Energy interconversions in photosynthesis

Energy Interconversions in Photosynthesis. Charles L. Bering. SUNY College of Technology, Utica, NY 13502. Although the overall equation of photosynth...
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Energy Interconversions in Photosynthesis Charles L. Bering SUNY College of Technology, Utica, NY 13502

Although the overall equation of photosynthesis 6C02 + 6H10 + light

-

+

CsHlzOs (glucose) 602

molecules necessarv to run the machine known as man are directly or indirectiy produced by plants. However, plants do not inaest larae fuel molecules themselves. but are canahle of synthesizing glucose and other such molecules from carbon dioxide and water, which are also waste products of animal metabolism (Fig. 1).However, alarge increase in free energy (a positive AG) is required for the production of glucose and oxygen. This endergonic process is accomplished through the use of an external energy source-sunlight. Thus our thermodynamic bookkeeping of life on earth is complete as long as the sun provides the external "push" that drives the whole cycle. Green plants are not the only organisms capable of photosvnthesis on earth. Althoueh thev oerform the bulk of the ihotosynthesis on the a ia&e percentage of photosvnthesis occurs in the world's oceans where nhvtonlankton . a i d algae not only provide oxygen, but also represent the beginning of the oceanic food chain. Photosynthetic bacteria, which are known to perform some very important roles in mineral cycles, and freshwater algae are also active phototrophs. Thus a more general equation for photosynthesis which accounts for all of the various photosynthetic organisms was proposed by Van Neil (5) ~~~

(1)

is familiar tomost people, the process is given rather superficial treatment in most introductory courses, probablydue to its broad multidisciplinary nature. Photosynthesis is studied by physicists interested in energy transfer processes, by engineers interested in photovoltaic cells, by agronomists interested in crop yields, and by many other scientists. Excellent books and reviews are available from all of these disciplines ( 1 4 ) .This paper limits itself to the areas pertinent to chemistry, particularly the energetics of the light reactions of photosynthesis. Bloenergetlcs If one were to burn a mole of glucose in air (oxygen) to produce carbon dioxide and water, a total of 2803 kJ would be released. This process, in proceeding from State A (glucose and oxygen) to State B (COs and HzO) is highly exothermic. Not only would there be a large enthalpy change (AH),there would also he an increase in the entropy (AS) of the system since there are more degrees of freedom in State B than in State A. The comhination of the enthalpy and entropy contributions is the change in free energy where T is the absolute temoerature. This equation aives an indication of the spontaneity of the reaction. If AG 2 0, the reaction is said to he exeraonic and will occur soontaneouslv. If AG > 0, the reaction is endergonic and n ~ & ~ o n t a n e o u s At 25"C, this free energy change, AGO, for the oxidation of glucose is -2870 kJImol. Humans are heterotrophic, highly complex organisms. They require reduced organic compounds-carbohydrates, fats, and protein-as a food source. Stored in these molecules is agreat amount of useful energy. Humans can tap the energy from these food sources a t ahout a 40% efficiency and can use it to build up their own complex molecules, to move about, to sense the environment, and to keep warm. Without this input of energy, they would soon die-or in thermodynamic terms, they would he a t equilibrium with their environment. Fortunately, this external source of energy, unlike money, does grow on trees-and elsewhere. The large, complex fuel

-

HzA + CO2

-+ A

C(H,O)

(3)

In this equation, H2A acts as a donor of reducing equivalents, and CO2 as a source of carbon. Normally, A is oxygen, but if one substitutes sulfur for A, one sees that hydrogen sulfide, HsS, can be used as a source of reducing power, with elementalsulfur as a byproduct. This type of reaction is seen with the family of green and purple sulfur bacteria (e.g., Chlorobium). The key ingredient for all photosynthetic organisms is sunlight as the external energysource. Let us begin with this source and work through the storage of available energy as adenosine triphosphate (ATP) or reducing power in the form of reduced nicotinamide adenine dinucleotide phos-

-u\/ SUN

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ORGANIC PRODUCTS

p G z q This feature presents relevam applications of chemistry to everyday life. i'b in(ormationpresented might be useddirecdy in class, posted on bulletin boards or otherwise used to slim* late student involvemem in activitiesrelated to chemistry. Contributions should be sent to the feature editors.

