por What is the epidermis of the onion?
The onion epidermis It 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 forceps.
The onion epidermis is ideal for studying cell morphology, hence the visualization of it is always one of the most frequent practices that are taught in the biology subject. In addition, the assembly of the preparation is very simple and economical.
The structure of the cells of the onion epidermis is very similar to that of human cells, since both are eukaryotic and have organelles such as the nucleus, the Golgi apparatus, and chromosomes, among others. Also, cells are surrounded by a plasma membrane.
Despite the similarities, it is necessary to clarify 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 making fresh preparations (that is, without dye) and the second is staining the sample with methylene blue, methyl acetate or lugol green.
Technique
Sample collection
A medium onion is taken, minced with a scalpel and the innermost layer is extracted. Using forceps, carefully remove the film that covers the concave part of the onion bulb.
fresco mounting
The membrane is placed on a slide and carefully spread. A few drops of distilled water are added and a coverslip is placed on top to be able to observe under a microscope.
colored mounting
It is placed on a watch glass or petri dish, hydrated with water, and spread as far as possible without damage.
It is covered with some coloring. For this you can use methylene blue, methyl acetate or lugol green. The stain will enhance visualization of cell structures.
The staining time is 5 minutes. Subsequently, it is washed with plenty of water to remove all the excess dye.
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, because 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 of all, the preparations must be focused at 4X to have a wide visualization of a large part of the sample.
In said sample, an area is chosen to pass to the 10X objective. At this magnification it is possible to observe the arrangement of the cells, but for greater details it is necessary to go to the 40X objective.
At 40X the cell wall and nucleus can be seen, and sometimes it is possible to distinguish vacuoles found in the cytoplasm. On the other hand, with the immersion objective (100X) it is possible to see granulations inside the nucleus, which correspond to the nucleoli.
In order to observe other structures, more sophisticated microscopes are needed, such as fluorescence or electron microscopes.
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 innermost layers.
organization levels
The various structures that make up the onion epidermis are divided into macroscopic and submicroscopic.
The microscopic ones 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 smaller (tiny) elements that make up large structures.
For example, with the light microscope the cell wall is visible, but not the microfibrils that make up the cellulose of the cell wall.
The level of organization of the structures becomes more complex as the study of ultrastructures progresses.
cells
The cells of the onion epidermis 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 observed much better if some colorant is applied.
By studying the cell arrangement, you can see that the cells are next to each other in close relationship, 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 an impermeable and closed 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 regularly shaped pores through which cells communicate with each other. These pores or microtubules are called plasmodesmata, and they traverse the pectocellulosic wall.
Plasmodesmata are responsible for maintaining the flow of liquid substances to maintain the tonicity of the plant cell, including 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. It is believed that they are related to cell differentiation.
Core
The nucleus of each cell will also look better defined by adding methylene or lugol’s blue to the preparation.
In the preparation you can see a well-defined nucleus 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 which case the plasmalemma is exposed.
vacuoles
The vacuoles are usually located in the center of the cell and are surrounded by a membrane called the tonoplast.
function of onion cells
Although the cells that make up the epidermis of the onion are vegetables, 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.
Their 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 in charge of 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 cells is influenced by the osmotic and pressure potentials. This means that the movement of water between the interior of the cells and the exterior will depend on the concentration of solutes and water that exists on each side.
The water will always flow to 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 lower than that of the interior, then the cells lose water and, therefore, become plasmolyzed.
This phenomenon is completely reversible and can be demonstrated in the laboratory by subjecting onion epidermis cells to different concentrations of sucrose and inducing water to enter or leave the cells.
References
Geydan T. Plasmodesmata: Structure and function. Act biol. Columbia
Plant physiology practice. Plant Biology Department. Recovered from uah.es.