A) Cytochrome
B) Phytochrome
C) Pheophytin
D) Plastocyanin
Correct Answer: A
Solution :
Cytochromes are proteins with characteristic strong absorption of visible light due to their iron-containing haem prosthatic groups. Like complex III (ubiquinone to cytochrome-c) of mitochondria, the cytochrome \[{{b}_{6}}f\] in chloroplast conveys electrons from a reduced quinone a mobile-lipid soluble carrier of two electrons (Q in mitochondria, \[P{{Q}_{B}}\] in chloroplasts) to a water soluble protein thatcarries one electron (cytochrome-c in mitochondria plastocyanin in chloroplast). In the photochemical reaction of photosystem II, the excited \[{{P}_{680}}\] transfer an electron to pheophytin within picoseconds giving it a negative charge. The pheophytin very rapidly passes its extra electron to protein bound plastoquinone. In photosystem-I, the excited \[{{P}_{700}}\] loses an electron to an acceptor \[{{A}_{0}}\] i.e. becomes \[{{P}_{{{700}^{+}}}}\]. The \[{{P}_{{{700}^{+}}}}\] is a strong oxidizing agent which quickly acquires an electron from plastocyanin (a soluble Cu containing electron transfer protein). Phytochrome is an amorphous photoreceptor chromoprotein or protein pigment which exists in two states i.e., blue and yellowish green, phytochrome is active above 550 nm. It regulates many developmental phenomenon in plants like germination of seeds, photomorphogenesis, photoperiodism etc.
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