Which diagram would best illustrate the boundary of a cell?

• A. Plant cell diagram
• B. Animal cell diagram
• C. Plasma membrane diagram
• D. Diffusion diagram

Answer: (C)

The boundary of a cell is defined by the plasma membrane, a phospholipid bilayer that encloses the cytoplasm and regulates what enters and leaves. A diagram of the plasma membrane best illustrates this boundary because it shows the actual barrier that separates the inside of the cell from the external environment, often with embedded proteins that form channels and transporters for moving substances across the boundary.

A plant cell diagram can include a cell wall outside the membrane, which is a rigid outer boundary for plant cells but not the active boundary that controls exchange with the environment. The diffusion diagram conveys how particles move across membranes but doesn’t depict the boundary itself. So the plasma membrane diagram most accurately represents the boundary that defines the cell’s internal environment.