The Invisible Living World: Beyond Our Naked Eye
Looking at real cells
Look at real onion cells under a microscope, and find their parts.
Every living thing is made of cells. Your teacher can show you two kinds.
Onion cells (Activity 2.2). Cut an onion and use forceps to pull the thin, see-through skin from the inside of one layer: the onion peel. Put it in a few drops of safranin, a red stain, for 30 seconds. The stain colours the cells pink so they stand out. Rinse off the extra stain.
Lay the peel flat on a glass slide, without folding or tearing it. Add a drop of glycerin (it stops the cells drying out and makes them clearer). Lower a thin coverslip with a needle so no air bubbles get trapped, wipe off extra glycerin with blotting paper, and look under a microscope. A simple foldable paper microscope will do.
You see nearly rectangular cells, packed tightly with no gaps, like bricks in a wall.
Cheek cells (Activity 2.3). Rinse your mouth, then gently scrape the inside of your cheek with the blunt end of a clean toothpick. Spread it in a drop of water on a slide, add a drop of methylene blue, a blue stain that makes the cells easier to see, then glycerin and a coverslip. You see polygon-shaped cells. They line the inside of your mouth.
Compare the two. Every cell has three main parts:
| Part | Where |
|---|---|
| cell membrane | the thin outer layer |
| nucleus | the round body in the middle, with its own thin membrane |
| cytoplasm | everything filling the space between the two |
The onion cells also have an extra, firm outer layer: the cell wall. Cheek cells do not.
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