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Photosynthesis.


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 is


6CO2 + 12H2O =C6H12O6 + 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.

Light Reaction

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.

Dark Reaction

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 dioxide.

Artificial Photosynthesis

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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