An unstained onion cell is a segment of onion that has not been dyed to make it easier to see beneath a microscope. All living things have cells that can be observed when placing them beneath a microscope. Adding a stain or dye makes it easier to see the cells.
Onions are multi-cellular plant organisms with translucent cells that are typically used to introduce students to plant anatomy. The onion cells contain cell parts that every student should be able to identify, such as a cell wall, cell membrane, cytoplasm and a nucleus. Onion cells have the additional advantage of being simple in structure and lacking chloroplasts seen in photosynthetic plants due to the fact that onions are fruiting underground portions of the plant.
Onion cells are often compared to human cheek cells due to the fact that human cheek cells are a relatively simple type of animal cell to obtain. Animal cells differ from plant cells in that plant cells have a cell wall that cannot be penetrated and usually have chloroplasts as well. Plant cells are also fixed shapes with large vacuoles or empty cavities inside them, while animal cells are irregular, have much smaller cavities and may have multiple small cavities within them.