Biomedical Engineering Reference
In-Depth Information
Archaeplastida is the superclass of the kingdom of plants. As the story
goes, oxygenic photosynthesis was discovered by cyanobacteria, about
2.5 billion years ago. Subsequently, the cyanobacteria refined the process,
and no organism other than cyanobacteria can be credited with oxygenic
photosynthesis (though other organisms use photosynthetic processes that do
not produce oxygen). All oxygenic photosynthesis in eukaryotes is accom-
plished with chloroplasts, symbiotic organelles descended from a captured
cyanobacteria. An ancient member of Class Archaeplastida found that by
engulfing a cyanobacteria, it could photosynthesize. This indentured relation-
ship between Arachaeplastida and cyanobacteria became permanent, and
every descendant of Class Arhaeplastida, including all the green plants, have
benefited from their ancestor's theft of a cyanobacteria. It is generally
believed that the acquisition of a cyanobacteria as a self-replicating
synthesizing organelle, occurred only once in earth's history. Chloroplast-
containing organisms other than those within Class Archaeplastida, seized
chloroplasts (not cyanobacteria) from green algae or other members of Class
Archaeplastida.
How do we know that chloroplast-containing organisms outside Class
Archaeplastida acquired chloroplasts from other eukaryotic organisms, and
not from cyanobacteria? By counting membranes around the chloroplast.
Chloroplasts in the Archaeplastida are lined by two layers, corresponding to
the inner and outer membranes of the original, indentured cyanobacteria.
The chloroplasts of non-Archaeplastida eukaryotes have three or four mem-
brane layers, suggesting that a member of Archaeplastida was engulfed,
and the membranes of the chloroplast and the Archaeplastida were entrapped
permanently in the host cell.
As aforementioned, the creation of a chloroplast, from a captured cyano-
bacteria, occurred once only, with all extant chloroplasts deriving from an
early union between a member of Class Archaeplastida and a cyanobacte-
rium. This assertion may be false,
in one specific instance.
Paulinella
chromatophora
, a member of the eukaryotic class Rhizaria, seems to have
captured its own cyanobacteria and created its own permanent chloroplast-
like organelle [86]. This conclusion is based, in part, on the dissimilarities
between the photosynthesizing organelles of
Paulinella chromatophora
, and
all other Eukaryotic chloroplasts.
Chloroplasts can be acquired as a temporary symbiont through a process
called kleptoplasty. The kleptoplastic cell captures a chlorplast from an algae
and uses the captured chloroplast for a short period (a few days to a few
months) until the chloroplast degenerates. Fresh chloroplasts can be obtained
by more of the same kleptoplastic behavior. Permanent chloroplasts are
never found in Class Animalia. Thanks to kleptoplasty, one member of Class
Animalia, the sacoglassan sea slug, has achieved a photosynthetic life style.
In the past, the algae were all classified as types of plants, primarily
because they contained chloroplasts, and they looked more like plants than