~

+

O2

piGKGl

Figure 1. Carbon cycle an earn.

Volume 62 Number 8 August 1985

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Figure 3. (a1 Absorption of light energy by atoms wilh promotion to excited state E'. (b) Emission of light by atoms returning to ground state. Figure 2. Electromagnetic waves. E = electrical component. B = magnetic component.

phate (NADPH). A t each step we willemphasize the conversion of energy from one form t o another.

Absorption

LigM a s Energy: Photochemistry

Standing in the noonday sun or grabhing a lightbulb that has heen on for a few hours should convince you that light is a form of energy that can be converted into beat. Light can also bleach the brilliant color of an automobile's finish or ruin a color photograph with time. Lasers use light energy to cut through metals or to destroy cancer cells. Also, consider a common exothermic reaction such as the burning of a log. Not only does the log provide warmth (heat energy), hut some of the energy stored in the complex molecules of the wood is released as light. Another varticularlv interestine exothermic vrocess that produces light as a prdduct is the &ermonuclea~fusionreaction, in which four hvdroeen - " atoms fuse to form a helium atom, releasing a large amount of e n e r b , including a quantum (photon, hu) of light 41'H -%&He+ 2+10@+ hn

(4)

This is the reaction that occurs, fortunately 93 million miles away, at the sun, but the energy from this reaction is felt throughout the solar system. I t may be a bit chauvinistic for us to s ~ e a kof lieht enerev. Actuallv. ".the fusion reaction in the sun produces a rangeof energies of which visible light is onlv a small part-the r art that our eves can perceive. The energy prodiced by the sun, called electromagnetic radiation, travels a t a speed (c) of 3 X 108 m/s in a vacuum. As seen below (Fig. 21, electromagnetic radiation is propagated as a wave, consisting of an electrical component, and, a t 90' to the electrical component, a magnetic-component. These waves have a characteristic length, A, measured from peak to peak, or from any point in one wave to the corresponding point on the next wave. For radiation propagating a t velocity c, the number of waves passing a given point in one second is the frequency, u, usually expressed in hertz (Hz), or cycles per second (s-1). . . At a constant velocitv. .. an increase in the frequency (waves per second) implies adecrease in the wavelength-thus frequency and wavelength are inversely relat-

The energy of light a t agiven frequency or wavelengthcan he determined by

where h is Planck's constant, 6.626 X 10-3%. Since frequency and energy are directly proportional, the energy of UV, X-ray, and gamma radiation is much higher than visible light. The energy required to strip away the valence electron of the sodium atom is 5.10 eV (the first ionization potential of sodium). The energy of blue light a t 450nm can he calculated as being 2.78 eV, which is ohviously 660

Journal of Chemical Education

Fluorescence

Figure 4. Emission of llght as fluorescence by excited molecules after relaxation to lowest vibrational state within upper electronic excited state.

