The Electron Transport
This section elaborates on the complex processes involved in the electron transport phase of photosynthesis, specifically focusing on how light energy is converted into chemical energy.
Overview of Photosystems
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Photosystem II (PS II): The process begins in PS II, where chlorophyll a absorbs light at 680 nm. This energy excites electrons, causing them to jump to a higher orbit. These electrons are then transferred to an electron acceptor and enter the electron transport system composed of cytochromes.
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Photosystem I (PS I): After passing through the electron transport chain, the electrons reach PS I, where they are re-excited by light at 700 nm before being transferred to another acceptor, NADP+, resulting in the formation of NADPH.
Splitting of Water
To sustain this flow of electrons, water molecules are split in a process associated with PS II, releasing O2, protons (H+), and replenishing electrons. The overall reaction can be summarized as:
2H2O → 4H+ + O2 + 4e−
Photophosphorylation
There are two types of photo-phosphorylation: non-cyclic and cyclic. In non-cyclic phosphorylation, both ATP and NADPH are produced when both PS II and PS I are operational. Conversely, cyclic photophosphorylation involves only PS I, where the electron returns to the same chlorophyll molecule, primarily producing ATP without NADPH.
Chemiosmotic Hypothesis
The ATP is synthesized based on the proton gradient created across the thylakoid membrane, resulting from the movement of electrons. The protons accumulate in the lumen of the thylakoids, and as they flow back into the stroma through ATP synthase, ATP is generated, supporting the biosynthetic processes in the stroma that require energy.
Conclusion
This electron transport process is vital for photosynthesis, as it converts light energy into chemical energy, specifically ATP and NADPH, which are essential for the subsequent reactions of the Calvin cycle where carbon fixation occurs.