heavy metals such as osmium tetroxide appear in section as two densely
Department, Sheffield Teaching Hospitals, UK .)
stained layers separated by an electron-translucent zone – the classic
unit membrane. The total thickness of each layer is about 7.5 nm. The
and distinct compartments within the cytoplasm. Cytomembranes
overall thickness of the plasma membrane is typically 15 nm. Freeze-
include the rough and smooth endoplasmic reticulum and Golgi appa-
fracture cleavage planes usually pass along the hydrophobic portion of
ratus, as well as vesicles derived from them. Organelles include lyso-
the bilayer, where the hydrophobic tails of phospholipids meet, and
somes, peroxisomes and mitochondria. The nucleus and mitochondria
split the bilayer into two leaflets. Each cleaved leaflet has a surface and
are enclosed by a double-membrane system; lysosomes and peroxi-
a face. The surface of each leaflet faces either the extracellular surface
somes have a single bounding membrane. There are also non-
(ES) or the intracellular or protoplasmic (cytoplasmic) surface (PS). The