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Ammonia as uncoupler of photosynthesis

Gary Cote gcote at radford.edu
Fri Nov 8 12:59:35 EST 2002


In my cell bio lab we do an experiment to test two compounds for their 
effects on light-dependent electron transport in chloroplasts.  One is
DCMU 
(diuron), an inhibitor of photosynthetic electron transport.  The other
is 
ammonia, an uncoupler.  The students are supposed to figure out which is 
which from their data.

There is no question that ammonia is an uncoupler, at least for the
better 
students.  However, my question is, how does it work?  I teach the
students 
that uncouplers collapse the proton gradient by moving protons across
the 
membrane.  DNP can move across the membrane both in the protonated and 
unprotonated (negative charged) form.  However, while ammonia should
cross 
membranes fairly easily, ammonium should not, being +charged.  The
students 
haven't called me on it yet, but I'd like to know the answer.  So far I 
haven't found anything in books.

Can ammonium cross membranes?  (From what I know of patch-clamping, no).

Does ammonia simply enter the thylakoids and soak up the protons, thus 
removing them from the gradient?

Do thylakoids leak ammonium?  (I understand they leak Mg2+ and Cl-, so 
there's no membrane voltage across them.)

Any enlightenment?

Gary

Dr. Gary Coté
Assistant Professor
Department of Biology
Box 6931
Radford University
Radford, VA 24142-6931

Ph: 540-831-5630
Fax: 540-831-5129
email: gcote at radford.edu
http://www.radford.edu:8800/~gcote/

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