A fluid-filled cavity within the cytoplasm and separated from it by a membrane, the * tonoplast . In newly formed cells, when division has ceased, vacuoles are formed from small detached parts of the endoplasmic reticulum. As fluid (cell sap) accumulates they enlarge and coalesce to form a single vacuole, pushing the protoplasm against the cell wall. The surface area of the wall is increased, by stretching and the formation of additional material, to accomodate the increased volume of the cell. The vacuolar sap is a solution of organic and inorganic compounds. These may include sugars, soluble polysaccharides, soluble proteins, amino acids, carboxylic acids, red, blue, and purple anthocyanins, and mineral salts. Starch grains, oil droplets, and crystals of various kinds may also be present. These constituents of the sap probably represent metabolic by-products and reserve food material. |
