The tiny, spherical, intracellular vacuole
of protozoa and freshwater
sponges is not a true excretory organ,
since ammonia and other nitrogenous
wastes of metabolism readily enter the
surrounding water by direct diffusion
across the cell membrane. The contractile
vacuole is an organ of water balance.
It expels excess water that freshwater
protozoa gain by osmosis. As
water enters the protozoan, the vacuole
grows and finally collapses, emptying
its contents through a pore on the surface.
The cycle is repeated rhythmically.
Although the mechanism for filling
the vacuole is not fully understood,
recent research suggests that contractile
vacuoles are surrounded by a network
of membranous channels populated
with numerous proton pumps (proton
pumps were described in connection
with the electron transport chain in
,and following). Proton
pumps apparently create H+ and
gradients that draw water into
the vacuole, forming an isosmotic solution.
These ions are excreted when the
Contractile vacuoles are common
in freshwater protozoa, sponges, and
radiate animals (such as hydra), but
rare or absent in marine forms of these
groups, which are isosmotic with seawater
and consequently neither lose
nor gain too much water.