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Photosynthesis - Coggle Diagram
Photosynthesis
Light reaction
Photosystem II
chromophores absorb light (roughly 680 nm),which is then directed to a specialist chlorophyll which generates a high energy electron, energy of chlorophyll a molecule increases by 42 kcal/mol, which donates electron to adjacent chlorophyll
the oxygen evolving complex splits water (meaning the complex has a more positive reduction potential than forming water: +1.2V)
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electrons are moved from the oxygen evolving complex and from the excitation of light by plastoquinone to the cytochrome b/f complex
plastocyanin then moves the electrons to photosystem I (which can also absorb light which increases energy of the electrons since lots lost for cytochrome)
the electrons are moved onto ferredoxin, then are used to reduce NADP+ to NADPH using ferredoxin NADP reductase, ehich use the energy in the electrons to drive the synthesis of NADP+
the energy of the reduced quinone provides the energy for the cytochrome bf to drive across the thylakoid membrane from the quinone and from the stroma to generate a proton-motive force
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Calvin cycle
Rubisco
takes CO2, H20 and ribulose 1,5-bisphosphate and converts it to a hexose sugar
it splits the ribulose 1, 5-bisphosphate and converts it to 2 molecules of phosphoglycerate
it can also react with oxygen to form an unusable molecule so the reaction is completed in anaerobic conditions (likely to be a carboxysome)
activation
it is activated when CO2 binds to lysine side chain in active site forming a carbamate group that binds to Mg2+
normal CO2 levels are too low so catalysed by rubisco activase which uses ATP to clear active site of inhibitors
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phosphoglycerate kinase uses ATP hydrolysis to make 1,3-bisphosphoglycerate
this is turned into glyceraldehyde 3-phosphate using glyceraldehyde 3-phosphare dehydrogenase which uses NADPH
1 glyceraldehyde 3-phosphate is added to the glucogenesis pathway which eventually makes glucose 6-phosphate
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Phosphoribulokinase uses ATP and ribulose 5-phosphate to make ribulose 1,5-bisphosphate
Chloroplasts
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genome
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shows less sequence diversity than mtDNA since event that gave rise to chloroplasts was more recent than mitochondria
many genes essential for chloroplast function have been transferred to nuclear genome, and these genes are synthesised on cytosolic ribosomes and then incorporated into chloroplast
chloroplast DNA can be manipulated by transformation or by CRISPR/Cas9 to engineer resistance to infection
photosystems
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photosystem I
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structure
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has 3 subunits consisting of 12 polypeptides, 96 chlorophyll molecules, 22 carotenoids, several phyloquinones and other components
most chlorophyll molecules are called antenna pigments that collect light energy and conduct it to the reaction centre (the site where an electron is excited and transferred through various steps to ferredoxin)
chlorpohyll are highly modified tetrapyrrole ring with a central MG2+ ion and an a polar phytol side chain
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the electrons are transferred to a quinone and then iron-sulphur complex, then to ferredoxin which transports the electrons to NADP
if there is no NADP, the electron can be recycled to the bf complex
photosystem II
absorbs light and generates high energy electrons which move down the ETC until it reaches photosystem I, along the way teh bf complex has pumped protons
also forms O2, H+ and electrons from H2O
Bacteriorhodsopin
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light
In the absence of light it is in all-trans retinal form but when it intercepts light it bends and turns to 13-cis retinal
This kink in structure opens the channel allowing protons to come through, it pushes a proton from the cytoplasm into the environment
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Vision uses uses a similar mechanism, such that G protein sequences opens ion channel openings that provide signals and there is a chloride transporter in bacteria that uses a similar mechanism
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