In eukaryotic cells, which structure is selectively permeable, allowing some substances to cross while restricting others?

• A. The cell wall
• B. The cytoskeleton
• C. Ribosomes
• D. The plasma (cell) membrane composed of a phospholipid bilayer

Answer: (D)

Verified step by step guidance

1. Understand the concept of selective permeability, which refers to a structure's ability to allow certain molecules or ions to pass through it while blocking others.
2. Identify the main structures listed: the cell wall, cytoskeleton, ribosomes, and the plasma (cell) membrane.
3. Recall that the cell wall provides structural support and protection but is generally permeable to many substances and does not regulate passage selectively.
4. Recognize that the cytoskeleton is involved in maintaining cell shape and intracellular transport, not in controlling substance passage across the cell boundary.
5. Know that ribosomes are the sites of protein synthesis and do not function as barriers or selective gates for molecules entering or leaving the cell, unlike the plasma membrane, which is composed of a phospholipid bilayer and is selectively permeable.