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LIGHT REACTION (PHOTOCHEMICAL PHASE) - Coggle Diagram
LIGHT REACTION (PHOTOCHEMICAL PHASE)
Electron Transport
Electrons get excited and are transferred to an electron acceptor when PSII absorbs red light of 680nm
The electron acceptor passes them to the electron transport chain (ETC) consisting of cytochromes
The electrons are then transferred to the pigments of PSI
Electrons in PSI also become excited when it absorbs red light of 700nm, and are transferred to another acceptor molecule
The electrons reduce NADP+ to NADPH+H+
Z scheme
Transfer of electrons from PSII to PSI and finally downhill to NADP+ is called as Z scheme
The name arose due to its zigzag shape which occurs as a result of the sequence in which the carriers are placed on a redox potential scale
Photolysis of water
The water splitting complex is in PSII's inner side of thylakoid membrane
PSII supplies electrons continuously by replacing lost electrons from water splitting
Thus, PSII provides electrons needed to replace those removed from PSI
Protons are used to reduce NADP to NADPH
Oxygen is liberated as by-product
Photo-phosphorylation
Synthesis of ATP from ADP by cells in the presence of light mitochondria and chloroplasts is called photo-phosphorylation
Non-cyclic photo-phosphorylation
Occurs when 2 PS work as a series through ETC
ATP and NADPH+H+ are synthesised
Electrons lost by PSII don't return, rater are passed on to NADP+
Cyclic photo-phosphorylation
Occurs in stroma lamellae where only PSI is functional
Electron is circulated within PSI and ATP synthesis occurs due to cyclic flow of electrons
Only ATP is synthesised
Occurs only when lights of wavelengths beyond 680nm are available
Chemiosmotic Hypothesis
Requires a membrane, proton pump, proton gradient, ATP synthase
Since, splitting of water occurs on the inner side of membrane, proton accumulation occurs in lumen
Primary electron acceptor in outer side of membrane transfers electron to H carrier.
So, this molecule removes a proton while transporting an electron
Protons, being necessary to reduce NADP+, are removed from stroma
Hence, protons in stroma decrease, but in lumen, protons are accumulated. This creates a proton gradient across the thylakoid membrane, decreasing pH of lumen
Breakdown of proton gradient leads to ATP synthesis by ATP synthase enzyme
ATP synthase enzyme
CF0
Trans-membrane channel
Carries facilitated diffusion of protons across membrane to stroma. This results in breakdown of proton gradient
CF1
Undergoes conformational change due to breakdown of proton gradient. This enables the enzyme to synthesis ATP