Photophosphorylation: Anoxygenic*# - Biology

Photophosphorylation: Anoxygenic*# - Biology

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Photophosphorylation an overview

Photophosphorylation is

the process of

transferring the energy from light into chemicals, particularly ATP. The evolutionary roots of photophosphorylation are likely in the anaerobic world, between 3 billion and 1.5 billion years ago, when life was abundant in the absence of molecular oxygen. Photophosphorylation probably evolved relatively shortly after electron transport chains (ETC) and anaerobic respiration provided metabolic diversity. The first step of the process involves the absorption of a photon by a pigment molecule. Light energy transfers to the pigment and promotes electrons (


-) into a higher quantum energy state—something biologists term an "excited state". Note the use of anthropomorphism here; the electrons are not "excited" in the classic sense and aren't suddenly hopping all over or celebrating their promotion. They are


in a higher energy quantum state. In this state, the electrons are colloquially said to

be "energized

". While in the "excited" state, the pigment now has a much lower reduction potential and can donate the "excited" electrons to other carriers with greater reduction potentials. These electron acceptors may become donors to other molecules with greater reduction potentials and, in doing so, form an electron transport chain.

As electrons pass from one electron carrier to another via red/ox reactions, enzymes can couple these exergonic electron transfers to the endergonic transport (or pumping) of protons across a membrane to create an electrochemical gradient. This electrochemical gradient generates a proton motive force (PMF). Enzymes can couple the exergonic drive of these protons to reach equilibrium to the endergonic production of ATP, via ATP synthase. As we will see in more detail, the electrons involved in this electron transport chain can have one of two fates: (1) they may return to their initial source in a process called cyclic photophosphorylation; or (2) they can transfer onto a close relative of NAD+ called NADP+. If electrons return to the original pigment in a cyclic process, the whole process can start over. If, however, the electron transfers onto NADP+ to form NADPH (**shortcut note—we didn't explicitly mention any protons but assume that they

are also involved

**), the original pigment must regain an electron from somewhere else. This electron must come from a source with a smaller reduction potential than the oxidized pigment and depending on the system there are different sources, including H2O, reduced sulfur compounds such as SH2 and even elemental S0.

What happens when a compound absorbs a photon of light?

When a compound absorbs a photon of light, the compound is said to leave its ground state and become "excited".

Figure 1. A diagram depicting what happens to a molecule that absorbs a photon of light. Attribution:Marc T. Facciotti (original work)

What are the fates of the "excited" electron? There are fourpossibleoutcomes, whichare schematically diagrammedin the figure below. These options are:

  1. Thee- can relax to a lower quantum state, transferring energy as heat.
  2. Thee- can relax to a lower quantum state and transfer energy into a photon of light—a process known as fluorescence.
  3. The energy can be transferred by resonanceto a neighboring molecule as thee- returns to a lower quantum state.
  4. The energy can change the reduction potential such that the molecule can become ane- donor. Linking this excitede- donor to a propere- acceptor can lead to an exergonic electron transfer.The excited state can be involvedin red/ox reactions.

Figure 2. What can happen to the energy absorbed by a molecule.

As the excited electron decays back to its lower energy state, it can transfer its energy in a variety of ways. While many so-called antenna or auxiliary pigments absorb light energy and transfer it to something known as a reaction center (by mechanisms depicted in option III in Figure 2), it is what happens at the reaction center that we are most concerned with (option IV in the figure above). Here a chlorophyll or bacteriochlorophyll molecule absorbs a photon's energy, and an electron is excited. This energy transfer suffices to allow the reaction center to donate the electron in a red/ox reaction to a second molecule. This starts the electron transport reactions. The result is an oxidized reaction center that must nowbe reducedto start the process again. How this happens is the basis of electron flow inphotophosphorylationand we describe this below.