not sufficient to ionize sodium atoms. However, the energy possessed by 243 nm radiation (ultraviolet) is sufficient to remove the 3s electron from sodium. Such radiation is often referred to as ionizing radiation. Ultraviolet light, in stripping electrons from molecules in living organisms can form free radicals which can lead to mutations or cancer, and Xrays and gamma rays are lethal a t high enough doses. Fortunately our atmosphere deflects or absorbs most of this radiation. At the lower frequency end of the spectrum, infrared, microwave, and radio frequencies do not have sufficient energy to strip electrons from molecules and are only capable of inducing vibrational and rotational changes (i.e., the warming effects of infrared), or changes in the nuclear spin state (as exploited in nuclear magnetic resonance). What happens then when visible radiation impinges upon an atom such as sodium in its ground state? Because the electrons in the sodium atom are in their lowest available enerw levels, onlv some external innut of enerw can vromot& electron from a ground state'orhital to some higher unoccupied orbital (Fig. 3). This input may he via collisions, heat, or light, and must exactly match the gap between the ground and excited states. Enerw ereater or less than this gap will be wasted as heat. whatgoes up must come down. Therefore, in about 10-12 s, the excited electron returns to the ground state with the emission of a quantum of light of the same frequency. (Atomic absorption and atomic emission analytical methods take advantage of this phenomenon to identify and quantitate certain elements.) The picture is a hit more complex for molecules because once the photon of light is absorbed and the excited state E* is formed, the molecule may relax to a lower vibrational or rotational substate within this excited electronic state before the electron can return to the ground state. The net result is that the emitted light is a t a lower energy than the absorbed light (Fig. 4). This process is commonly referred to as fluorescence. In the above transition diagrams, the total enerw .. chance from the excited state to theiround state is the same whether the path isemission of light at the same wavelen~th(same energy) or a t a longer wavelength (lower energyjwith the remainder given off as heat. The latter mode is known as ~

~

radiationless deexcitation. A third path of deexcitation is available in some circumstances-the excited molecule can perform a chemical reaction, or photochemistry. A common type of photochemical reaction is oxidation-reduction. Equation (7) involves three molecules-a sensitizer, S, a donor, D, and an acceptor, A. DSA %DS*A

- DS+A-

D+SA-

ChlOmphyll

0

Absorbance

(7)

The sensitizer is capable of absorbing light of a given wavelength. Upon absorption, an electron from S becomes excited to Si. In the presence of an appropriate acceptor molecule, A, this electron can be passed on, producing reduced A, (A-). This leaves the sensitizer in the cation form, S+. The reverse reaction, charge recombination, would occur very rapidlv . . unless some barrier is nresent to nrevent back reaction. One way to prevent charge recombination is to rapidly reduce S+. If the donor D r a ~ i d l vreduces S+. nroducine the final state, D+SA- of eqn. -(I),-there may bk no pa& for recombination. The above reaction scheme is the basis of artificial photovoltaic cells. In solid state terminolom. the charee - separa. tion produces holes and electrons, with holes migrating through D, and electrons migrating through A. T o obtain an electric current, the entire circuit would beconnected so that electrons would travel through a wire ultimately back to D+. Light would drive the circuit continuously. A similar process occurs in photosynthesis, but not for the purpose of producing an electric currentl-Let us now examine the initial charge separation events in photosynthesis and what chemical components allow this light-induced charge separation to he maintained long enough to perform photochemistry. Llght Energy into Eiectrlcal Potential Energy Artificial photovoltaic cells have not yet been able to match the efficiency of biological systems. The key to the success of biological photosynthesis lies in the architecture of the molecular components necessarv for ~hotosvnthesis and, particularly, their location in membranes. ~ i o l o ~ i c a l membranes consist of a fluid bilaver of amphipathie linids. particularly phospholipids. he amphipkhk nature of these lipids means that they have a polar, charged region a t one end of the molecule and a pair of long, nonpolar hydrocarbon tails (fatty acids esterified to glycerol) a t the other end. Thus phospholipids behave somewhat like detergents. The polar regions interact with water and the hydrocarbon tails interact tail-to-tail with a second layer of phospholipids. Thus a cross section of a memhrane would be the thickness of two phospholipids (a bilayer), arranged as an aqueous side, polar head groups, the hydrocarbon tails, polar head groups of the second phospholipid layer, and an inner aqueous side. This phospholipid bilayer makes up the matrix of a memhrane, and acts as a semi~ermeableharrier separating two aqueous compartments. he red blood cell membrane, for instance, separates extracellular fluid (blood plasma) from the aqueous intracellular cytoplasm, which has its own unique chemical composition. Embedded in this lipidmatrix of the membrane or loosely attached to the polar head regions are a number of proteins responsible for a variety of activities such as transport of nutrients or ions, receptors for hormones, and so on. In photosynthetic organisms, one set of proteins, pigments, and sensitizer moleciles embedded in a specific arrangement in specialized membranes make up the photosynthetic apparatus. In green plants and algae, the photosynthetic apparatus is located in intracellular, membrane-bound oreanelles called chloroplasts. An extensive array of stacked-and folded membranes winds its way throughout the inner compartment of chloroplasts. I t is this inner membrane system that contains the photosynthetic apparatus. In single-celled photosynthetic bacteria there are no chloroplasts, hut the components for

