Contractile Vacuole
Contractile Vacuole
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 Cellular Metabolism,and following). Proton pumps apparently create H+ and HCO− gradients that draw water into the vacuole, forming an isosmotic solution. These ions are excreted when the vacuole empties.
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.
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 Cellular Metabolism,and following). Proton pumps apparently create H+ and HCO− gradients that draw water into the vacuole, forming an isosmotic solution. These ions are excreted when the vacuole empties.
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.
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