Tag: photosynthesis in plants

Z scheme of light reaction is non cyclic fow of electron from PSII through quinone, cyt $b _6$ 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 non 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. Result is formation of ATP but no NADPH2. Stroma lamellae have PSI only but lack PSII and NADP reductse. Statements B an D are correct. Correct answer is A.

Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Reaction centre of it is 700 nm.

Photosystem II is the first link in the chain of photosynthesis. It captures photons and uses the energy to extract electrons from water molecules. First, when electrons are removed water molecules are broken into oxygen gas which bubbles away, and hydrogen ions, which are used to power ATP synthesis. Photosystem II is located in the thylakoids and stacks of thylakoids is known as grana. 

A photosystem is composed of chlorophyll 'a' molecule which can absorb the wavelength of 700 nm. The chlorophyll a molecule acts as a reaction centre. PS-I is involved in a non-cyclic type of photophosphorylation where there is a chain of electron carriers and acceptors that transmits the electron from one to another. The primary electron acceptor is the ferredoxin molecule which is composed of Fe-S protein

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.