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16.4Photosynthesis

Photosynthesis consists of reactions in which light energy is absorbed and converted to chemical energy to produce ATP and reducing power (light reactions), and reactions in which carbon dioxide is fixed as organic compounds using the ATP and reducing power produced by the light reactions (dark reactions). Photosynthesis proceeds in chloroplasts (organelles unique to plant cells), wherein the light and dark reactions occur locally in the thylakoid membrane and stroma (an aqueous space in chloroplasts), respectively. In the light reactions, light energy is absorbed and transported by antenna pigments*13 , and eventually photochemical reactions are triggered at the photochemical reaction center*14 , thus driving electron transport. The electron transport reaction of photosynthesis, in which ATP is synthesized through H+ transport, differs from the respiratory chain in that NADPH is synthesized by the breakdown of H2O. On the other hand, in the dark reactions, the carbon dioxide fixation reaction and the Calvin cycle (or saccharometabolic cycle) are driven by ATP and NADPH. In addition, oxygen is produced as a byproduct of photosynthesis. Incidentally, there was no oxygen in the atmosphere when Earth was created, and all the oxygen in the atmosphere today comes from photosynthesis.

*13 These pigments absorb light energy and transport it to the reaction center chlorophyll, but they do not undergo photochemical reaction.

*14 The core site containing chlorophyll a molecules triggering photochemical reactions (charge separation) on being excited by light energy.

16.4.1

Light reactions

The photosynthetic apparatus has several hundred molecules of antenna pigments, which absorb light energy and transfer it efficiently to the photochemical reaction center. Often, antenna pigments are color pigments other than chlorophyll a, contributing to the use of a wide range of light wavelengths. A special form of chlorophyll a, in the photochemical reaction center, releases electrons when excited by light to turn into chlorophyll a+. This reaction is called the photochemical reaction. Electrons with strong reducing power and chlorophyll a+ with strong oxidizing power drive the electron transport reaction of photosynthesis to transport H+ and reduce NADP+ *15 . The NADPH generated in this reaction is required in large amounts for carbon fixation, and water is used as the source that supplies the electrons. One key point of the light reaction is that because visible light does not have enough energy to carry out oxidization of water molecules and reduction of NADP+ at the same time, the two reactions are connected in tandem. Unlike electrical circuits, two identical photosystems cannot be directly connected in tandem, and are thus arranged as follows:

*15 Nicotinamide adenine dinucleotide phosphate (oxidized form). Coenzyme used mainly for biosynthetic reaction although its redox reaction is the same as NAD+.

WaterPhotochemical system IICytochrome b6f complexPhotochemical system INADP+

Figure 16-8 Photosynthesis system in chloroplast (top left), tykaloid membrane cross-section (top right), and tykaloid membrane (bottom)

Q is plastoquinone, and b6 and f are hemes (i.e., cytochrome cofactors). FeS represents iron sulfur cluster cofactors. Mn and Cu are manganese and copper atoms, respectively, carrying out reduction and oxidization. The cytochrome b6f complex and quinone cycle have the same basic structure as mitochondrial complex III. Some species are known to use cytochrome c instead of plastocyanin.

Chlorophyll a+ produced in photochemical system II deprives electrons from water molecules with its strong oxidizing power (E°′ = +1.1 to +1.2 V), whereas the electrons produced in photochemical system I reduce NADP+ with their strong reducing power (E°′ = –1.4 V). In this way, in the light reactions, electrons are transported between a wide range of redox potentials (–1.4 to +1.2 V) to carry out work. (Range of redox potentials in the respiratory chain is between –0.32 to +0.815 V). The cytochrome b6f complex, which connects the two photochemical systems, closely resembles the cytochrome bc1 complex (complex III of the respiratory chain) in structure and function. Plastoquinone, which conveys electrons to the cytochrome b6f complex from photochemical system II, also closely resembles ubiquinone of the respiratory chain and transports H+ through the quinone cycle. On the other hand, a characteristic unique to photosynthesis is the cyclic electron transport pathway circulating between photochemical system I and the cytochrome b6f complex (Figure 16-8). This pathway does not have an electron source, and hence is unable to stably supply NADPH. However, it contributes to ATP synthesis by driving the quinone cycle and transporting H+. In the electron transport pathway that connects photochemical systems I and II in tandem, the ratio of ATP synthesis and NADPH synthesis is fixed. When required, the cyclic pathway is activated to adjust the NADPH/ATP synthesis ratio according to demand. Like in the mitochondria, F-ATP synthase is also found in the thylakoid membrane of chloroplasts and functions to couple H+ transport and ATP synthesis. This enzyme is basically the same as that in mitochondria and eubacteria (Figure 16-7). *15 Nicotinamide adenine dinucleotide phosphate (oxidized form). Coenzyme used mainly for biosynthetic reaction although its redox reaction is the same as NAD+.

