Plant and Animal Cells Under a Microscope, Grade 9
Onion is a plant, and onion cells have no chloroplasts.
That single fact undoes most Grade 9 cell lessons. A class is told that plant cells contain chloroplasts, then hands an onion slide, then looks down the microscope and finds nothing green anywhere. Half of them decide they made the slide wrong. The other half decide the textbook lied.
Neither is true, and the reason is worth more than the list of differences it interrupts. This page covers how to focus a microscope without wrecking it, how to make a wet mount that shows cells instead of air bubbles, and the three differences between plant and animal cells you can genuinely see at 400x.
The three differences you can actually see
Textbooks list eight. On a school microscope you can see three of them. The rest need an electron microscope, and saying so is not a shortcut, it is honesty about what the instrument does.
| Structure | Plant cell | Animal cell | Visible at 400x? |
|---|---|---|---|
| Cell wall | Present | Absent | Yes. Straight edges, cells packed like bricks |
| Chloroplasts | In green parts only | Absent | Yes. Green discs, sometimes drifting |
| Large central vacuole | One big one | Small or none | Yes. It pushes everything else against the wall |
| Nucleus | Present | Present | Yes, but it is in both, so it is not a difference |
| Mitochondria, ribosomes | Present | Present | No. Too small, and no contrast |
Straight lines and neat rows mean a cell wall. No straight lines means no wall. That one sentence answers every "identify these cells" question in the topic.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 1 |
| Strand | Life and living |
| Topic | Topic 1, Cells as the basic unit of life |
| Status | Skills focus, not a formally assessed practical task |
| Marks | None prescribed. Our worksheet is 45 marks |
Why the onion has no chloroplasts
The onion you buy is a bulb, and a bulb grows underground. It never sees light, so it never photosynthesises, so its cells never develop chloroplasts. The green leaves above the soil have plenty. The storage bulb has none.
This is not a fault in the specimen. It is the specimen teaching you something. Chloroplasts are not a badge that every plant cell carries. They are equipment, and a cell only builds them if it has a use for them.
| Specimen | Cell wall | Chloroplasts | Why |
|---|---|---|---|
| Onion epidermis | Beautifully clear | None | Underground storage tissue |
| Pond weed or thin moss leaf | Clear | Dozens per cell | It is a leaf, and its job is photosynthesis |
Use both, in that order. The onion gives the cleanest walls a learner will ever see, and the pond weed gives the green. Ask the class to explain the difference before you tell them.
Focusing a microscope without breaking it
Get the order wrong and you drive a lens through a glass slide. At 400x the objective sits about a millimetre above the cover slip, and the coarse focus knob moves it much further than that.
- Carry the microscope with two hands, one under the base
- Click the lowest power objective into place
- Put the slide on the stage, specimen over the hole, clips holding it
- Watching from the side, not through the eyepiece, wind the objective down close to the slide
- Now look through the eyepiece and wind slowly up until the image appears
- Sharpen with the fine focus
- Set the light so the field is bright but not glaring
- Only now turn to a higher power, and use the fine focus only
Coarse focus on low power only. This is the rule that keeps a R1 990 instrument alive in a class of thirty.
Total magnification is the eyepiece multiplied by the objective. A 10x eyepiece with a 40x objective is 400x. A 4x eyepiece with a 40x objective is 160x, which is the exam question that catches learners who assume the eyepiece is always 10x.
The wet mount, step by step
What you need: a clean glass slide · a cover slip · a dropping pipette · water · iodine solution for plant tissue · a small piece of specimen · forceps · paper towel
- Lay the specimen flat in the centre of the slide. A folded piece of onion skin is two layers of cells on top of each other and will never focus
- One drop of water on the specimen
- One drop of stain if you are using one. Iodine for plant tissue, methylene blue for cheek cells
- Lower the cover slip on its edge, slowly, letting it fall the last little way. Dropping it flat traps air
- Draw off the excess with the corner of a paper towel at the far side
Air bubbles look exactly like cells, and learners draw them
A bubble is a perfect circle with a thick, dark, distinct rim. Cells are not perfectly round and their edges are thin. Bubbles also sit at a different focal depth, so they blur when the cells are sharp.