600

700

Wavelength (nm)

Flgure 5. Absorbance spechum of chlor~phylla in elher.

photosynthesis are located in the membranethat surrounds the cell. This membrane enlarges greatly under photosynthetic growth conditions and hecomes highly infolded or invaginated, producing a greater surface area in which photosynthesis can occur. The sensitizer in photosynthesis is chloro~hvll(bacteriorhlorophyll in bacteria). ~ h l o r o p h ~closely're~emhles ll the heme found in hemoglobin and cvrochromes. It ronsisu of a tetrapyrrole ring buthas a magnesium atom in the center of the ring, rather than iron as in heme. Clearly this sensitizer must absorb light to initiate photochemistry, and the absorbance spectrum of chlorophyll in ether is shown in Figure 5. The spectrum shows that chlorophyll absorbs well in the blue or red regions, but shows very little absorhance in between. For this reason, chlorophyll appears green as green light is selectively transmitted, while red and blue are filtered out or absorbed. Since sunlight consists of aU wavelengths in the visible spectrum, i t would appear that chlorophyll is not terribly efficient as a sensitizer since light in the 450450 nm region would appear to be wasted. This is not the case, however, as plants and algae also have a number of other accessory pigments, particularly carotenoids, which absorb light in the region of the spectrum where chlorophyll cannot. The accessory pigments cannot, however perform the charge separation that initiates photosynthesis. T o understand how light energy absorbed by an accessory pigment finds its way to the sensitizer (chloro~hvll)it is necessarv to discuss resonance (FRrster) energy transfer. We have seen that an excited moll ecnle (E") can lose its enerm in three wavs-as emitted lieht (fluorescence~,as heat (rtadiationless~deexcitationJand through an oxidation-reduction renrtion (~hotocbemistrv). A fourth method would be simply to pa& the energy"& another molecule. The excited molecule, E*, could pass the energy onto a second molecule, .M, producing E in the ground state and M*, an excited state. Note that this transfer does not involve photochemistry. I t does require that the absorption bands (energy levels) of E and Moverlap and also that the molecules are physically close to each other. The efficiency of this energy transfer decreases with increasing separation of M and E (distance r ) as r-6. Thus. if the distanre between M and E is doubled, energy transfer will only be ' a d as dficient as in the orlmnal separation. he accessory pigments in-photo&nthetic systems perform this enerw transfer to the chloro~hvllsensitizer. However, there are-also a large number of chiorophyll molecules that perform only resonance energy transfer. In fact, only about one chlorophyll out of 400 actually performs photochemistry. If we examine the nature of the photosynthetic apparatus, we find a closely packed array or aggregate of pigments and chlorophyll molecules complexed with proteins all embedded in the memhrane. This packed aggregate allows for efficient transfer of energy. The unique sensitizer Volume 62

