A student prepared some onion cells and viewed them using a microscope. The method included adding three drops of iodine solution to the layer of cells.

Why was iodine solution added to the layer of onion cells?

When viewed under a light microscope, many parts of a cell (like the cytoplasm, vacuole, and nucleus) are colourless and transparent, making them very difficult to see. A stain is a dye used to add colour to these structures. Iodine solution is a common stain used for plant cells. It stains the starch in the cytoplasm a blue-black colour and the nucleus a yellow/brown colour, making them stand out against the cell wall and allowing them to be observed clearly.