Simple photophosphorylation systems:anoxygenicphotophosphorylation

Early in the evolution of photophosphorylation, these reactions evolved in anaerobic environments where there was very little molecular oxygen available. Two sets of reactions evolved under these conditions, both directly from anaerobic respiratory chains as described previously. We know these as the light reactions because they require the activation of an electron (an "excited" electron) from the absorption of a photon of light by a reaction center pigment, such as bacteriochlorophyll. We classify the light reactions either as cyclic or as noncyclic photophosphorylation, depending upon the final state of the electron(s) removed from the reaction center pigments. If the electron(s) returns to the original pigment reaction center, such as bacteriochlorophyll, this is cyclic photophosphorylation; the electrons make a complete circuit. We diagram this in Figure 4. Consider adding a space." data-pwa-id="pwa-CE0968A762B033E23D2D75E1F8BE1A52" data-pwa-rule-id="WHITESPACE" data-pwa-suggestions=" (">(s) are used to reduce NADP+ to NADPH, the electron(s)are removedfrom the pathway and end up on NADPH; we refer to this process as noncyclic since the electrons are no longer part of the circuit. Here the reaction center mustbe re-reduced before the process can happen again. Therefore, an external electron sourceis requiredfornoncylicphotophosphorylation. In these systems reduced forms of Sulfur, such as H2S, which canbe used asan electron donor andis diagrammedin Figure 5. To help you better understand the similarities of photophosphorylation to respiration, we provided a red/oxtower that contains many commonly used compounds involved withphotosphosphorylation.

oxidized form



Eo´ (volts)

PS1* (ox)

PS1* (red)



ferredoxin (ox) version 1

ferredoxin (red) version 1



PSII* (ox)

PSII* (red)



P840* (ox)

PS840* (red)











ferredoxin (ox) version 2

ferredoxin (red) version 2










-0.42 (at [H+] = 10-7;pH=7)

NAD+ + 2H+




NADP+ + 2H+




Complex I

FMN (enzyme bound)




Lipoicacid, (ox)

Lipoicacid, (red)



FAD+ (free) + 2H+




Pyruvate + 2H+




FAD+ + 2H+ (bound)

FADH2 (bound)



CoQ (Ubiquinone - UQ + H+)




UQ + 2H+




Plastoquinone; (ox)

Plastoquinone; (red)



Ubiquinone; (ox)

Ubiquinone; (red)



Complex III Cytochrome b2; Fe3+

Cytochrome b2; Fe2+



Complex III Cytochrome c1; Fe3+

Cytochrome c1; Fe2+



Cytochrome c; Fe3+

Cytochrome c; Fe2+



Complex IV Cytochromea;Fe3+




1/2 O2 + H2O




P840GS (ox)

PS840GS (red)



Complex IV Cytochrome a3; Fe3+

Cytochrome a3; Fe2+







Cytochrome f; Fe3+

Cytochrome f; Fe2+



PSIGS (ox)

PSIGS (red)











1/2 O2 + 2H+





PSIIGS (red)



* Excited State, after absorbing a photon of light

GS Ground State,stateprior to absorbing a photon of light

PS1: Oxygenic photosystem I

P840: Bacterial reaction center containing bacteriochlorophyll (anoxygenic)

PSII: Oxygenic photosystem II

Figure 3. Electron tower that has a variety of common photophosphorylation components. PSI and PSII refer to Photosystems I and II of the oxygenic photophosphorylation pathways.

Cyclic photophosphorylation

In cyclic photophosphorylation thebacteriochlorophyllred molecule absorbs enough light energy to energize and eject an electron to formbacteriochlorophyllox. The electron reduces a carrier molecule in the reaction center whichin turnreduces a series of carriers via red/ox reactions. These carriers are the same carriers found in respiration. If the change in reduction potential from the various red/ox reactions are sufficiently large, H+ protons canbe translocatedacross a membrane. Eventually, the electron is used to reducebacteriochlorophyllox(making a complete loop) and the whole process can start again. This flow of electrons is cyclic and is therefore said to drive a processed called cyclic photophosphorylation. The electrons make a complete cycle: bacteriochlorophyll is the initial source of electrons and is the final electron acceptor.ATP is producedvia the F1F0 ATPase. The schematic in Figure 4demonstrateshow cyclic electrons flow and thus how cyclic photophosphorylation works.

Figure 4. Cyclic electron flow. The reaction center P840 absorbs light energy and becomes excited, denoted with an *. The excited electronis ejectedand used to reducean FeSprotein leaving an oxidized reaction center. The electronitstransferred to a quinone, then to a series of cytochromes, whichin turnreduces the P840 reaction center. The process is cyclical. Note the gray array coming from the FeS protein going to aferridoxin(Fd), also in gray. This represents an alternative pathway the electron can take and willbe discussedbelow in noncyclic photophosphorylation. Note that the electron that initially leaves the P840 reaction center is not necessarily the same electron that eventually finds its way back to reduce the oxidized P840.