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August 1985

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chlorophyll molecule is attached to membrane proteins that make up a protein called the reaction center. The absorbance of this particular chlorophyll, because of its unique environment in the protein, is a t the longest wavelength, or lowest energy of all the chlorophylls. Thus, energy transfer proceeds through the chlorophyll aggregate, ultimately to the reaction center chlorophyll where photochemistry (charge separation) is initiated. The pigment-chlorophyll aggregate responsible for light absorption and energy transfer is called the antenna complex. Since the final destination of the energy is the reaction center, this reaction center complex is often called the phototrap, as the light energy is finally trapped as chemical energy. Since we will now discuss oxidation-reduction reactions initiated by light, it is necessary to briefly clarify the terminology of oxidation-reductions in biological systems. Standard reduction potentials (SRP) are expressed as Eo (in volts) for half-reactions compared with the standard hydrogen electrode (E" = 0.0 V). T h e tabulated values are based on 1M (or 1 atm) concentration of all oxidants and reductants. However, many biochemical oxidation-reduction involve protons. Eo values with protons a t 1M concentration (pH 0.0) are rather meaningless for biochemical systems which are typically near neutral pH. Thus biochemists and biologists correct the SRP's to pH 7.0, and designate the values as En'.This can be illustrated by two examples. Cytochrome f undergoes a one-electron reduction of its heme iron as follows cyt f(Fe3+)+ e-

-

cyt f(Fe2f)E' = +0.365V

(8)

Since no protons are involved, the SRP remains the same throughout the pH scale, and Eo = Eo'. However, consider the reduction of the electron carrier plastoquinone to the hydroquinone This reaction is a two-electron, two-proton reduction, so one must account for the protons a t 10-7 M concentration. The value for Eo is +0.52V. Bv using the Nernst equation we can calculate the value for E;,. 1f p~astoquinoneand plastoquinone.Hz in eq. (9) are both a t 1M and H+ is 10-7 M, then

P680 and this plastiquinone prevent charge recombination. hut P6R07 is also rapidly reduced by electrons drawn from water. In fact, after tour photochemical events \turnovers) ha\.e occurred, a burst of 0' is seen. The overall equation fur oxygen evolution at P68O is The oxidizing equivalents appear to accumulate a t a manganese-containing protein which acts as an immediate donor of electrons to P680+ and as an active site for splitting water molecules. Thus water ultimately acts as a large reservoir of electrons for photosynthesis, with the oxygen being evolved as a waste product. The four protons produced are also very important as we will see in the next section. On the reducing side of photosystem 11, the electron is passed from the primary acceptor (the plastoquinone-nonheme iron complex) to a secondary acceptor, also a plastoquinone. This latter quinone plus a large pool of plastoquinones in the membrane act t o store two electons (plus two protons), thus constantly removing the electrons from the primary acceptor, and allowing photochemical charge separation to continue. From the large pool of plastoquinone, electrons are drained off and flow down a series of electron transport carriers with increasingly positive 1.:' values. This intermediate electron transpurt is exeryonic (AGO < 0)and occurs without any further input of light energy. Light acts to "pump" electrons to the top of this chain, but the electrons then travel downhill spontaneously. The electron carriers in this chain include some iron-sulfur (nonheme iron) proteins, cytocbrome bs,cytochrome f (as a hsf complex), and a blue, copper-containing protein known as plastocyanin. The free energy released in this electron transport process can he trapped as useful chemical energy as will he discussed below. Turning to the second photosynthetic reaction center, we find another chlorophyll sensitizer, P700, in a complex known as photosystem I. PI00 undergoes photochemical oxidation to P700+, reducing an acceptor, X which may be a tightly hound iron-sulfur protein known as ferredoxin. From there, the electron travels to a soluble (not tightly bound) ferredoxin. to a flavin-containine motein. and finallv to the pyridine n k e o t i d e , NADP+, whilh is thd ultimate electron acceptor in the light reactions of photosynthesis. Mean-

where R = gas constant, 8.314 J1mol.K; T = absolute temperature; n = number of electrons; and F = Faraday, 96,500 J/V-mol. Then

