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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 16.3Photosynthetic Stages and Light-Absorbing Pigments

Image plant.jpgWe now shift our attention to photosynthesis, the second main process for synthesizing ATP. In plants, photosynthesis occurs in chloroplasts, large organelles found mainly in leaf cells. The principal end products are two carbohydrates that are polymers of hexose (six-carbon) sugars: the disaccharide sucrose (see Figure 2-10) and leaf starch, a large, insoluble glucose polymer (Figure 16-33). Leaf starch is synthesized and stored in the chloroplast. Sucrose is synthesized in the cytosol from three-carbon precursors generated in the chloroplast and is transported from the leaf to other parts of the plant. Nonphotosynthetic (nongreen) plant tissues like roots and seeds metabolize sucrose for energy by the pathways described in the previous sections. Photosynthesis in plants, as well as in eukaryotic single-celled algae and in several photosynthetic prokaryotes (the cyanobacteria and prochlorophytes), also generates oxygen. The overall reaction of oxygen-generating photosynthesis,

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is the reverse of the overall reaction by which carbohydrates are oxidized to CO2 and H2O.

Figure 16-33. Structure of starch.

Figure 16-33

Structure of starch. This large glucose polymer and the disaccharide sucrose (see Figure 2-10) are the principal end products of photosynthesis. Both are built of six-carbon sugars.

Our emphasis is on photosynthesis in plant chloroplasts, but we also discuss a simpler photosynthetic process that occurs in green and purple bacteria. Although photosynthesis in these bacteria does not generate oxygen, detailed analysis of their photosynthetic systems has provided insights about the first stages in oxygen-generating photosynthesis — how light energy is converted to a separation of negative and positive charges across the thylakoid membrane, with the simultaneous generation of a strong oxidant and a strong reductant. In this section, we provide an overview of the stages in photosynthesis and introduce the main components, including the chlorophylls, the principal lightabsorbing pigments.

Photosynthesis Occurs on Thylakoid Membranes

Chloroplasts are bounded by two membranes, which do not contain chlorophyll and do not participate directly in photosynthesis (Figure 16-34). Of these two membranes, the outer one, like the outer mitochondrial membrane, is permeable to metabolites of small molecular weight; it contains proteins that form very large aqueous channels. The inner membrane, conversely, is the permeability barrier of the chloroplast; it contains transporters that regulate the movement of metabolites into and out of the organelle.

Figure 16-34. The structure of a leaf and chloroplast.

Figure 16-34

The structure of a leaf and chloroplast. The chloroplast is bounded by a double membrane: the outer membrane contains proteins that render it permeable to small molecules (MW < 6000); the inner membrane forms the permeability barrier (more...)

Unlike mitochondria, chloroplasts contain a third membrane — the thylakoid membrane — that is the site of photosynthesis. In each chloroplast, the thylakoid membrane is believed to constitute a single, interconnected sheet that forms numerous small flattened vesicles, the thylakoids, which commonly are arranged in stacks termed grana (see Figure 16-34). The spaces within all the thylakoids constitute a single continuous compartment, the thylakoid lumen. The thylakoid membrane contains a number of integral membrane proteins to which are bound several important prosthetic groups and light-absorbing pigments, most notably chlorophyll. Carbohydrate synthesis occurs in the stroma, the soluble phase between the thylakoid membrane and the inner membrane. In photosynthetic bacteria extensive invaginations of the plasma membrane form a set of internal membranes, also termed thylakoid membranes, or simply thylakoids, where photosynthesis occurs.

Three of the Four Stages in Photosynthesis Occur Only during Illumination

It is convenient to divide the photosynthetic process in plants into four stages, each occurring in a defined area of the chloroplast: (1) absorption of light, (2) electron transport leading to the reduction of NADP+ to NADPH, (3) generation of ATP, and (4) conversion of CO2 into carbohydrates (carbon fixation). All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1 – 3 are catalyzed by proteins in the thylakoid membrane. The enzymes that incorporate CO2 into chemical intermediates and then convert it to starch are soluble constituents of the chloroplast stroma (see Figure 16-34). The enzymes that form sucrose from three-carbon intermediates are in the cytosol.

