This is a vacuole with a single surrounding membrane called the tonoplast. The vacuole is the toxic waste dump for the plant cell. Phenolics, acids, and a range of nitrogenous wastes are held here, away from the cytosol. The water-soluble pigments are generally held here too. Chief among these are the family of pigments known as anthocyanins. These pigments are responsible for red-pink-blue-purple coloration in many (but not all) flowers and fruits.

The tonoplast (vacuole membrane) is an interesting membrane. It has transport proteins for a range of materials to be held inside the vacuole. These transport proteins are mostly active-transport carriers. If you think about why that is so, you will probably answer the question easily. I'll leave that for you to do on your own!

As a cell ages the onset of death is usually associated with tonoplast leakage and breakdown.

Another question to ask yourself is: why don't plants excrete their wastes? What advantage is there to a plant in keeping poisons in a sac inside the cell? The dangers are obvious, so evolution must be responding to the advantages of such an arrangement! What is that advantage?

Information provided by: http://koning.ecsu.ctstateu.edu