Plant and Animal Cells
Senior 1 · Biology
Plant and Animal Cells
Lesson 2 of 5
25 min
The inside of your cheek is lined with thin, flat cells. Chewing rubs the top layer away all the time, and new cells grow up from underneath to replace them, so you swallow thousands of your own cells every day without noticing. Because they come off so easily, cheek cells are the simplest human cells to study: a clean cotton bud collects hundreds of them in one gentle stroke.
You need: a clean glass slide and coverslip, a sterile cotton bud, methylene blue stain in a dropper bottle, a mounted needle, a paper towel, a light microscope, and a beaker of disinfectant.
Safety: use only your own cells and a new cotton bud. Put the used bud straight into the disinfectant. Methylene blue stains skin and clothes. Wash your hands when you finish.
Figure 1: Making a stained cheek cell slide.
Place the slide on the stage, find the cells with the ×10 objective (total magnification ×100) and then turn to the ×40 objective (×400). Draw two or three cells.
Cells are almost transparent, so without a stain you see only faint outlines. Methylene blue is taken up most strongly by the DNA in the nucleus, so the nucleus turns dark blue while the cytoplasm stays pale blue. Staining kills the cells, but it makes their parts easy to tell apart.
Figure 2: Cheek cells stained with methylene blue, at ×400.
| Part | What it looks like at ×400 | What it does |
|---|---|---|
| Cell membrane | a very thin line at the edge of the cell | holds the cell together and controls what enters and leaves |
| Cytoplasm | pale blue and slightly grainy | jelly in which most of the cell's chemical reactions happen |
| Nucleus | a dark blue oval, usually near the middle | holds the DNA; controls the cell's activities and its division |
| Small vacuoles | tiny clear spaces, not always seen | short-term stores of water and food |
| Mitochondria | too small to see clearly | release energy from food by respiration (lesson 4) |
Definition
Nucleus (plural nuclei): the largest organelle in the cell, surrounded by a nuclear membrane; it contains the DNA that controls the cell.
Cytoplasm: the jelly-like material between the cell membrane and the nucleus, in which the organelles sit.
Cell membrane: the thin layer around the cytoplasm of every cell. It is partially permeable: it lets some substances (water, oxygen) through but stops others.
Stain: a dye that colours some parts of a cell more than others so they can be seen. Example: methylene blue for animal cells.
Warning
An air bubble is not a cell. Beginners often draw air bubbles. A bubble is perfectly round, has a thick black rim and looks bright in the middle; a cheek cell is flat, irregular in shape, pale blue, and has a nucleus. Lowering the coverslip slowly at an angle (step 3) stops most bubbles forming.
Worked example 1: explaining the method
Explain the reason for each of these steps: (a) adding methylene blue; (b) lowering the coverslip at an angle; (c) starting with the ×10 objective.
Worked example 2: the magnification of a drawing
Gatete measures a cheek cell with an eyepiece scale: it is 60 µm wide. In his drawing the same cell is 45 mm wide. What is the magnification of his drawing?
Answer: the drawing is ×750. Magnification has no unit, and both sizes must be in the same unit before dividing.
Because every cheek cell has a nucleus full of DNA and can be collected without pain or blood, a cheek swab is the standard way to take a DNA sample. Forensic laboratories, including Rwanda's national forensic laboratory in Kigali, use cheek swabs to identify people and to settle questions of who is related to whom. The method is the one you used in step 2, done with a sterile swab.
Cheek Epithelial Cells: How to Prepare a Wet Mount Microscope Slide, Tami Guy. Compare her steps with Figure 1, and watch how slowly the coverslip is lowered.
Tip
Start low, go slow. Always find the cells with the lowest-power objective first, then turn to the next objective and focus with the fine focus knob only. The coarse knob can drive a high-power lens through the coverslip.
Practice: animal cells
Key points
Sources and credits
Figures 1 and 2, the practical method, the table, the worked examples and the practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