If the field is full of them, remake the slide. Lowering the cover slip on its edge is the whole fix.
An animal cell, and a note on school policy
The classic animal specimen is a cheek scraping stained with methylene blue. It works well and some schools do not allow it, which is a reasonable position.
| Method | Controls it needs |
|---|---|
| Cheek cells | One sterile swab per learner, used once. Straight into disinfectant afterwards. Nobody handles anybody else's slide |
| Prepared slide | None. The comparison still works, and a school with no stains can run the whole lesson this way |
Making the mount is the skill being taught, so use the real thing where you can. But a prepared slide of cheek epithelium teaches the plant-against-animal comparison perfectly well, and no lesson should be cancelled over it.
What you should see
| Specimen | At 400x |
|---|---|
| Onion epidermis | Brick-shaped cells in tidy rows, straight walls, a small stained nucleus in each, no green anywhere |
| Pond weed or moss | Green discs pressed against the walls. In a warm, living leaf they drift slowly around the edge, which is the cytoplasm moving |
| Cheek cells | Irregular flattened blobs, no straight edges, one dark nucleus, scattered rather than in rows |
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Everything is black | The objective is not clicked in, or the light is off or blocked | Turn the turret until it clicks. Check the lamp or the mirror |
| It will not focus at 400x | They started at high power | Focus at low power first, every time, then turn up |
| Perfect circles with thick dark rims | Air bubbles | Remake the mount, cover slip on its edge |
| Cells look crushed or burst | The cover slip was pressed down | It only needs its own weight |
| A dark band across the middle | The onion skin folded over | Re-lay it flat. This is the commonest onion problem |
| The image dries out and goes grainy | The water has evaporated | One drop of water at the edge of the cover slip. It draws underneath |
| No chloroplasts in the onion | Correct behaviour | Nothing to fix. This is the discussion |
| The pond weed shows no movement | The leaf is dying or cold | Collect it the same day and keep it in water |
| Cheek cells are invisible | Unstained, or too few on the slide | One drop of methylene blue, and scrape a little more firmly |
Safety
- Iodine and methylene blue stain skin, clothes and benches permanently. One drop is enough for any slide
- Cover slips are 0,13 mm of glass and they snap in the fingers. Hold them by the edges and sweep up breakages rather than picking them up
- Cheek cells: one swab per learner, used once, into disinfectant. Never share a slide
- A scalpel is not needed for an onion. A plastic knife does it
- Wash hands afterwards
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 45 marks. Part A is the microscope and the wet mount, including the magnification calculations. Part B is the plant against animal comparison, with the onion question
- Marking memorandum, with the full answers and the three answers that look right and score nothing
Related practicals
- The pH scale and indicators, Grade 9. Term 2, and the assessed practical task
- Reactions of acids with bases, carbonates and metals, Grade 9. The chemistry half of the year
- Choosing a school microscope. What the magnifications actually mean before you spend the money
What you need to run it
One thing on this list costs real money and the rest is small change.
The microscope is the practical. Without one there is no lesson, and no quantity of slides fixes that. Our student microscopes start at R1 990 for a 400x monocular, which is the magnification this topic is written around.
Then the consumables, which are genuinely cheap. Glass slides are R30 for fifty and cover slips are R16 for a hundred. A class of thirty gets through both, and the cover slips break, so buy them by the box rather than by the packet.
A glass dropping pipette at R10 does the water and the stain.
Iodine solution and methylene blue are not currently in our catalogue. We are sourcing both in dropper bottles. Until then, a school pharmacy sells iodine, and a school with neither stain can still run the whole lesson with a prepared slide set at R400 for twelve slides.
For the wider range, see microscope slides and cover slips.