Photosynthesis

Non-cyclic electron flow also involves PS1 and occurs on the thylakoid membrane, but it begins with another integral membrane protein complex called Photosystem II.  The first step is similar in both cyclic and non-cyclic electron flow.  The reaction center in PS2 holds an electron that becomes excited.  However, in PS2, the optimal light is P680 as opposed to the P700 of PS1.  After the electron in PS2 becomes excited, it moves into the second stage.

 

The next step is for the electron to move down the electron transport chain.  There is a protein called the cytochrome complex that is responsible for two jobs.  The first is to transfer the excited electrons to PS1.  The other, which is also vital to photosynthesis, is to pump protons into the lumen.  Referring back to the dam example, this accomplishes the goal of creating a concentration gradient so that protons will flow out of the ATP synthase.

 

While the protons are freeing ATP by travelling out of the lumen through the ATP synthase, the excited electrons reach PS1.  They get re-excited by P700 light in the reaction center, and then travel to an enzyme called NADP+ reductase.  Here, the reductase combines NADP+ and H+ with the two electrons to form NADPH, which is another form of energy for the cell.  A good way to think of non-cyclic electron flow is a dam that still creates electrical energy, but also uses water to make chemical energy.  Non-cyclic electron flow creates not one but two forms of usable energy for the cell.  One molecule of NADPH holds about three times the about of energy of a molecule of ATP.