White or Red? Which Onion is Better for Observing Plasmolysis
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The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (2/2020):
Few chemical compounds are as important to every living organism as oxidane, also known as dihydrogen monoxide H2O, which we more commonly refer to simply as water. Water exists in liquid form under standard conditions. In gaseous form, water is known as water vapor, and in solid form as ice. Due to its chemical properties, this compound can naturally exist on Earth in all three states of matter, which is truly a rarity.
Water is a common solvent for biologically significant compounds. It participates in most metabolic reactions, acts as a transport medium within organisms (including for metabolic products, nutrients, hormones, and enzymes). With its high specific heat and heat of vaporization, water allows for effective thermoregulation. The reasons above are just a few that make this substance an essential part of the diet of all known organisms, although their needs for it can vary greatly. Water makes up on average 70% of an adult human’s mass, including 60–70% of lymph, 95% of blood plasma, 90% of leaves, fruits, 20% of bones, 10% of tooth enamel, and fat tissue.
As for plants, they have developed mechanisms related to water management that differ from those in animals. Such management refers to the set of physiological processes that allow plants to maintain appropriate water content in their tissues. Proper hydration of tissues is necessary to ensure the continuity of metabolic processes, xylem and phloem transport, and to maintain the shape of cells and the plant itself. The pressure inside the cells is an important factor enabling cell growth. As a solvent, water enables the transport of assimilates and regulatory substances in the phloem, and mineral salts in the xylem. Water evaporation from the plant’s above-ground organs generates the so-called suction force, which is the driving force transporting large amounts of water even several dozen meters upwards, as seen in tall trees [1] [2].
In school, university, or even hobbyist biology laboratories, many experiments related to water management in various organisms can be conducted. An example is experiments demonstrating the principle of osmosis using biological membranes, as well as the construction of a model illustrating the suction force of leaves [3] [4]. Another phenomenon related to the issue of water transport is plasmolysis, which I have already mentioned in previous articles. However, from correspondence with dear readers, it appears desirable that I devote more space to the phenomenon itself, as well as the methods of its observation in a basic laboratory setting.
Experiment
Plasmolysis can be observed in various cells of many plant species. The most commonly used are the epidermal cells of the inner surface of the storage leaves of the common onion Allium cepa.
Although we all call this well-known vegetable "onion", the correct botanical name is bulb onion. This plant belongs to the Amaryllidaceae family Amaryllidaceae, which also includes many ornamental plants, such as the trumpet daffodil Narcissus pseudonarcissus [5].
Onion is one of the earliest cultivated plants and is no longer found in the wild [4]. The plant is probably native to Central Asia but is now found almost everywhere. Traces indicating the use of onions have been found in excavations in Palestine from the Bronze Age, dating back about 5000 years BC; these are drawings depicting the vegetable and its use for consumption [6].
The whole plant, especially its bulb, is rich in useful chemical compounds, including vitamins. It contains volatile oils, such as diallyl-propyl disulfide, which has a characteristic smell, as well as other sulfides and alkyl compounds, including substances with strong phytocidal properties, which is why onions have been used in folk medicine and phytotherapy [7].
Thanks to centuries of cultivation, many different varieties of onions have been developed, differing in appearance, taste, and nutritional value. Photo 1 shows white and red onion varieties.
Upon cutting them open, we can observe the internal structure of this fascinating plant organ (Photo 2).
The onion bulbus is a highly specialized underground shoot with a storage and perennation function, with its main part consisting of modified leaves. The plant’s stem is strongly shortened and takes the form of a so-called bulb plate, densely covered with thickened storage leaves that accumulate reserve substances, with the outer leaves usually dead and forming protective scales. The plant's above-ground part originates from the so-called apical bud.
As shown, both onions differ in color, but not in structure. It turns out that for plasmolysis observations, the red variety is more suitable. I think the reason for this will become clear in a moment.
To observe plasmolysis, it is best to cut a suitable fragment of the epidermis from the inner surface of the storage leaf in the area indicated by the arrow in Photo 3. These are the apical regions of the leaf, on the side opposite to the bulb plate.
As seen in Photo 4, in some cases, the epidermis is so easily separable from deeper layers of cells that it can be removed from the entire surface of the leaf in one piece. However, for experiments, it is recommended to collect smaller fragments.
The properly taken epidermal fragment should be one cell layer thick and quickly placed on a microscope slide with a drop of water, so that it does not dry out, and then covered with a coverslip. Observations should be conducted under transmitted light, in a bright field.
