Noncyclic photophosphorylation

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!

First we'll start with the cyclic events we just learned, but with a fundamental difference.  This time electrons excited by light and leaving PsI will not cycle back into the photosystem.  Instead, they will reduce NADP to NADPH.  This is diagrammed in the animation to the left.  Although it occurs n the thylakoid. membrane, I have ignored that for the present to emphasize what happens.  We have now stored some of the energy lost as the electron traveled down the ETS as the reduced coenzyme NADP.  So, light went into the system and NADPH came out.   There is a problem, though.  If the plant continually does this, there will be holes in the chlorophyll of PsI as electrons continually leave and do not return.  How do these electrons get replaced?

To solve this problem, another photosystem is required. It is called photosystem II (PsII), because it was discovered after PsI.  This name is confusing, because it is the first photosystem in the entire noncyclic system.  Let's see what happens to electrons excited by light striking PsII.  The animation shows that electrons go from PsII to PsI.  As they pass along the ETS some of the energy they lose is used to pump a proton (hydrogen ion) into the lumen of the thylakoid.  Since the pumped proton will become part of the proton gradient inside the thylakoid, it will ultimately generate ATP.  This will happen as protons escape from the thylakoid interior into the stroma of the chloroplast through CF1 particles.  (Click here and review how this happened in the noncyclic scheme.)  To summarize, light entered the system and ATP was generated as electrons moved from PsII to PsI.

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.  The oxygen leaves as waste.  Remember, oxygen is a waste product of photosynthesis.  (As we will see later, however, plants need oxygen, as do other eukaryotic organisms, for cellular respiration.)  The two atoms of hydrogen are then stripped of their electrons leaving behind their protons as part of the proton gradient inside the thylakoid.  The two electrons enter the chlorophyll in PsII. 

OK folks, that's the whole system.  Let's go through it one more time step by step starting with the splitting of water.

  1. 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.

  2. 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.

  3. 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.

  4. The system is noncyclic. Electrons never return to the place from which they started.

  5. 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. 

Cyclic Photophosphorylation


One light event  

Noncyclic Photophosphorylation

In Out
Two light events
Water Oxygen (as waste)


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copyright June B. Steinberg, 2000