In summary, the following relationship is established in the light reactions:

2H2O + 2NADP+ + light energy → O2 + 2NADPH + 2H+ (+ nATP)

The factor “n” (number of ATP molecules synthesized) varies between 3–5, depending on how the two electron transport pathways are combined.

column

Mechanism allowing photosynthesis to efficiently use visible light

Column Figure 16-3 Chlorophyll absorbance (tinted area) and three types of antenna pigments of algae

The green, orange, and red lights absorbed by phycoerythrin (–), phycocyanin (┉), and allophycocyanin (•–•–) are efficiently tranferred (→), in the end, to the red absorption band of chlorophyll a and used for photochemical reactions. The blue light absorbed by chlorophyll is converted (⇒) to red light energy in the molecule. Consequently, a broad range of visible light can be used for photosynthesis.

There are two important conditions for the various antenna pigments of photosynthesis to efficiently convey light energy to the photochemical reaction center. One is that the antenna pigment molecules need to be located close to each other to efficiently pass on the excitation energy. For this reason, they are incorporated into proteins and accumulated within the thylakoid membrane. The second is that since the shorter the wavelength, the higher the light energy, there needs to be a sophisticated mechanism to absorb short-wavelength light and transfer its energy to long-wavelength pigments. Pigments found in algae and cyanobacteria vary in color because the spectrum of sunlight differs in water depending on the conditions of absorption and scattering. As shown in Column Figure 16-3, the light energy absorbed by the respective pigments is efficiently transferred to chlorophyll a, and therefore, a wide range of visible light can be used in photosynthesis.

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16.4.2

Dark reactions

The dark reaction or carbon dioxide fixation in photosynthesis is called the Calvin cycle*16 , after the name of the person who discovered it. This cycle is basically divided into three reaction groups (see Column Figure 16-5). The first is a group of reactions in which ribulose 1,5-bisphosphate incorporates carbon dioxide (CO2) to produce two molecules of phosphoglycerate (top of Figure 16-9) and is catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (also known as RuBisCO* ). The second is a group of reactions that use ATP and NADPH to produce and release sugar phosphates from the cycle, consequently synthesizing starch and sucrose. The third is a pathway in which various sugar phosphates are connected by equilibrium reactions, regenerating ribulose 5-phosphate, which is a precursor of ribulose 1,5-bisphosphate. The sum of all these dark reactions, excluding regeneration of ribulose 1,5-bisphosphate, can be expressed as follows:

CO2 + 3ATP + 2NADPH + 2H+ → (CH2O) + 2NADP + 3ADP + 3H3PO4

Figure 16-9 Two reactions catalyzed by RubisCO

Carbon dioxide fixation (top) and reaction with oxygen (bottom)


(CH2O) corresponds to sugar (the entry/exit of water molecules has been omitted). In other words, in order to fix one molecule of CO2, three molecules of ATP and two molecules of NADPH are required.

*16 The Calvin−Benson cycle or C3 cycle to differentiate from the C4 plant reaction (C4 cycle), described later. It is also called the reductive pentose phosphate cycle because it is based mainly on pentose conversion.

