Question

Effect of DCMU and sodium azide on Chlamydomonas? We did an experiment where we had chlamydomonas untreated, with either DCMU or sodium azide, put it in like a chamber (half of the chamber is covered by aluminum so no light and the other side had light) and see what happened. Untreated Chlamydomonas move to either the side with light or side covered with aluminum and nothing happened to the sodium azide one but what does DCMU do?

Answer 1

DCMU is a very specific and sensitive inhibitor of photosynthesis. It blocks the plastoquinonebinding site of photosystem II, disallowing the electron flow from where it is generated, in photosystem II, to plastoquinone. This interrupts the photosynthetic electron transport chain in photosynthesis and thus reduces the ability of the plant to turn light energy into chemical energy (ATP and reductant potential).

DCMU only blocks electron flow from photosystem II, it has no effect on photosystem I or other reactions in photosynthesis, such as light absorption or carbon fixation in the Calvin cycle.

However, because it absorbs electrons oxidized from water in PS II, the electron "hole" of PS I cannot be satisfied, effectively shutting down "linear" photosynthesis by blocking the reduction of NADP⁺ to NADPH. At the same time, it was found that DCMU not only does not inhibit the cyclic photosynthetic pathway, but, under certain circumstances, actually stimulates it.