onion skin cell experiment

Onion Peel Cell Experiment

The onion peel cell experiment is very popular for observing a plant cell structure. Onion is a eukaryotic plant that contains multicellular cells. We know that the cell is a structural and functional unit of life that builds up living structures.

The bulb of an onion is formed from modified leaves . Like plant cells, onion cells have a rigid cell wall and a cell membrane enclosing the cytoplasm and nucleus.

Onion epidermal cells exist as a single layer that serves as a protective skin. It separates the thick, juicy scale leaves of the onion. Thus, the bulb of onion is formed from modified leaves.

The epidermal cell of an onion bulb is simple and transparent. Its microscopic observation introduces the general view of plant anatomy to the students.

Firm and medium-sized onions are generally used to visualize the onion’s epidermal cells. This post explains the theory, requirements, and procedure of the onion peel experiment. Also the observation, result and precautions of the experiment are also discussed.

Content: Onion Peel Cell Experiment

Requirements, observation, precautions.

The main objective of performing the onion peel cell experiment is to observe the arrangement and structural components of the onion epidermis. The following facts about the onion peel cell experiment play a significant role in educating students:

• The epidermis of the onion bulb is a single layer of tissue that is easy to separate. For this reason, onion peel is best for educational and experimental purposes to study the structure of plant cells.
• Due to the large size of onion cells, the cells can be examined under low magnification.
• It is also a simple experiment that the students can efficiently perform, plus they can practice how to use a microscope.

We need the following glassware and reagents to prepare a temporary slide of an onion peel.

Materials required to separate onion skin

• Medium-sized onion

Materials needed to stain and mount the onion peel

• Petri Plate
• Distilled Water
• Clean glass slide
• Blotting paper
• Compound microscope

An onion is a multicellular plant. The presence of a rigid cell wall and a large vacuole is a characteristic feature of a plant cell. Thus, onion being a plant, comprises features common to plant cells. Like plant cells, onion cells consist of a cell wall and cell membrane surrounding the cytoplasm, nucleus and a large vacuole.

• The cell wall is a rigid, protective coat covering the cell membrane, including all the internal components. The rigid cell wall maintains the shape of onion cells and contributes to the compact arrangement of the epidermal cells in onion.
• The cell membrane is interior to the cell wall surrounding the cytoplasm, including all the internal structures.
• The cytoplasm is the cell’s inner space that appears jelly-like. It moves the cytosolic material around the cell through cytoplasmic streaming.
• The nucleus is present near the periphery of the cytoplasm. It is the control centre of the cell and the largest organelle in the cell.
• The vacuole is large and prominently seen at the centre of the cell. It stores solid and liquid contents. The basic shape or size of a vacuole differs depending on the needs of the cell.

Video: Onion Peel Cell Experiment

Procedure of Onion Peel Cell Experiment

The steps to perform the onion peel cell experiment are as follows:

Steps to separate an onion peel

A. Take an onion, separate its outermost peel and chop it into two equal halves.

B. Then, take one fleshy scale leaf of a chopped onion bulb and split it into two.

C. Then carefully pull a thin, transparent epidermal peel from the convex surface of the scale leaf using forceps.

D. Then, wash the separated peel in the Petri plate containing water. You can cut the onion peel into small rectangular pieces using a blade.

Steps to stain and mount an onion peel

E. After that, transfer the onion peels into the Petri plate containing diluted safranin stain. Leave the peels undisturbed for about 3 minutes.

F. Finally, rinse the extra stain of the peel by again dipping it in the Petri plate containing water.

G. With the help of a brush or forcep, transfer the peel to the centre of a clean glass slide.

H. Then, to mount the onion peel, add a drop of glycerine over the centre of the slide. Glycerine prevents the peel from drying up.

I. After that, carefully mount a cover slip over the centre of the prepared slide by slowly lowering it with a needle. During this stage, you need to avoid the entry of any air bubbles.

