Higher plants have evolved two photosystems to capture and transform the sun’s energy into the chemical energy stored in ATP and NADPH. Each photosystem contains a reaction center and an electron acceptor. The reaction center is where the electron transfer reaction occurs. The reaction center of photosystem II is called P680, and the reaction center of photosystem I is called P700. The reaction centers are named for the wavelength of light that causes the maximum loss of electrons from the reaction center pigments.

Chlorophyll and other pigment molecules are located in the thylakoid membrane of plant chloroplasts. The molecules are arranged into clusters called antenna complexes. To maximize the range of wavelengths that can be absorbed, the antenna complexes contain pigments that absorb different wavelengths of light. When photons of light strike molecules in the antenna complexes, their electrons get excited to higher energy levels. This excitation energy is transferred from molecule to molecule until it reaches the reaction center.

The reaction center contains special pigment molecules that trap the excitation energy. In the reaction center, an excited electron is transferred from a chlorophyll molecule to an electron acceptor. This is an oxidation-reduction reaction.

In photosystem II, the electron lost from the reaction center is replaced with an electron that comes from water. When two water molecules are split, four electrons and four protons are released. It requires a lot of energy to break the bonds in a water molecule—much more energy than a single photon of light contains. A manganese-containing complex in photosystem II absorbs four photons, one at a time, and passes four electrons, one at a time, back to the reaction center, P680. Four protons are pumped into the lumen of the thylakoid and, after the fourth photon is absorbed, molecular oxygen is released. The four electrons and the four protons originate from two water molecules.

The excited chlorophyll electrons are transferred from the electron acceptors of photosystem II to photosystem I through an electron transport chain. The soluble molecule plastoquinone, or PQ, acts as the electron carrier from photosystem II to the cytochrome b6f complex. The cytochrome b6f complex pumps protons into the thylakoid lumen as it transfers electrons from plastoquinone to plastocyanin.

Plastocyanin, or PC, is a one-electron carrier, soluble protein similar to cytochrome c in the mitochondria. Plastocyanin transfers electrons from the cytochrome b6f complex to the reaction centers of photosystem I.

In order for photosystem I to accept an electron from plastocyanin, it must first lose an electron. As in photosystem II, the electrons of antenna molecules of photosystem I absorb photons of light and become excited. The excitation energy is transferred to the P700 reaction center, which loses an electron to an acceptor. The electron-deficient P700 reaction center quickly acquires an electron from plastocyanin.

The P700 electron is passed through a series of electron carriers within photosystem I until it reaches it reaches ferredoxin. Ferredoxin is a soluble, iron-sulfur protein. The reduced ferredoxin delivers electrons to the coenzyme FAD in the ferredoxin-NADP reductase complex.

FAD can donate one or two electrons and makes an excellent bridge between one-electron donors, such as ferredoxin, and two-electron acceptors, such as NADP. The reduced form of FAD, FADH2, transfers two electrons to NADP+. To transfer two electrons from P700 to NADP, photosystem I must absorb two photons.

How many photons does it take to transfer four electrons from two water molecules to two NADP+ molecules? Enter your answer and click Submit.

Yes, that’s correct.

No, that’s incorrect.

Four photons are absorbed by photosystem II to split the water molecules, form molecular oxygen, and release four electrons. Four more photons are required to complete the transfer of electrons to two NADP+ in photosystem I. Chloroplasts must absorb eight photons to evolve one molecule of molecular oxygen.

These reactions are often described as the Z-scheme. The Z-scheme series of reactions are noncyclic. Electrons flow from water to NADP and molecular oxygen is evolved. The noncyclic flow of electrons produces one molecule of NADPH and one molecule of ATP for every two electrons that move through this transport chain.

Scientists observed that chloroplasts always absorbed more than eight photons per oxygen molecule produced. They found that not all the electrons flowed to NADP+, but some were returned via the cytochrome b6f complex. This flow is cyclic because the electrons are returned to the starting reactant. Only photosystem I participates in cyclic electron flow.

A photon of light is absorbed by the P700 reaction center and an electron is transferred from chlorophyll to the electron transport chain. Protons are pumped into the thylakoid lumen as the electrons spontaneously flow from the excited chlorophyll to molecules with higher affinities for electrons. The electron then returns to the electron-deficient chlorophyll in the P700 reaction center. During this cycle, no oxygen is evolved and no NADP+ is reduced. This cyclic flow of electrons generates the additional ATP needed to convert carbon dioxide into glyceraldehyde 3-phosphate.

Now let’s see how the protons in the lumen are used to make ATP.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education