Onion Cells (Onion peel) Under a Microscope – Onion Epidermal Cell Wall

Onion cells are plant cells obtained from the thin epidermal layer of an onion (Allium cepa). This layer is peeled from the inner surface of onion scale and placed on a clean glass slide. The cells are transparent, so iodine stain is generally used for better observation.

Under low magnification, onion cells appear as closely arranged rectangular cells. The arrangement looks like a brick wall. Each cell is surrounded by a thick and clear cell wall, which gives the cell a fixed box-like shape.

A large central vacuole is present inside the cell and occupies most part of the cell. The nucleus is generally seen near the outer side because it is pushed by the large vacuole. The cytoplasm is present as a thin layer around the vacuole.

Chloroplasts are not observed in onion bulb cells. The onion bulb grows under the soil and does not receive sufficient light for photosynthesis. Red onion cells may show natural colour due to anthocyanin pigments present inside the vacuole.

Objectives of Onion Cell Microscopy Experiment

• To observe the arrangement and basic structure of onion epidermal cells.
• To identify the important parts of a plant cell such as cell wall, nucleus, cytoplasm, and large central vacuole.
• To learn the preparation of a temporary wet mount slide by using a thin onion peel.
• To develop skill in handling, focusing, and observing the specimen under a light microscope.
• To understand the use of stains like iodine, safranin, or methylene blue for increasing the contrast of transparent cells.
• To draw and record the observed structures of onion cells (Allium cepa) correctly.
• To compare the onion plant cells with animal cells and study the basic structural differences between them.

What do Onion Cells Look Like Under the Microscope?

• Onion cells appear as a continuous and closely packed sheet under the microscope. The arrangement looks similar to a brick wall.
• Individual cells are large and mostly rectangular or elongated in shape. Some cells may also appear hexagonal.
• A thick and clear cell wall is seen around each cell. It gives a fixed and box-like shape to the onion cell.
• A large central vacuole is present inside the cell. It occupies most of the internal cell space and appears transparent.
• The nucleus appears as a dark-stained oval or round structure. It is usually present near the outer side of the cell because of the large vacuole.
• One or more dark nucleoli may be observed inside the nucleus under higher magnification.
• The cytoplasm is present as a thin granular layer between the cell boundary and central vacuole.
• Chloroplasts are absent in onion epidermal cells of Allium cepa. The onion bulb grows underground and these cells do not perform photosynthesis.

How to Observe Onion Cells under a Microscope?

Requirements

• Onion bulb – A fresh and firm onion bulb is used as the specimen. Red onion can also be used because its cells contain natural pigment.
• Compound light microscope – It is used to observe the onion epidermal cells. The microscope should have low and high-power objective lenses such as 4X, 10X, and 40X.
• Glass slide – A clean and scratch-free glass slide is required for placing the onion peel.
• Coverslip – A thin coverslip is used to cover the specimen and keep it flat on the slide.
• Scalpel or blade – It is used to cut the onion and remove a small piece from its inner surface.
• Forceps – Fine forceps or tweezers are used to peel the thin epidermal membrane of Allium cepa.
• Mounted needle or brush – It is used to spread the onion peel properly and lower the coverslip without forming air bubbles.
• Dropper or pipette – It is used to add water, glycerine, or stain over the specimen.
• Distilled water or glycerine – It is used as a mounting medium. It keeps the onion peel moist and prevents drying during observation.
• Staining solution – Iodine solution, safranin, or methylene blue is used to increase the contrast of the cell structures.
• Blotting paper – It is used to remove excess water or stain from the sides of the coverslip.
• Petri dish or white tile – It may be used for cutting and handling the onion specimen safely.

Stain Used for Observation of Onion Cells under a Microscope

1. Iodine solution – It is the most commonly used stain for onion epidermal cells. It increases the contrast and makes the nucleus, cell wall, and cytoplasm more clearly visible.
2. Safranin – It stains the cell structures red or pink. It is also used for clear observation of the onion peel.
3. Methylene blue – It stains the nucleus dark blue. It helps in identifying the nucleus from the surrounding cytoplasm.

For observation of onion cells (Allium cepa), iodine solution is generally preferred. Only one or two drops of stain should be used. Excess stain should be removed with the help of blotting paper.