J. Using a piece of blotting paper, remove extra glycerine from the margins of a cover slip.

K. Observe the temporary slide under the compound microscope.

• First, turn on the microscope’s light and ensure the low objective lens is in line with the optical tube. Then, place the prepared slide on the stage of a microscope.
• Looking from the side (not through an eyepiece), lower the tube using the coarse focus knob until the end of the objective lens is just above the cover glass. During this stage, do not crack the cover glass, or the objective lens may get damaged.
• Now look through the eyepiece and adjust the smaller, fine focusing knob to move the optical tube upwards until an image comes into focus.
• Then, swap the objective lens to a high objective lens so that you can notice the cells at greater magnification.
• Prepare an observation table for the cells as seen under a microscope and note the features listed in the observation table.
• Shape of cells: Rectangle
• Arrangement of cells: Compact
• Inter-cellular spaces : Absent
• Nucleus : Present (at the cell’s periphery)
• Stained portions : Cell wall and nucleus are darkly stained with less-stained cytoplasm
• Unstained portions : Cell membrane and Vacuole

• Do not overstain the onion skin.
• Avoid the folding of the peel.
• The glass slide and cover slip should be dry and clean.
• Put a coverslip carefully to avoid any air bubbles.
• Using blotting paper, remove the extra glycerine.

Therefore, the onion peel cell experiment is an engrossing activity that can help a student to observe and study the plant cell structure . Students can prepare the temporary slide and observe the differences between the slide with stained onion skin and the slide without any stain. The microscopic observation of onion peel cells will allow students to closely examine each component of onion cells.

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Observing Onion Cells Under The Microscope

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.

Requirements

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.

Methylene blue

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.

Preparation

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.

Observations

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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Onion Cells Under a Microscope ** Requirements, Preparation and Observation

How to obtain a thin layer of onion cells, how to prepare a wet mount slide, observations.

View Epidermal Cells

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How To Prepare an Onion Cell Slide

Peeling Back the Layers of Nature's Microcosm

Microscopy offers a fascinating glimpse into the intricate world of cells, where the seemingly ordinary onion serves as a gateway to understanding cellular structure. In this comprehensive guide, we delve into the process of preparing an onion cell slide, exploring the steps involved and the significance of staining in revealing the hidden details of these microscopic wonders.

How Do You Make an Onion Cell?

The first step in unlocking the secrets of onion cells lies in the preparation of a microscope slide. Begin by selecting a fresh onion bulb and carefully peeling away the outer layers to expose the translucent inner tissue. Using a sharp blade or knife, slice a thin section of the onion, ensuring a clean and uniform cut to facilitate microscopic examination.

Once the section is obtained, transfer it to a glass microscope slide using fine forceps or a dropper. Add a small drop of water to the slide to help flatten and hydrate the onion tissue, making it easier to observe under the microscope. Gently lower a coverslip onto the specimen, taking care to avoid trapping air bubbles that could distort the image.

Why Is It Easier to See the Onion Cells After They Are Stained?

While the natural transparency of onion cells allows for basic observation under a microscope, staining techniques enhance contrast and reveal specific structures with greater clarity. Staining involves the application of colored dyes or chemical solutions that selectively bind to cellular components, highlighting their presence and distinguishing them from the surrounding tissue.

In the case of onion cells, staining is particularly advantageous due to the presence of cell walls that can obscure internal structures. By treating the specimen with a suitable stain, such as iodine or methylene blue, the nuclei and other organelles within the cells become more prominent, enabling detailed examination and analysis.

Staining of Onion Cell Nuclei

Among the various cellular components, the nucleus stands out as a vital hub of genetic information and regulatory processes. Staining techniques play a crucial role in visualizing onion cell nuclei, shedding light on their morphology, distribution, and function within the larger cellular context.

