Observing Onion Cells under a Microscope is a great introduction to the microscope. Make sure to subscribe for free lab observation sheets and visit How to Make Amazing Observations with a Microscope for more microscope lessons.

You are watching: Why use iodine to stain onion cells

If you are studying life science or biology this year,

one of the easiest labs in cell biology is observing onion cells under a microscope.

I thought it would be helpful to share how I help students to see an example of a plant cell.

Today’s objective: Observing Onion Cells under a Microscope

The goals for this lesson are to:

Make a wet mount slide.Observe an onion cell under the microscope.Record our observations.

Materials Needed for Observing Onion Cells

You will need to purchase some science materials for this lab exercise.

The nice thing about purchasing the appropriate supplies is that you’ll have them on hand the next time you want to use your microscope.

I like to buy supplies from Amazon and Home Science Tools depending on the availability and price.

Iodine– to stain the cells so it can be seen under the microscope. Not all cells can be stained with iodine, but onion cells take it up nicely so that there is contrast under the microscope.Onion– Peeled to one layer. If it’s any thicker, you won’t be able to see through it under the microscope.Microscope– A simple light microscope will be sufficient. We use a digital microscope.Lab Sheet– We made our own for recording some simple observations.

Staining Onion Cells

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Since onion peels are translucent,

you’ll need to stain the onion cells before you observe them under the microscope.

There are different types of stains depending on what type of cell you are going to look at.

Iodine– dark stain that colors starches in cells. In an onion cell, it will make the cell wall more visible. It provides some contrast for viewing under a microscope.Methylene Blue– a blue stain that will color blood, bacteria, acidic or protein rich cell structures like nucleus, ribosomes, and endoplasmic reticulum.Eosin Y– a pink or red stain that colors blood, plants, and alkaline animal cell structures like the cytoplasm.
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The Microscopic Life Kit from Home Science Tools contains both Eosin Y and Methylene Blue for staining many types of cells.

Preparing the Onion Cells for the Microscope Slide


Iodine is a great choice because it’s a household product, but more sophisticated stains will allow you to see more structures in the onion cell.

The procedure is fairly simple.

You just need to be careful as you work to get a good peel of an onion.

You want a single layer of cells or you won’t see much under the microscope.

In order to see the cells well, the light needs to pass through easily.

Peel Your Onion– You want one layer of it and it should be super thin.Place the Onion Peel onto the Slide– You’ll want to smooth out any wrinkles with forceps or the end of your pipettePut One Drop or Two of Iodine– onto the top of the onion cell. If you are using Methylene blue, you’ll need to apply the stain next to the cover slip after it is down. Go light because too much will mean you can’t see the cell well. Drop the Cover Slip– over the onion cells by placing one end of the cover slip into the iodine and dropping the other side down. This helps to prevent bubbles.Check for Bubbles– if you see one, you can try and remove it by lightly tapping with the bulb end of the pipette or placing a tissue on the liquid at the edge of the cover slip close to the bubble.Observe under the Microscope– Place the slide on the stage of the microscope and observe under all three levels of magnification.

Observing & Recording Your Observations

To prepare for recording your observations, have your students create a data sheet. Having your students make their own data sheet helps them to organize information. I usually give the following directions.

We go over what we will be recording and make a list.Remind them to include the objectives or the goal of the lab along with the materials.Think of the best way to record the information we need to write down. Remember that even if it’s not the way you would do it, all that is important is that everything is written down and that it makes sense to your student. You will see growth in this area as they get older.
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In this case, we want to record as much as we can about what we see. For example:

The goal of the lab exerciseList the materials we will use.Make a place to draw what you see under each level of magnification.Label the places with 10x, 60x, and 200xLeave a space for writing a conclusion describing what we able to see.

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Teacher Hint: You will not be able to see the individual organelles with a standard light microscope. The most noticeable item will be the cell wall. In general, you will see a group of neat rectangles with an outer layer (the cell wall). The higher the magnification, the more defined the cell wall will become and the field of view will be smaller so you will see less rectangles.