Onion Skin Cells Under the Microscope: Making and Observing a Wet Mount

Observing plant cells is a classic middle school biology experiment — and for many students, it is the first time they truly learn how to use a microscope. This guide walks through the whole process: making a temporary wet mount of onion scale-leaf epidermis, adjusting the light, focusing, and finally recognizing the cell wall and the nucleus.

Once students master this routine, they will be able to observe any wet mount with confidence.

Safety first (read this section before you start)

• You will use a razor blade or forceps to cut and peel the onion. Work slowly, keep the blade pointing away from you, and do this with an adult present to avoid cuts.
• Slides and coverslips are thin glass that breaks easily and can cut your hands. Handle them gently, and wrap and discard any broken glass immediately.
• Cutting an onion may make your eyes sting and water — that is normal. Wash your hands after the experiment.

Materials and equipment

• A microscope (a standard classroom light microscope);
• An onion (you will use the inner epidermis);
• Slides, coverslips, forceps, a razor blade or dissecting needle;
• A dropper, clean water, and dilute iodine solution (for staining — optional but recommended);
• Absorbent paper (a paper napkin works) and a piece of gauze.

Making the wet mount

The whole procedure fits into six little words: wipe, drop, peel, spread, cover, stain.

1. Wipe: clean the slide and coverslip with a piece of gauze so that dust and fingerprints do not spoil the view.
2. Drop: use the dropper to place one drop of clean water in the center of the slide.
3. Peel: break off a small piece of onion and use the forceps to peel a thin, transparent layer of epidermis from the inner side of the scale leaf (the thinner the better — a thick piece will be impossible to see through).
4. Spread: place the peel into the water drop and gently flatten it out with the forceps or dissecting needle so it does not curl or fold.
5. Cover: pick up the coverslip with the forceps, touch one edge to the edge of the water drop first, then lower it slowly to keep air bubbles out.
6. Stain (optional): place a drop of dilute iodine solution at one edge of the coverslip and hold a piece of absorbent paper at the opposite edge to draw it through. The iodine soaks across the specimen and makes the nucleus stand out clearly.

Using the microscope, step by step

1. Carry and place the microscope: hold the arm with one hand and support the base with the other, and set the microscope slightly to the left on the bench.
2. Adjust the light: rotate the nosepiece so the low-power objective lines up with the hole in the stage, then adjust the diaphragm and mirror until you see a bright, evenly lit white field through the eyepiece.
3. Place the slide: put the wet mount on the stage, hold it with the stage clips, and center the specimen over the opening.
4. Focus: watching from the side, turn the coarse focus knob to lower the body tube slowly toward the slide; then look through the eyepiece and slowly raise it until the image appears, and finish with the fine focus knob for a sharp image.
5. Switch to high power (optional): first move the cells you want to study to the center of the field under low power, then rotate to the high-power objective and touch up the focus with the fine focus knob only.

What you will see and record

Once the image is sharp, students will see many neatly arranged, regular "rectangular boxes" — these are the onion epidermal cells. Have them draw a simple cell diagram and label:

• Cell wall: the clear boundary around the outside of each cell, separating the cells from one another;
• Cytoplasm: the fairly transparent material inside the cell wall;
• Nucleus: a small dark dot after staining, usually one per cell;
• Sometimes you can also see a large, transparent vacuole in the center.

A few observations deserve their own notes — they show up on every test, and they only really make sense once you have done them yourself:

• The image is upside down. A microscope forms an inverted image: up-down and left-right are both reversed. So when the specimen appears in the upper left of the field, move the slide toward the upper left and the image will shift to the center. Remember the rule: move the slide toward where the image is.
• Total magnification = eyepiece power × objective power. A 10× eyepiece with a 10× objective gives 100×; switch to a 40× objective and you get 400×. Note that this is magnification of length, not of area or volume.
• Higher magnification means fewer cells and a darker field. Going from 100× to 400×, the number of cells in view drops noticeably and the image gets dimmer. The fix is to open the diaphragm wider or switch to the concave mirror to add light — not to reach for the coarse focus knob.
• How to tell air bubbles from cells. A bubble has a thick, dark edge with a bright center, it is round, and it moves or changes shape when you gently press the coverslip; cells sit in orderly rows with thin, even boundaries. This is the mix-up beginners make most often.

