Contractile Vacuoles

Contractile vacuoles are believed to have evolved as single celled organisms evolved cuticles. Cuticles are not highly permeable to water and ions. A good example of single celled organisms, also called a protozoan, that have contractile vacuoles include the freshwater Amoeba and Paramecium. These contractile vacuoles are somewhat different. In Amoeba, the contractile vacuole floats freely in the cytoplasm. When full, the vacuole combines with the plasma membrane and the contents are released into the environment.

• How does water enter the contractile vacuole of the Amoeba?
• Do marine protozoans have contractile vacuoles?

The contractile vacuole of the Paramecium is much different. Often there are two contractile vacuoles that are found in specific locations underneath the the cell membrane. Each contractile vacuole is connected to the outside by an excretory pore. The contractile vacuole is a membrane surrounded by several rays or ampullae. In turn, the ampullae are connected to feeder canals which collect water from the endoplasmic reticulum.

To move the water out of the organism, the vacuole per se and the ampullae are surrounded by contractile fibrils made up of actin and mitochondria. When the fibrils of the ampullae contract, water is forced into the vacuole. When the fibrils of the vacuole contract, the ampullae temporarily disconnect to prevent backflow and the water is forced through the excretory pore.

• How is water forced into the vacuole, and not into the feeder canals, when the ampullae contract?
• Design a simple experiment to prove that the contractile vacuole serves in osmotic regulation of a Paramecium.
• When will you know that the environment is isosmotic with the cytoplasm of the Paramecium?
• Design an experiment to prove that the mitochondria are used in the contraction of the contractile vacuole.

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