Figure 3.1: The outline of onion cells are visible under a light microscope. ( 9 )Observing Cells
Most cells are so tiny that you can’t see them without the help of a microscope. The
microscopesthatstudentstypicallyuseatschoolarelightmicroscopes. RobertHookecreated
a primitive light microscope in 1665 and observed cells for the very first time. Although the
light microscope opened our eyes to the existence of cells, they are not useful for looking at
the tiniest components of cells. Many structures in the cell are too small to see with a light
microscope.
When scientists developed more powerful microscopes in the 1950s, the field of cell biology
grew rapidly. A light microscope passes a light beam through a specimen, but the more
powerfulelectron microscopepasses a beam of electrons through the specimen, allowing
a much closer look at the cell (Figure3.2).
Transmission electron microscopes(TEM), which pass an electron beam through some-
thing, areusedtolookataverythinsectionofanorganismandallowustostudytheinternal
structure of cells.Scanning electron microscopes(SEM), which pass a beam of electrons
across the surface of something, show the details of the shapes of surfaces, giving a 3D image.
Electron microscopes showed many small structures in the cell that had been previously
invisible with light microscopes. One drawback to using an electron microscope is that it
only images dead cells. A light microscope can be used to study living cells.