AP BIOLOGY

Detailed explanation-1: -Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot.

Detailed explanation-2: -The cell membrane is called selectively permeable as it only allows specific molecules to pass. Only specific molecules like water and gaseous molecules can pass through the cell membrane directly. It stops the flow of other molecules towards the two sides.

Detailed explanation-3: -The selectively-permeable cell membrane protects the cell. It maintains the balance in the internal environment of the cell by transporting the toxic substances outside the cell and bringing the nutrients inside the cell.

Detailed explanation-4: -Cell membrane or plasma membrane is described as a selectively permeable membrane.