This organelle will let some materials pass through, but keeps other materials out.

• A. Selectively Permeable Membrane
• B. Nucleus
• C. Chloroplast
• D. Ribosome

Answer: (A)

Selective permeability is the ability of a membrane to let some things pass through while keeping others out. The cell membrane does this work. Its phospholipid bilayer forms a barrier that blocks most charged or large molecules, yet small nonpolar molecules like oxygen and carbon dioxide can diffuse across. Water moves by osmosis, and ions or larger molecules often need specific transport proteins or energy to cross through channels or pumps. This control helps the cell maintain a stable internal environment. The other structures mentioned have different roles—nucleus stores genetic material, chloroplast carries out photosynthesis, and ribosome builds proteins—but they don’t regulate what moves across the cell boundary the way the membrane does.