Possible NB Discussion Point

The figure of cyclic photophosphorylation above depicts the flow of electrons in a respiratory chain. How does this process help generate ATP? Why might running the process in a cyclical fashion be advantageous for a cell?

Noncyclic photophosphorylation

In cyclic photophosphorylation, electrons cycle from bacteriochlorophyll (or chlorophyll) to a series of electron carriers and eventually back to bacteriochlorophyll (or chlorophyll); there is theoretically no net loss of electrons and they stay in the system. In noncyclic photophosphorylation, electrons leave from the photosystem and red/ox chain and eventually end up on NADPH. That means there needs to be a source of electrons, a source that has a smaller reduction potential than bacteriochlorophyll (or chlorophyll) that can donate electrons tobacteriochlorophyllox to reduce it. From looking at the electron tower in Figure 3, you can see what compounds can reduce the oxidized form of bacteriochlorophyll. The second requirement is that, when bacteriochlorophyll becomes oxidized by ejecting its excited electron, it must reduce a carrier that has a greater reduction potential than NADP/NADPH (see the electron tower). Here, electrons can flow from energized bacteriochlorophyll to NADP forming NADPH and oxidized bacteriochlorophyll. The system loses electrons and they end up on NADPH; to complete the circuit,bacteriochlorophylloxis reducedby an external electron donor such as H2S or elemental S0.

Noncyclic electron flow

Figure 5. Noncyclic electron flow. In this example, the P840 reaction center absorbs light energy and becomes energized; the emitted electron reduces aFeSprotein and reducesferridoxin. Reducedferridoxin(Fdred) can now reduce NADP to form NADPH.The electrons are now removedfrom the system, finding their way to NADPH. The electrons need tobe replacedon P840, which requires an external electron donor. Here, H2S serves as the electron donor.

We note that for bacterial photophosphorylation pathways, for each electron donated from a reaction center [remember only one electron is actually donated to the reaction center (or chlorophyl molecule)], the resulting output from that electron transport chain is either the formation of NADPH (requires two electrons) or ATP can be made but NOT not both. The path the electrons take in the ETC can have one or two outcomes. This puts limits on the versatility of the bacterial anoxygenic photosynthetic systems. But what would happen if a process evolved that used both systems? More precicely, a cyclic and noncyclic photosynthetic pathway which could form both ATP and NADPH from a single input of electrons? A second limitation is that these bacterial systems require compounds such as reduced sulfur to act as electron donors to reduce the oxidized reaction centers, but they are not necessarily widely found compounds. What would happen if a chlorophyll ox molecule would have a reduction potential higher (more positive) than that of the molecular O2/H2O reaction? Answer: a planetary game changer.


Photosynthesis is a process by which chlorophyll-containing cells synthesize carbohydrate from carbon dioxide & water with the help of solar energy & the byproduct is oxygen. It occurs in the Chloroplast. During the light reaction, light energy is converted into chemical energy in the form of ATP & NADPH. This ATP & NADPH is used in the dark reaction to fix CO2in the stroma. Photophosphorylation occurs in the thylakoids (grana) & involves Photosystem I & II. A Photosystem is a group of closely associated pigments molecules that help in converting solar energy into chemical energy. Also referred as Light-harvesting complexes. It is of 2 types, Non- Cyclic & Cyclic.

Noncyclic photophosphorylation:

  • Both photosystem I & II are involved.
  • Photosystem I & II are excited simultaneously with solar energy.
  • Electrons are passed through electron transport chain between photosystem I & photosystem II.
  • Electron acceptors are Pheophytin (primary acceptor), Plastoquinone, Cytochrome complex & Plastocyanin.
  • They are arranged in decreasing energy concentration (downhill).
  • Photolysis of water occurs.
  • Oxygen is the by-product.
  • Responsible for synthesis of both ATP & NADPH.

