Why does the reduction of the dye DCPIP work best using the chloroplasts whose outer envelopes have been damaged during preparation?

NADP is the cofactors in photosynthesis which loses electrons,becomes reduced(NADPH) in photosynthesis. It transfer  electrons  in reduced form(NADPH) into the stroma of the chloroplast. The H is splits into e- and H+

These electrons for the ETC chains, generates the PMF for pumping H+ as protrons into the thylakoid  intramembranes.

Outer membrane covers  the intramembrane space, between these two membranes layers. The protons in the intramembrane space generate the electrochemical gradient for the supply of  the energy used by ATPase synthase for ATP synthesis from phosphorylation with ADP.

The chloroplast is made up of both the outer and inner membrane,separated by the intramembrane space. The walls of the intramembrane space is guided by the outer envelope. 

Therefore with the outer envelope removed, the intramembrane is exposed,more protons(H+) are released, but can not be held in the outer envelope to form electrochemical gradients,beacause the intramembrane space is now permable.

Thus   the rate of reduction of DCPIP is faster.The  color changes to colorless increases when reduced , from blue color.

Explanation:

i think it's c