Lab 3: Onion-skin cells under the microscope
In this laboratory lesson a thin skin of onion is studied under the microscope. Equipment: a slide, a cover glass, forceps, a pipette, a microscope, filter paper, iodine solution and an onion. The teacher cuts the onion into four pieces with a scalpel. A drop of water is put on a clean slide with the pipette; with forceps a very thin skin is peeled from the inner side of an onion piece and laid flat in the drop. One drop of the iodine solution, given by the teacher, is added – it makes the nucleus show up better. The cover glass is lowered slowly from one edge (so no air bubbles stay), and excess water is soaked up with filter paper. Look first with the small objective, then the large one. You see tightly packed elongated cells with their cellulose walls, cytoplasm, a nucleus and a vacuole. Onion-skin cells have no chloroplasts because light does not reach the inner fleshy layers of the onion bulb. Tiny pores in the cell wall let water and minerals pass from cell to cell.
With the teacher, prepare the onion-skin slide, view it under the microscope and draw 3–4 cells in the notebook, labelling the parts (wall, cytoplasm, nucleus, vacuole). Write a conclusion: which features of a plant cell did you see? Safety: only the teacher uses the scalpel; iodine solution is handled only as the teacher directs, it stains skin and clothes and must never be tasted; tell the teacher if a slide breaks; wash your hands with soap afterwards.