Depending on that, whether the basic genetic material (DNA) is
located “freely” in cytoplasm or in a well defined nucleus encircled by a
double membrane, the cells are divided into prokaryotic (protocists) and
eukaryotic (eucists). Bacteria and unicellular blue-green algae are typical
prokaryotic cells. All the other cells — either independently existing, or
included in the composition of multicellular organisms — are eukaryotic.
Irrespective of some differences and peculiarities the cells possess
similar internal organization. Each cell possesses an envelope, nucleus or
nuclear equivalent (nucleoid) and cytoplasm, which plays the role of “internal
skeleton” of the cell. In the cytoplasm (Fig. 2–53) there are situated cell
organelles (from Latin organella — organ, instrument) connected in a common
system. Some of them perform also autonomous functions.
image
Figure 2–53. Cytoplasmic structure in a cell of onion root (Allium cepa), fixed through OsO₄. The dark particles represent ribosomes (After Frey-Wissling and Mühlethaler, 1965).
Cell Wall and Cell Envelope
The cell walls and cell envelope separate the cells from the surroundings or
from the other cells in multicellular organisms, thus creating conditions
necessary for carry out of the life processes in them. They are peculiar
biological filters, selectively permeable for inorganic and organic
substances necessary for their metabolism. They also excrete the waste
products.
In 1931 W. Lewis discovered the phenomenon pinocytosis (Greek:
pino — drink and kýtos — cell) considered to be one of the basic
mechanisms of penetrating into the cells of high-molecular compounds —
proteins, carbohydrates, sugar-protein complexes, etc. It was shown
diagrammatically, that this process is realized through forming bubbles by
making concave the cell membrane. Once formed, the bubbles pass in the
cytoplasm and are transported in it (Fig. 2–54 A). Latter on pinocytosis was