Comparing Prokaryotic and Eukaryoti

Comparing Prokaryotic and Eukaryotic Cells
All cells share certain basic features: They are all bounded by
a selective barrier, called the
plasma membrane
. Inside all cells
is a semifluid, jellylike substance called
cytosol
, in which
subcellular components are suspended. All cells contain
chromosomes
, which carry genes in the form of DNA. And all
cells have
ribosomes
, tiny complexes that make proteins ac-
cording to instructions from the genes.
A major difference between prokaryotic and eukaryotic cells
is the location of their DNA. In a
eukaryotic cell
, most of the
DNA is in an organelle called the
nucleus
, which is bounded by
a double membrane (see Figure 6.8, on pp. 100–101). In a
prokaryotic cell
, the DNA is concentrated in a region that
is not membrane-enclosed, called the
nucleoid
(Figure 6.5)
.
The word
eukaryotic
means “true nucleus” (from the Greek
eu
,
true, and
karyon
, kernel, here referring to the nucleus), and
the word
prokaryotic
means “before nucleus” (from the Greek
pro
, before), reflecting the fact that prokaryotic cells evolved
before eukaryotic cells.
The interior of either type of cell is called the
cytoplasm
; in
eukaryotic cells, this term refers only to the region between the
nucleus and the plasma membrane. Within the cytoplasm of a
eukaryotic cell, suspended in cytosol, are a variety of organelles
of specialized form and function. These membrane-bounded
structures are absent in prokaryotic cells. Thus, the presence or
absence of a true nucleus is just one aspect of the disparity in
structural complexity between the two types of cells.
Eukaryotic cells are generally much larger than prokary-
otic cells (see Figure 6.2). Size is a general feature of cell struc-
ture that relates to function. The logistics of carrying out
cellular metabolism sets limits on cell size. At the lower limit,
the smallest cells known are bacteria called mycoplasmas,
which have diameters between 0.1 and 1.0 μm. These are
perhaps the smallest packages with enough DNA to program
metabolism and enough enzymes and other cellular equip-
ment to carry out the activities necessary for a cell to sustain
itself and reproduce. Typical bacteria are 1–5 μm in diameter,
about ten times the size of mycoplasmas. Eukaryotic cells are
typically 10–100 μm in diameter.
Metabolic requirements also impose theoretical upper limits
on the size that is practical for a single cell. At the boundary of
every cell, the
plasma membrane
functions as a selective
barrier that allows passage of enough oxygen, nutrients, and
wastes to service the entire cell
(Figure 6.6)
. For each square mi-
crometer of membrane, only a limited amount of a particular