The osmolarity lab provides a clear, easy technique for identifying and quantifying cell response to changes in osmotic conditions. Contractile vacuoles in Tetrahymena are large and visible even with the fairly low quality microscopes generally available in high school biology labs, and changes are readily captured for detailed analysis using the single frame, time lapse, and movie options available with low cost digital cameras. Tetrahymena contractile vacuoles gather and expel water in periodic fashion, with the rhythm and rate of contraction dependent on environmental factors. The lab addresses membrane permeability and osmoregulation in a free-swimming cell under a variety of conditions. The exercise can be varied according to class level, from simply observing changes to collecting and graphing data on rates of contraction under different conditions. It also invites student-designed experiments that alter the cell environment in specific ways.

Contractile Vacuoles
The contractile vacuole is an organelle that is very important for osmoregulation in many cells, including Tetrahymena. The contractile vacuole helps to control the relative concentration of water and solutes in the cell by collecting water from within the cell (diastole stage) and periodically pumping it out into the environment by a mechanism involving membrane contraction (systole stage). The rhythmic pulsing of the contractile vacuole is easily visible under the light microscope, and the size of the contractile vacuole as well as the rate of contraction can be affected by environmental changes. This clip shows many Tetrahymena cells in which the contractile vacuoles are evident. Following the cells inside the circles will allow you to see complete contractile vacuole cycles in a live cells.
Module Protocols

Middle School, High School

Relevant Concepts

Water and Chemistry of Life; Physiological Regulation; Relationship of Structure to Function; Homeostasis; Feedback Mechanisms; Structural similarity between Single cell and Multicellular Organisms

Next Generation Science Standards Relationships

High School:

HS-LS4-5 Evaluate the evidence supporting claims that changes in environmental conditions may result in: (1) increase in the number of individuals of some species, (2) the emergence of new species over time, and (3) the extinction of other species.

Middle School:

MS-LS1-1 Conduct an investigation to provide evidence that living things are made of cells, either one cell or many different number and types of cells.

MS-LS1-2 Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function. 

MS-LS2-1 Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem. 


●Dunham Philip B. 1964. The Adaptation of Tetrahymena to a High NaCl Environment. Biol.Bull. 126 (3):pp. 373-390.

●Gounaris EG, Bardopoulou H, Kotinis KK. 1990. Biochemical and physiological responses of Tetrahymena pyriformis during adaptation to high glycerol concentrations. Microbiologica 13 (3):239-246.

●Gounaris EG and Kotinis KK. 1990. Physiological response of Tetrahymena pyriformis during adaptation to elevated external salinity. Microbiologica 13 (1):73-76.

●Morrison GA and Tomkins AL. 1973. Determination of mean cell size of Tetrahymena in growing cultures. J.Gen.Microbiol. 77 (2):383-392.

●Orias E, Hamilton EP, Flacks M. 1979. Osmotic shock prevents nuclear exchange and produces whole-genome homozygotes in conjugating Tetrahymena. Science 203 (4381):660-663.

●Rifkin JL. 1973. The role of the contractile vacuole in the osmoregulation of Tetrahymena pyriformis. J.Protozool. 20 (1):108-114.

●Stoner LC and Dunham PB. 1970. Regulation of cellular osmolarity and volume in Tetrahymena. J.Exp.Biol. 53 (2):391-399.

See our glossary for the terms used in the modules.