Encyclopedia of Evolution.pdf - Online Reading Center

acteria, evolution of Of all the cells and organisms in the
world, bacteria and archaebacteria are the: (1) smallest, (2)
simplest, (3) most numerous, (4) most metabolically diverse,
and (5) oldest. Although for many decades scientists classified
archaebacteria in the same category as bacteria, most now recognize
archaebacteria to be an evolutionary lineage as distinct
from the bacteria (also called eubacteria, or true bacteria) as
they are from the complex organisms (see tree of life).
Smallest. The typical eubacterial or archaebacterial cell
is about 10 times smaller than the typical cell of complex
organisms, in each dimension; therefore the typical bacterium
has a volume a thousand times smaller than that of a typical
cell of a complex organism. There are exceptions. The largest
known bacterium is Thiomargarita namibiensis, which is onetwenty-fifth
of an inch (1 mm) in diameter, which is larger
than many complex cells.
Simplest. Eubacteria and archaebacteria are prokaryotic,
which means they do not have a nucleus (Greek, karyo).
Human, plant, fungus, and protist cells are eukaryotic (see
eukaryotes, evolution of) because they have a nucleus.
Most eubacteria and archaebacteria have:
• a cell wall. In eubacteria, the wall consists mostly of a
combination of protein and polysaccharide called peptidoglycan.
Because the Gram stain indicates the presence of
peptidoglycan, the Gram positive bacteria are those with a
thick cell wall, and the Gram negative bacteria have thinner
cell walls. Archaebacteria do not have peptidoglycan walls.
• a cell membrane, which regulates the molecules that can
enter and leave the cell. The cell membranes of bacteria
resemble those of eukaryotic cells, but the cell membranes
of archaebacteria have a different chemical composition
from those of all other cells.
• cytoplasm, the liquid portion in which the molecules are
dissolved
• ribosomes, which make proteins. Bacteria and archaebacteria
have smaller ribosomes than those of eukaryotic cells.
B
Although bacteria and archaebacteria do not have the
complex structures found in the eukaryotic cell, they do carry
out most of the same chemical processes. Prokaryotes do not
have nuclei, but they do have DNA and use it in a manner
similar to eukaryotic cells (see DNA [raw material of evolution]).
Prokaryotes do not have mitochondria, but many
bacteria carry out the same reactions that mitochondria do.
Some bacteria have numerous internal membranes and others
have simple flagella that allow them to swim. Most bacteria
function as single cells, although some can form multicellular
aggregates. The shapes of bacteria are relatively simple: round
ones are called cocci, rod-shaped ones are called bacilli, and
spiral ones are called spirilla (if they have a central axis, they
are spirochetes).
Most numerous. Bacteria and archaebacteria are the
most numerous cells in the world. A typical spoonful of soil
can contain billions of bacteria.
Most metabolically diverse. Bacteria and archaebacteria
are the most diverse cells in the world, in terms of the metabolic
reactions that they perform. Even bacteria that have
similar appearances can have very different types of metabolism
(see table on page 38):
• Many bacteria obtain energy the same way that animal
cells do: by digesting organic molecules. Some of them
live in the presence of oxygen and release energy through
aerobic respiration, like most animal cells; some live in the
absence of oxygen and rely on fermentation (see respiration,
evolution of).
• Some bacteria (in particular, the cyanobacteria) use sunlight
energy to make food molecules in the same way that
plants do. They obtain electrons by splitting water molecules
(H2O), and release oxygen gas into the air (see photosynthesis,
evolution of).
• Some bacteria obtain energy using processes of which no
eukaryotic cell is capable. Some photosynthetic bacteria
use hydrogen sulfide (H2S) rather than water (H2O) as an