The organelle responsible for storing water and helping to maintain the shape of a plant cell is the vacuole.
Vacuoles are large, membrane-bound sacs that can occupy a significant portion of a plant cell’s interior. They primarily store water, but they also hold nutrients, waste products, and other substances. When the vacuole is filled with water, it creates turgor pressure against the cell wall. This pressure is crucial for keeping the plant upright and maintaining its structure. Without adequate water in the vacuole, a plant cell becomes flaccid, leading to wilting and loss of support for the plant as a whole.