What part of the cell encloses the protoplasm and permits soluble substances to enter and leave the cell?

• A. Nucleus
• B. Cytoplasm
• C. Cell Membrane/Structure
• D. Mitochondria

Answer: (C)

The cell membrane (plasma membrane) is the outer boundary that encloses the protoplasm. It acts as a selective barrier, allowing some substances to cross while restricting others, through channels, carriers, and pumps embedded in the lipid bilayer. This permeability control enables soluble substances to enter and leave the cell by diffusion, osmosis, or active transport, helping maintain the cell’s internal environment. The nucleus is the genetic control center, the cytoplasm is the internal fluid containing organelles, and mitochondria are energy producers; none of these serve as the gatekeeper for all solutes the way the cell membrane does.