One commonly used stain for highlighting nuclei in onion cells is acetic orcein, a dye that binds specifically to DNA, the genetic material housed within the nucleus. After fixing the onion tissue with ethanol or formaldehyde to preserve cellular structures, immerse the specimen in a solution of acetic orcein for a brief period.

The stain permeates the nucleus, imparting a distinctive reddish-purple hue to the DNA-rich regions, while the surrounding cytoplasm and cell walls remain relatively unaffected. Under the microscope, the contrast between stained nuclei and the background allows for precise localization and characterization of these essential cellular organelles.

The Hidden Beauty of Onion Cells

The preparation and staining of onion cell slides offer a captivating journey into the microscopic world, where seemingly mundane plant tissue reveals its intricate beauty and complexity. By following the steps outlined in this guide and harnessing the power of staining techniques, researchers and enthusiasts alike can gain valuable insights into the structure and function of onion cells, paving the way for deeper understanding and exploration of the cellular world.

Onion Cell Under Microscope

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Onion peel under microscope

This experiment is for beginners to learn how to make a slide of a specimen, an onion peel, and view it under a microscope.

Prepare an unstained, stained temporary mount of an onion peel and study its cells.

Driving Questions

• What does a cell, in this case, a plant cell look like?
• Why does the onion smell so pungent, but only when cut?

All living things are made of cells. An onion cell is a multicellular (consisting of many cells) plant organism. An onion cell, like all plant cells, contains the following essential parts:

• A thick cell wall made of cellulose. Yes, the same thing that makes up your cello tape. It’s the cellulose that makes the cell rigid.
• A cell membrane is within the cell wall and holds everything else inside.
• A large vacuole , which is an extensive reservoir of water and ions inside the cell
• It is present at the center of the cell. Cytoplasm surrounds it.
• Cytoplasm , sandwiched between the vacuole and the cell wall. This is also primarily water, but also other organic elements. Within the cytoplasm rest other organelles (smaller organs).
• The nucleus is also present in the cytoplasm. This is the command center of the cell; its DNA resides here.

The characteristic onion “smell” comes from the organic molecules in the cytoplasm and the vacuole, _when combined, which can only happen when the cell is cut - either via a knife, or a rodent, insect, etc. The scent is a defense mechanism to ward off attackers!

This workshop will show the cell, its cell wall, and the nucleus.

Prerequisite

Anthony Van Leeuwenhoek (try pronouncing that one without murdering it!) first used the microscope to look at the micro-cosmos of the world of the extremely tiny. In this workshop, you will use the foldscope , a highly affordable microscope similar to the one used by Anthony. Here is the user guide to using the foldscope . Please get familiar with it before doing this experiment.

The Foldscope Deluxe Individual Kit contains a LED module that can be attached to the foldscope. Doing so shines light through the specimen, illuminating it brightly. Then, using magnetic “couplers” (also included in the kit) attached to your phone camera, you can snap on the foldscope to your phone and take a picture using the phone’s camera.

All pictures of onion cells in this post have been taken using the LED module.

Materials and Equipment

• A foldscope. This is conveniently available on Amazon and other online places.
• Glass slides. Get a pack of 100, since you will not stop at one specimen.
• Coverslips. These are very thin square glass pieces, that you “slip” on your specimen.
• Tweezer. Very useful to pick and move small things around.
• Water and clear tape.
• Dropper, to drop water droplets on the slide.
• Knife, to cut the onion.
• Tissue paper, to blot out excess water.
• Journal to draw what you see.
• Safranin stain solution.
• Surgical gloves.
• Onions of course. One is enough to make a dozen slides or more.

Safety Instructions

• Be careful when you are using the knife to cut out the onion peel. If you are careless, this could turn into a microscopy lesson for your red blood cells, and that would not be a good thing.
• If you choose to stain your onion peel, then you must use surgical gloves when handling the Safranin staining solution.

Prepare the onion peel slide

Use water to clean the slides and cover slips. Use the knife to cut a small square section of the onion.

