CK12 Life Science

such as algae and plankton, but also giant seaweeds that can grow to be 200 feet long (an
alga protist is shown inFigure2.30). Plants, animals, fungi, and protists might seem very
different, but remember that if you look through a microscope, you would find cells with a
membrane-bound nucleus in all them.

Figure 2.30: This microscopic alga is a protist in the domain Eukarya. ( 21 )

The cells of the two other domains - theArchaeaand theBacteria- do not have a nucleus.
All the cells in both domains are tiny, microscopic one-celled organisms that can reproduce
without sex by dividing in two. The difference between the archaea and the bacteria is in
their cell walls. Also, archaea often live in extreme environments like hot springs, geysers,
and salt flats, while bacteria are abundant and live almost everywhere. A teaspoon of soil
can contain 100 million to a billion individual bacteria. Bacteria obtain energy in lots of
different ways. Some infect plants and animals and cause disease. Others break down dead
organisms. Thecyanobacteriaphotosynthesize, likeplants. Infact, theancestorsoftoday’s
cyanobacteria invented photosynthesis more than two billion years ago.

Table 2.2: Three domains of life: Bacteria, Archaea, and Eukarya

Archaea Bacteria Eukarya

Multicelluar No No Yes

Cell Wall Yes, without pepti-

doglycan

Yes, with peptido-

glycan

Varies. Plants and

fungi have a cell

wall; animals do not.

Nucleus (DNA in-

side a membrane)

No No Yes