic, plant or animal, the forms a protective barrier between the exterior environment and the interior of the cell.

The correct answer for the fill in the blank is Cell membrane.

Cell membrane ( also called plasma membrane) is the membrane around cytoplasm of the cell. It is present in the cells of all living organisms. It forms a protective barrier of the cell as it separates interior components from the external environment of the cell.

It is a selectively permeable membrane as it controls the transport of substances in and out of the cell. By doing this, it does not allow the harmful substances to enter the cell.

not sure if this what   you'r looking for would be percent composition