Remove the first layer (its a leaf!) and then further cut it into two or four parts. The smaller the specimen the better.

Use the tweezer to pull out one thin peel of the onion. This peel is exactly one cell thick. Don’t pull in any additional tissue.

Place the peel in water and let it soak for 5-10 mins. Soaking the peel in water allows the cells to get hydrated so they become easier to look that.

Take the peel out using the tweezer and then place it on the glass slide. The, add a drop of water. The water will keep the onion peel hydrated for a longer time. It will also help in the next step.

Take the cover slip and place it at an angle to the slide, and then gently lower it on to the slide. You can use the tweezer to lower the cover slip. The important part is to ensure there are no air bubbles when the cover slip is lying flat. If there are bubbles, don’t panic - just remove the cover slip and try again.

Press the cover slip and soak out any excess water from the side using the tissue paper. The water will now ensure the onion peel is now stuck in between the glass slide and the cover slip. Turn the slide over and check that the cover slip does not fall off.

(Optional) For added protection, put a clear tape on the cover slip so the slip does slip and slide around when you are putting it into the slide.

Mount the slide on to the microscope

Insert the slide into the foldscope, with the cover slip facing down . Incorrectly inserting the slide is the most common mistake beginner foldscopers will make. If you do not insert it correctly, you will see something, but not quite what you want to see.

Using the focusing ramp to get a sharp image of the onion cell. If you need to, try pressing on the paper surrounding the lens to get it to focus even more sharply. Then admire the building block of life.

Pan around to see different parts of the onion peel.

Note how the water on top of the peel further magnifies the cell. The “fatty” looking cells aren’t really fat, they just look that way due to the water on top.

If you see a lot of cells appear “thick” as shown below, try to soak out the excess water.

The nucleus usually is at the periphery of the cell (i.e. along the cell wall).

Draw in your journal

Microscopy is, first and foremost, an observation sport. So you have to be able to see as closely as you can. Take out your journal and make a drawing of what you see. Make notes, if you need to.

The nucleus can be seen more clearly if the onion peel is dipped in a solution of safranin. This is because the nucleus has chromatin which attracts color. (CHECKME). To stain the onion peel follow the steps below:

• Put on your surgical gloves throughout these steps.
• Make a mix of Safranin and water.
• Drop the onion peel into water and soak it for 5-10 mins, as before.
• Take out the peel and drop it into the Safranin solution.
• Take it out and drop it into clear water again to take out any excess color.
• Place the peel on the slide as before and place the cover slip on top.

You should now see the nucleus more clearly in some of the cells now.

Points to Ponder

• What does the cell look like if it dried up (by keeping it out in the sun, or using a heat gun)?
• Why is the nucleus of the onion cell at its periphery (most of the times)?
• Why does safranin stain the nucleus a deep shade of pink, but rest of the cell remains a lighter shade of pink?
• Is it possible to see the vacuole using the foldscope?
• In the picture below, it appears as though a single cell has two nuclei. Is that possible?

• Glycerine can be used instead of water; this is to keep the onion cell moist for a longer period of time. It is hygroscopic, which means it can absorb moisture from the air to keep the specimen hydrated. However, both are almost the same in terms of image clarity.
• 1% solution means 1g in 100ml.
• 0.1% solution would mean 0.1g, i.e. 100mg in 100ml
• Or 20mg in 20ml of distilled water.
• The growing tips of onion roots are similarly used as classic subjects when observing meiosis.
• Things to experiment next with: apples, potatoes, and elodea leaves.
• Observe leaf epidermal peel of Tradescandia/Rhoeo/ Bryophyllum, onion root tip.
• https://static1.squarespace.com/static/611ab82db3f068166494869d/t/61280ba0a898c41cf289d11d/1630014369469/FS_lesson-sbs-ANIMAL.pdf
• https://sciencing.com/cell-structure-onion-5438440.html
• https://www.youtube.com/watch?v=jElKNfJloo0
• About safranin
• The onion cell nucleus may vary from its text-book shape …
• Cells are like factories .
• https://www.youtube.com/watch?app=desktop&v=Tfy1mOT-gEQ

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School Science/How to prepare an onion cell slide

Tissue from an onion is a good first exercise in using the microscope and viewing plant cells. The cells are easily visible under a microscope and the preparation of a thin section is straight forward. An onion is made of layers, each separated by a thin skin or membrane. In this exercise, you will make a wet mount on a microscope slide and look at the cells of the onion membrane magnified by the high power, compound microscope .

