Correct Response: B. As the central vacuole of a plant cell fills with water, the total osmotic pressure within the cell increases. The cell begins to expand until it meets resistance from the rigid cell wall. It is the pressure of the cell pushing against the cell wall that provides the overall structural rigidity in a plant. Thylakoids (A) are flattened sacs that divide the interior of the chloroplast into chambers and do not supply structural rigidity. Facilitated diffusion (C) uses specialized proteins to move large molecules or charged particles (ions) across the cell membrane. These molecules are not part of the plant cell's structure. The cytoskeleton (D) consists of different proteins making up filaments and tubules, which do provide some support within the cell but do not contribute to the overall rigidity of a plant to the extent that osmotic pressure within the central vacuole does.
