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being passed to NADP from ferredoxin, may be passed to the cytochrome complex and then back to P700. Through this process, ATP may be generated, which is known as cyclic photophosphorylation, and electrons' flow begins and ends with P700. The only constant supply of oxygen gas in atmosphere is photosynthesis through diffuses out of the leaves into the atmosphere. The current 20% oxygen content in the atmosphere is the result of 3.5 billion years of photosynthesis. Cyanobacteria are the first photosynthetic organisms that began to accumulate oxygen, in ancient time, when there was no oxygen. Now most of the living organisms depend on oxygen for cellular respiration, and therefore, it is the energy that maintains life. The flow of electrons and moves from water to NADPH is powered by sunlight energy. The result of the light reactions is ATP and NADPH that are needed to drive the biochemical reactions or dark

12 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

The atmosphere considers carbon dioxide as the main source, which produces sugar through photosynthesis process. Carbon dioxide occupies approximately 0.035% of earth's atmosphere, which enters the leaf by diffusing through the stomata. Carbon fixation reactions, which reduces CO2 to the sugar, are sometimes known as Calvin cycle or C3 pathway. These reactions do not use light directly but utilize the ATP and NADPH produced through the light reactions sometimes referred as light-independent reactions or dark reactions. The dark reactions take place in stroma of chloroplast where many reactions were catalyzed by enzymes. This pathway was worked out by Melvin Calvin, in association with Andrew Benson and James Bassham, during the late 1940s and early 1950s. The pathway is named in honor of Calvin, who received a Nobel Prize for his work in 1961. The following discussion will be limited to the main events of the Calvin cycle. The end product of this pathway is the synthesis of a six-carbon sugar, which requires the input of carbon dioxide. To produce single molecule of a six-carbon sugar, it needs to join six molecules of CO2 by six turns of the cycle. The first process is the cooperation of CO2 with ribulose-1, 5-bisphosphate (RUBP), a five-carbon sugar with two phosphate groups. The enzyme ribulose bisphosphate carboxylase (RUBISCO) catalyzes carboxylation reaction. The most abundant protein on the Earth is RUBISCO (12.5–25% of total leaf protein). Carboxylation produces an unstable six-carbon transitional that immediately splits into two molecules of a three-carbon compound, with one phosphate group known as phosphoglyceric acid (PGA), or phosphoglycerate. In total, 12 molecules of PGA is yielded from six turns of the cycle, and these molecules of phosphoglyceraldehyde (PGAL) come from the conversion of about 12 PGA molecules. 12 NADPH and 12 ATP are required for this step, which supply the energy for this reaction. Six molecules of RUBP were regenerated from 10 of the 12 glyceraldehyde phosphate molecules in a complex series of interconversions that require six more ATP and allow the cycle to continue. Two molecules of glyceraldehyde phosphate are the net gain from six turns of the Calvin cycle; these are converted into one molecule of fructose-1, 6-bisphosphate, which is converted to glucose. The glucose produced is converted into starch, sucrose, or a variety of other products, thus completing the conversion of solar energy into chemical energy. All steps of photosynthesis process can be concluded in simple

reaction in the Calvin cycle [6].

2. The Calvin cycle (dark reactions)

Nimir Eltyb Ahmed Nimir1,2,3\* and Zhou Guisheng1,2

\*Address all correspondence to: nimir1000@gmail.com

1 Joint International Research Laboratory of Agriculture and Agri-Product Safety, The Ministry of Education of China, Yangzhou, China

2 Key Lab of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou, Jiangsu Province, China

3 Faculty of Agriculture, University of Khartoum, Khartoum, Sudan
