Photosynthesis in Plants

Light reactions occur inside thylakoids. It is dependent upon light. It involves photolysis of water (breaking of water into $H _{2}$ and $O _{2}$  and production of assimilation power (NADPH and ATP). Electron released during photolysis of water are  picked up by $P _{680}$ photocentre of photosystem II. From here electrons passes over series of carrier which include PQ, cytochrome b-f complex, and PC. While passing over cytochrome complex, the electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron losses sufficient energy for creation of proton gradient and ATP from ADP and inorganic phosphate. From PC electron is picked by the trap center $P _{700} $of phtotsystem I, which pushes out electron after absorbing light energy. Electron passes over carriers Fes, ferredoxin and NADP reductase which gives electron to $NADP^+$ for combing with $H^+$ to produce NADPH.
$NADP^+ 2e^- + H^{+} \xrightarrow[NADP \,\, reductase]{} NADPH $ , So, the correct answer is 'ATP, hydrogen and O2'.

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis and splitting of water is associated with it. Water is split into H+, [O] and electrons, creating O$ _{2}$ which is one of the net products of photosynthesis. The electrons needed to replace those removed from PS I are provided by PS II.

A photosynthetic reaction centre is a protein that is the site of the light reactions of photosynthesis. The reaction centre contains pigments such as chlorophyll and phaeophytin. These absorb light, promoting an electron to a higher energy level within the pigment which are located at centre of photosynthetic apparatus. so, 'the correct option is - It is located at centre of photosynthetic apparatus'.

• A very small part of the electromagnetic spectrum can be seen by the human eye i.e., between the wavelengths 390 and 780 nm.
• This part of the electromagnetic spectrum is called a visible light spectrum.
• Within the spectrum the longer the wavelength of the radiation, the slower the vibration of the photons and the less energy each photon contains. 
  Hence, correct option is B.

Photosystem- II is involved in non-cyclic photophosphorylation with a reaction center P${ _6}$${ _8}$${ _0}$. The electrons excited from P${ _6}$${ _8}$${ _0}$ are accepted by plastoquinoine. 

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the grana of the thylakoid membrane of plants, algae, and cyanobacteria.

Photosynthetic bacteria posses only one type of photosystem and are mostly anaerobic and contains two type of pigment bacteriochlorophylls and bacterioviridin while cyanobacteria or blue-green algae posses two photosystems  PS I and PS II.

Photosystem I is one of the two membrane-bound photosystems of plants, algae and cyanobacteria that facilitate light-determined electron transport from water to NADPH. It utilizes absorbed light for electron transport from plastocyanin on the lumenal side to ferredoxin on the stromal side of the thylakoid membrane. In plants, this special integral membrane complex consists of more than 15 protein subunits, approximately 175 chlorophyll molecules, 2 phylloquinones and 3 Fe$ _4$S$ _4$ clusters. Whereas Photosystem II (of cyanobacteria and green plants) is as many as 35 chlorophyll a, 12 beta-carotene, two pheophytins, two plastoquinone, two heme, one bicarbonate, 20 lipid molecules and other ionic clusters. Thus Photosystem I contains more chlorophylls and accessory pigments.

DCMU or Dichlorophenyl dimethyl urea inhibits PS II in photosynthetic plants by blocking electron transfer from plastoquinone to cytochrome. DCMU binds to and blocks the site of plastoquinone, thus hinders the path and growth of plants. It is used as herbicide and algicide. It is also used in studying photosynthetic activity.

Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H⁺ + 1/2 O₂ + 2e⁻ by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H⁺and 1/2O₂ are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin. The deficit of electrons is replenished by taking electrons from another molecule of water. 

Photosystems refer to PSI and PSII. Photosystem II contains chlorophyll a, as well as up to 50% chlorophyll b. It probably evolved later as a supplement to Photo I. It is needed to capture enough energy to do the biosynthetic reactions of the dark reaction. Its reaction centre is a molecule called P680 which absorbs light maximally at 680 nm. Similarly, PSI or P700 absorbs light at 700nm. A reaction centre comprises several (>10 or >11) protein subunits, that provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters and electron acceptors for transduction in the electron transport chain. 

Leghemoglobin supports the centralization of free oxygen in the cytoplasm of contaminated plant cells to guarantee the best possible capacity of root knobs. Its capacity is to help give oxygen to the breathing harmonious bacterial cells in root nodules of leguminous plants in a way practically equivalent to hemoglobin transporting oxygen to breathing tissues in creatures.

• Leghaemoglobin is an oxygen carrier and hemoprotein found in the nitrogen-fixing root nodules of leguminous plants. 
• Leghaemoglobin reduce the amount of free oxygen, and thereby to protect the nitrogen-fixing enzyme nitrogenase, which is irreversibly inactivated by oxygen.
• Hence The pigments essential for $N _2$ fixation by leguminous plants is leghaemoglobin is a true statement.
• So, the correct answer is 'True'.