Cyclic photophosphorylation:

  • Occurs in outer thylakoids & in stroma lamellae.
  • They possess Photosystem I.
  • Photosystem II & NAD reductase enzyme is absent.
  • Only photosystem I take part in cyclic photophosphorylation.
  • Only responsible for ATP formation.
  • No splitting of water occurs.

A Chemiosmotic hypothesis for ATP formation:

  • Noncyclic & Cyclic photophosphorylation creates a proton gradient between stroma & thylakoid lumen.
  • Reasons for proton gradient:
In Noncyclic:

1). Photolysis of water at photosystem II releases H + into the thylakoid lumen. 2). Plastoquinone in the electron transport chain is a hydrogen acceptor, it takes H + from the stroma & releases them in thylakoid lumen. 3). NADP + to NADPH by taking H + from the stroma.

Photosynthetic Structures in Eukaryotes and Prokaryotes

In all phototrophic eukaryotes, photosynthesis takes place inside a chloroplast, an organelle that arose in eukaryotes by endosymbiosis of a photosynthetic bacterium (see Unique Characteristics of Eukaryotic Cells). These chloroplasts are enclosed by a double membrane with inner and outer layers. Within the chloroplast is a third membrane that forms stacked, disc-shaped photosynthetic structures called thylakoids (Figure 2). A stack of thylakoids is called a granum, and the space surrounding the granum within the chloroplast is called stroma.

Figure 2. (a) Photosynthesis in eukaryotes takes place in chloroplasts, which contain thylakoids stacked into grana. (b) A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. (credit: scale bar data from Matt Russell.)

Photosynthetic membranes in prokaryotes, by contrast, are not organized into distinct membrane-enclosed organelles rather, they are infolded regions of the plasma membrane. In cyanobacteria, for example, these infolded regions are also referred to as thylakoids. In either case, embedded within the thylakoid membranes or other photosynthetic bacterial membranes are photosynthetic pigment molecules organized into one or more photosystems, where light energy is actually converted into chemical energy.

Photosynthetic pigments within the photosynthetic membranes are organized into photosystems, each of which is composed of a light-harvesting (antennae) complex and a reaction center. The light-harvesting complex consists of multiple proteins and associated pigments that each may absorb light energy and, thus, become excited. This energy is transferred from one pigment molecule to another until eventually (after about a millionth of a second) it is delivered to the reaction center. Up to this point, only energy—not electrons—has been transferred between molecules. The reaction center contains a pigment molecule that can undergo oxidation upon excitation, actually giving up an electron. It is at this step in photosynthesis that light energy is converted into an excited electron.

Different kinds of light-harvesting pigments absorb unique patterns of wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear the corresponding color. Examples of photosynthetic pigments (molecules used to absorb solar energy) are bacteriochlorophylls (green, purple, or red), carotenoids (orange, red, or yellow), chlorophylls (green), phycocyanins (blue), and phycoerythrins (red). By having mixtures of pigments, an organism can absorb energy from more wavelengths. Because photosynthetic bacteria commonly grow in competition for sunlight, each type of photosynthetic bacteria is optimized for harvesting the wavelengths of light to which it is commonly exposed, leading to stratification of microbial communities in aquatic and soil ecosystems by light quality and penetration.

Once the light harvesting complex transfers the energy to the reaction center, the reaction center delivers its high-energy electrons, one by one, to an electron carrier in an electron transport system, and electron transfer through the ETS is initiated. The ETS is similar to that used in cellular respiration and is embedded within the photosynthetic membrane. Ultimately, the electron is used to produce NADH or NADPH. The electrochemical gradient that forms across the photosynthetic membrane is used to generate ATP by chemiosmosis through the process of photophosphorylation, another example of oxidative phosphorylation (Figure 3).

Figure 3. This figure summarizes how a photosystem works. Light harvesting (LH) pigments absorb light energy, converting it to chemical energy. The energy is passed from one LH pigment to another until it reaches a reaction center (RC) pigment, exciting an electron. This high-energy electron is lost from the RC pigment and passed through an electron transport system (ETS), ultimately producing NADH or NADPH and ATP. A reduced molecule (H2A) donates an electron, replacing electrons to the electron-deficient RC pigment.

