Demonstrating Osmosis and Anthocyanins Using Purple Onion

Abstract

Wet mounts of white onion cells are widely used in introductory biology to demonstrate plant cell structure. We have found that purple onion cells show cellular structure more clearly and can also be used to directly observe osmotic changes in cells under a microscope rather than by resorting to use of models. These studies are simple to perform, require only ordinary equipment, and use easily obtainable materials. Students are asked to make tear preparations and wet mounts of the outer, purple epidermis of the purple onion bulb's leaf scales. The vacuoles of these cells contain an anthocyanin pigment which delineates the large, central plant cell vacuole and makes it possible to observe the positions of cell and vacuolar membranes which are usually very difficult to discern in white onion preparations. In addition, the position of the cell wall in relation to these membranes and to the nucleus are clear in the purple onion (fig. 1). Students may also be given a feel for the condition of plasmolyzed and non-plasmolyzed cells. Finally, the study of purple onion epidermis is open-ended and can be used as a starting point for other projects such as investigation of vacuolar pigments as pH indicators.