- Vacuoles are surrounded by a vacuole membrane known as tonoplast or vacuolar membrane.
- It resembles the plasmalemma, but it differs in function and is often thinner.
- The plasmalemma controls the entry and exist of solutes into the cytoplasm, the tonoplast transports solutes into and of the vacuoles and thus controls the water potential of the cell.
- This is especially important for example, in the guard cells of the stomatal apparatus. Potassium and other ions are pumped in or out of guard cells to swell or shrink, closing or opening of stomates.
- The tonoplast is derived from the ER, but recent evidences suggest that this may be via Golgi apparatus, as in case with the plasmalemma.
- In some cases ER may small (dilate) directly to form vacuoles.