Think about It

Non-Cyclic and Cyclic Photophosphorylation | Photosynthesis

In this a photon of light is involved to excite electron in chlorophyll b or other accessory pigments of photosystem II. Energy of two such excited electrons is accepted by an oxidized plastoquinone forming completely reduced plastoquinone and electron-deficient chlorophyll b (Chl b).

Chl b then accepts an electron from a water molecule. In this step, Mn ++ and CI sup ions are needed. Consequently, water loses electron, produces oxygen (O2) and yields reduced plastoquinone.

plastoquinone + 2H ⇋ Reduced plastoquinone

The reduced plastoquinone then donates the electrons, one at a time, to cytochrome b6. From the latter, the electron may fall to cytochromeƒand then to plastocyanin and P 700 in pigment system I. P 700 can accept an electron only if it has just lost one of its own when excited by the energy in a second photon of light.

Ferredoxin is the direct acceptor of the electron lost by the excited P 700 molecule and is reduced. Two molecules of reduced ferredoxin then transfer their electrons to NADP reducing it to NADPH2 and itself is oxidized (Fig. 13-24).

Ferredoxin (Fe +2 ) + NADP + 2e − ⇋ 2 Ferredoxin (Fe +3 ) + NADPH2

In this process excess electron energy which results from the absorption of light quanta is used in the synthesis of ATP, from ADP and Pi, probably at one location between cytochrome b6 and cytochrome ƒ. In this scheme both Pigment systems participate in driving an electron from water to NADPH in an unidirectional manner.

When a herbicide dichlorophenyldimethylurea (DCMU) is added to the suspended chloroplasts, electron flow and ATP formation are blocked. DCMU inhibits the carrier chain link between PSI and PSII. If a reducing agent is added, ATP formation remains blocked but NADPH is produced. Obviously, NADPH formation is a function of PSI whereas ATP formation requires both PSI and PSII.

Comparison # Cyclic Photophosphorylation:

Here no ‘outside’ electron donor is needed. Chlorophyll absorbs photon of light of enough energy and an electron with high energy state (e – ) is produced which reduces ferredoxin and the cytochrome system (cyt. b6), respectively.

The cytochrome reduction is coupled to ATP formation. The electron at a low energy level returns to chlorophyll ultimately. Clearly the electron flow was from the electron donor and back to the same compound (Fig. 13-12).

There are three types of photosynthetic bacteria (green, purple sulphur, purple non-sulphur). The former two types are autotrophic and use H2S as electron donor for CO2 reduction. However, purple non-sulphur bacteria are photoheterotrophic and use succinate or malate and not H2S for CO2 reduction.

Bacteria do not use water and therefore produce no oxygen. The reductant in bacteria is NADH + H + and lack plastocyanin. With the production of ATP and NADPH2, the light reaction of photosynthesis is completed (Fig. 13-15, 13-16) and these two products are now utilized to reduce CO2 to the carbohydrate level in the dark phase.

Some photosynthesis bacteria have cyclic photophosphorylation as the only source of ATP. As will be seen cyclic photophosphorylation is carried out by photosystem I only. Details are clear in the following figure (Fig. 13.24). This can occur under anaerobic condition.

In brief it may be stated that biochemistry of oxygen evolution mechanisms is still a challenging problem for the biochemists and much remains to be understood.

The Oxygenic and Anoxygenic Photosynthesis

For photosynthesis to continue, the electron lost from the reaction center pigment must be replaced. The source of this electron (H2A) differentiates the oxygenic photosynthesis of plants and cyanobacteria from anoxygenic photosynthesis carried out by other types of bacterial phototrophs. In oxygenic photosynthesis, H2O is split and supplies the electron to the reaction center. Because oxygen is generated as a byproduct and is released, this type of photosynthesis is referred to as oxygenic photosynthesis. However, when other reduced compounds serve as the electron donor, oxygen is not generated these types of photosynthesis are called anoxygenic photosynthesis. Hydrogen sulfide (H2S) or thiosulfate (S2O3 2 -) can serve as the electron donor, generating elemental sulfur and sulfate (SO4 2 -) ions, respectively, as a result.

