One of the easiest, simplest, and also fun ways to learn about microscopy is to look at onion cells under a microscope. As a matter of fact, observing onion cells through a microscope lens is a staple part of most introductory classes in cell biology – so don’t be surprised if your laboratory reeks of onions during the first week of the semester.
Think of it this way: preparing onion samples is easy, since onions are widely available and easy to source, not to mention peeling an onion isn’t all that hard (if you don’t factor in the inevitable tears). Studying onion cells is also a great way to learn about plant cells in general.
Below is a brief guide on how to observe onion cells under a microscope.
What do onion cells look like under the microscope?
Studying cell tissues from an onion peel is a great exercise in using light microscopes and learning about plant cells, since onion cells are highly visible under a microscope, especially when stained correctly.
Onions are multicellular plant organisms, which basically means that they are made up of many cells that are uniform in size and shape. This is unlike animal cells, which appear a lot more differentiated. The uniformity of onion cells are due to the existence of the cellulose, which helps the cell maintain its shape.
Onions are also made up of several layers, with a membrane or thin skin separating each layer. The main onion cell structures are quite easy to observe under medium magnification levels when using a light microscope. The cells look elongated, similar in appearance- color, size, and shape- have thick cell walls, and a nucleus that is large and circular in shape.
How to observe onion cells under a microscope
Onions are composed of several layers separated by thin membranes. In this activity, we will be using these thin membranes to observe onion cells in great detail. Obtaining a thin membrane from a bulb of onion is easy with the use of a pair of tweezers.
For best results, use small, firm onions. And, for the most amount of detail, which you will be able to appreciate when using fluorescence or advanced microscopy techniques, use the layers located halfway between the center and outside of the onion.
Before we proceed with the experiment, you need to first gather all the necessary materials that you will use to successfully look at onion cells.
If you don’t have some of these materials ready, consider buying them from your local pharmacy or through Amazon. These microscopy tools are affordable and highly useful especially if you plan on doing more microscopic observations in the future.
For this activity, you will need:
- One onion peeled to a single layer (make sure that the layer isn’t too thick, or else you won’t be able to see the onion cell structure)
- Specimen stain (in this activity, we will be using iodine, which is easily accessible and works well with onions to produce clear contrast)
- A set of blank microscope slides to mount the specimen on
- Cover slips to keep the specimen in place and protect the microscope lenses from coming into contact with the specimen
- An eye dropper or pipette to create a wet mount (you can choose between disposable and reusable droppers)
- Your lab sheet to take down your notes and observations on
- A digital microscope or any simple light microscope
As we mentioned above, iodine is the best stain to use when looking at onion cells. That said, there are other types of stains that can be used based on the type of cell that will be observed under the microscope, and some of these can be used on onions as well. Here is how common stains differ from one another:
Iodine is a dark stain which targets the starch content of plant cells in order to color them and provide better contrast. When it comes to onion peel cells, iodine enhances the visibility of the cell wall. Keep in mind that onion cells don’t contain as much starch as other plants, especially potatoes. Still, an iodine stain can bind well with the little starch granules in an onion cell.
You can also use methylene blue, which, as the name suggests, is a blue colored stain. It works particularly great on protein rich or acidic cell structures, such as the endoplasmic reticulum, ribosomes, and nucleus, thereby making it a common choice in staining bacteria and blood cells.
Another example is Eosin Y, which is a red or pink stain used to color plant cells, blood cells, and animal cell structures that are more alkaline by nature, including the cytoplasm.
The most important step here is to adequately and properly stain the onion peel cell sample so that you can see the individual cells and the cell structure under the microscope. That, and slicing the onion into a single thin layer that allows ample light to easily pass through.
This is because onions are translucent, so you’ll want the added contrast that stains provide to enhance the detail of the different cell parts. But, you also don’t want too much contrast or thickness, or light won’t pass through the sample and you won’t be able to see anything through the microscope eyepiece.
- Cut the onion then peel off the epidermal layers, which are membrane-like skins located between each onion layer. This thin membrane is best for studying onion epidermal cells at low magnifications.
- Alternatively, peel the onion into one super thin layer. If you’re not sure on whether your sample is good, make a few peelings and experiment with each one when viewing.
- Carefully mount the onion peel on top of the microscope slide, making sure to smooth out wrinkles using the end of the pipette or with the use of your forceps.
- Using the pipette or dropper, apply one or two drops of iodine over the onion sample. Then, carefully place the cover slip over the stained sample, starting by dropping one end then the other end in order to prevent bubbles.
