While it’s true that the bulb of an onion is composed of modified onion leaves, they don’t have quite the same functionality as the leafy, above ground section of the plant.
For instance, the leafy parts of the onion above soil are, as you’d expect, responsible for photosynthesis, and contain chloroplasts; however, the two leaf types are not entirely unrelated.
During photosynthesis, trace amounts of glucose are produced, which is then, in turn, converted into a starch. This starch then makes its way down through the stem of the onion, towards the sub-terra section of the plant, and finds rest within the bulb.
The cells and pigment responsible for photosynthesis (chloroplasts and chlorophyll) are not present in the bulb of an onion.
These interesting details and differences, alongside the unique cellulose cell structure of these vegetables, is what makes them such an interesting specimen to view through a microscope.
In this article, I’ll be discussing how you can orchestrate the perfect onion cell observation. Let’s get to it!
How To Obtain Your Onion Sample
For observing onion cells through a microscope, we only need a very small specimen, so we need to know how to extract such a subject from the bulk of an entire onion. Thankfully, you don’t need any super “sciency” equipment to do this.
Simply grab a pair of tweezers, and peel a sliver from any of the onion layers. A thin membrane is all that’s required.
Preparing Your (Wet) Slide
The best way to set up your onion specimen is by using a wet slide. For the uninitiated, a wet slide is just a normal glass slide that suspends the subject on a tiny droplet of moisture, making examination nice and easy. Here’s what you’ll need to get started.
- Your onion membrane (of course)
- Glass slide for microscopy
- A single needle
- Some blotting paper
- Iodine solution
- A dropper for said solution
- A little bit of water
- A cover slip
- And your trusty microscope
With all your supplies gathered, you’re almost ready to begin, but before you get started in earnest, it’s good practice to ensure that the optical tube has descended to the point at which the objective lens has the closest proximity to the slide as the microscope will permit.
Set this up in the lowest magnification setting, and rather than adjusting while peering into the eyepiece, it’s best to just address the scope from the side.
The reason we do all this is to prevent accidental damage to the slide and the eyepiece if something goes awry during the observation. Simply put, this setup makes focusing up far easier and safer.
Now let’s take a look at the process…
- Step 1. Use your dropper to drip a small tear of water onto the middle of your glass slide.
- Step 2. Layout your onion membrane across this central droplet of water. The water helps to keep the specimen nice and flat.
- Step 3. Using your dropper again, drip some iodine solution on top of the onion membrane. Methylene blue will also work.
- Step 4. Ever so delicately, lay down your cover slip on to the membrane and glass slide, then gently press it down.
- Step 5. If there are any air bubbles in the cover slip, use your needle to remove them.
- Step 6. Use your blotting paper to buff away any excess moisture after pressing the cover slip down on the slide.
- Step 7. Load your prepared slide onto the stage of your microscope. Make sure you’re in the lowest magnification.
- Step 8. Use your coarse adjustment control to find acuity.
- Step 9. Increase objective magnification x1.
- Step 10. Use your fine adjustment control to regain focus.
- Step 11 & Beyond. Repeat steps 9 & 10 until you have reached the desired magnification and clarity. To distinguish discrete parts of an onion’s cellular architecture, you’ll need a microscope with magnification facilities that extend to at least 40x.
If you like, you can prepare a second slide without using the iodine solution. It can be more interesting than you’d think comparing the difference between stained and unstained slides.
We’ll discuss why we stain the primary slide in just a sec, but first, let’s talk about what you can expect to see through your eyepiece when everything is set up correctly.
Observing Onion Cells
- The First thing you’ll notice about your specimen is the interlocking rectangular cell structure. These are the largest general structures but are by no means the only thing you’ll see.
- You’ll also be able to distinctly make out the cell walls.
- The nucleus will also be very clear thanks to the stain.
- Look a little closer, and you’ll see some central vacuoles, spaces within the cellular cytoplasm that contain fluids.
- Within the cell walls, you may also observe some granules, but what exactly are these mysterious floaties?
Well, an onion’s layers are composed in part of rudimentary sugars, or carbohydrates if you want to be technical about it. A select few of these carbs get filed into storage as small granules of starch.
Now, normally, you wouldn’t be able to see these granules, but, as you may already be aware, iodine loves starch. In fact, it loves starch so much that it can’t help but to give it a big hug and bind to it.
Much like a bucket of paint falling on an invisible man, the starch granules are then visible for us to see through our eyepiece — hooray!
That said, it’s best not to set your expectations too high when it comes to these ephemeral granules, as there aren’t going to be all that many of them.
This is simply because, comparatively speaking, onions just don’t contain all that much starch. A potato, for example, will be much rifer with this particular substance.
Another thing to note before you begin, just so you’re not disappointed by their absence after the fact, although onions are indeed plants, you won’t be able to spot any chloroplasts.
As touched upon at the beginning of this guide, the chloroplasts are not needed in the bulb, as the bulb will be planted in the dark, dank ground, away from the reach of sunlight.
Instead, chloroplasts along with their chlorophyll are situated in the leafy upper regions of the onion, the parts that are exposed to sunlight.
One of the most astounding things that you’ll take away from this observation is just how dissimilar onion cells (or any plant cells for that matter) are to animal cells.
If, for example, you’ve observed any animal cheek cells, you’ll be well aware of their almost free form orchestration.
With animal cells, everything is all a bit looser and less organized. Onion cells, in contrast, seem meticulously mapped out into rectangular shapes.
The reason for this stark difference between plant and animal cells is that the former enjoy the benefit of a cell wall composed of cellulose.
This matter helps to hold everything together in its proper place — Plants are well regimented life forms, even on a cellular level!
That’s not to say that you’ll see rows of perfect rectangles when you peer through your eyepiece. Perhaps it’s more accurate to describe what you’ll see as a set of elongated, irregular scales.
Still, the difference between the onion cells and animal cells is like night and day.
Observing onion cells through a microscope is the perfect way to introduce yourself or your students to the differences between plant and animal cells.
It’s a fun and easy observation task that costs very little to do, yet it provides pretty astonishing insights into the biology of various life forms on planet Earth.
With the difference in results between stained and non-stained cells, this study is also a brilliant way to learn about scientific approach, and how small changes in execution can garner extremely different results.
So, go on; grab those onions! But prepare to shed a tear or two (a worthy price for scientific glory).
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