Maximum Yield Australia/New Zealand March/April 2019 | Page 74

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FACTS ON
CHLOROPLASTS
by Philip McIntosh
No organelle is as closely
identified with what it
means to be a plant than
the chloroplast.
FROM GREEK, CHLOROS means green and plastos means formed.
Indeed, chloroplasts are well-formed small green entities that in some ways
look and act like miniature cells within the cell.
IT IS PROBABLY one of the most universally known biological facts that
plants are green because they contain chlorophyl, and chlorophyll is critical
for photosynthesis.
OF COURSE, CHLOROPLASTS contain chlorophyll,
but there is so much more to them than that. Estimates put the
number of proteins to be found at work in a chloroplast to
be between 3,000 to 5,000.
WHERE DO ALL those proteins come from?
Some come from the relatively small genome (120,000
to 150,000 base pairs) of the chloroplast itself, which
means the rest are coded for by the nuclear genome
and must be transported into the chloroplast.
AS IS TRUE for mitochondria, chloroplasts possess
their own DNA which is much like that of bacteria.
THAT AND OTHER evidence suggests that, as is also
the case for mitochondria, chloroplasts originated as prokaryotic
cells that were engulfed by early eukaryotes, eventually co-evolving
into a symbiotic relationship.
CHLOROPLASTS, UNLIKE MOST other organelles, possess two
membranes instead of the usual one. As you might guess, there is one other
organelle enclosed by a double membrane (hint: it starts with an m).
INSIDE THE CHLOROPLAST double membrane is an aqueous
liquid called the stroma. The stroma is where the Calvin cycle takes place.
THE CALVIN CYCLE is a series of light-independent reactions that
produce carbon compounds using carbon dioxide as a carbon source.
THE LIGHT-DEPENDENT REACTIONS take place in stacks
of flattened membranes called grana, where electromagnetic energy is
absorbed by chlorophyll and converted into chemical energy which is then
used to drive the Calvin cycle.
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