- Alternatively, you may also use methylene blue, in which case you have to first cover the onion peel with the cover slip, then apply the methylene blue next to it. Do not apply too much stain or it will ruin the contrast of the sample.
The procedure for viewing onion cells is relatively simple and easy, especially if you are using a digital microscope, which should be easy and convenient to use.
- Start by “polishing off” your prepared specimen slide. First, check it for bubbles- if you notice any, grab your pipette, and using the bulb end, lightly tap the slide until the bubble disperses. You can also hold a tissue at the edge of the slide to absorb excess stain and close any bubbles.
- Afterwards, carefully mount the prepared and stained onion cell slide onto the microscope stage. Make sure that the cover slip is perfectly aligned with the microscope slide, and that any excess stain has been wiped off. Secure the slide on the stage using the stage clips.
- Finally, observe the mounted slide through the eyepiece, starting with the lowest magnification, and adjusting the focus, condenser, and illumination as needed. Take note of your observations before moving on to the second magnification, repeating the same steps, then study the onion cells under the highest magnification and compare.
Writing down your observations on a data sheet will make it easier to organize information, form comparisons, and better understand the significance of what you’re seeing at different magnifications.
Like all plant cells, an onion peel cell consists of different parts, including the cell wall and cell membrane. There is also the cytoplasm, and the nucleus, which is located at the cytoplasm’s periphery. You will also be able to see the vacuole, which is prominently visible at the cell’s center.
FAQS About Onion Cells Under Microscope
What do onion cells look like?
As we mentioned earlier, onion peel cells should appear uniform in size and shape. At low magnifications, you should be able to see the clusters of onion cells, and at medium magnification, these cells become even more visible.
At this point, you will easily be able to distinguish the cytoplasm and plasma membrane. The nucleus should also be clearly visible if the staining procedure is done properly.
What are the parts of an onion cell?
Understanding the different parts of an onion cell (or any cell, for that matter) means understanding their specific functions in keeping the organism alive. When it comes to onion cells, this means learning about the different parts of an onion plant, the function of each part, and how the onion cells contribute to these functions.
An onion plant is made up of leaves and a bulb- the leaves grow above ground to receive sunlight, while the bulb grows underground.
Chloroplast and chlorophyll
The leaves contain chloroplast, and these leaves are where the magic of photosynthesis happens. This process produces glucose, which is then converted to starch granules, and these granules go to the onion bulb (which are essentially modified leaves) for storage, and are later processed as energy. An onion also produces other simple sugars or carbohydrates.
All this means that chloroplasts and chlorophyll, which are needed for photosynthesis, are only present in the leaves of the onion cells, and are absent in the onion bulb. Hence, when observing onion epidermal cells, which come from the bulb, you won’t see either of these.
Instead, what you will see are thick cell walls made of cellulose. The cell wall is what maintains the shape of the cell and acts as its protective layer against fungi and viruses which can easily harm the cell’s sensitive tissues.
Other cell parts
Aside from the cell wall, you will also see other parts of the cell: the cell membrane, the cytoplasm that surrounds the prominent vacuole at the center of the cell, and the large, circular nucleus at the cytoplasm’s periphery.
What microscope do you need to observe cork cells?
As with most, if not all, plant cells, using a simple low power light microscope is sufficient to be able to see most of the minute details of each individual cell part. So, you have the option of using your preferred light microscope, or whatever you have on hand. In this procedure, we will be using a digital microscope.
What magnification do you need to see onion cells?
Now, one thing to keep in mind is that while standard light microscopes are enough for an introductory study of onion (or other plant) cells, a simple optical tool won’t allow you to observe individual organelles. At most, you will be able to see cell clusters as a group of rectangular structures with a prominent outer layer, which is the cell wall.
So, if you want to see more detail and be able to distinguish various parts of the cell, you will need to observe your sample at higher magnifications of at least 40x. At the very least, this will offer a more defined view of the cellulose. You can also use more advanced microscopy techniques to enjoy “better” views of the cells, or even zoom into the nucleus.
Understand, however, that as you use higher power objectives, you will see less cells due to the reduced field of view, so instead of looking at cell clusters and how they appear or behave amongst one another, you will only be able to observe perhaps a single cell at a time.
Observing onion cells under a microscope is a fun and easy activity for students and hobbyists alike. Onion epidermal cells appear as a single thin layer and look highly organized and structured in terms of shape and size. Certain parts of the cell are also clearly distinguishable with or without staining, making the activity even easier and more interesting.
Studying onion cell structures will also help you understand the nature of plant cells, and to an extent, how they differ from animal cells. Moreover, by seeing various cell parts and how they are positioned relative to each other, you will also have a better understanding of their functions.
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