Which structure functions as a barrier in plant cells?

The cell wall is the correct answer because it is a rigid structure that surrounds the cell membrane in plant cells, providing support and protection. It functions as a barrier, helping to maintain the cell's shape and prevent the entry of harmful pathogens while allowing certain substances to pass through. The composition of the cell wall primarily includes cellulose, which contributes to its strength and durability.

In contrast, the cell membrane, while also a barrier, is a flexible and selectively permeable membrane that regulates what enters and exits the cell. The nucleus is an organelle that contains genetic material and does not serve as a barrier to the cell's exterior. The cytoplasm is the gel-like substance inside the cell where organelles reside, but it does not function as a barrier to the external environment. Thus, the distinct characteristics and role of the cell wall solidify its position as the key protective barrier in plant cells.