Note there is a -60 mVIpH unit dependency for the twoelectron, two-proton transfer. Let us now examine the electron transfer reactions in photosynthesis, beginning with oxygen-evolving green plants and algae. Figure 6 shows the widely accepted Zscheme of Hill and Bendall, with a scale of E0' values for comoarison of the various comnonents. Note that in ereen plants and algae, elecrron transfer is linear, with water ultimatelv beine uxidized (on the left side of the srheme) and NADP+ beiig reduced (on the right). Also, note that there are two light events, or two different phototraps, from Hz0 oxidation to NADP+ reduction. On the water-oxidizing side of the Z-scheme the chlorophyll sensitizer molecule has a red absorbance peak a t 680 nm and is designated P680. The whole complex on the water-oxidizing side of the Z-scheme is known as photosystem 11. Upon absorption of light energy, P680* is formed and undergoes oxidation to P680+ while reducing an acceptor, which happens to he a plastoquinone complexed with a nonheme iron. The one-electron reduced nroduct is a semiauinone free radical and has been detected by electron spin resonance. The membrane location of the

.~

662

~

~

Journal of Chemical Education

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Figure 6. 2-scheme of elecnon transport in plants and algae. Photochemical events are indicated bv broad vertical arrows. P680. P700 = reaction center ~hlo~ophyils. 0 = p8aslaquonone. nanhsme iron prmary acceptor. PO * planoqdinone. be = cymchrame be. I = crochromef: PC = Plaslocyanm X 1 ghlly oound lerredox n: Fa = fenedox n Fp = f avoprotein. ~

~

~

~

while, P700+ is reduced by electrons from plastocyanin and the intermediate electron transfer system. If we examine the SRP's of thevarious comoonents. we see that the electrons used come from water oxkation (E" for H ~ O I O=~+0.82 V) and eventuallv reduce NADP+ (Eo' for k E"' for this proN~DP+/NADPH= - 0.32 V). ~ h overall cess, 1.14 V, is clearly unfavorable. Two photons of light, however, supply the energy needed to drive this unfavorable process. In photosystem I1 water is oxidized and a bound plastoquinone (Eo' E -0.1 V) is reduced. Given an overall Eo' of -3.93 V for this step, let us see if a photon of light a t 680 nm has sufficient energy to drive this reaction. From eqn. (11) we calculate AGO of +89.75 kJ1mol. From eqn. (6)the energy of one photon of light a t 680 nm is found to be 2.92 X 10-l9 J, or 176.60 kJdeinstein (an einstein is Avogadro's number of photons). Clearly there is sufficient energy in 680 nm light to drive this reaction. A similar calculation for photosystem I (En' for P700/P700f = +0.44V. ED'for hound ferredoxin = -0.55 V) shows a similar result: However, the reduced product of photosystem I1 cannot reduce NADP+. Thus two photosystems are required in pbotosynthesis-photosystem 11, which produces a weak reductant but a strong oxidant (capable of oxidizing water) and photosystem I producing a weak oxidant but a strong reductant (capable of reducing NADP+).

Figure 7. Electron wansport in photosymhetic bacteria. The photochemical event is indicated bv a broad vertical arrow. P865 = reaction center bacteric~hlorophyll: Fe.0 = ubiquinone, nonheme iron primaryacceptor:UQ = ubiquinone: Cyl b = cylochrome b: Cyl C2 = cytochrame CI.

Fieure 7 shows the electron transoort scheme for the our" ple, nonsulfur bacterium Rhodopseudomonas capsulata. Note that there is only one photosystem, and that the electron transport scheme is cyclic. Also, the sensitizer molecule is bacteriochlorophyll and has an absorbance maximum in the near infrared, a t 865 nm. The primary electron acceptor for P865 is a quinone called ubiquinone which is also complexed to a nonheme iron. Also in analogy with photosystem I1 in plants and algae, a pool of ubiquinone accepts electrons from-the primaryacceptor. ~ l e c t r o n sthen through a series of carriers including a cytochrome b and a cytochrome cs, which also acts as the electron donor to the oxidized phototrap, thus completing the electron transfer cycle. Bacteria also have an antenna aeereeate consistine of bacter"" iochlorophyll molecules and, in wild-type strains, carotenoids as accessory pigments.