• First add a few drops of water or solution on the microscope slide to avoid dryness and wilting
• Take a small piece of onion and using tweezers, peel off the membrane from the underside (the rough side).
• Place the membrane flat on the surface of the slide.
• Add a drop of Iodine solution to the onion skin
• Using a pin, lower the thin glass cover slip or cover glass onto the slide. Make sure there are no air bubbles (Fig. 1).
• Make sure the lowest power objective lens (the shortest lens if there are several present) is in line with the optical tube, and the microscope light is turned on. Then place the prepared slide onto the stage of the microscope.
• Looking from the side (NOT through the eyepiece), lower the tube using the coarse focus knob until the end of the objective lens is just above the cover glass. Do this carefully so as not to crack the cover glass (and possibly damage the objective lens).
• Now look through the eyepiece and turn ONLY the smaller, fine focusing knob to move the optical tube upwards until an image comes into focus. The cells should look something like lizard skin.
• Swap the objective lens for a higher powered one so that you can see the cells at greater magnification. You should be able to make out a nucleus in each cell.
• Be very careful; these dyes can stain your skin and clothes. Could be dangerous if it is on you.

Proper use of the microscope - intended to prevent damage to the objective lenses - requires that the following techniques be followed:

• Never use the coarse focus knob while looking through the eyepiece. The point of focus will be very near the cover glass. Looking from the side, lower the optical tube until the objective lens is as close as you can get it to the cover glass without actually touching it. Starting with the low power objective lens is the fastest way to achieve proper focus.
• Initially, slowly focus back (turn the fine focus knob to raise the optical tube) while looking through the eyepiece. Once the specimen comes into focus, you can make fine adjustments up or down with the fine focus knob without fear of damaging the slide or the microscope.
• If the specimen does not focus, raise the tube a little with the coarse focus knob and attempt to focus again with the fine focus knob. Once the object is in focus, switching objective lenses should be possible without any further coarse adjustments.

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03. Preparation and scientific drawing of a slide of onion cells

• 01:15 What do air bubbles look like under the microscope?
• 01:32 What cell structures do you observe?
• 01:51 What units will you use to record the cell size?

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microscope slide and cover slip

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mounted needle

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• Cell (Biology): An Overview of Prokaryotic & Eukaryotic Cells

The Cell Structure of an Onion

Where Is Starch Stored in Plant Cells?

Onions have a long history of human use, originating in southwestern Asia but having since been cultivated across the world. Their strong odor — actually a defense mechanism — and unique structure belie a complex internal makeup, composed of cell walls , cytoplasm , and the vacuole . As onions can be found in almost every grocery store or easily grown independently, educators tend to use them when students learn about plant biology, thanks also in part to their easily-seen cell walls.

Plant Cells and Animal Cells

Plant cells are distinct from animal cells : plant cells have rigid cell walls, rather than the more flexible cell membranes of animal cells. The cell walls are high in cellulose, a material that gives rigidity to the cell and which, when accumulated in large amounts in many cells, provides the strength and rigidity of everything from flower stems to tree trunks. Plant cells have one large vacuole — a large open area central to the cell which is used as a reservoir for water and ions, and in certain cases for storage of toxins. While animal cells may have vacuoles, they are not present as a single, large, central reservoir but as several lesser reservoirs distributed through the cell. Plant cells also have chloroplasts: these are organelles containing chlorophyll in systemic arrays to capture light and convert it to glucose.

