Vacuoles are essentially membrane-bound sacs found in the cytoplasm. In animal cells, vacuoles are relatively small, and are used as temporary storage areas for materials and for transport purposes. In plant cells, however, there is generally a large single central vacuole. Comprising approximately 90% of an mature plant cell, the central vacuole provides structure and support to the cell by maintaining turgor pressure, which is essentially fluid pressure that keeps the cells rigid.
They are necessary to cell functions in many different ways such as maintaining cell structure and storing nutrients, waste products, and many other substances. Below is a picture of a central vacuole in a cell.
As seen in the figure above, the central vacuole comprises a large percentage of a plant cell's volume. It can compromise anywhere from 30% to 90% of the cells volume. The central vacuole itself is consists of two main bodies; the tonoplast and cell sap.
The tonoplast is the membrane that encloses the vacuole and separates it from the cytoplasm. Like many other cell membranes, the tonoplast contains transport proteins that move substances into and out of the central vacuole.
Cell sap is the solution of chemicals and molecules that fills the vacuole. Cell sap is made up of water, enzymes, inorganic ions, salts and also stores organic acids, sugars, amino acids, lipids, oligosaccharides, storage proteins, etc. It could also include pigments, which determine the color of flowers. Some plant cells also store waste in the central vacuole. Others have enzymes that will break down biological molecules. Still others will contain molecules that act as a chemical defense, like an immune system response.
Also, the cell sap contains toxins that have been removed from the cystol. In some cases, these toxins help to deter predators.
As stated earlier, the primary function of the central vacuole in plant cells is to maintain the turgor of the cell. Turgor is the pressure exerted on the cell wall by the contents of the cell. The central vacuole determines the amount of turgor based on it water content as a result of osmotic pressure. When there is enough water, the central vacuole will swell, which increases turgor pressure; without water, however, the vacuole will shrink, which reduces turgor pressure. This is why when plants are not watered, they wilt; the reduction in turgor causes the plant cell to lose its rigidity.
Central vacuoles also have other functions. They act both as temporary stores for reserve materials and as final stores for waste products. However, these substances also contribute to maintaining turgor; the solutes play a large role in determining osmotic pressure. The central vacuole contains selective transport proteins that allow ions to pass in and out in order to adjust solute concentrations inside and outside the vacuole which maintains osmotic pressure. The more concentrated the cell sap, the more water will come into the central vacuole, which will increase turgor pressure; the opposite is true of less concentrated solution.
Also, the selective transport proteins of the central vacuole transport ions into the vacuole to maintain the pH which allows some enzymes to perform their functions. Also, the central vacuole helps to push the contents of the cytoplasm closer to the cell membrane, allowing the chloroplasts to get more exposure to light. Below is a diagram of osmosis (Refresher: Osmosis is the net movement of water from a low concentration of solute to a high concentration of solute.).
Citations, Additional Resources
http://www.biologie.uni-hamburg.de/b-online/e23/23.htm#tono http://en.wikipedia.org/wiki/Vacuole http://en.wikipedia.org/wiki/Turgor