The science behind it, and where to go next

An onion is a plant, and its epidermis is made of many plant cells packed tightly together. The reason you can peel off a layer thin enough to observe directly is that the inner epidermis is only one cell thick, so light passes through it easily.

Iodine stains the nucleus because it reacts with substances in the cell to produce color, making the nearly colorless, hard-to-spot nucleus stand out against its surroundings. That is what staining is for in microscopy.

Onion epidermal cells show us the basic structures of a plant cell: cell wall, cell membrane, cytoplasm, nucleus, plus the vacuole that plants are known for. If students go on to observe human cheek cells, they will find those have no cell wall and no regular shape — and that comparison is exactly how they come to understand the similarities and differences between animal and plant cells.

Why can't we see the cell membrane? Someone asks this in almost every class. The membrane is really there, but it presses tightly against the inside of the cell wall and is extremely thin and almost transparent, so a light microscope cannot resolve it. To make it visible, add a drop of concentrated salt water to the mount: as the cell loses water, the cytoplasm shrinks inward and the membrane pulls away from the wall. This is called plasmolysis, and the boundary that peels away is the cell membrane.

Why can't we see chloroplasts? Onion scale leaves grow underground, where no light reaches them, so they contain no chloroplasts and look colorless. To see chloroplasts, use an Elodea leaf or the lower epidermis of a spinach leaf instead — the little green grains are easy to spot. It is also a neat piece of evidence that the statement "all plant cells have chloroplasts" is wrong.

Why must we use the inner epidermis? The outer epidermis of an onion scale leaf has several layers of cells and often carries purple pigment, so light cannot get through; the inner epidermis is a single, almost transparent layer of cells, so light can pass through and form an image. Any specimen for a microscope must be thin and transparent — that is the common requirement for every wet mount.

Troubleshooting common problems

• The field is completely dark: most likely the light was never adjusted properly, or the objective is not lined up with the stage opening — redo the light adjustment.
• It never comes into focus no matter what: the specimen may be too thick or full of bubbles; peel a thinner layer and make a new mount.
• Lots of dark-edged circles: those are air bubbles, not cells; re-cover the mount, tilting the coverslip and lowering it slowly.
• A dark speck in the field that never moves: if it stays put when you move the slide, the dirt is not on the specimen. Rotate the eyepiece — if the speck turns with it, the eyepiece is dirty; if nothing moves it, the objective is dirty. Clean with lens paper only, never fingers or ordinary tissue.
• Cells stacked on top of each other, no single cell visible: the peel is too thick. Lightly score a small square on the inner epidermis with the razor blade first, then lift one corner of the square with the forceps and peel — you will get a single layer much more easily.
• It was sharp a moment ago and now it's blurry: you probably bumped the stage or the body tube. Keep a steadying hand on the slide and make only fine-focus adjustments while observing.
• Everything turns dark after staining: too much iodine. The right way is to drop at one edge and draw with absorbent paper at the other, letting the iodine seep across the specimen — not to pour iodine straight onto it.

When a student can make a mount, adjust the light, and focus on their own — and can name each structure and say what it does — this observation lab has done its job.

Tips for teachers and parents

• Time and grouping: two class periods; two students per microscope.
• Before hands go on equipment: razor blades are stored and handed out by the teacher; slides break easily.

The step most often skipped. Demonstrate the six-word routine — wipe, drop, peel, spread, cover, stain — once before students start. Peeling too thick a layer is the number one cause of failure; have students score a small square on the inner epidermis first and lift from a corner. And before anyone switches to high power, insist on centering the target in the field of view.

How to know they really understand. A student meets the bar when they can make the mount and focus independently, point out the cell wall and nucleus, and state the rule "move the slide toward where the image is."