Vacuoles

In plant cells, vacuoles are typically large, fluid-filled sacs, each bounded by its own single unit membrane called the tonoplast. Inside, a vacuole holds cell sap - essentially a solution containing sugars, amino acids, mineral salts, various waste chemicals, and, in some plants, pigments called anthocyanins (beetroot cells, for instance, store their characteristic red-purple anthocyanin pigment inside their vacuoles). When a plant cell is placed in water, its overall expansion is chiefly controlled by the vacuole, since water moves into the vacuole by osmosis. This is also the vacuole's single most important physiological role: by drawing in water, the vacuole builds up an internal water pressure called turgor pressure, and it is this turgor pressure, pushing outward against the surrounding cell wall, that keeps the whole plant structurally upright and rigid - lose that turgor (as happens when a plant wilts) and the structure collapses. Beyond turgor regulation, vacuoles function as the cell's dedicated storage and sequestration compartment, and the chapter gives several concrete, named examples of exactly what different plants store inside them: sugar in sugar beet and sugar cane; malic acid in apple; various acids in citrus fruits; the flavonoid pigment c …