lesson
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If you slice an onion or swab your cheek and place it directly under a light microscope, you will mostly see a blurry, transparent glare. Most living cells are nearly 70% water, making their internal organelles almost invisible without special preparation.
To study living tissue, biologists prepare a temporary mount (also called a wet mount), which suspends a paper-thin specimen in liquid between a flat glass microscope slide and a thin glass coverslip.
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How do we prepare plant cells so they are thin enough for light to pass through them?
Mounting Plant Cells: Onion Epidermis
Light microscopes rely on light transmitting directly through a specimen. If your plant sample is several cell layers thick, the cells stack up, blocking light and creating an unreadable dark clump.
To get a single layer of cells, peel the transparent epidermal layer from the concave inner curve of an onion bulb scale using forceps, and lay it flat without folds in a drop of water.
We then add a drop of iodine solution; this stain binds to cellular starch and glycogen, turning the cell walls, cytoplasm, and nucleus a crisp golden-brown so their boundaries become clearly defined.
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Plant cells have rigid walls that keep them intact, but what happens when we prepare soft, wall-less animal cells?
Mounting Animal Cells: Cheek Epithelium
Animal cells lack a rigid cell wall, so they are loose and easily shed. You can collect human epithelial cells (the protective outer surface layer) by gently scraping the inside of your cheek with a sterile cotton swab or blunt flat toothpick.
Swirl the swab into a small drop of water on the slide to disperse the cells, then add a drop of methylene blue—a positively charged stain that binds strongly to negatively charged DNA, turning the oval nucleus deep blue while leaving the surrounding cytoplasm pale.