Light energy absorbed by these pigments of the antenna complex is "passed to the reaction center chlorophyll molecules from which it passes along an electron-transport chain" (Blankenship, 215). Photosystems II contains a kind of chlorophyll a (P680) which shows maximum light absorption at a wavelength of about 684 nm. When activated by sunlight, a pair of electrons become excited and leave photosystem II and is replaced by electrons from the photolysis of H2O, summarized as 2H2OO2+4H++4e-. Molecular oxygen is then released and the protons pass into the lumen (the central space that remains in a cell that has lost its living contents) of the thylakoid (an elongated, flattened fluid-filled sac that forms the basic unit of the photosynthetic membrane system). Then, the electrons pass through what is known as an electron transport chain (ETC) in the thylakoid membrane. It is here that the electrons are given more power by sunlight to create even higher energy levels.

They then pass through a second ETC which involves the protein ferredoxin, being a "group of red-brown proteins which contain non-heme iron with sulfur at the active site and function as electron carriers" (Gregory, 223). This extremely complex sequence which today's engineers are attempting to duplicate for use in solar-generated energy systems is also known as noncyclic electron flow (NEF) which is used to drive ATP synthesis (cyclic photophosphorylation). ATP synthesis which is linked to the non-cyclic electron transport chain (NETC) is known as non-cyclic photophorphorylation.

From this point on, the light reactions are utilized to reduce carbon dioxide to carbohydrate. Carbon dioxide is then fixed via a combination of with the 5-carbon sugar ribulose biphosphate (RuBP) which forms two molecules of phosphoglyceric acid (PGA), "a complex lipid similar to acylglycerides, the major foundation of the membranes of cells" (Blankenship, 216).

Thus, this reaction is catalyzed by the enzyme known as ribulose biphosphate carboxylase, and in a series of other reactions, PGA is converted to a succession of carbon sugar phosphates collectively known as the Calvin cycle, named after Melvin Calvin, "an American biochemist best-known for his experiments with the dark reactions of photosynthesis via radioactive carbon" (Bacon, 237). These by-products and chemicals are then used in the...

This is known as the carbon cycle in which plants are viewed as "carbon sinks that remove carbon dioxide from the atmosphere and oceans by fixing it into organic chemicals" via the influence of sunlight (Farabee, Internet). Conversely, other living things produce carbon dioxide from respiration and osmosis which in the end completes the carbon cycle.
However, due to recent findings concerning the so-called Greenhouse Effect, a.k.a. global warming and climate change, CO2 levels in the Earth's atmosphere are now higher than ever before in recorded history, due to the burning of fossil fuels like oil, petroleum and wood. Exactly how the Earth's atmosphere and the planet itself will ultimately be affected by this huge increase in CO2 remains unknown, but it is possible that the science of chemistry may eventually provide an answer by arriving at some type of system or process which could effectively eliminate a good amount of this built-up CO2 outside of actually ceasing the burning of fossil fuels.

Bibliography

Bacon, K.E. Photosynthesis: Photobiochemistry and Photobiophysics. New York:

Kluwer Academic Publishers, 2003.

Blankenship, Robert E. Molecular Mechanisms of Photosynthesis. New York: Wiley-Blackwell, 2002.

Farabee, M.J. "Photosynthesis." 2007. Internet. Retrieved April 9, 2008 at http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html.

Gregory, R.P.F. The Biochemistry of Photosynthesis. New York: John Wiley & Sons,

Silverstein, Alvin. Photosynthesis. New York: Lerner Publishing Group, 2003.