-

-

Electrical Potential Energy Into Chemical Energy: The Chemlosmotlc Hypothesis

It would appear a t first glance that nothing useful is produced by the cyclic electron transport system of photosynthetic bacteria. Since these organisms do thrive in light,

clearly some useful product is formed. I t would also appear that in the intermediate electron transport chain in green plants and algae, a large amount of free energy is wasted as electrons flow from Q- to P700. In fact, for one electron transported from Q (Eo' = -0.1 V) to P700 (E"' = +0.44 V), the free energy change calculated from eqn. (11) is -52.11 kJs/mol. In bacteria. the same free enerevchanee is calculated for an electron flowing from Q- b a z to P&. Of course this enerev is not wasted but is traooed in a storable chemical compound, adenosine triphos&ate (ATP). ATP is the "energy currency" of all biological systems. The free energy of hydrolysis of ATP ATP + Hz0

-

ADP (adenosine diphosphate) +Pi

(12)

is -30.54 kJlmol. Many biosynthetic reactions are nonspontaneous and do not occur unless thev are couoled to ATP hydrolysis. For example, the free energy change'for the phosphorylation of glucose glucose + Pi

-

glucose4-phosphate

(13)

is +12.97 kJ/mol. When this reaction is coupled with ATP hydrolysis, the overall reaction glucose + ATP

-

glucose-6-phosphate+ ADP

(14)

has a AGO of -17.57 kJs/mol, and is spontaneous. In addition to being used in couoled biosvnthetic reactions. ATP is also used as the energ; source for locomotion, tiansport across membranes, and many other processes. Many heterotrophic organisms synthesize ATP by trapping the energy of oxidation of food molecules in reduced compounds such as nicotinamide adenine dinucleotide (NADH). The electrons are then transferred from NADH t o oxygen, forming water. This process, known as respiration, occurs in intracellular organelles known as mitochondria. T h e S R P of NAD+/NADH is t h e s a m e a s t h a t of NADPf/NADPH-the two differ only by a single phosphate-so respiration may he viewed energetically as the reverse of the light reactions of photosynthesis. However, there are numerous similarities when one examines the mechanism oi ATI' formation ill respirntion and phorosynrhesis. .4TP synthesis in both sysrenn is coupled to elrctnm transport through a system of electron carriers uniquely arranged in a membrane. The model for the mechanism of coupling electron transport with phosphorylation is the chemiosmotic hypothesis proposed by Mitchell (6). The chemiosmotic hvoothesis states that electron transnort causes a .. undirectional "pumping" of protons across the membrane. The result is that the free energy from electron transport is converted into an electrochemical gradient, or protonmotive force, consistina of a transmembrane electrical ootential and pH gradient ( i p ~ ) T. o further understand 'the coupling mechanism, we must examine the membrane-bound nature of the electron transport chain. The Mitchell model requires an enclosed membrane vesicle or sac (Fig. 8) and an asymmetric arrangement of the electron carriers across the membrane. The inner membrane systems of both mitochondria and chloroplasts include an inner and an outer aqueous compartment. In chloroplasts, let us consider the reduction of plastoquinone, which involves two electrons and two protons. Because of the asymmetry of membrane components, this reduction takes place a t one face of the membrane (the outside) and the protons used are taken from the outer aqueous compartment. In the subseauent redox reaction. c~tochromebi undergoes a one-electron reduction as plasto: w i n o n e H z is oxidized, hut protons are not involved. Cvtochrome & is located hear the inner face of the membraie, and, since protons are not involved in be reduction, the protons from plastoquinone.Hz are deposited a t the inner aqueous compartment. Clearly as electron transport proceeds, protons are removed from the outside compartment and are accumulated on the inside, leading to the production Volume 62

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in turn drives ATP synthesis. Under certain experimental conditions, one can synthesize ATP without electron transport. Jagendorf (7), in a classical experiment using chloro;last innel membranes, created an artificial prorongri~dient acres* the rnernhrunc in the dark and then coupled the d~ssination of that eradient with ATP svnthesis. Thus he was able to synthesrze a major product df the light reactions of ~hotosvnthesis(ATP) without electron transport or lieht! Using ietergents, onecan damage the integrit; of the membrane causine dissipation of the proton eradient while not affecting electron &ansport. w i t h detergent treatment no ATP synthesis occurs, though electron transport continues. Thus for ATP synthesis, light-driven electron transport merely acts to produce the required proton gradient.

