Question

why the addition of the uncouple ammonium chloride changes the rate of electron flow

Answer 1

The rates of electron transport in the thylakoid membranes of isolated, illuminated chloroplasts are enhanced in the presence of ammonium chloride. Uncoupling reagents like ammonium chloride prevent the formation of a proton gradient across the thylakoid membrane and consequently remove a constraint on the rate of electron transport.

In intact chloroplasts, the potential energy in the proton gradient is used to convert ADP and P_i to ATP as the protons return to the stroma through the CF₀CF₁ complexes. Consequently, photophosphorylation (ATP synthesis) is said to be coupled to electron transport in the thylakoid membrane. Reagents such as ammonium chloride are called uncouplers because they dissipate the proton gradient leaving no energy for ATP synthesis. In solution, the NH₄⁺ ion dissociates to NH₃ (ammonia) and H⁺. Because it is not charged, the ammonia freely crosses the membrane to the lumen, where it reassociates with protons to again form an ammonium ion. As a result, free protons are sequestered in the lumen, and no gradient is generated. When a steep proton gradient is present, it is more difficult for the reduced plastoquinones within the membrane to become oxidized because they have to give up the protons against the concentration gradient. As the proportion of reduced plastoquinones in the membrane increases, there are fewer oxidized plastoquinones to accept electrons from PS II, so the rate of electron efflux from PS II declines. This is probably the step that limits the rate of electron transport through the redox pathway at light saturation because when the proton gradient is abolished, the rate of electron transport can increase dramatically.