Chloroplast
"Chloroplast use light to generate ATP and Sugars"
Plant cells and cells of other eukaryotic organisms that carry out
photosynthesis typically contain from one to several hundred
chloroplasts. Chloroplasts use light to generate ATP and sugars.
This bestows an obvious advantage on the organisms that possess
them: They can manufacture their own food. Chloroplasts contain
the photosynthetic pigment chlorophyll, which gives most plants
their green color.
The chloroplast, like the mitochondrion, is surrounded by
two membranes (figure below). However, chloroplasts are larger and
more complex than mitochondria. In addition to the outer and
inner membranes, which lie in close association with each other,
chloroplasts have closed compartments of stacked membranes
called grana(singular,granum), which lie inside the inner
membrane.
A chloroplast may contain a hundred or more grana, and
each granum may contain from a few to several dozen disk-shaped
structures called thylakoids. On the surface of the thylakoids are
the light-capturing photosynthetic pigments.Surrounding the thylakoid is a fluid matrix
called the stroma. The enzymes used to synthesize glucose during
photosynthesis are found in the stroma.
Like mitochondria, chloroplasts contain DNA, but many
of the genes that specify chloroplast components are also
located in the nucleus. Some of the elements used in photosynthesis, including the specific protein components necessary to
accomplish the reaction, are synthesized entirely within the
chloroplast.
Other DNA-containing organelles in plants, called
leucoplasts, lack pigment and a complex internal structure. In
root cells and some other plant cells, leucoplasts may serve as
starch-storage sites. A leucoplast that stores starch (amylose) is
sometimes termed an amyloplast. These organelles—
chloroplasts, leucoplasts, and amyloplasts—are collectively
called plastids. All plastids are produced by the division of
existing plastids.
Endosymbiosis
"Mitochondria and Chloroplasts arose by Endosymbiosis"
Symbiosis is a close relationship between organisms of different
species that live together. The theory of endosymbiosis proposes
that some of today’s eukaryotic organelles evolved as a consequence of a symbiosis arising between two cells that were originally each free-living. One cell, a prokaryote, was engulfed by and
became part of another cell, which was the precursor of modern
eukaryotes (figure ).
Possible origins of eukaryotic cells. Both
mitochondria and chloroplasts are thought to have arisen by
endosymbiosis, whereby a free-living cell is taken up but not
digested. The nature of the engulfing cell is unknown. Two
possibilities are shown. The engulfing cell (top) is an archaean
that gave rise to the nuclear genome and cytoplasmic contents.
The engulfing cell (bottom) consists of a nucleus derived from an
archaean in a bacterial cell. This could arise by a fusion event or
by engulfment of the archaean by the bacterium.
According to this endosymbiont theory, now widely accepted
by biologists, the engulfed prokaryotes provided their hosts with
certain advantages associated with their special metabolic abilities. Eventually many of the genes of the prokaryote transferred to
the host eukaryotic chromosome.
Two key eukaryotic organelles are believed to be the descendants of these endosymbiotic prokaryotes: mitochondria, which
are thought to have originated as bacteria capable of carrying out
oxidative metabolism, and chloroplasts, which apparently arose
from photosynthetic bacteria.
Posts
