The lumen of a lit thylakoid is rich in protons…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The lumen of a lit thylakoid is rich in protons. (a) Name three things that make the lumen gain protons or the stroma lose them. (b) In which of the three does the stroma lose a proton and the lumen gain nothing? (c) What happens to the gradient when the light is switched off?
Need a reminder? The key ideaShow
Chemiosmosis: a membrane, a proton pump, a proton gradient and ATP synthase. Protons collect in the lumen (water splitting, the carrier that moves protons across, and NADP reductase using stroma protons). ATP synthase: CF0 is a channel in the membrane, CF1 sticks into the stroma and makes ATP. Each pH unit is a factor of ten in the proton concentration. The ATP and NADPH go to the stroma.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The three things. (i) Water is split in the lumen and sets protons free there. (ii) A carrier takes a proton from the stroma and gives it up in the lumen as it passes on an electron. (iii) NADP reductase uses a proton from the stroma to make NADPH.
Step 2. The one that only takes from the stroma. That is NADP reductase.
Step 3. In the dark. The light reactions stop, no more protons are added, and the protons flow out through ATP synthase until the gradient is gone.
Answer: (a) splitting of water, the proton-carrying carrier, NADP reductase; (b) NADP reductase; (c) the gradient fades away.
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