Question

Can you Please Solve this ASAP

4. Answer the following questions about photoinhibition and the xanthophyll cycle:
a) What is photoinhibition? How are reactive oxygen species (ROS) involved in photoinhibition?
b) How can photoinhibition “protect” the photosynthetic apparatus from further damage?
c) How does the xanthophyll cycle operate? How do xanthophylls contribute to the defense against excess light?

Answer 1

a) Ans- Photoinhibition is defined as the light dependent decrease in photosynthetic rate which may occur whenever the photon flux is in excess of that required for photosynthesis. In Photoinhibition under high light , ROS production is unavoidably associated with energy transfer and electron transport in PSII. In the energy transfer, the singlet oxygen is formed by the energy transfer from triplet chlorophyll to O2. In electron transport, ROS are formed by the consecutive one electron reduction of O2 and by the converted H2O on the PSII electron acceptor and donor sides respectively. Under moderate stress, when scavenging system maintains ROS level low , ROS serves as signalling molecules which activate on acclimation response and programmed cell death.

b) Ans- The very efficiency of light driven electron transfer process poses considerable problems for the stability of the photosynthetic apparatus. Photoinhibition occurs continuously when plants or cyanobacteria are exposed to light and the photosynthesizing organisms must therefore, continuously protect further damage occuring in chloroplasts and cyanobacteria consists of degradation and synthesis of D1 protein followed by activation of the reaction centre. Elicited by a relatively low luminal pH plants have developed a rapid response to excess energy by which it is given off as heat and damage is reduced.

c) Ans- The xanthophyll cycle operates by involving the enzymatic removal of epoxy groups of xanthophylls to create epoxidized xanthophylls. These enzymatic cycle found to play a key role in non photochemical quenching that reduce the amount of energy that reaches the photosystemic reaction. Three carotenoids pigments are involved in xanthophylls cycle which plays a direct photoprotective role acting as a lipid protective anti-oxidant and by stimulating non photochemical quenching within light harvesting proteins. Xanthophylls play a role in the non-radiative dissipation of excess absorbed light energy measured as non photochemical quenching of chlorophyll fluorescence. Mutants with a defect of either alpha or beta branch of carotenoids biosynthesis exhibited less non photochemical quenching but we're still able to tolerate high light. The LHCs absorb and transfer excitation energy to be photosynthetic reaction centres to drive electron transport , these reactions convert light energy into chemical energy that is used to fix atmospheric CO2 into sugars.