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Demonstrating Osmosis and Anthocyanins Using Purple Onion

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Demonstrating Osmosis and Anthocyanins Using Purple Onion Demonstrating Osmosis and Anthocyanins Using Purple Onion Anne A. Kamrin Joan S. LaVan The Baldwin School BrynMawr, Pennsylvania 19010 Wet mounts of white onion cells usually very difficult to discern in mounting solutions by capillarity are widely used in introductory white onion preparations. In addi- using paper towels, and can reflood biology to demonstrate plant cell tion, the position of the cell wall in the preparations with other solu- Downloaded from http://online.ucpress.edu/abt/article-pdf/46/2/116/41228/4447791.pdf by guest on 01 October 2021 structure.We have found that pur- relationto these membranesand to tions, thereby changing the en- ple onion cells show cellular struc- the nucleus are clear in the purple vironmentof the cells. Forexample, ture more clearly and can also be onion (fig. 1). Students may also be a cell mounted in 2% NaCl will used to directly observe osmotic given a "feel" for the condition of plasmolyze (fig. 2); when it is changes in cells under a microscope plasmolyzed and non-plasmolyzed reflooded with distilled water it will rather than by resorting to use of cells. Finally, the study of purple return to a more normal condition models. These studies are simple to onion epidermisis open-ended and (fig. 3). Continuing to add distilled perform, require only ordinary can be used as a starting point for water will produce a slight bulging equipment, and use easily ob- other projectssuch as investigation as a result of increased turgor tainable materials. of vacuolar pigments as pH pressure. Students are asked to make tear indicators. Students can actually watch the preparationsand wet mounts of the changes occurringin cells within a of time. The idea that outer, purple epidermis of the pur- Procedures short period ple onion bulb's leaf scales. The osmosis involves movement of vacuoles of these cells contain an The tear preparations are water across- membranes is em- anthocyaninpigment which deline- mounted in distilled water as well phasized because the anthocyanin ates the large, central plant cell as in NaCl solutions of varying pigment remains within cell vacuole and makes it possible to osmotic concentration to vacuoles, and its color intensity observe the positions of cell and demonstrateplasmolysis. Then, the changes due to changes in pigment vacuolar membranes which are students can remove the various concentration as water moves into . z/42 O FIGURE1. Normalcondition of purple onion epidermal cells. Gray shaded areas show pigment-filled central vacuoles. Nuclei can be observed in some cells. lOOX ii e ISHr 116 THE AMERICAN BIOLOGY TEACHER, VOLUME 46, NO. 2, FEBRUARY 1984 .. .. .. K jP s;$S. }. wf LS_bh) Eil ;;:;s~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. ... i , * . .. .. ... .......................................-z$.v~~~~._:,.,z,,$.w;.....................................*,,t . .... Ri l ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-i=^,ICau Downloaded from http://online.ucpress.edu/abt/article-pdf/46/2/116/41228/4447791.pdf by guest on 01 October 2021 FIGURE2. Condition of purple onion epidermal cells in 2% NaCl FIGURE3. Condition of plasmolyzed purple onion epidermalcells solution. Concentration of pigment in the central vacuoles of rehydratingfollowing flooding with distilled water. The vacuoles plasmolyzed cells can be seen. Some cytoplasmic strands are also become rounded. Some areas of the cell walls show the position of visible. lOOX the plasmodesmata. lOOX or out of the cell, as is shown in 4.5 they appearblue-purple. Above thocyanin pigments that all have figures 1-3. The rate at which these neutrality,they tend to breakdown similar basic structures, with dif- changes occur can be observed. and appear green. These color ferent active groups which produce These observations readily lead to changes can be demonstratedusing variationsin color (Clevenger1964). a discussion of the causes of the purple onion cells from the This can lead to a discussion of the wilting, turgor, and the role of the previous preparations. The addi- role these pigments play in the col- cell wall in preventing the cytolysis tion of 1N acetic acid (pH 2.4) will oration of flowers, fruits, and of plant cells. cause vacuoles to take on a reddish leaves, as well as the effect that the hue, while addition of O.1N boric external environment has on the Studies of Anthocyanin acid (pH 5.2) will render the colors produced. vacuoles bluish. The use of am- Formore advanced students with monia or strong bases to elicit the some backgroundin chemistry,fur- basic color change is to be avoided Reference ther studies of anthocyanin since it produces an irreversible CLEVENGER, S. 1964. Flower pigments can be undertaken. An- change in the pigment, thus pigments. Scientific American 210(6): thocyanin pigments are pH in- destroying its usefulness as an 84-92. dicators. They generally appear to indicator. be red-purple in solutions around Students should be informedthat pH 2, while is solutions above pH there are a number of different an- HOW-TO-DO-IT 117 .
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