All living things are composed of cells, and new cells arise only from preexisting ones. Cells are the fundamental unit of all living things.
Cells are small to maintain an optimal surface area to volume ratio.
The compound light microscope passes light rays through specimens and uses glass lenses to view objects. The transmission electron microscope passes electrons through a specimen, focuses with magnetic lenses, and projects images onto a fluorescent screen or photographic film. In the fluid-mosaic model, the plasma membrane is a bilayer of phospholipids with embedded or attached proteins. Glycoproteins and glycolipids serve as identification markers. Embedded proteins may serve as hormone receptors, transport channels, and enzymes in metabolic reactions.