The cell is possibly the most important concept in biology since it is the basic unit of life. Every living organism is made up of cells (or just one in the case of bacteria). All the animals, trees and plants are made up of cells that share many similar characteristics.
An onion is a multicellular (consisting of many cells) plant organism.As in all plant cells, the cell of an onion peel consists of a cell wall, cell membrane, cytoplasm, nucleus and a large vacuole. The presence of a cell wall and a large vacuole are indicators that help identify plant cells. As in all animal cells, the cells of the human cheek do not possess a cell wall. A cell membrane that is semi-permeable surrounds the cytoplasm. The vacuole in an an animal cell is smaller in size, or absent. The absence of a cell wall and a prominent vacuole are indicators that help identify animal cells.
This video describes the preparation of stained temporary mounts of onion peel and human cheek cells and their structure.