Science of Water : Concepts and Applications

Water Biology 129
TABLE 5.2
Comparison of Typical Prokaryotic and Eukaryotic Cells
Characteristic Prokaryotic Eukaryotic
Size 1–10 µm 10–100 µm
Nuclear envelope Absent Present
Cell wall (animals) Usually Present (plants)/absent
Plasma membrane Present Present
Nucleolus Absent Present
DNA Present (single loop) Present
Mitochondria Absent Present
Chloroplasts Absent Present (plants only)
Endoplasmic reticulum Absent Present
Ribosomes Present Present
Vacuoles Absent Present
Golgi apparatus Absent Present
Lysosomes Absent Often present
Cytoskeleton Absent Present
The presence of these membrane-bound organelles points to the signifi cant difference between
prokaryotes and eukaryotes. Although eukaryotes use the same genetic code and metabolic processes
as prokaryotes, their higher level of organizational complexity has permitted the development
of truly multicellular organisms.
√ Important Point: An enormous gap exists between prokaryotic and eukaryotic cells:
… prokaryotes and eukaryotes are profoundly different from each other and clearly represent a marked
dichotomy in the evolution of life … The organizational complexity of the eukaryotes is so much
greater than that of prokaryotes that it is diffi cult to visualize how a eukaryote could have arisen from
any known prokaryote.
—C.P. Hickman et al. (1997)
Prokaryotic and eukaryotic cells also have their similarities. All cell types are bounded by a
plasma membrane that encloses proteins and usually nucleic acids such as DNA and RNA. Table 5.2
shows a comparison of key features of both cell types.
√ Interesting Point: Plant cells can generally be distinguished from animal cells by (1) the presence
of cell walls, chloroplasts, and central vacuoles in plants and their absence in animals; and
(2) the presence of lysosomes and centrioles in animals and their absence in plants.
BACTERIA
The simplest wholly contained life systems are bacteria or prokaryotes, which are the most diverse
group of microorganisms. As mentioned, they are among the most common microorganisms in
water, are primitive, unicellular (single-celled) organisms, possessing no well-defi ned nucleus, and
present a variety of shapes and nutritional needs. Bacteria contain about 85% water and 15% ash or
mineral matter. The ash is largely composed of sulfur, potassium, sodium, calcium, and chlorides,
with small amounts of iron, silicon, and magnesium. Bacteria reproduce by binary fi ssion.
√ Note: Binary fi ssion occurs when one organism splits or divides into two or more new
organisms.