ONION EPIDERMIS: MICROSCOPE OBSERVATION, ORGANIZATION - BIOLOGY - 2024

The epidermis of the onion is the superficial tunic that covers the concavity of each layer that makes up the onion bulb. It is a very thin and transparent film that can be visualized if it is carefully removed with a tweezers.

The epidermis of the onion is ideal for studying cell morphology; Hence, the visualization of it is always one of the most frequent practices that are dictated in the subject of Biology. Furthermore, the assembly of the preparation is very simple and inexpensive.

A. Onion epidermis seen at 10X. B. Onion epidermis seen at 40X. Viascos, from Wikimedia Commons / Laurararas, from Wikimedia Commons

The structure of the cells of the epidermis of the onion is very similar to that of human cells, since both are eukaryotic and have organelles such as nucleus, Golgi apparatus and chromosomes, among others. Likewise, cells are surrounded by a plasma membrane.

Despite the similarities, it should be clarified that there are obviously important differences, such as the presence of a cell wall rich in cellulose that is absent in human cells.

Microscope observation

There are two techniques to observe the epidermis of the onion with an optical microscope: the first is by making fresh preparations (that is, without dye) and the second by staining the sample with methylene blue, methyl acetate green or lugol.

Technique

Taking the sample

Take a medium onion, chop it with a scalpel and remove the innermost layer. With a tweezers, the film that covers the concave part of the onion bulb is carefully removed.

Fresco mounting

The membrane is placed on a slide and carefully spread. A few drops of distilled water are added and a cover object is placed on top to be observed under a microscope.

Colored mount

It is placed in a watch glass or in a Petri dish, hydrated with water and spread as much as possible without damaging.

It is covered with some coloring; For this you can use methylene blue, methyl acetate green or lugol. The stain will improve the visualization of cell structures.

The staining time is 5 minutes. Later it is washed with abundant water to eliminate all the excess colorant.

The stained film is taken to a slide and carefully stretched to place the coverslip on top, taking care that the film is not folded or bubbles remain, since under these conditions it will not be possible to observe the structures. Finally, the slide is placed under the microscope for observation.

Microscope visualization

First, the slides must be focused at 4X to have a wide visualization of much of the sample.

In this sample, a zone is chosen to pass the 10X objective. In this magnification it is possible to observe the arrangement of the cells, but for more details it is necessary to go to the 40X objective.

At 40X the cell wall and nucleus can be seen, and sometimes vacuoles found in the cytoplasm can be distinguished. On the other hand, with the immersion objective (100X) it is possible to see granulations inside the nucleus, which correspond to the nucleoli.

To be able to observe other structures, more sophisticated microscopes are needed, such as the fluorescence microscope or the electron microscope.

In this case it is advisable to make preparations with onion epidermis obtained from the intermediate layers of the bulb; that is, from the central part between the outermost and the innermost.

Organization levels

The various structures that make up the epidermis of the onion are divided into macroscopic and submicroscopic.

Microscopic are those structures that can be observed through the light microscope, such as the cell wall, the nucleus, and the vacuoles.

On the other hand, submicroscopic structures are those that can only be observed with electron microscopy. These are the smallest elements that make up large structures.

For example, with the light microscope the cell wall is visible but the microfibrils that make up the cellulose of the cell wall are not.

The level of organization of structures becomes more complex as the study of ultrastructures progresses.

Cells

The cells of the epidermis of the onion are longer than they are wide. In terms of shape and size, they can be highly variable: some have 5 sides (pentagonal cells) and others 6 sides (hexagonal cells).

Cellular wall

The light microscope shows that the cells are delimited by the cell wall. This wall is much better observed if some colorant is applied.

By studying the cell arrangement, it can be seen that cells are closely related to one another, forming a network in which each cell resembles a cell.

It is known that the cell wall is composed mainly of cellulose and water, and that this hardens as the cell reaches its full maturation. Therefore, the wall represents the exoskeleton that protects and provides mechanical support to the cell.

However, the wall is not a closed, waterproof structure; quite the opposite. In this network there are large intercellular spaces and in certain places the cells are linked by pectin.

Throughout the cell wall there are regular pores with which each cell communicates with neighboring cells. These pores or microtubules are called plasmodesmata and pass through the pectocellulosic wall.

Plasmodesmata are responsible for maintaining the flow of liquid substances for the maintenance of the tonicity of the plant cell, which include solutes such as nutrients and macromolecules.

As the cells of the onion epidermis elongate, the number of plasmodesmata decreases along the axis and increases in the transverse septa. These are believed to be related to cell differentiation.

Core

The nucleus of each cell will also be better defined by adding methylene blue or lugol to the preparation.

In the preparation, a well-defined nucleus can be seen located on the periphery of the cell, slightly ovoid and surrounded by cytoplasm.

Protoplasm and plasmalemma

The protoplasm is surrounded by a membrane called the plasmalemma, but it is hardly visible unless the protoplasm is retracted by adding salt or sugar; in this case the plasmolemma is exposed.

Vacuoles

Vacuoles are usually located in the center of the cell and are surrounded by a membrane called the tonoplast.

Cell function

Although the cells that make up the epidermis of the onion are plants, they do not have chloroplasts, since the function of the vegetable (bulb of the onion plant) is to store energy, not photosynthesis. Therefore, the cells of the onion epidermis are not typical plant cells.

Its shape is directly related to the function they fulfill within the onion: the onion is a tuber rich in water, the cells of the epidermis give the onion its shape and are responsible for retaining water.

In addition, the epidermis is a layer with a protective function, since it serves as a barrier against viruses and fungi that can attack the vegetable.

Water potential

The water potential of the cells is influenced by the osmotic and pressure potentials. This means that the movement of water between the inside of the cells and the outside will depend on the concentration of solutes and water that exists on each side.

The water will always flow towards the side where the water potential is lower, or what is the same: where the solutes are more concentrated.

Under this concept, when the water potential of the exterior is greater than that of the interior, the cells hydrate and become turgid. On the other hand, when the water potential of the exterior is less than that of the interior, then the cells lose water and, therefore, are plasmolyzed.

This phenomenon is completely reversible and can be demonstrated in the laboratory by subjecting the cells of the epidermis of the onion to different concentrations of sucrose and inducing the entry or exit of water from the cells.

References

1. Wikipedia contributors. "Onion epidermal cell." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, Nov. 13, 2018. Web. 4 Jan. 2019.
2. Geydan T. Plasmodesmos: Structure and function. Acta biol. Colomb. 2006; 11 (1): 91-96
3. Plant physiology practice. Department of Plant Biology. Available at: uah.es
4. De Robertis E, De Robertis EM. (1986). Cellular and molecular biology. 11th edition. Editorial Ateneo. Buenos Aires, Argentina.
5. Sengbusch P. The Structure of a Plant Cell. Available at: s10.lite.msu.edu