20.1 Light Absorption in Chapter 20 Photosynthesis and Carbohydrate Synthesis in Plants

20.1 Light Absorption

Photophosphorylation (ATP synthesis driven by light) resembles oxidative phosphorylation in that electron flow through a series of membrane carriers is coupled to proton pumping, producing the proton motive force that powers ATP formation. The processes are compared in Figure 20-2. In oxidative phosphorylation, the electron donor is NADH and the ultimate electron acceptor is $O_{2}$ $upper O Subscript 2$ , forming $H_{2} O$ $upper H Subscript 2 Baseline upper O$ . In photophosphorylation, electrons flow in the opposite direction: $H_{2} O$ $upper H Subscript 2 Baseline upper O$ is the electron donor and NADPH is formed. How is this endergonic process possible?

A two-part figure shows the chemiosmotic mechanism for A T P synthesis in a chloroplast in part a and in a mitochondrion in part b. — FIGURE 20-2 The chemiosmotic mechanism for ATP synthesis in chloroplasts and mitochondria. (a) Movement of electrons through a chain of membrane-bound carriers in the chloroplast membrane is driven by the energy of photons absorbed by the green pigment chlorophyll. Electron flow leads to the movement of protons and positive charge across the membrane, creating an electrochemical potential. This electrochemical potential drives ATP synthesis by the membrane-bound ATP synthase, which is fundamentally similar in structure and mechanism to (b) the mitochondrial machinery for oxidative phosphorylation of mitochondria. In mitochondria, the force that moves electrons through the complexes is a large difference in the reduction potentials of electron donor and acceptor. In both systems, the energy made available by electron transfer is captured as a transmembrane proton gradient, which drives ATP synthesis by an ATP synthase.

Part a shows a green oval surrounded by a yellow thylakoid membrane. A wavy yellowish-range oval points from light to a purple structure embedded in the top of the thylakoid membrane. This purple structure lies horizontally in the membrane with two rounded protrusions extending down into the oval. To the lower left, two blue arrows point from H 2 O. One arrow points from H 2 O to O 2 and the second arrow points to e minus, from which another arrow points into the left-hand rounded protrusion of the purple structure. Accompanying text reads, light converts H 2 O to a good e minus donor. The purple structure is immediately adjacent to a light purple structure to its right adjacent to a light red structure, all aligned in the top center of the membrane. The light purple structure has two vertical rectangles joined at the top and bottom, where they flare outward to the left and right below the membrane. The light red structure is a horizontal rectangle with a rounded protrusion extending above the membrane at its upper left. Beneath the words electron transport chain, red highlighted H plus is shown above an arrow that points down through the right side of the purple structure, and through the left-hand flare of the light purple structure to point at red highlighted H plus in the light green interior of the oval. A blue arrow points from the light red structure to text above the membrane reading, N A D P plus (e minus acceptor). Text above reads, electron carriers pump H plus in as electrons flow to N A D P plus. The outside of the thylakoid membrane is lined with minus symbols and the interior is lined with plus symbols. Many red H plus are in the center of the oval. A small white circle is in the center left of the oval. Text in the center of the oval reads, energy of e minus flow is stored as electrochemical potential. A T P synthase is shown at the bottom center of the thylakoid membrane with a square portion embedded in the membrane that has a vertical rectangular portion to its right and short stalk below to a rounded piece from which a long strand bends back down to run along the right side and end adjacent to the vertical rectangular portion on its right. Red highlighted H plus inside the oval has an arrow pointing through the vertical rectangle of A T P synthase before curving right across the strand running along the right side of A T P synthase to end at red highlighted H plus outside of the oval. A D P plus P I to the left has a curved arrow through the rounded portion of A T P synthase to an orange oval labeled A T P. Text below reads, A T P synthase uses electrochemical potential to synthesize A T P. Part b shows a light orange oval surrounded by an inner mitochondrial membrane. Four structures are shown embedded across the top center of the membrane and are labeled electron carriers (respiratory chain). The left-hand structure is pink and has a horizontal piece in the membrane that bends down to a rounded lobe at the lower right. The second structure is tan and has a vertical portion that crosses the membrane attached to a rounded portion below. The third structure is blue and has a rounded portion above that crosses the membrane, two small horizontal protrusions beneath the membrane, and then a wider region below with a small invagination in its center. The fourth structure is green and roughly rectangular with two small dents on the left and right top and a small triangular cutout at the bottom. Many red highlighted H plus are shown outside of the oval. The outer surface of the inner mitochondrial membrane is lined with plus symbols and the interior surface is lined with minus symbols. A blue arrow points from a reduced e minus donor within the oval to e minus, from which a blue arrow points to the rounded lobe of the pink structure that starts the respiratory chain. Text above reads, reduced substrate (fuel) donates e minus. A red arrow points from red highlighted H plus below the membrane between the tan structure on the left and the blue structure on the right as it crosses the membrane to indicate red highlighted H plus above the membrane. A blue arrow points from the right-hand green structure to O 2 (e minus acceptor) below, within the oval. Text above reads, electron carriers pump H plus out as electrons flow to O 2. A white circle is in the left center of the oval. Text inside the oval reads, energy of e minus flow is stored as electrochemical potential. A T P synthase is shown with the same shape as in part a but inverted so that the rectangular portion is in the membrane below and the rounded portion is within the oval above. A red arrow points from red highlighted H plus below through the vertical rectangle at the right side of A T P synthase, within the membrane, then crosses the long piece that runs down the right side to end at H plus above the membrane. Text below reads, A T P synthase uses electrochemical potential to synthesize A T P.

