Berg J.M., Tymoczko J.L., Stryer L. Biochemistry

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Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in

photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase

to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps

protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP

synthase to generate ATP in the matrix.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.4. A Proton Gradient Across the Thylakoid Membrane Drives ATP Synthesis

Figure 19.26. Cyclic Photophosphorylation. In this pathway, electrons from reduced ferredoxin are transferred to the

cytochrome bf complex rather than to ferredoxin-NADP

reductase. The flow of electrons through cytochrome bf pumps

protons into the thylakoid lumen. These protons flow through ATP synthase to generate ATP. Neither NADPH nor O

generated by this pathway.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis

19.5. Accessory Pigments Funnel Energy Into Reaction Centers

A light-harvesting system that relied only on the chlorophyll a molecules of the special pair would be rather inefficient

for two reasons. First, chlorophyll a molecules absorb light only at specific wavelengths (see Figure 19.6). A large gap is

present in the middle of the visible regions between approximately 450 and 650 nm. This gap corresponds to the peak of

the solar spectrum, so failure to collect this light would constitute a considerable lost opportunity. Second, even in

spectral regions where chlorophyll a absorbs light, many photons would pass through without being absorbed, owing to

the relatively low density of chlorophyll a molecules in a reaction center. Accessory pigments, both additional

chlorophylls as well as other classes of molecules, are closely associated with reaction centers. These pigments absorb

light and funnel the energy to the reaction center for conversion into chemical forms. Indeed, experiments in 1932 by

Robert Emerson and William Arnold on Chlorella cells (unicellular green algae) demonstrated that only 1 molecule of

was produced for 2500 chlorophyll molecules excited.

19.5.1. Resonance Energy Transfer Allows Energy to Move from the Site of Initial

Absorbance to the Reaction Center

How is energy funneled from an associated pigment to a reaction center? We have already seen how the absorption of a

photon can lead to electron excitation and transfer (Section 19.2). Another reaction to photon absorption, not leading to

electron transfer, is more common. Through electromagnetic interactions through space, the excitation energy can be

transferred from one molecule to a nearby molecule (Figure 19.27). The rate of this process, called resonance energy

transfer, depends strongly on the distance between the energy donor and the energy acceptor molecules; an increase in

the distance between the donor and the acceptor by a factor of two typically results in a decrease in the energy-transfer

rate by a factor of 2

= 64. For reasons of conservation of energy, energy transfer must be from a donor in the excited

state to an acceptor of equal or lower energy. The excited state of the special pair of chlorophyll molecules is lower in

energy than that for single chlorophyll molecules, allowing reaction centers to trap the energy transferred from other

molecules (Figure 19.28).

19.5.2. Light-Harvesting Complexes Contain Additional Chlorophylls and Carotinoids

Chlorophyll b and carotenoids are important light-harvesting molecules that funnel energy to the reaction center.

Chlorophyll b differs from chlorophyll a in having a formyl group in place of a methyl group. This small difference

moves its two major absorption peaks toward the center of the visible region. In particular, chlorophyll b efficiently

absorbs light with wavelengths between 450 and 500 nm (Figure 19.29).

Carotenoids are extended polyenes that also absorb light between 400 and 500 nm.

The carotenoids are responsible for most of the yellow and red colors of fruits and flowers, and they provide the

brilliance of fall, when the chlorophyll molecules are degraded to reveal the carotenoids.

In addition to their role in transferring energy to reaction centers, the carotenoids serve a safeguarding function.

Carotenoids suppress damaging photochemical reactions, particularly those including oxygen, that can be induced by

bright sunlight. Indeed, plants lacking carotenoids are quickly killed on exposure to light and oxygen.

The accessory pigments are arranged in numerous light-harvesting complexes that completely surround the reaction

center. The 26-kd subunit of light-harvesting complex II (LHC-II) is the most abundant membrane protein in

chloroplasts. This subunit binds seven chlorophyll a molecules, six chlorophyll b molecules, and two carotenoid

molecules. Similar lightharvesting assemblies exist in photosynthetic bacteria (Figure 19.30).

19.5.3. Phycobilisomes Serve as Molecular Light Pipes in Cyanobacteria and Red Algae

Little blue or red light reaches algae living at a depth of a meter or more in seawater, because such light is absorbed by

water and by chlorophyll molecules in organisms lying above. How can photosynthetic organisms survive under such

conditions? Cyanobacteria (blue-green algae) and red algae contain large protein assemblies called phycobilisomes that

enable them to harvest the green and yellow light that penetrates to their ecological niche. Phycobilisomes are bound to

the outer face of the thylakoid membrane, where they serve as light-absorbing antennas to funnel excitation energy into

the reaction centers of photosystem II. They absorb maximally in the 470- to 650-nm region, in the valley between the

blue and far-red absorption peaks of chlorophyll a. Phycobilisomes are very large assemblies (several million daltons) of

many phycobiliprotein subunits, each containing many covalently attached bilin prosthetic groups, as well as linker

polypeptides (Figure 19.31). Phycobilisomes contain hundreds of bilins. Phycocyanobilin and phycoerythrobilin are the

two most common ones.

