The cell is the unit of structure in all living things. The cell is the unit of function of all living things. All cells come from preexisting cells. The light microscope can only be used to observe the cell wall, cell membrane, nucleus, and chloroplasts under ordinary conditions. The problem with diffraction requires the use of microscopes utilizing a shorter wavelength such as the electron microscope.
Specimens are usually first fixed and then embedded with wax or resin before sectioning.
Uptake of I-131 by the thyroid gland indicates the metabolism level of that gland. All membranes, nuclear, cytoplasmic, mitochondrial, endoplasmic, etc. appear to display a similar structural pattern. The outer triplets are microtubules, but the pair of tubules in the center are NOT microtubules. Certain simple organisms lack cellular separations. Viruses lack cellular organization.