Onion and cheek cells under a microscope: what you can see

Why these two cells

Onion skin and cheek cells are the classic first microscope lab because they are easy to get and show the difference between a plant cell and an animal cell. A thin layer peeled from inside an onion bulb is one cell thick. Cheek cells are scraped gently from the inside of the mouth by the student, following the teacher’s instructions and lab safety rules.

What onion cells look like

Onion skin cells look like a wall of long, brick-shaped boxes. The clear outlines are the cell walls. With a stain such as iodine, each cell shows a round nucleus, usually near one edge, and sometimes a nucleolus. You will not see chloroplasts, because the onion bulb grows underground and its cells do not photosynthesize. That surprises many students, who expect every plant cell to be green.

What cheek cells look like

Cheek cells are flat, irregular and rounded, scattered or folded over each other. There is no cell wall, so the outline is the cell membrane. A stain such as methylene blue makes the nucleus stand out as a dark spot in the middle. The rest of the cell is cytoplasm.

Why you cannot see most organelles

OpenStax notes that most light microscopes in a college biology lab magnify cells up to about 400 times and resolve details down to about 200 nanometers. Ribosomes are far smaller than that, and the ER, Golgi and lysosomes are too small or too clear to pick out without special stains or an electron microscope. That is why cell diagrams are drawings that combine what many kinds of microscopes have shown.

• Visible: cell wall (onion), cell membrane outline, nucleus, cytoplasm.
• Sometimes visible: nucleolus, a vacuole in some plant cells.
• Not visible in a basic lab: ribosomes, ER, Golgi, most mitochondria.

Drawing what you see

Lab reports usually ask for a drawing, not a copy of a textbook diagram. Draw only what you actually see, in pencil, with the magnification written beside it, and label the cell wall or membrane, the nucleus and the cytoplasm. Then compare your drawing with the plant and animal cell diagrams on this site and explain the difference.