Photosynthesis in Plants

Cyclic photophosphorylation is carried out by PS I only. This process takes place in stroma lamellae membrane. An external source of electrons is not required. Photolysis of water does not take place. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It does not involve the formation of NADPH. It operates under low light intensity, anaerobic conditions or when $CO _{2}$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to the cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NADP^{+}$ and is cycled back to the PS I complex through the electron transport chain.

The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compound. One of the first steps in this complex process depends on chlorophyll and other pigment molecules. Chlorophyll is the green pigment molecule that make plants appear green. In photosynthetic plant cells, chlorophyll molecules are embedded in stacked membranes (thylakoids) contained in special membrane-bound organelles called chloroplast. The chlorophyll molecules are arranged in discrete units called photosystems. Each one of it contains hundreds of pigment molecules arranged into an "antenna complex" surrounding a reaction center.

Since carotene (alpha and beta), chl A, chl B are all found in both photosystem, the one which is only present in PS II is xanthophyll.

Photosystem II has P680 reaction center wherein oxygen atoms of two water molecules bind to a cluster of manganese atoms. These Mn atoms are bound to the reaction center and associated with an enzyme that splits water, removes electrons one at a time. These electrons fill the holes which are left in the reaction center due to released to light-energized electrons. Immediately after removal of four electrons from the two water molecules, O2 is released. The absence of PS II results in cyclic photophosphorylation by PSI that does not have water splitting complex and no oxygen is released. Phytochromes are blue-green, proteinaceous pigments that are involved in physiological responses to light. Phycocyanin is the photosynthetic pigment of cyanobacteria. Thus, the correct answer is option B.

Chloroplasts are eukaryotic cell organelles. They are covered by the double membrane and also have an internal membrane system comprised of thylakoid membranes. The thylakoid membranes are of two types. The membranes which make grana are called granal lamellae. The other thylakoid membranes are called stromal membranes . The non-appressed membranes (stromal membranes and edges of granal membranes) are in direct contact with the stroma. The PS- I is abundant in non-appressed membrane whereas PS- II is abundant in granal membranes.

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 

PS II (Photosystem II) is associated with the P680 reaction center, not P700. P700 is the reaction center for PS I (Photosystem I).

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.