Procedure for Observation of Onion Cells under a Microscope

1. Take a fresh onion bulb and cut it into small pieces with the help of a blade or scalpel.
2. Remove one fleshy scale leaf from the onion bulb. A thin transparent layer is present on its inner concave surface.
3. Hold the thin layer with fine forceps and peel it carefully. This thin layer is referred to as the onion epidermal peel.
4. Take a clean and scratch-free glass slide. Place one or two drops of distilled water or 50% glycerine at the centre of the slide.
5. Transfer the onion epidermal peel into the drop of water. The peel should not become folded during this process.
6. Spread the peel properly with the help of a mounted needle, fine brush, or forceps. Remove all wrinkles and keep the peel as a single flat layer.
7. Add one or two drops of iodine solution, safranin, or methylene blue over the onion peel. The stain increases the contrast of the transparent cell structures.
8. Keep the peel in the stain for about 30 to 60 seconds. Excess stain can be removed by washing gently with distilled water.
9. Take a clean coverslip and hold it at an angle of about 45°. Touch one edge of the coverslip near the onion peel.
10. Lower the coverslip slowly with the help of a mounted needle. In this step, air bubbles should not be trapped under the coverslip.
11. Remove the excess water or staining solution from the margins of the coverslip by using blotting paper. Do not press the coverslip strongly.
12. Place the prepared slide on the stage of a compound light microscope. Fix it properly with the help of stage clips.
13. Turn on the light source and adjust the diaphragm so that sufficient light passes through the specimen.
14. Start the observation by using the lowest power objective lens, generally 4X objective lens.
15. Bring the objective lens near the slide by observing from the side. Do not allow the objective lens to touch the coverslip.
16. Look through the eyepiece and use the coarse adjustment knob to bring the onion cells into rough focus.
17. Use the fine adjustment knob to obtain a sharp and clear image. Adjust the light if the specimen appears too bright or dark.
18. Find a clear area where the onion peel is flat and cells are present in a single layer.
19. Change to the 10X objective lens for better observation. Focus the specimen again by using the fine adjustment knob.
20. Use the 40X objective lens for detailed observation of the cell structures. Only the fine adjustment knob should be used at this magnification.
21. Observe the rectangular arrangement of onion cells (Allium cepa). Identify the cell wall, cytoplasm, nucleus, and large central vacuole.
22. Draw the observed onion cells and label the visible cell structures properly.

Temporary Mount of Onion Peel

Temporary mount of onion peel is a simple slide preparation method used to observe onion epidermal cells under microscope. In this method, a thin peel of onion is placed on a glass slide with a drop of stain and covered with a cover slip. It is called temporary mount because the slide is prepared for short time observation only.

The inner epidermis of onion (Allium cepa) is mostly used for this mount. It is thin, transparent and made up of single layer of cells. Due to this, the rectangular onion cells can be seen clearly under a compound microscope.

In temporary mount preparation, the onion peel should be very thin and should not be folded. A drop of iodine solution or safranin is added to stain the cell parts. The stain makes the cell wall, cytoplasm, and nucleus more visible.

After staining, the cover slip is placed slowly over the onion peel. It should be placed at an angle to avoid air bubbles. Air bubbles can disturb the observation and the cells may not be seen clearly.

The prepared slide is then placed on the microscope stage. At first, the slide is observed under low power objective lens. After getting the clear view, high power objective lens is used to observe the cell wall, nucleus, cytoplasm, and large vacuole.

Temporary mount of onion peel is commonly used in biology practical because it is easy to prepare. It helps to study the basic structure of plant cell and also helps to learn the use of microscope, slide, stain, and cover slip.

Observation of Onion Cells Under Microscope

Onion cells under microscope are seen as rectangular and elongated cells. The cells are arranged side by side in a single layer. They appear like brick wall or honeycomb-like pattern.

The epidermal cells of onion (Allium cepa) are transparent in nature. So, without staining the cell boundary can be seen, but internal parts are not very clear. After staining with iodine or safranin, the nucleus, cytoplasm, and cell wall become more visible.

The following observations are seen under microscope-

Cell shape – The cells are rectangular, elongated and closely packed with each other.

Cell wall – The cell wall is thick and clearly visible. It forms the outer boundary of each cell and gives fixed shape to the cell.

Cell membrane – The cell membrane is present just inside the cell wall. It is thin and not very clearly visible under ordinary light microscope.

Cytoplasm – The cytoplasm is seen as a thin layer along the inner side of the cell. It surrounds the large central vacuole.

Nucleus – The nucleus is small and round. It is generally present near the side of the cell because the large vacuole pushes it towards the periphery.

Vacuole – The vacuole is large and present at the centre of the cell. It occupies most of the cell space and contains cell sap.

Chloroplast – Chloroplast is not seen in onion bulb epidermal cells. It is absent because onion bulb is mainly a storage organ and it does not perform photosynthesis.

