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Q.
Which is an electron carrier common to both photosynthesis and respiration?
Delhi UMET/DPMTDelhi UMET/DPMT 2006
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, $ PQ_{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.