Tag: photosynthesis in higher plants

There are two photosystems PS I and PS II. Photosynthetic bacteria have only PS I and absence of PSII makes them carry out cyclic photophosphorylation wherein $H _2S$ serves as an electron donor, rather than water, and there is no release of oxygen. The low light intensity and anaerobic conditions favor cyclic photophosphorylation. The correct answer is A. 

There are two types of photosystems, the photosystem I (PS- I) and the photosystem II (PS II). These are the structural and functional protein components in the photosynthesis. The photosytem I is present in the green and purple bacteria that conduct the bacterial photosynthesis where as the photosystem II is present in the cyanobacteria that conduct the normal plant photosynthesis.

Ferredoxin is an iron-containing protein, associated with photosystem I. The ferredoxin accepts electrons from photosystem I and passes them to oxidized NADP to form reduced $NADPH _2$. The enzyme responsible for reduction of NADP is called as NADP ferredoxin oxidoreductase.

During photosynthesis light energy is converted to chemical energy of glucose. Light energy is absorbed by a pair of chl a molecules, of PS I or PS II, this excited chl a molecule releases one e$^-$, that moves through the ETS of the PSI and PSII releasing energy at each level during the transport, used for the formation of ATP 

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