A sap vacuole refers to the fluid found within the central vacuole of a cell and is more commonly known as cell sap. Cell sap, according to Biology-Online, is a dilute fluid made up of water, amino acids, glucose and salts.
Cell sap accumulates over time within the central vacuole, which is enclosed by a membrane known as the tonoplast. The central vacuole matures slowly as smaller vacuoles derived from the endoplasmic reticulum and Golgi apparatus fuse together. The central vacuole is extremely selective about the materials allowed to pass through it, thus it contains fluid vastly different from the cytoplasm. This fluid is known as cell sap.
Cell sap contains pigments that give flowers their distinctive color. It also contains plant waste, which tastes bitter to insects and animals to deter them from eating the plant. According to Reference.com, cell sap is used as a storage place for useful materials, such as proteins. It also provides mechanical support for non-woody plants. In other words, the fluid helps the cell remain filled with liquid, which helps the plant stand upright. When the vacuole is filled with liquid, it exerts pressure against the cell wall, making the plant more rigid and sturdy. Additionally, cell sap helps plant cells in the process of osmosis, which is the process of transferring a liquid solvent through a membrane. Finally, the materials within the cell sap, such as glucose and amino acids, help the plant grow.