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.

Objective

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?

Background

Cells

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.

Steps

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.

Observe

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.

Staining

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?

Notes

  • 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.
  • If you use safranin powder, you need to make a 0.1% staining solution.
    • 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.

Next Steps

  • 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.

References

Stay Qurious.