Vacuoles are storage centers for food and waste in cells.  Some vacuoles are storage areas for water.  Plant cells have a large vacuole that helps maintain the structure of the plant cell by storing water.  Vacuoles are distributed throughout the cell's cytoplasm.  Most are spaced equidistantly between the cell membrane, the nucleus, and the cell's other large organelles.  Since they serve as storage centers, they have priority placing by the cell's powerhouses that create energy for the cell, the mitochondrion in animal cells, and the chloroplast in plant cells.  In some microbial animals, such as the amoeba, the vacuoles are able to maintain water balance within the amoeba by contracting, or squeezing the water out.  Such vacuoles are said to be contractile vacuoles.