are prokaryotes, as are the archaea.
This means that they are small
organisms, with no nucleus. They
are very tiny -- and very tough.
Although bacteria reproduce by binary
fission, and so change slowly, over the
millenia they have adapted to almost
every possible environment on
earth. They are in the soil, in
the air, in the water. They are
found in the digestive tracts of animals
and people, in our mouths, on our
skin. Sometimes they cause
disease, but most bacteria are not
harmful to us.
One very important group of bacteria is
the cyanobacteria. They are found
as the earliest bacterial fossils, and
today still look very like their ancient
ancestors. Cyanobacteria release
oxygen into our water and
atmosphere. They gradually added
oxygen to the atmosphere on
earth. As a result, complex
life forms were able to
Bacteria make major contributions to the
produce oxygen by
photosynthesis, in ponds and streams
and in the cells of plants, where they
have become the chloroplasts in the
bacteria become decomposers, breaking
down dead leaves, twigs, and other
organic material so that the molecules
can be recycled by other life forms.
bacteria are nitrogen fixing.
The bodies of our life forms are built
up almost entirely of hydrogen,
carbon, oxygen and nitrogen.
These are plentiful elements with
small atoms, but they have to be put
together. The plants capture
carbon from the air and build it into
sugars. Nitrogen, however, is
difficult to capture in a useable
form, and it is nitrogen fixing
bacteria that can do this.
Leguminous plants, such as peas and
beans, have special nodules in their
roots where nitrogen fixing bacteria
can live, These plants are often
used in crop rotation, because the
leave the soil enriched with nitrogen
in a form that plants can use.
Bacteria have mechanisms for surviving
difficult times. Some kinds of
bacteria can form spores, which are hard
little packets that enclose their
DNA. When the conditions that they
are living in make life very difficult,
the bacteria enclose their DNA in a
spore instead of just dying. The
spores are very tough. When conditions improve, the
bacterium goes back to being actively alive. No one knows how long
spores can live. Spores recovered
from ice or even amber may have survived
for millions of years. Scientists
are continuing to work on this
Each domain consists of groups of life
forms that are related to each other and
have characteristics in common. We
are looking at these domains now because
they are the most fundamental divisions
of life forms. These domains
describe the characteristics of every
living creature that we know
about. Learning about them is an
important part of this unit.
the domain that you would like to read
1996,1997, 1998, 1999,
2000, 2002, 2003.
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