Absorption of Light

The initial step in photosynthesis is the absorption of light by chlorophylls attached to proteins in the thylakoid membranes. Like cytochromes, chlorophylls consist of a porphyrin ring attached to a long hydrocarbon side chain (Figure 16-35). They differ from cytochromes (and heme) in containing a central Mg2+ ion (rather than Fe atom) and having an additional five-membered ring. The energy of the absorbed light is used to remove electrons from an unwilling donor (water, in green plants), forming oxygen,

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and then to transfer the electrons to a primary electron acceptor, a quinone designated Q, which is similar to CoQ.

Figure 16-35. The structure of chlorophyll a, the principal pigment that traps light energy.

Figure 16-35

The structure of chlorophyll a, the principal pigment that traps light energy. Chlorophyll b differs from chlorophyll a by having a CHO group in place of the CH3 group (green). In the porphyrin ring, a highly conjugated system, electrons are delocalized (more...)

Electron Transport

Electrons move from the quinone primary electron acceptor through a chain of electron transport molecules in the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP+, reducing it to NADPH (see Figure 16-4). The transport of electrons is coupled to the movement of protons from the stroma to the thylakoid lumen, forming a pH gradient across the thyla-koid membrane (pHlumen < pHstroma), in much the same way that a proton-motive force is established across the mitochondrial inner membrane during electron transport (see Figure 16-2).

Thus the overall reaction of stages 1 and 2 can be summarized as

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Many photosynthetic bacteria do not use water as the donor of electrons. Rather, they use molecules such as hydrogen gas (H2) or hydrogen sulfide (H2S) as the ultimate source of electrons to reduce the ultimate electron acceptor (NAD+ rather than NADP+).

Generation of ATP

Protons move down their concentration gradient from the thylakoid lumen to the stroma through the F0F1 complex which couples proton movement to the synthesis of ATP from ADP and Pi. This use of the proton-motive force to synthesize ATP is identical with the analogous process occurring during oxidative phosphorylation in the mitochondrion (see Figures 16-28 and 16-30).

Carbon Fixation

The ATP4− and NADPH generated by the second and third stages of photosynthesis provide the energy and the electrons to drive the synthesis of polymers of six-carbon sugars from CO2 and H2O. The overall balanced equation is written as

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The reactions that generate the ATP and NADPH used in carbon fixation are directly dependent on light energy; thus stages 1 – 3 are called the light reactions of photosynthesis. The reactions in stage 4 are indirectly dependent on light energy; they are sometimes called the dark reactions of photosynthesis because they can occur in the dark, utilizing the supplies of ATP and NADPH generated by light energy. However, the reactions in stage 4 are not confined to the dark; in fact, they primarily occur during illumination.

Each Photon of Light Has a Defined Amount of Energy

Quantum mechanics established that light, a form of electromagnetic radiation, has properties of both waves and particles. When light interacts with matter, it behaves as discrete packets of energy (quanta) called photons. The energy of a photon, ϵ, is proportional to the frequency of the light wave: ϵ = hγ, where h is Planck’s constant (1.58 × 10−34 cal·s, or 6.63 × 10−34 J·s), and γ is the frequency of the light wave. It is customary in biology to refer to the wavelength of the light wave, λ, rather than to its frequency, γ. The two are related by the simple equation γ = c ÷ λ, where c is the velocity of light (3 × 1010 cm/s in a vacuum). Note that photons of shorter wavelength have higher energies.

Also, the energy in 1 mol of photons can be denoted by E = Nϵ, where N is Avogadro’s number (6.02 × 1023 molecules or photons/mol). Thus

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The energy of light is considerable, as we can calculate for light with a wavelength of 550 nm (550 × 10−7 cm), typical of sunlight:
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or about 52 kcal/mol, enough energy to synthesize several moles of ATP from ADP and Pi if all the energy were used for this purpose.

Chlorophyll a Is Present in Both Components of a Photosystem

The absorption of light energy and its conversion into chemical energy occurs in multiprotein complexes, called photosystems, located in the thylakoid membrane. A photosystem has two closely linked components, an antenna containing light-absorbing pigments and a reaction center comprising a complex of proteins and two chlorophyll a molecules. Each antenna (named by analogy with radio antennas) contains one or more light-harvesting complexes (LHCs). The energy of the light captured by LHCs is funneled to the two chlorophylls in the reaction center, where the primary events of photosynthesis occur.

