Tag: photosynthesis in higher plants

The oxygen evolution is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere.

In the light reaction of photosynthesis, chlorophyll molecule absorbs light energy, results in the splitting of water molecules, results in the release of protons, electrons and nascent oxygen. This process is called Photolysis of water. It occur in the lumen of thylakoid.

So, the correct option is ‘photolysis of water’

Photolysis of water occurs at grana i.e., lumen side of grana thylakoid membrane with the help of water splitting complex or OEC (oxygen evolvong complex). This step is associated with PS- II of Z- scheme.

This part of photosynthesis occurs in the granum of a chloroplast where light is absorbed by chlorophyll; a type of photosynthetic pigment that converts the light to chemical energy. This reacts with water (H2O) and splits the oxygen and hydrogen molecules apart. In the first stage of photosynthesis light energy is absorbed by chlorophyll in plant cells. This light energy is then used to produce ATP and to split water into hydrogen and oxygen. The splitting of water is referred to as photolysis. 

Photosystems are functional and structural units of protein complexes involved in photosynthesis, that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. There are 2 kinds of photosystems - photosystem I and II. In photosystem I, the reaction center is P-${ _7}$${ _0}$${ _0}$. In photosystem II, the reaction centers are P-${ _6}$${ _8}$${ _3}$. Thus, the correct answer is option A. 

P$ _{680}$ is the primary donor present in photosystem II. Structurally,  it's chlorophyll pigment dimer is present at the center of photosystem II. 680 suggests that the absorption is maximum at 680nm in red light. P$ _{680}$ or primary donor receives energy either by absorbing light or by excitation of electrons present in nearby chlorophyll. The excited electrons get captured by electron acceptor present in photosystem II, which is pheophytin and oxidized P$ _{680}$ is then reduced by electron generated from water during oxygenic photosynthesis.

Ferredoxin is the iron-containing, soluble compound in chloroplasts that helps in electron transportation and is the constituent of PS I which asses electrons to reductase complex that helps in the reduction of NADP$^+$ to NADPH which is a strong reducing agent.

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.