The geometrical arrangement of phycobiliproteins in phycobilisomes (Figure 19.32), as well as their spectral properties,

contributes to the efficiency of energy transfer, which is greater than 95%. Excitation energy absorbed by phycoerythrin

subunits at the periphery of these antennas appears at the reaction center in less than 100 ps. Phycobilisomes are

elegantly designed light pipes that enable algae to occupy ecological niches that would not support organisms relying

solely on chlorophyll for the trapping of light.

19.5.4. Components of Photosynthesis Are Highly Organized

The complexity of photosynthesis, seen already in the elaborate interplay of complex components, extends even to the

placement of the components in the thylakoid membranes. Thylakoid membranes of most plants are differentiated into

stacked (appressed) and unstacked (nonappressed) regions (see Figures 19.1 and 19.3). Stacking increases the amount of

thylakoid membrane in a given chloroplast volume. Both regions surround a common internal thylakoid space, but only

unstacked regions make direct contact with the chloroplast stroma. Stacked and unstacked regions differ in the nature of

their photosynthetic assemblies (Figure 19.33). Photosystem I and ATP synthase are located almost exclusively in

unstacked regions, whereas photosystem II is present mostly in stacked regions. The cytochrome bf complex is found in

both regions. Indeed, this complex rapidly moves back and forth between the stacked and unstacked regions.

Plastoquinone and plastocyanin are the mobile carriers of electrons between assemblies located in different regions of

the thylakoid membrane. A common internal thylakoid space enables protons liberated by photosystem II in stacked

membranes to be utilized by ATP synthase molecules that are located far away in unstacked membranes.

What is the functional significance of this lateral differentiation of the thylakoid membrane system? The positioning of

photosystem I in the unstacked membranes also gives it direct access to the stroma for the reduction of NADP

. ATP

synthase, too, is located in the unstacked region to provide space for its large CF

globule and to give access to ADP. In

contrast, the tight quarters of the appressed region pose no problem for photosystem II, which interacts with a small polar

electron donor (H

O) and a highly lipid soluble electron carrier (plastoquinone).

19.5.5. Many Herbicides Inhibit the Light Reactions of Photosynthesis

Many commercial herbicides kill weeds by interfering with the action of photosystem II or photosystem I. Inhibitors of

photosystem II block electron flow, whereas inhibitors of photosystem I divert electrons from the terminal part of this

photosystem. Photosystem II inhibitors include urea derivatives such as diuron and triazine derivatives such as atrazine.

These chemicals bind to the Q

site of the D1 subunit of photosystem II and block the formation of plastoquinol (QH

Paraquat (1,1

-dimethyl-4-4 -bipyridinium) is an inhibitor of photosystem I. Paraquat, a dication, can accept electrons

from photosystem I to become a radical. This radical reacts with O

to produce reactive oxygen species such as

superoxide and hydroxyl radical (OH·). Such reactive oxygen species react with double bonds in membrane lipids,

damaging the membrane (Section 18.3.6).

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.27. Resonance Energy Transfer. Energy, absorbed by one molecule, can be transferred to nearby molecules

with excited states of equal or lower energy.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.28. Energy Transfer from Accessory Pigments to Reaction Centers. Light energy absorbed by accessory

chlorophyll molecules or other pigments can be transferred to reaction centers, where it drives photoinduced charge

separation. The green squares represent accessory chlorophyll molecules and the red squares carotenoid molecules; the

white squares designate protein.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.29. Absorption Spectra of Chlorophyll. A and B .

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.30. Structure of a Light-Harvesting Complex.

Eight polypeptides, each of which binds three chlorophyll

molecules (green) and a carotenoid molecule (red), surround a central cavity that contains the reaction center.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.31. Structure of a Phycobilisome Subunit.

This protein, a phycoerythrin, contains a phycoerythrobilin

linked to a cysteine residue. The inset shows the absorption spectrum of a phycoerythrin.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.32. Structure of a Phycobilisome. (A) Electron micrograph of phycobilisomes from a cyanobacterium

(Synechocystis). (B) Schematic representation of a phycobilisome from the cyanobacterium Synechocystis 6701. Rods

containing phycoerythrin (PE) and phycocyanin (PC) emerge from a core made of allophycocyanin (AP) and

allophycocyanin B (APB). The core region binds to the thylakoid membrane. [(A) Courtesy of Dr. Robley Williams and

Dr. Alexander Glazer; (B) after a drawing kindly provided by Dr. Alexander Glazer.]

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.5. Accessory Pigments Funnel Energy Into Reaction Centers

Figure 19.33. Location of Photosynthesis Components. Photosynthetic assemblies are differentially distributed in the

stacked (appressed) and unstacked (nonapressed) regions of thylakoid membranes. [After a drawing kindly provided by

Dr. Jan M. Anderson and Dr. Bertil Andersson.]