Found in all photosynthetic organisms, both eukaryotic and prokaryotic, chlorophyll a is the principal pigment involved in photosynthesis, being present in both antennas and reaction centers. In addition to chlorophyll a, antennas contain other light-absorbing pigments: chlorophyll b in vascular plants, and carotenoids in both plants and photosynthetic bacteria (Figure 16-36). The presence of various antenna pigments, which absorb light at different wavelengths, greatly extends the range of light that can be absorbed and used for photosynthesis.

Figure 16-36. The structure of β-carotene, a pigment that assists in light absorption by chloroplasts.

Figure 16-36

The structure of β-carotene, a pigment that assists in light absorption by chloroplasts. β-Carotene, which is related to the visual pigment retinal (see Figure 21-47), is one of a family of carotenoids containing long hydrocarbon chains (more...)

One of the strongest pieces of evidence for the involvement of chlorophylls and β-carotene in photosynthesis is that the absorption spectrum of these pigments is similar to the action spectrum of photosynthesis (Figure 16-37). The latter is a measure of the relative ability of light of different wavelengths to support photosynthesis.

Figure 16-37. Photosynthesis at different wavelengths.

Figure 16-37

Photosynthesis at different wavelengths. (a) The action spectrum of photosynthesis in plants; that is, the ability of light of different wavelengths to support photosynthesis. (b) The absorption spectra for three photosynthetic pigments: chlorophyll (more...)

When chlorophyll a (or any other molecule) absorbs visible light, the absorbed light energy raises the chlorophyll a to a higher energy state, termed an excited state. This differs from the ground (unexcited) state largely in the distribution of electrons around the C and N atoms of the porphyrin ring (see Figure 16-35). Excited states are unstable, and will return to the ground state by one of several competing processes. For chlorophyll a molecules dissolved in organic solvents, such as ethanol, the principal reactions that dissipate the excited-state energy are the emission of light (fluorescence and phosphorescence) and thermal emission (heat). The situation is quite different when the same chlorophyll a is bound to the unique protein environment of the reaction center.

Light Absorption by Reaction-Center Chlorophylls Causes a Charge Separation across the Thylakoid Membrane

The absorption of a quantum of light of wavelength ≈680 nm causes a chlorophyll a molecule to enter the first excited state. The energy of such photons increases the energy of chlorophyll a by 42 kcal/mol. In the reaction center, this excited-state energy is used to promote a charge separation across the thylakoid membrane: an electron is transported from a chlorophyll molecule to the primary electron acceptor, the quinone Q, on the stromal surface of the membrane, leaving a positive charge on the chlorophyll close to the luminal surface (Figure 16-38). The reduced primary electron acceptor becomes a powerful reducing agent, with a strong tendency to transfer the electron to another molecule. The positively charged chlorophyll, a strong oxidizing agent, will attract an electron from an electron donor on the luminal surface. These potent biological reductants and oxidants provide all the energy needed to drive all subsequent reactions of photosynthesis: electron transport, ATP synthesis, and CO2 fixation.

Figure 16-38. The primary event in photosynthesis.

Figure 16-38

The primary event in photosynthesis. After a photon of light of wavelength ≈680 nm is absorbed by one of the many chlorophyll molecules in one of the light-harvesting complexes (LHCs) of an antenna (only one is shown), some of the absorbed energy (more...)

The significant features of the primary reactions of photosynthesis are summarized in the following model, in which P represents the chlorophyll a in the reaction center, and Q represents the primary electron acceptor:

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According to this model, the ground state of the reaction-center chlorophyll, P, is not a strong enough reductant to reduce Q; that is, an electron will not move spontaneously from P to Q. However, the excited state of the reactioncenter chlorophyll, P*, is an excellent reductant and rapidly (in about 10−10 seconds) donates an electron to Q, generating P+ and Q. This photochemical electron movement, which depends on the unique environment of both the chlorophylls and the acceptor within the reaction center, occurs nearly every time a photon is absorbed. The acceptor, Q, is a powerful reducing agent capable of transferring the electron to still other molecules, ultimately to NADP+. The powerful oxidant P+ can remove electrons from other molecules to regenerate the original P. In plants, the oxidizing power of four molecules of P+ is used, by way of intermediates, to remove four electrons from H2O to form O2:

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Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield — the amount of photosynthesis per absorbed photon — is the same for all wavelengths of visible light shorter than 680 nm.

