2. Function
They are used for transport and storage of nutrients and waste products. Vacuoles also permit plant cells to grow to a volume much larger than animal cells. These cells do not grow by filling the cells with cytoplasm, which would be very costly in terms of maintenance and synthesis, but by instead they fill up the vacuoles with water.
As stated
earlier, vacuoles can store a wide variety of substances, in particular
those that are essential but also harmful if present in big quantities
in the cytoplasm. Vacuoles of certain specialized cells can store
products such as rubber and opium. Ordinary molecules, such as
Sodium, can also be found in vacuoles. As a matter of fact, since
the vacuole is a much greater volume than the cytoplasm, most of the
cellular sodium in stored in the vacuole. Other molecules stored
in vacuoles are resposible for the interaction of the plant with
animals or with other plants. Examples would be Pigment, which
adds color to a flower thus attracting insects. Defense chemicals
are also stored in the vacuoles, such as poisonous alkaloids.
Contractile Vacuoles
There are
also small vacuoles found in many freshwater organisms. These
vacuoles
periodically expand within the cell, filling with water (from
cytoplasm) and
then suddenly contracting, thus expelling its contents to the exterior
of the
cell. This
mechanism probably removes the water which continually enters the cell
by
osmosis from the environment, or with food.
(http://io.uwinnipeg.ca/~simmons/cm1503/vacuoles.htm)
Sources: Dr. Kent
Simmons's Biology Page
Very good website created
by a
professor at the University of Winnipeg.