The osmolarity lab provides a clear, easy technique for identifying and quantitating 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 to student-designed experiments altering the cell environment in specific ways and recording and analyzing the results.


Contractile vacuoles

(licensed under Creative Commons by-nc-nd 4.0)


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

Glossary of Terms

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:

Middle School: | |

NYS Science Curriculum Guideline Relationships

Key Ideas | | | | |


Evaluate the evidence supporting claims that changes in environmental conditions may result in: (1) increases in the number of individuals of some species, (2) the emergence of new species over time, and (3) the extinction of other species.
Conduct an investigation to provide evidence that living things are made of cells, either one cell or many different numbers and types of cells.
Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function.
Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
The central purpose of scientific inquiry is to develop explanations of natural phenomena in a continuing and creative process.
Beyond the use of reasoning and consensus, scientific inquiry involves the testing of proposed explanations.
The observations made while testing proposed explanations, when analyzed using conventional and invented methods, provide new insights into natural phenomena.
Living things are both similar to and different from each other and from nonliving things.
Organisms maintain a dynamic equilibrium that sustains life.
Plants and animals depend on each other and their physical environment.