Vacuole means actually empty space. Plant cells have very large vacuoles that can take up most space of the cell and push all the other organelles towards the cell wall.
The vacuole is surrounded by an active transport membrane which is called the tonoplast.The vacuoles are, however, not empty. They are responsible for the trugor or pressure in the cells that keeps plants upright. The driving force of the trugor is the absorption of water via osmosis.
Water can cross through the tonoplast, or membrane around the vacuole, and move into the vacuole by osmosis. This causes the cell to swell and create a pressure against the cell wall (trugor pressure). The vacuoles do also function as a storage organelle. They store many types of molecules, for example the pigments in the petals of flowers; or in lemon fruits, the organic acids that are build up during the citric acid cycle, are stored temporarily in the vacuoles so that the cytosol does not become too acidic and denature (disable) other enzymes that can not function at a low pH. Another group of molecules that are stored in vacuoles are amino acids. They have to be fast and easily accessible and if stored in the cytosol they would also lower the pH too much.
Vacuoles from the appropriate plant can also store substances like rubber or opium. This makes vacuoles a very useful organelle and the tonoplast have to be a dynamic and active membrane to let all these substances in and out of the vacuole.
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