A level biology question - ammonium hydroxide

I suspect you are doing the AQA practical 8. The wording of this practical is very confusing. The action of ammonium on photosynthesis is complex and not just the inhibition of an enzyme as indicated in the title of the practical. It is probably the ammonium ions interrupt the action of photosystem (PSII) by destroying the manganese cluster of the oxygen evolving cluster. This then prevents the splitting of water so there are no electrons available to return PSII to its ground state (PSII donates electrons to PSI). What you need to understand from this practical is that DCPIP is used as an electron acceptor instead of NADP. You can follow this process as DCPIP is decolourised when it is reduced. Ammmonium ions interrupt the production of electrons (i.e. it diminishes the reducing activity of the chloroplasts) and so will slow the rate of DCPIP decolourisation. What I find a little sad is that no mention is made of this being an investigation of the Hill reaction as this should lead students on to investigate how this part of photosynthesis was elucidated.

Hope that helps.