Prokaryotic (Gr. pro, before, + karyon, nucleus) cells are found only in bacteria. These cells are small (1–5 m long), typically have a cell wall outside the plasmalemma, and lack a nuclear envelope separating the genetic material (DNA) from other cellular constituents. In addition, prokaryotes have no histones (specific basic proteins) bound to their DNA and usually no membranous organelles.
In contrast, eukaryotic (Gr. eu, good, + karyon, nucleus) cells are larger and have a distinct nucleus surrounded by a nuclear envelope (Figure 2–1). Histones are associated with the genetic material, and numerous membrane-limited organelles are found in the cytoplasm. This book is concerned exclusively with eukaryotic cells.
The ultrastructure and molecular organization (right) of the cell membrane. The dark lines at the left represent the two dense layers observed in the electron microscope; these are caused by the deposit of osmium in the hydrophilic portions of the phospholipid molecules.
The human organism has about 200 different cell types, all derived from the zygote, a single cell formed by fertilization of an oocyte by a spermatozoon. The first cellular divisions of the zygote originate cells called blastomeres, which are able to form all cell types of the adult. Through this process, called cell differentiation, the cells synthesize specific proteins, change their shape, and become very efficient in specialized functions. For example, muscle cell precursors elongate into spindle-shaped cells that synthesize and accumulate myofibrillar proteins (actin, myosin). The resulting cell efficiently converts chemical energy into contractile force.
The main cellular functions performed by specialized cells in the body are listed in Table 2–1.