The largest organelle in a plant cell is the central vacuole. This membrane-bound structure can occupy up to 90% of the cell's volume and serves multiple essential functions. The central vacuole stores water, nutrients, and waste products, playing a vital role in maintaining turgor pressure, which helps keep the plant upright and structurally sound. It also aids in the storage of compounds that can be harmful to predators or pathogens, thereby contributing to the plant's defense mechanisms. Additionally, the central vacuole is involved in cell growth and homeostasis by regulating the internal environment of the cell.