Water is a poor donor of electrons; its standard reduction potential is 0.816 V, compared with $- 0.320 V$ $negative 0.320 upper V$ for NADH, a good electron donor. Photosynthesis requires the input of energy in the form of light to create a good electron donor and a good electron acceptor. Electrons flow from the electron donor through a series of membrane-bound carriers, including cytochromes, quinones, and iron-sulfur proteins, while protons are pumped across a membrane to create an electrochemical potential. Electron transfer and proton pumping are catalyzed by a membrane complex that is homologous in structure and function to Complex III of mitochondria. The electrochemical potential so produced is the driving force for ATP synthesis from ADP and $P_{i}$ $upper P Subscript i$ , catalyzed by a membrane-bound ATP synthase complex closely similar to that of mitochondria and bacteria.

Chloroplasts Are the Site of Light-Driven Electron Flow and Photosynthesis in Plants

In photosynthetic eukaryotic cells, both the light-dependent and the ${CO}_{2}$ $CO Subscript 2$ -assimilation reactions take place in chloroplasts (Fig. 20-3), organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, chloroplasts are surrounded by two membranes: an outer membrane that is permeable to small molecules and ions, and an impermeable inner membrane that bears specific transporters for a variety of ions and metabolites. The space enclosed by the inner membrane is called the stroma in chloroplasts and is analogous to the mitochondrial matrix; it is an aqueous phase containing most of the soluble enzymes required for the ${CO}_{2}$ $CO Subscript 2$ -assimilation reactions. Throughout the stroma is a highly organized set of topologically continuous internal membranes, forming a single compartment or lumen. This complex membrane system forms flattened sacks called thylakoids. Granal thylakoids are disk-like pouches arranged in stacks; they are connected by stromal thylakoids, which are flatter and spiral around a stack of grana. The thylakoid membranes provide a large area for the machinery of photophosphorylation — the photosynthetic pigments and enzyme complexes that carry out the light-dependent reactions and ATP synthesis. Traffic across these membranes is also mediated by specific transporters.

A two-part figure shows a schematic diagram of a chloroplast in part a and a colorized electron micrograph of a chloroplast in part b. — FIGURE 20-3 Chloroplast structure. (a) Schematic diagram. (b) Colorized electron micrograph at high magnification, showing the highly organized thylakoid membrane system.

In 1937, Robert Hill found that when leaf extracts containing chloroplasts were illuminated, they (1) evolved $O_{2}$ $upper O Subscript 2$ and (2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction

2 H_{2} O + 2 A \overset{light}{\to} 2 {AH}_{2} + O_{2}

$2 normal upper H 2 normal upper O plus 2 normal upper A right-arrow Overscript light Endscripts 2 upper A upper H 2 plus normal upper O 2$

where A is an artificial electron acceptor, or Hill reagent. One Hill reagent, the dye 2,6-dichlorophenolindophenol, is blue when oxidized (A) and colorless when reduced $({AH}_{2})$ $left-parenthesis AH Subscript 2 Baseline right-parenthesis$ , making the reaction easy to follow.

A figure shows the oxidized and reduced forms of 2,6-dichlorophenolindophenol.