Photo 5 shows that the epidermal cells of both varieties do not differ in shape; they are elongated polygons. Inside, one can observe barely visible nuclei pressed against the inner surface of the cell membrane by large vacuoles (the cytoplasm forms a barely discernible layer). However, an interesting difference can be observed: the protoplast of the white onion cells is almost colorless, while in the red variety, it has a distinct purple hue due to the presence of anthocyanin pigments. This natural coloration makes the image of the red onion cells much more contrastive than in the white variety, which significantly aids microscopic observations, particularly during plasmolysis. For this reason, using the red onion variety is highly recommended for school experiments.
To observe plasmolysis, the cells should be placed in a relatively concentrated glucose or sucrose solution (a hypertonic solution relative to the cell’s interior) for several minutes, after which the observations should be repeated. A clear change will be observed: the vacuole and the entire protoplast will significantly shrink and start to clearly detach from the cell wall (Photo 6).
The difference will become even more apparent when comparing Photo 6 with Photo 5B. Plasmolysis can be reversed (as long as the cell membranes are not too damaged during the process) by transferring the preparation to a hypotonic solution, i.e., one with a lower concentration than inside the protoplast.
Explanation
Plasmolysis results in the loss of turgor, which is the tension in the cell wall caused by the hydrostatic pressure inside the cell. The effect of turgor is the rigidity of plant tissues and the ability to maintain the shape and stiffness of even those elements that lack typical mechanical tissues.
Osmosis is a phenomenon that occurs when solutions of different concentrations are separated by a semipermeable membrane. The membrane has the characteristic that solvent molecules can pass through it relatively easily, while solute molecules can do so only to a much lesser extent. Osmosis spontaneously occurs from the side of lower solute concentration to the side of higher concentration, leading to an equalization of both concentrations. As a result, the volume of the more concentrated solution increases, while the less concentrated solution decreases. But what mechanism drives this phenomenon?
The key to understanding osmosis is the fact of diffusion, which is a spontaneous process of spreading molecules or energy in any medium at temperatures above absolute zero, caused by the chaotic collisions of molecules in the system. Introducing a semipermeable barrier that separates solutions of different concentrations allows for the observation of interesting patterns, which can be better understood with the help of Figure 1.
Initially (Fig. 1.A), different concentrations of solute (yellow circles) exist on both sides of the semipermeable barrier (gray), with the left side having a lower concentration than the right. Only solvent molecules (blue) can pass through the barrier, while solute molecules cannot. In this situation, solvent molecules contact the barrier more frequently on the side with lower concentration because the opposite side has more solute molecules competing for space. This causes more solvent molecules to move through the membrane from the less concentrated solution to the more concentrated one than in the opposite direction. This process is called osmosis (black arrow). After some time, the volumes of the solutions change: the less concentrated solution decreases, meaning its concentration increases, while the more concentrated one increases, meaning its concentration decreases, as shown in Fig. 1.B.
Plasmolysis results from osmosis when biological membranes act as the semipermeable barrier. If a cell is placed in a solution more concentrated than its interior, water flows out through biological membranes such as the membrane around the vacuole or the cell membrane, causing the cell to dehydrate and the protoplast volume to shrink. Moving a cell that has undergone plasmolysis into a hypotonic solution causes water to flow back into the cell and restore turgor, a process called deplasmolysis. Both plasmolysis due to cell dehydration and deplasmolysis due to membrane rupture can sometimes lead to cell death.
References:
- [1] Kacperska A., Water Management, in: Kopcewicz J., Lewak S., Plant Physiology, Wydawnictwo Naukowe PWN, Warsaw, 2002, pp. 192-227 back
- [2] Szweykowska A., Plant Physiology, Wydawnictwo Naukowe Uniwersytetu im. Adama Mickiewicza w Poznaniu, Poznań, 1997, pp. 41-59 back
- [3] Ples M., Niezwykłe bariery - o błonach półprzepuszczalnych i osmozie (eng. Fascinating Barriers: Exploring Semipermeable Membranes and Osmosis), Biologia w Szkole (eng. Biology in School), 3 (2017), Forum Media Polska Sp. z o.o., pp. 52-58 back
- [4] Ples M., Transport wody - jak to robią rośliny? (eng. Water Transport: How Do Plants Do It?), Biologia w Szkole (eng. Biology in School), 5 (2017), Forum Media Polska Sp. z o.o., pp. 59-63 back
- [5] Krześniak L. M., Herbal Medicine Kit, Sport and Tourism Publishing House, Warsaw, 1988 back
- [6] Onion History, in the service: https://www.onions-usa.org, available online: https://www.onions-usa.org/all-about-onions/history-of-onions/ [accessed on 22.02.2020] back
- [7] Nowiński M., The History of Medicinal Plants and Crops, State Agricultural and Forestry Publishing House, Warsaw, 1983, pp. 195 back
All photographs and illustrations were created by the author.
Marek Ples