*17 The name “RubisCO” is the abbreviation of the underlined letters of ribulose 1,5-bis phosphate carboxylase/oxygenase.

column

Comparison of electron transport reactions between the respiratory chain and photosynthesis

To summarize the two mechanisms (Column Figure 16-4), part of their reaction pathways are very similar (tinted area in the figure); however, the starting and final materials are almost completely opposite. Electrons flow downhill in the respiratory chain from the lower to higher redox potential, that is, NADH (–0.32 V) to oxygen (+0.815 V), whereas in photosynthesis they are transferred uphill from H2O (+0.815 V) to NADPH (–0.32 V). This means that the photochemical system uses light energy to supply energy to electrons. This electron transport reaction is coupled to H+ transport, and F-ATP synthase uses the electrochemical gradient to synthesize ATP. In the respiratory chain, about three ATP molecules are synthesized per NADH molecule, while in photosynthesis, one NADPH molecule and approx. 1.5 ATP molecules are synthesized. However, when the cyclic electron transport pathway in the light reactions starts working, more ATP is synthesized. In photosynthesis, carbon dioxide fixation is carried out using both NADPH and ATP.

Column Figure 16-4 Electron transport in order of reaction in respiration and photosynthesis

See Figures 16-4 and 16-8 for details of each reaction.
The tinted area suggests homologous portions. Q is ubiquinone (CoQ) in respiration and plastocyanin (PQ) in photosynthesis. Cytochromes b, b6, c1, and f are homologous proteins. Cytochrome c and plastocyanin are small metal-binding proteins and are functionally homologous.

Respiration
Dehydrogenase → NADH → complex I → CoQ → complex III (cytochrome bc1) → cytochrome c → complex IV → O2

Photosynthesis
H2O → photosystem II → PQ → cytochrome b6f → plastocyanin → photosystem I → NADPH → dark reaction

One major characteristic of the carbon dioxide fixation reaction in photosynthesis lies in the unique nature of RuBisCO. As its name suggests, RuBisCO has both carboxylase and oxygenase activities; when oxygen concentration is high and carbon dioxide concentration is low, it incorporates O2 instead of CO2 and wastefully uses ribulose 1,5-bisphosphate (bottom of Figure 16-9). The reactivity of RuBisCO with CO2 is more than 100 times higher than its reactivity with O2; however, since the oxygen concentration in the atmosphere is 500 times higher than that of CO2, the oxygenase activity in chloroplasts cannot be ignored. A byproduct generated in the oxygenase reaction, phosphoglycolate, is converted back to phosphoglycerate using ATP and NADPH. This pathway is called photorespiration, because CO2 is released during the process. In addition, because reactions catalyzed by RuBisCO are much slower than those of other metabolic enzymes, the amount of RuBisCO required is several hundred times more than that of other enzymes that catalyze reactions before and after those catalyzed by RuBisCO. As a result, RuBisCO proteins account for over half the water-soluble proteins found in chloroplasts, making them the most abundant proteins on earth. These mysterious characteristics of RuBisCO reflect the high CO2 concentration and absence of O2 in the atmosphere of primitive earth when phototrophs first appeared, explaining why CO2 gas, which is difficult to differentiate from oxygen gas, was adopted as a substrate.

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16.4.3

C3 and C4 plants

For many plants, the initial carbon dioxide fixation product in the Calvin cycle is phosphoglycerate (with 3 carbon atoms), whereas in other plants, carbon dioxide is fixed as malic acid or aspartic acid (with 4 carbon atoms). Those in the former group, which includes rice, spinach, and trees, are called C3 plants, and those in the latter, which includes corn, are known as C4 plants.

In C4 plants, photosynthesis occurs in a roundabout way: carbon dioxide is fixed once by phosphoenolpyruvate carboxylase with a bicarbonate ion (HCO3: against which oxygen does not compete) as the substrate, but other enzymes release carbon dioxide in the cell again, and finally RuBisCO refixes the carbon dioxide for good. Although the C4 photosynthetic reaction uses extra energy (ATP), oxygenase activity of RuBisCO is not induced even when extracellular carbon dioxide concentration is low, thus allowing efficient photosynthesis. In this way, C4 plants have thrived over tens of millions of years, reducing the carbon dioxide concentration in the atmosphere from approx. 1% to the current 0.03%. In-depth research on the application of C4 photosynthesis has been actively carried out, with the aim of increasing the productivity of C3 plants such as rice.

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Carbon dioxide fixation route (Calvin cycle)

Reactions regulated by light (→) proceed unidirectionally. Excluding these reactions and carbon dioxide fixation reactions catalyzed by RuBisCO, all other reactions are reversible (↔). C5 and C3 represent the number of carbon atoms in the molecules.

Column Figure 16-5 Simplified diagram of the Calvin cycle

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