Question

1. A number of herbicides have PS II or PS I as targets and interfere with the normal steps in the exchange of reduced electron carriers between PS II + PS I + cytochrome bf. One class of herbicides bind to subunit of PS II and are competitive inhibitors of ubiquinone.

a) What will be the consequences of that competitive inhibition, in terms of electron flow involving PS II?

b) Those herbicides have a deleterious effect on the plant because of an increase in inappropriate oxidation reactions carried out by activated chlorophyll. Do you think that is a direct effect (occurs within PS II) or indirect effect (chlorophylls that are part of PS I) of herbicide binding? Why do you think so? Another class of herbicides bind to PSI. These have different targets, but in all cases the redox reactions in PS I that involve electron movement between Fe 2+, ferredoxin and NADP+ are inhibited. As a consequence of this [Fe 2+] builds up and an electrons are transferred to O2 molecule.

c) What O2 species could be made from that electron transfer and why would that be detrimental?

d) Is there another reason why there would be build-up of Fe 2+ concentration?

Answer 1

A). Once an inhibitor binds with the PSII, photolysis preocess will be afftected and no more elctron will be available for cyclic reaction of PSI. Elctron flow will be stopped

B) Deleterious effect occurs indirectly due to the binding of herbicide to the PSII, This is beacuse, it assist the PSI for their electron flow properly. Once the elctron supply stops from PSII, electrons filling distrubs significantly. However, sudden death does not occur beacuse of the electron flow in cyclic electron transport system that keep the plant alive for some duration. During PSII inhibition, highly reactive molecules generate that indirectly dry the cell and disrupt the lipid and protein membrane.

C). H₂O₂ and O^2- are veing generated that attach on DNA and harmful mutation is responsible for deletrious effect.

D). Fe2+ enviornment is biould up during the herbicide inhibition because, anoxygeneic environment occur and allows the Fe2+ into Fe3+by ejecting one lectron into the Z scheme.

The extended question of section b is not clear...so many quaetion have been asked in b section creating confusion.