We've seen one way that plants convert light energy to chemical energy, but there is a more complicated system involving two photosystems. Here the electrons do not cycle back to the chlorophyll. This system is noncyclic. I'm going to try to explain this the way I do it in a lecture hall. I hope it works in this medium. Here goes!
To solve this problem, another photosystem is required.
We have now solved the problem of replacing electrons that left the chlorophyll of PsI. Remember, these electrons were used to reduce NADP to NADPH. We solved one problem, but now we have another one. You probably guessed what the difficulty is. Electrons from PsII are steadily moving to PsI. What do we do about the holes left in the chlorophyll of PsII?
The source of electrons for PsII is water! Water is split into H2 and oxygen.
OK folks, that's the whole system. Let's go through it one more time step by step starting with the splitting of water.
Water is split into hydrogen and oxygen. The hydrogen atoms are stripped of their electrons. The electrons move into the chlorophyll in PsII, leaving behind the protons (hydrogen ions) as part of the thylakoid gradient. The oxygen leaves as waste.
Light strikes PsII. Electrons are excited and leave the oxidized chlorophyll. The electrons travel down the ETS in the thylakoid membrane, losing energy as they go. Some of the lost energy is used to pump a proton into the proton gradient inside the lumen of the thylakoid. The electrons end up in the chlorophyll of PsI.
Light strikes PsI. Electrons are excited, leave the oxidized chlorophyll in PsI and travel down an ETS. The electrons use some of their energy to reduce NADP to NADPH.
The system is noncyclic. Electrons never return to the place from which they started.
The light energy which struck the chlorophyll has been stored as chemical energy in both ATP and NADPH.
Now I want to give you the entire process of noncyclic photophosphorylation in one animated diagram. There are some things to note as you view this picture. First of all the protons (H+) would all be moving around very rapidly, but this would make it impossible to follow the steps in the process. Secondly, the outer membrane of the chloroplast is not shown. Thirdly, the events actually happen simultaneously, not progressively as I have shown them. The animation is intended to demonstrate the steps in the process. As you watch it, think about what you have learned and follow each event as it occurs.
It is often convenient to study this in terms of what goes in and what comes out.
|One light event|
|Two light events|
|Water||Oxygen (as waste)|
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© copyright June B. Steinberg, 2000