When a leaf extract supplemented with the dye was illuminated, the blue dye became colorless and $O_{2}$ $upper O Subscript 2$ was evolved. In the dark, no $O_{2}$ $upper O Subscript 2$ evolution or dye reduction took place. This was the first evidence that absorbed light energy causes electrons to flow from some electron donor (now known to be $H_{2} O$ $upper H Subscript 2 Baseline upper O$ ) to an electron acceptor. Moreover, Hill found that ${CO}_{2}$ $CO Subscript 2$ was neither required nor reduced to a stable form under these conditions; $O_{2}$ $upper O Subscript 2$ production could be dissociated from ${CO}_{2}$ $CO Subscript 2$ reduction. Several years later, Severo Ochoa showed that ${NADP}^{+}$ $NADP Superscript plus$ is the biological electron acceptor in chloroplasts, according to the equation

2 H_{2} O + 2 {NADP}^{+} \overset{light}{\to} 2 NADPH + 2 H^{+} + O_{2}

$2 normal upper H 2 normal upper O plus 2 upper N upper A upper D upper P Superscript plus Baseline right-arrow Overscript l i g h t Endscripts 2 NADPH plus 2 normal upper H Superscript plus Baseline plus normal upper O 2$

To understand this photochemical process, we must first consider the more general topic of the effects of light absorption on molecular structure.

Visible light is electromagnetic radiation of wavelengths 400 to 700 nm, a small part of the electromagnetic spectrum (Fig. 20-4), ranging from violet to red. The energy of a single photon (a quantum of light) is greater at the violet end of the spectrum than at the red end; shorter wavelength (and higher frequency) corresponds to higher energy. The energy, E, in a single photon of visible light is given by the Planck equation:

E = h ν = h c / λ

$upper E equals h nu equals h c slash lamda$

where h is Planck’s constant $(6.626 \times 10^{- 34} J • s)$ $left-parenthesis 6.626 times 10 Superscript negative 34 Baseline upper J bullet s right-parenthesis$ , ν is the frequency of the light in cycles/s, c is the speed of light $(3.00 \times 10^{8} m/s)$ $left-parenthesis 3.00 times 10 Superscript 8 Baseline m slash s right-parenthesis$ , and λ is the wavelength of the light in meters. The energy of a photon of visible light ranges from 150 kJ/einstein for red light to ∼300 kJ/einstein for violet light.

A figure shows the spectrum of electromagnetic radiation with a close-up of the visible range that also includes the energy of photons across the range of wavelengths. — FIGURE 20-4 Electromagnetic radiation. The spectrum of electromagnetic radiation, and the energy of photons in the visible range. One einstein is $6.022 \times 10^{23}$ $6.022 times 10 Superscript 23$ photons.

FIGURE 20-4 Electromagnetic radiation. The spectrum of electromagnetic radiation, and the energy of photons in the visible range. One einstein is $6.022 \times 10^{23}$ $6.022 times 10 Superscript 23$ photons.

A horizontal bar is labeled type of radiation and the wavelengths of the different types of radiation are shown below. From left to right, the regions are as follows: Gamma rays: wavelength < 1 n m; X-rays: wavelength beginning at the < 1 n m cutoff and ranging to 100 n m; U V: wavelength 100 n m to 380 n m; Visible light: wavelength 380 to 750 n m; Infrared: wavelength 750 n m to < 1 mm; Microwaves: wavelength beginning at the < 1 m m cutoff and ranging to 1 m; Radiowaves: thousands of meters. The visible light range is shown in close-up as a horizontal bar ranging in wavelength from 380 to 750 n m. Energy (k J/Einstein) is shown below the bar. The bar is black at the far left for a short distance, then transitions into a violet region with an energy of 300 near its center. Past a wavelength of 430, it transitions into blue and then transitions into cyan at wavelength 500 and energy 240. All data are approximate. It transitions rapidly into green up to wavelength up to wavelength 560, where a short yellow region starts. Yellow transitions into orange at a wavelength of 600 and energy of 200, then transitions into red at a wavelength of 650. The spectrum reaches an energy of 170 near the right end of the red spectrum, then ends at a wavelength of 750.

WORKED EXAMPLE 20-1 Energy of a Photon

The light used by vascular plants for photosynthesis has a wavelength of about 700 nm. Calculate the energy in a “mole” of photons (an einstein) of light of this wavelength, and compare this with the energy needed to synthesize a mole of ATP.

SOLUTION:

The energy in a single photon is given by the Planck equation. At a wavelength of $700 \times 10^{- 9} m$ $700 times 10 Superscript negative 9 Baseline m$ , the energy of a photon is

\begin{matrix} E & = & h c / λ \\ = & \frac{[(6.626 \times 10^{- 34} J • s) (3.00 \times 10^{8} m/s)]}{(7.00 \times 10^{- 7} m)} \\ = & 2.84 \times 10^{- 19} J \end{matrix}