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis

19.6. The Ability to Convert Light Into Chemical Energy Is Ancient

The ability to convert light energy into chemical energy is a significant evolutionary advantage. Indeed,

photosynthesis arose early in the history of life on Earth, which began about 3.5 billion years ago. Geolog- ical

evidence suggests that oxygenic photosynthesis became important approximately 2 billion years ago. Anoxygenic

photosynthetic systems existed even earlier (Table 19.1). The photosynthetic system of the nonsulfur purple bacterium

Rhodospeudomonas viridis has most features common to oxygenic photosynthetic systems and clearly predates them.

Green sulfur bacteria such as Chlorobium thiosulfatophilum carry out a reaction that also seems to have appeared before

oxygenic photosynthesis and is even more similar to oxygenic photosynthesis than that of R. viridis . Reduced sulfur

species such as H

S are electron donors in the overall photosynthetic reaction:

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the

domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated

through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life

do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-

cytochrome c oxidoreductase and cytochrome bf family are present in both respiratory and photosynthetic electron-

transport chains. These components were the foundations on which light-energy-capturing systems evolved.

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis 19.6. The Ability to Convert Light Into Chemical Energy Is Ancient

Table 19.1. Major Groups of Photosynthetic Prokaryotes

Bacteria Photosynthetic electron donor O

use

Green sulfur H

, H

S, S Anoxygenic

Green nonsulfur Variety of amino acids and organic acids Anoxygenic

Purple sulfur H

, H

S, S Anoxygenic

Purple nonsulfur Usually organic molecules Anoxygenic

Cyanobacteria H

O Oxygenic

II. Transducing and Storing Energy 19. The Light Reactions of Photosynthesis

Summary

Photosynthesis Takes Place in Chloroplasts

The proteins that participate in the light reactions of photosynthesis are located in the thylakoid membranes of

chloroplasts. The light reactions result in (1) the creation of reducing power for the production of NADPH, (2) the

generation of a transmembrane proton gradient for the formation of ATP, and (3) the production of O

Light Absorption by Chlorophyll Induces Electron Transfer

Chlorophyll molecules

tetrapyrroles with a central magnesium ion absorb light quite efficiently because they are

polyenes. An electron excited to a high-energy state by the absorption of a photon can move to nearby electron

acceptors. In photosynthesis, an excited electron leaves a pair of associated chlorophyll molecules known as the special

pair. The functional core of photosynthesis, a reaction center, from a photosynthetic bacterium has been studied in great

detail. In this system, the electron moves from the special pair (containing bacteriochlorophyll) to a bacteriopheophytin

(a bacteriochlorophyll lacking the central magnesium ion) to quinones. The reduction of quinones leads to the generation

of a proton gradient, which drives ATP synthesis in a manner analogous to that of oxidative phosphorylation.

Two Photosystems Generate a Proton Gradient and NADPH in Oxygenic

Photosynthesis.

Photosynthesis in green plants is mediated by two linked photosystems. In photosystem II, excitation of P680, a special

pair of chlorophyll molecules located at the interface of two similar subunits, leads to electron transfer to plastoquinone

in a manner analogous to that for the bacte-rial reaction center. The electrons are replenished by the extraction of

electrons from water at a center containing four manganese ions. One molecule of O

is generated at this center for each

four electrons transferred. The plastoquinol produced at photosystem II is reoxidized by the cytochrome bf complex,

which transfers the electrons to plastocyanin, a soluble copper protein. From plastocyanin, the electrons enter

photosystem I. In photosytem I, excitation of the special pair P700 releases electrons that flow to ferredoxin, a powerful

reductant. Ferredoxin-NADP

reductase, a flavoprotein located on the stromal side of the membrane, then catalyzes the

formation of NADPH. A proton gradient is generated as electrons pass through photosystem II, through the cytochrome

bf complex, and through ferredoxin-NADP

reductase.

A Proton Gradient Across the Thylakoid Membrane Drives ATP Synthesis

The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP.

The ATP synthase of chloroplasts (also called CF

-CF

) closely resembles the ATPsynthesizing assemblies of bacteria

and mitochondria (F

-F

). If the NADPH:NADP

ratio is high, electrons transferred to ferredoxin by photosystem I can

reenter the cytochrome bf complex. This process, called cyclic photophosphorylation, leads to the generation of a proton

gradient by the cytochrome bf complex without the formation of NADPH or O

Accessory Pigments Funnel Energy into Reaction Centers

Light-harvesting complexes that surround the reaction centers contain additional molecules of chlorophyll a, as well as

carotenoids and chlorophyll b molecules, which absorb light in the center of the visible spectrum. These accessory

pigments increase efficiency in light capture by absorbing light and transferring the energy to reaction centers through