During observation, the onion peel should appear as a thin single layer. If the peel is folded or air bubbles are present, the cells may not be seen clearly. A clean temporary mount gives clear rectangular cells with visible cell wall, nucleus, cytoplasm, and vacuole.

Labeled Diagram of Onion Cell Under Microscope

The labeled diagram of onion cell under microscope shows the main parts of an onion epidermal cell. It is generally rectangular or brick-like in shape. The cells are arranged side by side in a single layer.

The onion epidermal cell of onion (Allium cepa) is a typical plant cell. It has a clear cell wall, cell membrane, cytoplasm, nucleus, and a large vacuole. These parts are seen more clearly when the onion peel is stained with iodine or safranin.

The following are the main labels in onion cell diagram-

Cell wall – It is the outer thick and rigid boundary of the onion cell. It gives definite shape and support to the cell.

Cell membrane – It is a thin membrane present just inside the cell wall. It surrounds the cytoplasm and controls the movement of substances.

Cytoplasm – It is a thin layer of jelly-like material present inside the cell. In onion cell, the cytoplasm is pushed towards the side due to large vacuole.

Nucleus – It is a small round structure present near the edge of the cell. It becomes clearly visible after staining and controls the activities of the cell.

Vacuole – It is a large central space present inside the onion cell. It occupies most part of the cell and contains cell sap.

In the diagram, the cell wall is the most visible part because it forms the boundary of each rectangular cell. The nucleus is generally seen at the side and not in the centre. The chloroplast is absent in onion bulb cells because onion bulb is a storage organ and it does not perform photosynthesis.

Onion Cells Under Microscope at Different Magnifications

Onion cells under microscope can be observed at different magnifications. At each magnification, the visibility of cell wall, cytoplasm, nucleus, and vacuole becomes different. The onion epidermis of onion (Allium cepa) is thin and transparent, so it is suitable for low and high power observation.

The magnification depends on the eyepiece lens and objective lens of the microscope. If the eyepiece is 10x, then total magnification is calculated by multiplying eyepiece magnification with objective magnification.

The following are the common observations at different magnifications-

At 40x magnification – The onion peel appears as a thin sheet of cells. The cells are seen in group and arranged side by side. The rectangular shape of the cells can be identified, but internal parts are not clearly visible.

At 100x magnification – The cell wall becomes more clear. The cells appear elongated and brick-like. The arrangement of cells in a single layer can be seen properly. The cytoplasm may be seen as a thin layer near the cell wall.

At 400x magnification – The internal parts of the onion cell become more visible. The nucleus can be seen clearly after staining with iodine or safranin. The large vacuole occupies most part of the cell and pushes the nucleus towards the side.

In unstained onion cells, the cell parts are not very clear because the cells are almost colourless. After staining, the nucleus and cell boundary become more distinct. So, stained onion peel gives better observation at high magnification.

The onion bulb cells do not show chloroplasts at any magnification. This is because onion bulb is a storage part and it does not carry out photosynthesis. So, under different magnifications, mainly cell wall, cytoplasm, nucleus, and vacuole are observed.

Why Onion Bulb Cells Do Not Have Chloroplasts?

Onion bulb cells do not have chloroplasts because onion bulb is an underground storage part of the plant. It remains below the soil and it does not receive direct sunlight. So, it does not perform photosynthesis.

Chloroplasts are the green plastids which contain chlorophyll. They are mainly present in the green parts of the plant such as leaves and young stem. These parts receive sunlight and prepare food by photosynthesis.

The onion bulb of onion (Allium cepa) is mainly used for storage of food. Its epidermal cells are thin and transparent. These cells protect the inner fleshy scale leaves and store food materials, but they are not mainly involved in food preparation.

Due to the absence of chloroplasts, onion bulb cells appear colourless under microscope. Only the cell wall, cell membrane, cytoplasm, nucleus, and large vacuole are observed. The nucleus becomes more clear when the onion peel is stained with iodine or safranin.

So, chloroplasts are not seen in onion bulb epidermal cells. This is because the onion bulb is a storage organ and not a photosynthetic organ. But green leaves of onion plant may contain chloroplasts because they are exposed to sunlight and carry out photosynthesis.

Difference Between Onion Cell and Cheek Cell Under Microscope

Onion cell and cheek cell are commonly observed under microscope to study the difference between plant cell and animal cell. Onion cell is a plant cell, whereas cheek cell is an animal cell. Both cells have cell membrane, cytoplasm, and nucleus, but their shape and outer covering are different.

