Delphinidin

In a mature living plant cell, as much as 90% or more of the volume may be taken up by one or two large central vacuoles that are bounded by vacuolar membranes (tonoplasts). The vacuolar membranes, which constitute the inner boundaries of the living part of the cell, are similar in structure and function to plasma membranes.

The vacuole evidently received its name because of a belief that it was just an empty space; hence its name has the same Latin root as the word vacuum (from vacuus—meaning “empty”). Vacuoles, however, are filled with a watery fluid called cell sap, which is slightly to moderately acidic. Cell sap, which helps to maintain pressures within the cell, contains dissolved substances, such as salts, sugars, organic acids, and small quantities of soluble proteins. It also frequently contains watersoluble pigments.

These pigments, called anthocyanins, are responsible for many of the red, blue, or purple colors of flowers and some reddish leaves. In some instances, anthocyanins accumulate to a greater extent in response to cold temperatures in the fall. They should not be confused, however, with the red and orange carotenoid pigments confined to the chromoplasts.

Yellow carotenoid pigments (carotenes) also play a role in fall leaf coloration. Sometimes, large crystals of waste products form within the cell sap after certain ions have become concentrated there. Vacuoles in newly formed cells are usually tiny and numerous. They increase in size and unite as the cell matures. In addition to accumulating the various substances and ions mentioned above, vacuoles are apparently also involved in the recycling of certain materials within the cell and even aid in the breakdown and digestion of organelles, such as plastids and mitochondria.