Prokaryotic and Eukaryotic Cells
The Prokaryotic Cell
Of all the types of cells revealed by the microscope, bacteria have the simplest structure
and come closest to showing us life stripped down to its essentials. Indeed, a bacterium
contains essentially no organelles—not even a nucleus to hold its DNA. This property—the
presence or absence of a nucleus—is used as the basis for a simple but fundamental
classification of all living things. The terms “bacterium” and “prokaryote” are often used
interchangeably, although we will see that the category of prokaryotes also includes another
class of cells, the archaea (singular archaeon), which are so remotely related to bacteria that
they are given a separate name.
Prokaryotes are typically spherical, rodlike, or corkscrew-shaped as shown in Figure 2. A
prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus.
They are also small—generally just a few micrometers long, although there are some giant
species as much as 100 times longer than this. Prokaryotes often have a tough protective coat,
or cell wall, surrounding the plasma membrane, which encloses a single compartment
containing the cytoplasm and the DNA.
In the electron microscope, the cell interior typically appears as a matrix of varying
texture, without any obvious organized internal structure as shown in Figure 3 below. The cells
reproduce quickly by dividing in two. Under optimum conditions, when food is plentiful, many
prokaryotic cells can duplicate themselves in as little as 20 minutes. In 11 hours, by repeated
divisions, a single prokaryote can give rise to more than 8 billion progeny (which exceeds the
total number of humans presently on Earth). Thanks to their large numbers, rapid growth rates,
and ability to exchange bits of genetic material by a process akin to sex, populations of
prokaryotic cells can evolve fast, rapidly acquiring the ability to use a new food source or to
resist being killed by a new antibiotic.
This figure shows the generalized structur
of a prokaryotic cell. All prokaryotes have
chromosomal DNA localized in a nucleoid,
ribosomes, a cell membrane, and a cell
wall. The other structures shown are
present in some, but not all, bacteria.
, Figure 1. Generalized structure of a prokaryotic cell.
Figure 2. Bacteria come in different shapes and sizes. Figure 3. The bacterium Escherichia coli (E. col
Prokaryotes are the Most Diverse and Numerous Cells on Earth
Most prokaryotes live as single-celled organisms, although some join
together to form chains, clusters, or other organized multicellular structures. In
shape and structure, prokaryotes may seem simple and limited, but in terms of
chemistry, they are the most diverse and inventive class of cells. Members of this
class exploit an enormous range of habitats, from hot puddles of volcanic mud to
the interiors of other living cells, and they vastly outnumber all eukaryotic
organisms on Earth. Some are aerobic, using oxygen to oxidize food molecules;
some are strictly anaerobic and are killed by the slightest exposure to oxygen.
The mitochondria—the organelles that generate energy in eukaryotic cells—
are thought to have evolved from aerobic bacteria that took to living inside the
anaerobic ancestors of today’s eukaryotic cells. Thus our own oxygen-based
metabolism can be regarded as a product of the activities of bacterial cells.
Virtually any organic, carbon-containing material—from wood to petroleum
—can be used as food by one sort of bacterium or another. Even more remarkably,
some prokaryotes can live entirely on inorganic substances: they can get their
carbon from CO2 in the atmosphere, their nitrogen from atmospheric N2, and their
oxygen, hydrogen, sulfur, and phosphorus from air, water, and inorganic minerals.
Some of these prokaryotic cells, like plant cells, perform photosynthesis, using
energy from sunlight to produce organic molecules from CO2. In either case, such
The Prokaryotic Cell
Of all the types of cells revealed by the microscope, bacteria have the simplest structure
and come closest to showing us life stripped down to its essentials. Indeed, a bacterium
contains essentially no organelles—not even a nucleus to hold its DNA. This property—the
presence or absence of a nucleus—is used as the basis for a simple but fundamental
classification of all living things. The terms “bacterium” and “prokaryote” are often used
interchangeably, although we will see that the category of prokaryotes also includes another
class of cells, the archaea (singular archaeon), which are so remotely related to bacteria that
they are given a separate name.
Prokaryotes are typically spherical, rodlike, or corkscrew-shaped as shown in Figure 2. A
prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus.
They are also small—generally just a few micrometers long, although there are some giant
species as much as 100 times longer than this. Prokaryotes often have a tough protective coat,
or cell wall, surrounding the plasma membrane, which encloses a single compartment
containing the cytoplasm and the DNA.
In the electron microscope, the cell interior typically appears as a matrix of varying
texture, without any obvious organized internal structure as shown in Figure 3 below. The cells
reproduce quickly by dividing in two. Under optimum conditions, when food is plentiful, many
prokaryotic cells can duplicate themselves in as little as 20 minutes. In 11 hours, by repeated
divisions, a single prokaryote can give rise to more than 8 billion progeny (which exceeds the
total number of humans presently on Earth). Thanks to their large numbers, rapid growth rates,
and ability to exchange bits of genetic material by a process akin to sex, populations of
prokaryotic cells can evolve fast, rapidly acquiring the ability to use a new food source or to
resist being killed by a new antibiotic.
This figure shows the generalized structur
of a prokaryotic cell. All prokaryotes have
chromosomal DNA localized in a nucleoid,
ribosomes, a cell membrane, and a cell
wall. The other structures shown are
present in some, but not all, bacteria.
, Figure 1. Generalized structure of a prokaryotic cell.
Figure 2. Bacteria come in different shapes and sizes. Figure 3. The bacterium Escherichia coli (E. col
Prokaryotes are the Most Diverse and Numerous Cells on Earth
Most prokaryotes live as single-celled organisms, although some join
together to form chains, clusters, or other organized multicellular structures. In
shape and structure, prokaryotes may seem simple and limited, but in terms of
chemistry, they are the most diverse and inventive class of cells. Members of this
class exploit an enormous range of habitats, from hot puddles of volcanic mud to
the interiors of other living cells, and they vastly outnumber all eukaryotic
organisms on Earth. Some are aerobic, using oxygen to oxidize food molecules;
some are strictly anaerobic and are killed by the slightest exposure to oxygen.
The mitochondria—the organelles that generate energy in eukaryotic cells—
are thought to have evolved from aerobic bacteria that took to living inside the
anaerobic ancestors of today’s eukaryotic cells. Thus our own oxygen-based
metabolism can be regarded as a product of the activities of bacterial cells.
Virtually any organic, carbon-containing material—from wood to petroleum
—can be used as food by one sort of bacterium or another. Even more remarkably,
some prokaryotes can live entirely on inorganic substances: they can get their
carbon from CO2 in the atmosphere, their nitrogen from atmospheric N2, and their
oxygen, hydrogen, sulfur, and phosphorus from air, water, and inorganic minerals.
Some of these prokaryotic cells, like plant cells, perform photosynthesis, using
energy from sunlight to produce organic molecules from CO2. In either case, such