Photosystems have been classified into two types: photosystem I (PSI) and photosystem II (PSII). Cyanobacteria and plant chloroplasts have both photosystems, whereas anoxygenic photosynthetic bacteria use only one of the photosystems. Both photosystems are excited by light energy simultaneously. If the cell requires both ATP and NADPH for biosynthesis, then it will carry out noncyclic photophosphorylation. Upon passing of the PSII reaction center electron to the ETS that connects PSII and PSI, the lost electron from the PSII reaction center is replaced by the splitting of water. The excited PSI reaction center electron is used to reduce NADP + to NADPH and is replaced by the electron exiting the ETS. The flow of electrons in this way is called the Z-scheme.

If a cell’s need for ATP is significantly greater than its need for NADPH, it may bypass the production of reducing power through cyclic photophosphorylation. Only PSI is used during cyclic photophosphorylation the high-energy electron of the PSI reaction center is passed to an ETS carrier and then ultimately returns to the oxidized PSI reaction center pigment, thereby reducing it.

What is Noncyclic Photophosphorylation

Noncyclic photophosphorylation refers to the synthesis of ATP during the light reaction of photosynthesis where an electron donor is required and oxygen is produced as a byproduct. Both photosystem I (P700) and photosystem II (P680) are used in noncyclic photophosphorylation. The high energy electrons expelled from P680 pass through the ETS and return to P700. At P700, these electrons are taken up by NADP + , producing NADPH. At P680, photolysis occurs, splitting water to replace the released electrons of P680. During this process, oxygen is produced as a byproduct. The noncyclic photophosphorylation is shown in figure 2.

Figure 2: Noncyclic Photophosphorylation

Generally, noncyclic photophosphorylation occurs in plants, algae, and cyanobacteria. During noncyclic photophosphorylation, both ATP and NADPH are produced.

Photophosphorylation in general

For any organism, the general process of phototrophy is going to be the same. A photosystem antennae absorbs light and funnels the energy to a reaction center, specifically to a special pair of chlorophyll/bacteriochlorophyll molecules. The molecules become excited, changing to a more negative reduction potential (i.e. jumping up the electron tower). The electrons can then be passed through an electron transport chain of carriers, such as ferredoxin and cytochromes, allowing for the development of a proton motive force. The protons are brought back across the plasma membrane through ATPase, generating ATP in the process. Since the original energy from the process came from sunlight, as opposed to a chemical, the process is called photophosphorylation. If the electrons are returned to the special pair of chlorophyll/bacteriochlorophyll molecules (cyclic photophosphorylation), the process can be repeated over and over again. If the electrons are diverted elsewhere, such as for the reduction of NAD(P) (non-cyclic photophosphorylation), then an external electron source must be used to replenish the system.

Energy Transduction in Anaerobic Bacteria


Anaerobic or facultatively anaerobic bacteria are able to grow in the absence of molecular oxygen by fermentation, anaerobic respiration, anoxygenic photosynthesis , and some other membrane-dependent reactions. Fermentation uses substrate-level phosphorylation for adenosine diphosphate phosphorylation, whereas the other processes rely on the formation of a H + or Na + potential over the membrane and a membrane-potential-driven ATP synthase. In growth reactions providing only a small free energy change, the latter reactions and use of a membrane potential is the preferred mechanism for energy conservation. Fermentation reactions supply products of biotechnological interest like short chain fatty acids, alcohols, H2, CO2, and methane. Major anaerobic electron acceptors are nitrate, nitrite, sulfate, CO2, and metal oxides. Many of the anaerobic electron transport chains use redox loop enzymes for generating the H + potential over the membrane.

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The difference between cyclic and noncyclic photophosphorylation is mainly due to the following factors: Type of photosynthesis: Cyclic photophosphorylation occurs during anoxygenic photosynthesis, while noncyclic photophosphorylation occurs in oxygenic photosynthesis. ATP synthesis: ATP synthesis during the cyclic electron flow of anoxygenic photosynthesis is known as cyclic photophosphorylation. ATP production during the noncyclic electron flow of &hellip

Reaction centre refers to the site of photosynthetic reactions. Chlorophyll and pheophytin are the pigments found in a reaction centre. It comprises protein pigments that mediates light absorption and excitation of an electron to the higher energy state. Photosynthetic organisms like green plants, many bacteria and algae have membrane-bound protein complexes or reaction centres that &hellip

Watch the video: Natures smallest factory: The Calvin cycle - Cathy Symington (May 2022).