Tag: photosynthesis in higher plants

The plant needed enough energy to do the biosynthetic reactions. Its reaction centre is a molecule called P680 which absorbs light maximally at 680 nm.

Z scheme of light reaction is non cyclic fow of electron from PSII through quinone, cyt b6b6 f complex and plastocyanin to NADPH via PSI; it does not return to PSII. NADP reductase enzyme is located on stromal side of PSI; Z scheme includes both PSI and PSII. Absence of PS-II leads to cyclic photophosphorylation in which electrons are passed from PS-I, via electron transfer chain, to back to PS-I with formation of ATP. Cyclic photophosphorylation does not pass electrons to NADP; rather electrons are passed back to PSI. PS I is involved in both cyclic and non cyclic photophosphorylation. Correct answer is B.

Two main photosynthetic pigment chlorophyll a and chlorophyll b absorb light in violet blue (400 -500 nm) and red (620-700nm) region of visible spectrum. Neither of these pigments absorbs light between 500 to 600 nm (green light) causing least photosynthesis in this wavelegnth. Thus, the correct answer is option B.

Photosynthetic bacteria are anaerobic and only one type of photosystem is found in them but cyanobacteria or blue-green algae have both systems PSI and PSII.  It has two types of pigments bacteriochlorophyll which differs from chlorophyll a, in having one pyrrole ring with two hydrogens and bactericidin.

PSI passes electron to ferredoxin on stromal side of lumen. Ferredoxin (iron sulfur protein) receives electrons from PSI and reduces to NADP+ to form NADPH. This light reaction is catalyzed by the membrane-bound enzyme NADP reductase. Both B and C are correct answers. 

Photosynthetic bacteria are anaerobic and only one type of photosystem is found in them but cyanobacteria or blue-green algae have both systems PSI and PSII. In PS I contain only one chlorophyll that is chlorophyll a while PS II contains various yellowish carotenoids and phycocyanin that differ in colour in different species.

Photosystems are the functional and structural unit of photosynthesis that contains chlorophyll and various electron carriers that helps in energy liberation.

Photolysis of water is carried out to fulfill the need of electrons that are given by photosystem II to photosystem I in the Z scheme. The splitting of water is associated with PSII in which water is split into H$^+$, [O] and electrons and this also creates oxygen, one of the net product of photosynthesis.

The constituents of PS I is located in the thylakoid membrane of stroma and the outer surface of grana, contain a number of light-harvesting complexes and a reaction centre complex, which is a protein complex with two special chlorophyll a molecules and a primary electron acceptor.

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of co-enzymes and co-factors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.