Cell Walls Give Structure

Cell walls in plants are rigid, compared to other organisms. The cellulose present in the cell walls forms clearly defined tiles. In onion cells the tiles look very similar to rectangular bricks laid in offset runs. The rigid walls combined with water pressure within a cell provide strength and rigidity, giving plants the necessary structure to resist gravity and pressure. The cell walls and the pressure from water contained in both cytoplasm and more particularly in the vacuole are what give onion its solid substance and crisp snap.

Sandwiched between the vacuole and the cell wall is the cytosol. The cytosol is primarily water, salts and a variety of organic molecules serving various functions in relation to the cell and the greater organism. Within the cytosol are organelles : organic structures that serve as factories, communications centers and other functional elements in the management of the cell metabolism. Also floating within the cytosol are inclusions consisting of a range of elements, starches, proteins and other molecules used as building blocks for a range of functions. Also contained within the cytoplasm of the plant cell is the nucleus , containing the primary genetic material of the plant.

Vacuoles contain the necessary water, ions and a range of organic molecules produced by the plant, in many cases including those for pigment, or chemicals that produce the distinctive scent or flavor of a plant. In onions, the vacuole is very large and distinct. The characteristic scent of onions is produced by the combination of flavor precursors present as organic molecules in the cytoplasm, and a secondary organic chemical, the enzyme allinase, which is contained and restricted to the vacuole of the onion. Only when the onion is damaged by cutting, bruising, the attack of insects or rodents or through some similar mechanical destruction are the precursors and the allinase combined, forming a powerful scent. Similarly, in red onions, the coloration of the onion is contained within the vacuole.

A Classic Subject for Study

Onion cells are among the most common choices for cell studies in early biology classes. Easily obtained, inexpensive, they offer samples with no difficult technique required. The thin layer of skin found on the inside of an onion scale (one layer of onion) lifts off without effort and can be wet mounted on a slide with no need for extreme skill. Likewise, the cells are large, regular, easily seen and conform very well with the standard generic elements of all plant cells. The growing tips of onion roots are similarly used as classic subjects when observing meiosis, for similar reasons of easy access and easy handling by novices. Onions, coupled with apples, potatoes and elodea leaves, are among the most reliable and useful of lab subjects when teaching new students the fundamentals of biology and the foundational skills of working in a biology lab.

Related Articles

Cell wall composition of the six kingdoms, the importance of plant cells, cell characteristics, what are the chemicals in cornstarch, how to measure the acidity of fruits, how to compare the cells of plants, animals & unicellular..., the difference between hard & soft wheat, wet cell battery vs. dry cell battery, list of cell organelles & their functions, what is urethane, what plants have thc in them, commercial uses of sodium polyacrylate, how does a dp cell work, how to measure the voltage in fruits, 7 types of connective tissue, how to make a model of a plant cell in a plastic bag, types of organisms that are made of plant cells.

• Onion Cells, animation
• Plant Cells vs. Animal Cells

About the Author

Peg Robinson's first sale was in Pocket Books' 1999 "Strange New Worlds." Her credits include award-winning "Helixsf," and "Cicada Magazine." Her novela, "Tonino and the Incubus," qualified for the 2007 Nebulas. She graduated with honors in religious studies from UCSB. She's currently in an M.A./Ph.D. program in mythological studies at Pacifica Graduate Institute.

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Preparing a slide of an onion cell and measuring a cell

You will need:

Microscope slide

Dropper pipette

Clear plastic ruler

Dilute iodine solution.