$StartLayout 1st Row 1st Column upper E 2nd Column equals 3rd Column h c slash lamda 2nd Row 1st Column Blank 2nd Column equals 3rd Column StartFraction left-bracket left-parenthesis 6.626 times 10 Superscript negative 34 Baseline upper J bullet s right-parenthesis left-parenthesis 3.00 times 10 Superscript 8 Baseline m slash s right-parenthesis right-bracket Over left-parenthesis 7.00 times 10 Superscript negative 7 Baseline m right-parenthesis EndFraction 3rd Row 1st Column Blank 2nd Column equals 3rd Column 2.84 times 10 Superscript negative 19 Baseline upper J EndLayout$

An einstein of light is Avogadro’s number of photons $(6.022 \times 10^{23})$ $left-parenthesis 6.022 times 10 Superscript 23 Baseline right-parenthesis$ ; thus the energy of one einstein of photons at 700 nm is given by

\begin{matrix} (2.84 \times 10^{- 19} J/photon) (6.022 & \times & 10^{23} photons/einstein) \\ = & 17.1 \times 10^{4} J/einstein \\ = & 171 kJ/einstein \end{matrix}

$StartLayout 1st Row 1st Column left-parenthesis 2.84 times 10 Superscript negative 19 Baseline upper J slash photon right-parenthesis left-parenthesis 6.022 2nd Column times 3rd Column 10 Superscript 23 Baseline photons slash einstein right-parenthesis 2nd Row 1st Column Blank 2nd Column equals 3rd Column 17.1 times 10 Superscript 4 Baseline upper J slash einstein 3rd Row 1st Column Blank 2nd Column equals 3rd Column 171 kJ slash einstein EndLayout$

So, a “mole” of photons of red light has about five times the energy needed to produce a mole of ATP from ADP and $P_{i}$ $upper P Subscript i$ (30.5 kJ/mol).

When a photon is absorbed, an electron in the absorbing molecule (chromophore) is lifted to a higher energy level. This is an all-or-nothing event: to be absorbed, the photon must contain a quantity of energy, called a quantum, that exactly matches the energy of the electronic transition. A molecule that has absorbed a photon is in an excited state, which is generally unstable. An electron lifted into a higher-energy orbital usually returns rapidly to its lower-energy orbital; that is, the excited molecule decays to the stable ground state, giving up the absorbed quantum as light or heat or using it to do chemical work. Light emission accompanying decay of excited molecules, fluorescence, is always at a longer wavelength (lower energy) than that of the absorbed light (see Box 12-1). An alternative mode of decay, central to photosynthesis, involves direct transfer of excitation energy from an excited molecule to a neighboring molecule. Just as the photon is a quantum of light energy, so the exciton is a quantum of energy passed from an excited molecule to another molecule in a process called exciton transfer.

Chlorophylls Absorb Light Energy for Photosynthesis

The most important light-absorbing pigments in the thylakoid membranes are the chlorophylls, green pigments with polycyclic, planar structures resembling the protoporphyrin of hemoglobin, except that ${Mg}^{2 +}$ $Mg Superscript 2 plus$ , not ${Fe}^{2 +}$ $Fe Superscript 2 plus$ , occupies the central position (Fig. 20-5a; compare to Fig. 5-1). The four inward-oriented nitrogen atoms of chlorophyll are coordinated with the ${Mg}^{2 +}$ $Mg Superscript 2 plus$ . All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV, and chlorophylls also have a fifth five-membered ring not present in heme.

A four-part figure shows the structures of primary and secondary photopigments as follows: part a showing chlorophylls italicized a end italics and italicized b end italics; part b shows beta-carotene; part c shows lutein; and part d shows phycoerythrobilin and phycocanobilin. — FIGURE 20-5 Primary and secondary photopigments. (a) Chlorophylls a and b and bacteriochlorophyll are the primary gatherers of light energy. (b) β-Carotene (a carotenoid) and (c) lutein (a xanthophyll) are accessory pigments in plants. (d) Phycoerythrobilin and phycocyanobilin (phycobilins) are accessory pigments in cyanobacteria and red algae. The conjugated bond systems in these molecules (alternating single and double bonds, shaded) have delocalized electrons that are easily excited by photons with the wavelengths of visible light.

