Unlike plastids, vacuoles are single membrane ( tonoplast) sacs filled with cell sap
90% of the volume of a cell is made of up of water either within the vacuole or within the protoplasm
The sap itself contains water, salts, sugars, proteins and is acidic
Functions: although a simple structure, it can serve a number of valuable functions....
a. cheap growth: water is generally quite available - does not require the expenditure of N for plasma. Growth can be rapid.
b. storage: vacuoles can be used to hold proteins as in seeds or malic acid in CAM photosynthetic cells. If the cell is specialized they can hold appropriate materials
c. they can serve as garbage dumps: unlike our vascular system which can empty wastes into kidneys or other excretory organs, wastes remain in the plant cell. These wastes however can be converted into what are termed secondary metabolites, which can act as herbivory deterrents; i.e. nicotine, alkaloids
d. they can also hold pigments, but unlike the fat-soluble carotenoids in the plastids, these watery sacs contain only water soluble anthocyanin pigments.. for example the blues and reds we associate with fall colors.
e. they can serve as lysosmes, to break down macromolecules which can then be recyled..