The onion epidermal cell of onion (Allium cepa) is rectangular and arranged in a regular pattern. It has a thick cell wall outside the cell membrane. Due to this cell wall, onion cells have fixed shape and appear like brick-like structures under microscope.

Cheek cells are obtained from the inner lining of human cheek. These cells are irregular or rounded in shape. They do not have cell wall, so their shape is not fixed. Only the cell membrane forms the outer boundary of cheek cell.

In onion cell, a large vacuole is present at the centre. This vacuole pushes the nucleus towards the side of the cell. In cheek cell, the vacuole is absent or very small and not clearly seen under ordinary microscope.

Onion bulb cells do not contain chloroplasts because onion bulb is a storage organ. Cheek cells also do not contain chloroplasts because they are animal cells and animals do not perform photosynthesis.

The main differences are as follows-

Onion cell – It is a plant cell. It is rectangular in shape and has a distinct cell wall. It has large vacuole and the nucleus is present near the side.

Cheek cell – It is an animal cell. It is irregular or rounded in shape and does not have cell wall. It has cell membrane, cytoplasm, and nucleus, but large central vacuole is absent.

So, under microscope onion cell can be identified by its fixed rectangular shape and thick cell wall. Cheek cell can be identified by its irregular shape and absence of cell wall.

Onion Peel Cell and Onion Root Tip Cell Difference

Onion peel cell and onion root tip cell are both obtained from onion (Allium cepa), but they are used for different microscopic studies. Onion peel cell is used to study the general structure of plant cell. Onion root tip cell is mostly used to study mitosis and cell division.

Onion peel cells are taken from the thin inner epidermis of onion bulb. These cells are rectangular, transparent and arranged in a single layer. They show clear cell wall, cytoplasm, nucleus, and large vacuole under microscope.

Onion root tip cells are taken from the growing tip of onion root. These cells are small, closely packed and actively dividing. The root tip contains meristematic cells, so different stages of mitosis such as prophase, metaphase, anaphase, and telophase can be observed after proper staining.

In onion peel cell, the main aim is to observe plant cell structure. The nucleus is generally present near the side because the large vacuole pushes it towards the periphery. The cells do not show active cell division in normal observation.

In onion root tip cell, the main aim is to observe cell division. The chromosomes become visible during mitosis after staining with acetocarmine, aceto-orcein, or safranin. The cells are not large and rectangular like onion peel cells.

The main differences are as follows-

Onion peel cell – It is taken from the onion bulb epidermis. It is rectangular, transparent and non-dividing in normal observation. It is used to study cell wall, nucleus, cytoplasm, and vacuole.

Onion root tip cell – It is taken from the growing root tip. It is small, dense and actively dividing. It is used to study mitosis, chromosomes, and different stages of cell division.

So, onion peel cell is suitable for studying basic plant cell structure. Onion root tip cell is suitable for studying cell division because root tip is a region of active growth.

Precautions

• A fresh and thin onion epidermal peel should be used. Thick or damaged peel should be avoided.
• The onion should be cut carefully with a blade or scalpel. Cutting should be done on a stable surface and away from the body.
• Safety goggles and gloves should be used while handling iodine, safranin, or methylene blue. These stains may irritate or stain the skin.
• The glass slide and coverslip should be handled gently. They are thin and may break by applying excess pressure.
• A clean, dry, and scratch-free glass slide should be used. Dust, oil, or fingerprints may appear like cell structures under the microscope.
• The epidermal peel of Allium cepa should be placed as a single flat layer. Folds and overlapping of cells should be avoided.
• A mounted needle or fine brush should be used to remove wrinkles from the onion peel. The peel should not be torn during this process.
• Only one or two drops of staining solution should be added. Excess stain makes the specimen dark and hides the internal structures.
• Excess staining solution should be removed with the help of blotting paper. The coverslip should not be pressed strongly.
• The coverslip should be lowered slowly at an angle of about 45°. This prevents the formation of air bubbles under the coverslip.
• The onion peel should not be allowed to dry. Water or glycerine should be added when the specimen begins to dry.
• Observation should always start with the low-power objective lens. The specimen should be focused first by using the coarse adjustment knob.
• The objective lens should not touch the coverslip. Otherwise, the slide may break and the objective lens may also become damaged.
• Under high magnification, only the fine adjustment knob should be used. The coarse adjustment knob should not be used.
• The light and diaphragm should be adjusted properly. Excess light may make the transparent onion cells difficult to observe.
• Broken slides and coverslips should not be touched directly. They should be discarded carefully in a proper sharps container.

Onion Cells Under a Microscope Video

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