Photosynthesis is a process by which organisms
containing chlorophyll i.e. green plants, algae, and some
bacteria they capture energy in the form of light and convert
it to chemical energy. Virtually all the energy available
for life in the Earth's biosphere which the zone in which
life can exist.
A quite generalized, unbalanced chemical equation for photosynthesis
+ 6H2O + 6O2
Above the arrow we have to write Light and below we have
to write chlorophyll.
Photosynthesis consists of two stages - a series of light-dependent
reactions that do not depend on temperature and a series
of temperature-dependent reactions that are light-independent.
The rate of the first series, called the light reaction,
can be increased by increasing light intensity (within certain
limits) but not by increasing temperature. In the second
series, called the dark reaction, the rate can be increased
by increasing temperature (within certain limits) but not
by increasing light intensity.
The first step in photosynthesis is the absorption of light
by pigments. Chlorophyll is the most important of these
because it is essential for the process. It captures light
energy and transforms it into chemical energy through a
series of reactions. Different forms of chlorophyll and
other pigments known as carotenoids absorb slightly different
wavelengths of light and pass the energy to a form of chlorophyll
called chlorophyll A for the completion of the transformation
process. These accessory pigments thus broaden the spectrum
of light energy that can be fixed through photosynthesis.
Photosynthesis takes place within cells, in organelles
called chloroplasts (in the leaves of plants) that contain
the chlorophyll and other chemicals, mainly enzymes, necessary
for the various reactions. The chemicals involved are organized
into units of the chloroplasts called thylakoids, and the
pigments are embedded in the thylakoids in subunits called
photo systems. Light is absorbed by the pigments, raising
their electrons to higher energy levels.
Two photo systems., numbered I and II, are recognized.
Light energy is first trapped by photo system II, and the
energized electrons are boosted to an electron receptor.
They are replaced in photo system II by electrons from water
molecules, and oxygen is released. The energized electrons
are passed along an electron transport chain to photo system
I, and energy-rich adenosine triphosphate, or ATP, is generated
in the process. Light energy absorbed by photo system I
is then passed to its reaction centre, and energized electrons
are boosted to its electron acceptor.
The dark reaction takes place in the stroma (matrix) of
the chloroplast, where the energy stored in the ATP is used
to reduce carbon dioxide to organic carbon. This is accomplished
through a series of reactions known as the Calvin cycle,
driven by the energy in the ATP . At each turn of the cycle
one molecule of carbon dioxide enters and is initially combined
with a five-carbon sugar called RuBP to form two molecules
of a three-carbon compound called PGA . Three turns of the
cycle—each of which consumes one molecule of carbon
Were chemists able to duplicate photosynthesis by artificial
means, resulting systems would have enormous potential for
tapping solar energy on a large scale. Much research is
now being devoted to improve this effort. An artificial
molecule that remains polarized sufficiently long to react
usefully with other molecules has not yet been perfected,
but its prospects are promising.
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