Plant and Animal Cells
Senior 1 · Biology
Plant and Animal Cells
Lesson 3 of 5
25 min
Snap a fresh scale of an onion from Kimironko market and it breaks with a crack, like a thin board. The lining of your cheek, made of animal cells, is soft and floppy. The difference lies in the cells. Every plant cell is wrapped in a stiff box, and inside the box a large sac of watery sap presses outwards. Under the microscope you can see both.
Safety: cut away from your fingers. Iodine solution irritates the eyes and stains skin, so wear goggles and wash off any spills.
Mount a whole leaf of a moss, or of the water plant Elodea (pondweed), in a drop of water with no stain. These leaves are so thin that light passes straight through them. Look for the small green discs inside each cell.
Figure 1: Onion epidermis cells stained with iodine solution.
Definition
Cell wall: a strong, fully permeable layer of cellulose outside the cell membrane of a plant cell. It gives shape and support.
Vacuole: a sac in the cytoplasm surrounded by a membrane. A plant cell has one large, permanent vacuole filled with cell sap.
Chloroplast: an organelle containing the green pigment chlorophyll, which absorbs light for photosynthesis, the process in which a plant makes glucose from carbon dioxide and water.
| Seen at ×400 | Onion epidermis cell | Moss or Elodea leaf cell |
|---|---|---|
| Cell wall | yes, clear | yes, clear |
| Nucleus | yes, once stained | often hidden by chloroplasts |
| Large vacuole | yes | yes |
| Chloroplasts | none: the scale grows underground, out of the light | many: the leaf is in the light |
Warning
Not every plant cell is green. Chloroplasts are found only in plant cells that receive light, mainly in leaves and young stems. Root cells, onion scales and the inside of a cassava root have none. So "no chloroplasts" does not mean "animal cell": look for a cell wall and a large vacuole instead.
Worked example 1: estimating the length of a cell
At ×100 the circular field of view of a school microscope is 1.8 mm across. Uwase counts 6 onion cells lying end to end across the middle of it (Figure 2). Estimate the length of one cell in micrometres.
Figure 2: Six onion cells across a field of view 1.8 mm wide.
Answer: about 300 µm. It is an estimate: the cells are not all the same length, so counting along several lines and taking the mean gives a better value.
Worked example 2: plant or animal?
A cell has a thick outer layer that stains with a cellulose stain and a large central vacuole, but no chloroplasts. Mugisha says it must be an animal cell because it is not green. Is he right?
Answer: no. It is a plant cell, probably from a root, a tuber or a bulb.
Leafy vegetables such as isombe (cassava leaves) and dodo (amaranth) look fresh in the morning at the market and droop by the afternoon: their vacuoles lose water in the sun, the cells go soft, and the leaves wilt. A sprinkle of water lets the vacuoles fill again. The cellulose of cell walls is also everywhere around you: it is the main material of wood, paper and cotton cloth, and in your food it is the fibre (roughage) that you will meet in unit 7.
How to Prepare a Microscope Slide (Onion and Cheek Cell), Ms Beh. Watch the onion part closely: notice how thin the epidermis is, and compare the finished onion slide with the cheek cell slide from lesson 2.
Tip
Memory hook: Onion takes Iodine (two vowels together); Mouth cells take Methylene blue.
Practice: plant cells
Key points
Sources and credits
Figures 1 and 2, the practical methods, 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.
