The vacuolar membrane is a flexible membrane that encloses the vacuolar, a fluid-filled organelle within plant cells. This sacs are large organelles that are noted to take up 30 to 90 percent of the total cell volume, and they have several functions within the cell, including storing fluid.
The membrane of the vacuole is referred to as the tonoplast. This membrane is unique among other cellular membranes due to its incredible flexibility. When the vacuole contains only a small amount of fluid, the membrane is small enough to contain this fluid without empty space. When the cell begins to take on water, however, the membrane is able to stretch rapidly to accommodate so much fluid that this single organelle can then take up 95 percent of the cell by volume. During this process, the tonoplast does not lose its integrity or its ability to continue functioning as an active membrane, allowing fluid to move back and forth through it.
The capability of the vacuole membrane to expand so effectively and rapidly allows the organelle to fulfill its function of creating hydrostatic pressure within the cells, providing rigidity to the plant structure. The strength and flexibility of the membrane also permits the organelle to perform the important role of storing nutrients for the plant.