Science of Water : Concepts and Applications

128 The Science of Water: Concepts and Applications
FIGURE 5.3 Bacterial cell.
Capsule
Inclusions
Nuclear material
Cell membrane
Cytoplasm
Mesosome
Ribosomes
Cell wall
Flagellum
Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular
forms. Some bacteria grow in fi laments, or masses of cells, but each cell in the colony is identical
and capable of independent existence. Prokaryotes are capable of inhabiting almost every place on
the Earth, from the deep ocean to the edges of hot springs to just about every surface of our bodies.
√ Important Point: It is often stated that prokaryotic cells are among the most primitive forms
of life on the Earth. However, it is important to point out that primitive does not mean they are
outdated in the evolutionary sense because primitive bacteria may seem little changed, and thus
may be viewed as well adapted.
As mentioned, prokaryotes are distinguished from eukaryotes on the basis of nuclear organization,
specifi cally their lack of a nuclear membrane. Also, prokaryotes are smaller and simpler than
eukaryotic cells. Again, prokaryotes also lack any of the intracellular organelles (i.e., internal cell
structures, each of which has a specifi c function within the cell) and structures that are characteristic
of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and
the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have
three architectural regions: appendages called fl agell a and pili—proteins attached to the cell surface;
a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic
region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions.
Eukaryotic cells evolved about 1.5 billion years ago. Protists, fungi, plants, and animals have
eukaryotic cells—all plants and animals are eukaryotes. They are larger, as much as 10 times the
size of prokaryotic cells and most of their genetic material is found within a membrane-bound
nucleus (a true nucleus), which is generally surrounded by several membrane-bound organelles.