Essential Cell Biology 5th edition

The Prokaryotic Cell
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(A)
H
S
V
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Figure 1–12 Some bacteria are
photosynthetic. (A) Anabaena cylindrica
forms long, multicellular chains. This light
micrograph shows specialized cells that
either fix nitrogen (that is, capture N 2
from the atmosphere and incorporate
it into organic compounds; labeled H ),
fix CO 2 through photosynthesis (labeled
V ), or become resistant spores (labeled
S) that can survive under unfavorable
conditions. (B) An electron micrograph of
a related species, Phormidium laminosum,
shows the intracellular membranes where
photosynthesis occurs. As shown in these
micrographs, some prokaryotes can have
intracellular membranes and form simple
multicellular organisms. (A, courtesy of
David Adams; B, courtesy of D.P. Hill and
C.J. Howe.)
(B)
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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
ECB5 e1.11/1.12
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 CO 2 in the atmosphere, their
nitrogen from atmospheric N 2 , 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 CO 2 (Figure 1–12); others
derive energy from the chemical reactivity of inorganic substances
in the environment (Figure 1–13). In either case, such prokaryotes play
a unique and fundamental part in the economy of life on Earth, as other
living organisms depend on the organic compounds that these cells generate
from inorganic materials.
Plants, too, can capture energy from sunlight and carbon from atmospheric
CO 2 . But plants unaided by bacteria cannot capture N 2 from the
atmosphere. In a sense, plants even depend on bacteria for photosynthesis:
as we discuss later, it is almost certain that the organelles in the plant
cell that perform photosynthesis—the chloroplasts—have evolved from
photosynthetic bacteria that long ago found a home inside the cytoplasm
of a plant-cell ancestor.
The World of Prokaryotes Is Divided into Two Domains:
Bacteria and Archaea
Traditionally, all prokaryotes have been classified together in one large
group. But molecular studies have determined that there is a gulf within
the class of prokaryotes, dividing it into two distinct domains—the bacteria
and the archaea—which are thought to have diverged from a common
prokaryotic ancestor approximately 3.5 billion years ago. Remarkably,
DNA sequencing reveals that, at a molecular level, the members of these
two domains differ as much from one another as either does from the
eukaryotes. Most of the prokaryotes familiar from everyday life—the species
that live in the soil or make us ill—are bacteria. Archaea are found
not only in these habitats but also in environments that are too hostile
for most other cells: concentrated brine, the hot acid of volcanic springs,
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Figure 1−13 A sulfur bacterium gets its
energy from H 2 S. Beggiatoa, a prokaryote
that lives in sulfurous environments, oxidizes
H 2 S to produce sulfur and can fix carbon
even in the dark. In this light micrograph,
yellow deposits of sulfur can be seen inside
two of these bacterial ECB5 e1.12/1.13 cells. (Courtesy of
Ralph S. Wolfe.)