Thin, flexible barrier around a cell; regulates what enters and leaves the cell.

• A. Cell membrane
• B. Cytoplasm
• C. Nucleus
• D. Cell wall

Answer: (A)

Think about what forms the boundary of a cell and how the cell controls what goes in and out. The thin, flexible barrier around a cell is the cell membrane, a phospholipid bilayer that is selectively permeable. It acts as a gatekeeper, letting some substances pass easily while using channels and transport proteins to move others, so essential nutrients stay inside and waste products can exit. This regulation keeps the cell’s internal environment stable and functioning.

The cytoplasm is the jelly-like material inside the cell where many metabolic processes occur; the nucleus houses the cell’s genetic material and controls activities by directing DNA-based processes; the cell wall provides rigid support and protection in plants, fungi, and some bacteria and is not the flexible barrier that regulates movement across the cell boundary.