Phdosynfhetlc Electron Transport Chaln Figure 8 Schematic of photosynthetic electron transport synthe~isvia transmembrane pH gradient

coupled

to ATP

Conclusion Equatiun ( I ) shuws the overall equation fur photospthesis. Onlv part of the reartions involved have I w n discusstd in this paper-the so-called light reactions. In green plants and algae, the overall Equation (1)shows the overall equation for photosynthesis. Only part of the reactions involved have heen discussed in this paper-the so-called light reactions. In green plants and algae, the overall reaction is

+ NADP' + ADP + Pi + hu

2 H20

of a pH gradient. Another reaction, the photooxidation of water. occurs a t the inner face of the membrane. As mentioned above (eqn. (lo)), four protons are released in the evolution of molecular oxygen. Since this occurs a t the inside of the membrane, the released protons further contribute to the pH gradient. A similar undirectional proton "pumping" also occurs in mitochondria1 electron transport although the direction is reversed (protons are "oum~ed" to the outer . . compartment). In Figure 8, a schematic of the enclosed membrane vesicle is shown with electron transport leading to a ApH, lower inside. This gradient, a source of energy like water behind a dam, then collapses through a specific channel in a large enzyme complex known as the coupling factor, CF1, or ATP synthetase. CFt enzymatically couples the "downhill" flow of protons with the nonspontaneous formation of ATP from ADP and . ohosuhate. much like water flowine throueh tur. hines can generate electricity. The exact mecLanismof this enzvmatic "turbine" is unknown but mav involve conformational changes of the CF1 induced by pr&onations. In mitachondria there is a verv similar c o u.~ l i n zfactor (called FI) which synthesizes the proton gradient. Thus we see that electron transport produces aprotongradient which

usin in^

684

Journal of Chemical Education

-

02

+ NADPH + ATP

(15)

The main products of photosynthesis are NADPH, which is used as a reductant in hiosvnthetic reactions. and ATP. Oxygen is a waste product (for which we are grateful). The fixation of carbon (as Cog) into carbohydrate is a very nonspontaneous biosynthetic process. T h e energy t o drive this process is supplied by NADPH and ATP and occurs in a complex series of reactions originally detailed by Calvin and co-workers (8) and not discussed here. We have seen, however, that beginning with light energy from a thermonuclear fusion reaction a t the sun, phototrophic organisms can trap some of this energy in a stable chemical form, with several energy interconversions along the way. Literature Clted I11 Clayton,R.K.,"MolcculsrPhysi~~inPhatosynthnis,"BlalsdeliPubiiahingCo.,Noru York, 1965. (21 Bs1d.A. J..Scienee, 207, I39 (19801. (31 Govindjee IEdifor). "Pbotrnynthenis: Energy Conversion by Plante and Bacteria," Academic PI~II.N e w York, 1982. I 0 ZeliVh, I., Chrm. Eng. News, 28 (February 5,19791. (5) Van Niel,C.B.,Aduen.Enzymol., 1,263 (19411. (6) Mitchell,P.,"ChemiosmoticCauplinginOxidativeandPhotosynthetiePhosphar~I~tion." Glynn Research. Bodmin. Cornwall,England. 1966. (7) Jagendorf.A. T., and Uribe E.. R o c . N a l l . Arod. Sci. USA.. 56.170 (1966). I81 Bassham, J. A , and Calvin M. "The Path of Carbon in Photosynthesis." PrentieeHall,Englewnod. Cliffs. NJ. 1951.