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

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

Middle School: MS-LS1-1 | MS-LS1-2 | MS-LS2-1

NYS Science Curriculum Guideline Relationships

Key Ideas 1.1 | 1.2 | 1.3 | 4.1 | 4.5 | 4.6