The chlorophyll a molecules within reaction centers are capable of directly absorbing light and initiating photosynthesis. However, even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sun, ≈1.2 × 1020 photons/m2/s), each reaction-center chlorophyll a absorbs about one photon per second, which is not enough to support photosynthesis sufficient for the needs of the plant. To increase the efficiency of photosynthesis, especially at more typical light intensities, organisms utilize additional light-absorbing pigments.

Light-Harvesting Complexes Increase the Efficiency of Photosynthesis

As noted earlier, each reaction center is associated with an antenna, which contains several light-harvesting complexes (LHCs), packed with chlorophyll a and, depending on the species, chlorophyll b and other pigments. LHCs promote photosynthesis by increasing absorption of 680-nm light and by extending the range of wavelengths of light that can be absorbed (see Figure 16-37).

Photons can be absorbed by any of the pigment molecules in each LHC. The absorbed energy is then rapidly transferred (in <10−9 seconds) to one of the two chlorophyll a molecules in the associated reaction center, where it promotes the primary photosynthetic charge separation (see Figure 16-38). Within an LHC are several transmembrane proteins whose role is to maintain the pigment molecules in the precise orientation and position that are optimal for light absorption and energy transfer, thereby maximizing the very rapid and efficient process known as resonance transfer of energy from antenna pigments to reaction-center chlorophylls. As depicted in Figure 16-39a, some photosynthetic bacteria contain two types of LHCs: the larger type (LH1) is intimately associated with a reaction center; the smaller type (LH2) can transfer absorbed light energy to an LH1. Figure 16-39b shows the structure of the subunits that make up the LH2 complex in Rhodopseudomonas acidophila. Surprisingly, the molecular structures of plant light-harvesting complexes are completely different from those in bacteria, even though both types contain carotenoids and chlorophylls in a clustered geometric arrangement within the membrane.

Figure 16-39. Light-harvesting complexes from the photosynthetic bacterium Rhodopseudomonas acidophila.

Figure 16-39

Light-harvesting complexes from the photosynthetic bacterium Rhodopseudomonas acidophila. (a) Schematic depiction of the cylindrical LHCs and the reaction center as viewed from above the plane of the membrane. Each LH2 complex consists of nine subunits (more...)

Although antenna chlorophylls can transfer absorbed light energy, they cannot release an electron. As we’ve seen already, reaction-center chlorophylls are able to release an electron after absorbing a quantum of light. To understand their electron-releasing ability, we examine the structure and function of the reaction center in bacterial and plant photosystems in the next section.

SUMMARY

  •  The principal end products of photosynthesis are polymers of six-carbon sugars: starch and sucrose. The overall reaction of oxygen-generating photosynthesis is 6 CO2 + 6 H2O → 6 O2 + C6H12O6.
  •  Chloroplasts are surrounded by a permeable outer membrane and an inner membrane that forms the permeability barrier; neither of these membranes participates in photosynthesis. In the chloroplast interior, the thylakoid membrane is folded into numerous flattened vesicles; the light-capturing and ATP-generating reactions of photosynthesis occur on this membrane (see Figure 16-34).
  •  Photosynthesis in plants can be described in four stages, which occur in specific parts of the chloroplast.
  •  In stage 1, light is absorbed by chlorophyll a molecules bound to reaction-center proteins in the thylakoid membrane. The energized chlorophylls donate an electron to a quinone on the opposite side of the membrane, creating a charge separation (see Figure 16-38). In green plants, the positively charged chlorophylls then remove electrons from water, forming oxygen.
  •  In stage 2, electrons move from the quinone through a chain of electron transport molecules in the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP+, reducing it to NADPH. Transport of electrons is coupled to the movement of protons across the membrane from the stroma to the thylakoid lumen, forming a pH gradient across the thylakoid membrane.
  •  In stage 3, movement of protons down their electrochemical gradient through F0F1 complexes powers the synthesis of ATP from ADP and Pi.
  •  In stage 4, the ATP and NADPH generated in stages 2 and 3 provide the energy and the electrons to drive the fixation of CO2 and synthesis of carbohydrates. These reactions occur in the thylakoid stroma and cytosol.
  •  Chlorophyll a is the only light-absorbing pigment in reaction centers. Associated with each reaction center are multiple light-harvesting complexes (LHCs), which contain chlorophylls a and b, carotenoids, and other pigments that absorb light at multiple wavelengths.
  •  Energy is transferred from the LHC chlorophyll molecules to reaction-center chlorophylls by resonance energy transfer (see Figure 16-39).
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21598