Cell sap is the liquid contained within a plant cell vacuole. The chemical composition of this liquid differs significantly from the materials contained outside the vacuole in the surrounding cytosol. This difference aids in the transfer of materials across the vacuole's membrane, called the tonoplast.
The vacuole and the cell sap inside it perform a variety of vital functions, such as storing nutrients and mineral salts, sequestering and isolating toxic metabolites, enabling waste disposal and providing structural support. Because of their relatively large size in plant cells, the vacuoles can take up as much as 30 to 80 percent of the cell's volume. The ability of the liquid-filled vacuole to maintain turgor pressure against the cell's outer wall is what enables it to help support structures such as flowers and leaves. When plants do not obtain sufficient water, the turgor pressure exerted by the vacuoles drops and the plant wilts.
Cell sap may contain pigments that account for a flower's recognizable colors. The cell sap may also contain enzymes that will react with other enzymes in the outside cytosol if the vacuole's membrane is broken. The new chemicals produced by the reaction between enzymes can be toxic to certain animals and represent a protective device that encourages herbivores to move on to another food source. The release of the enzyme syn-propanethial-S-oxide from cut onions is one example. In garlic plants, the combination of the enzymes alliin and alliinase will react to form allicin when a vacuole is broken.