Question

pls tell me the meaning of underlined sentences..... ( this is from the topic if electron transport).... see the picture

Answer 1

The Z scheme describes the oxidation and reduction changes during light reaction. The vertical line that is there in the image you have provided represents the reduction potential of a particular species. More negative the reduction potential is more easily it donates electrons. The electrons are not used  up as they pass the chain rather the electrons are removed from the water and it is passed on or donated to lower oxidised form of P680 (pigment of photosystem PS I. must be written in the book). Absorption of electron excites
P680 to P680* which now goes to more active reducing species that is it donates electrons to a  molecule NADP+. The addition of electrons reduces it to NADPH. Again this electron can go back in a cyclic form.

P.S : The redox potential is the affinity of a species towards electrons, as i have already written  more negative the reduction potential is more easily it can donate electrons and vice versa