The part of a plant cell that primarily helps maintain its shape is the cell wall. The cell wall is a rigid layer made mostly of cellulose that surrounds the cell membrane and provides mechanical support and protection, helping the cell keep its shape.
Additionally, the central vacuole plays a crucial role by maintaining turgor pressure inside the cell. When filled with water, the vacuole swells and pushes the cell membrane against the cell wall, making the cell firm and aiding in shape maintenance. If the vacuole loses water, the cell becomes flaccid and may lose its shape.
In summary, the plant cell's shape is maintained by the rigid cell wall and the turgor pressure generated by the central vacuole working together.