Preparing the onion slide

What to do:

• Slice an onion in two, lengthwise.
• Remove one of the thick leaf-like structures from inside.
• Pull away a piece of the thin papery lining of its inner surface.
• Using scissors, cut a small square of this lining, about 5 mm x 5 mm.
• Place this square on the centre of a slide.
• Add a drop of dilute iodine solution – make sure the solution spreads below as well as above the square of onion skin. The iodine acts as a stain to make the structures in the cell easier to see.
• Carefully lower a cover slip over the onion skin. Try to avoid trapping air bubbles.
• Place the slide on the microscope stage. Examine first using the low power. Focus carefully.
• Choose an area of the slide where the cells can be clearly seen. Switch to high power and refocus.
• Look for the structures shown in the photographs in Resource 1 .

Measuring the onion cell

• Place the ruler on the microscope stage under the low power objective lens.
• Move the ruler so the edge with the scale can be focused in the centre of the field of view of the microscope, as in Diagram 1 below.

• Use the scale to measure the field of view of your microscope.
• The diameter of the field of view in Diagram 1 is approximately 5 mm.
• You can use the measurement of the field of view in your microscope to estimate the size of objects viewed with the same objective lens.
• The cell viewed in Diagram 2 would be about 2 mm long if viewed with the microscope with the field of view shown above.

• Estimate the length and width of your onion cell using this method.

Using a microscope

The main parts of a light microscope are shown below

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How to use an onion for your osmosis lab

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Cell Structure

Instructor prep, student protocol.

• Visualizing Onion and Cheek Cells

Learning Objectives

What are the common features of all cells.

All cells share common features such as having a plasma membrane, a cytoplasm, DNA, and ribosomes. A plasma membrane is a phospholipid bilayer that surrounds the cell. Inside the plasma membrane, the cell is filled with a gel-like fluid called cytoplasm that contains organic molecules, salts, and other materials that are vital for the cell’s functions. All cells use DNA as the genetic material, which is the hereditary blueprint to construct cellular structures and products. Finally, all cells use ribosomes to synthesize their protein products.

What are the two main types of cells? How do ecologists calculate the biodiversity in a local community?

There are two types of cells based on the location of their genetic material: prokaryotic and eukaryotic.

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells are small in size and lack a distinct nucleus and organelles. Eukaryotic cells are relatively larger, contain a distinct nucleus, and various organelles with specialized functions.

What are the differences between plant and animal cells?

Plant cells have chloroplasts, which are used for photosynthesis and often contain the green pigment chlorophyll. Additionally, they are surrounded by cell walls, which are rigid outer layers made of cellulose to support growth and water retention. Because they need to store large amounts of water to maintain the water pressure in the cell, they have larger vacuoles than animal cells. In addition, plant cells also have another type of specialized storage organelles called plastids, which contain pigments and photosynthetic products, such as starch.

How do we visualize cells?

Most cells are too small to see with the naked eye and can only be observed with a microscope. However, most cells are also transparent, therefore they first need to be stained selectively based on their molecular composition. Next, they are mounted on glass slides to be observed with a microscope.

List of Materials

• Microscope Slides 10
• Glass coverslips 10
• Toothpicks 10
• Glass droppers 15
• Petri dishes 10
• Stop watch 5
• Blotting paper 1 roll/station
• Safranin O Solution 1 bottle/station
• Methylene blue solution 1 bottle/station
• Glycerol 1 bottle/station
• Immersion oil 1 bottle/station
• Lens cleaning paper 1 bottle/station
• Onion bulbs -1 Dependent on the lab size
• Compound Microscopes (with 100x Oil Immersion Objective) -1 Dependent on the lab size
• Immediately before the experiment, wash and peel onion bulbs for the class.
• Remove the entire brown outer skin and cut the onion in half with a knife. Pull apart the layers of the onion. The thin, nearly transparent film layers within the onion will be used by the students.
• Place the onion film into a Petri dish and then place this into the refrigerator until the students arrive.
• Bring out the microscopes and set them on the lab bench.
• Set the microscopes to the lowest magnification and plug them into an electrical outlet.
• Clean the lenses with lens paper or laboratory wipes and test the knobs to make sure the lenses focus properly.
• Make sure that the stage can be moved.