Part a shows the structure of chlorophyll italicized a end italics as a complex ring structure with a chain extending to the lower left. A note indicates the substituent that differs in chlorophyll italicized b end italics. The ring structure contains M g in the center and its outer boundary of alternating double and single bonds is highlighted. There are four symmetrical partial six-membered rings around M g with five-membered rings located between them at the upper left, upper right, lower right, and lower left. The five-membered rings are numbered with Roman numerals. The outline of the molecule is highlighted as part of the conjugated double bond system except for the five-membered ring at the lower left labeled Roman numeral 4. All of the five-membered ring at the upper right labeled Roman numeral 2 is highlighted. A phytol side chain extends from the a two-carbon chain bonded tot he bottom vertex of five-membered ring Roman numeral 4 at the lower left side. The detailed structure follows. The top center partial six-membered ring has a double bond at its upper left side, a single bond at its upper right side, a double bond at its right side shared with a five-membered ring to its right labeled Roman numeral 2, N substituted for C at its lower right vertex with an arrow to M g in the center that forms its lower right side, a left side shared with the right side of a five-membered ring to its left labeled Roman numeral 1, and N at its lower left vertex with an arrow to M g in the center that forms its lower left side. All parts of the ring except for the left side bond, N at the lower left vertex, and the arrows to M g are highlighted as part of the conjugated bond system. Five-membered rings are to the left and right of this central ring. The top left-hand ring is labeled Roman numeral 1. Shared with the top center ring, it has N at its lower right vertex with an arrow to M g in the center. It has a bottom single bond that is not highlighted that is shared with the left center six-membered ring located below. It has highlighted single bonds at its lower left and upper right sides, a highlighted double bond at its upper left side, a left side vertex bonded to C H 3, and a top vertex bonded to C H double bonded to C H 2 above with a callout indicating that this substituent is C double bonded to O to the lower left and to C H 3 above in bacteriochlorophyll. The top right-hand ring is labeled Roman numeral 2. It has a highlighted double bond on its left side shared with the top center six-membered ring; highlighted N at the lower left vertex shared with the top center and right side six-membered rings; a highlighted single bond at its bottom side that forms the top of the right-hand six-membered ring below; a highlighted single bond on its lower right side; a right side vertex bonded to C H 2 C H 3; a highlighted double bond on its upper right side with a callout reading, saturated bond in bacteriochlorophyll; a top vertex bonded to C H 3 with a callout indicating that this substituent is replaced with C H O in chlorophyll italicized b end italics, and a highlighted single bond on its upper left side. The right center six-membered ring has highlighted N at its upper left vertex shared with the top center six-membered ring and five-membered ring Roman numeral 2 that has an arrow to M g at the center that forms its upper left side, a highlighted top bond shared with the five-membered ring Roman numeral 2 above, a highlighted double bond at its upper right side, a highlighted single bond at it slower right side, a nonhighlighted single bond at its bottom side shared with five-membered ring Roman numeral 3 at the lower right below, and N at the lower left vertex shared with five-membered ring Roman numeral 3 below and the bottom center six-membered ring that has an arrow to M g in the center that forms its lower left side. The bottom right five-membered ring labeled Roman numeral 3 shares its top bond with the right-hand six-membered ring above, shares N at its upper left vertex with the six-membered rings at the right center and bottom center, has a highlighted double bond on its upper right side, has a right side vertex bonded o C H 3, has a highlighted bond at its lower right side, has a bottom vertex bonded to a vertical bond that forms the right side of a five-membered ring below, has a highlighted double bond at its lower left side, and has a bond at its left side shared with the center six-membered ring to its left that extends to N at its upper left vertex shared with the six-membered rings above and to its left. The five-membered ring below has a highlighted upper right vertex shared with five-membered ring Roman numeral 3 above, a lower right vertex double bonded to O, lower left vertex hashed wedge bonded to H and solid wedge bonded to C bonded to O C H 3 and double bonded to O, and a highlighted upper left vertex shared with the bottom center six-membered ring above. The bottom center six-membered ring has a nonhighlighted bond on its right side shared with five-membered ring Roman numeral 3, N at its upper right side vertex shared with five-membered ring Roman numeral 3 and the right center six-membered ring that has a red arrow to M g in the center that forms its upper right side, a highlighted bond at its lower right side, a highlighted double bond at its lower left side, a highlighted bond at its left side that extends to N shared with five-membered ring Roman numeral 4 to the left and the left center six-membered ring from which a red arrow extends to M g in the center to form its upper left side, and a lower left vertex connected to a bond that forms the lower right side of five-membered ring Roman numeral 4 to the left. Five-membered ring Roman numeral 4 at the lower left has a highlighted vertical right side bond shared with the six-membered ring to its right, N at its upper right vertex shared with the six-membered rings to its right and above, a highlighted double bond at its top side shared with the six-membered ring above, a single bond at its upper left side, a left side vertex hashed wedge bonded to C H 3 and solid wedge bonded to H, a single bond at its lower left side, and a bottom vertex hashed wedge bonded to H and solid wedge bonded to C H 2 bonded to C H 2 bonded to C double bonded to O bonded to O that bonds to a 17-carbon zigzag chain. The right side six-membered ring has a top single bond shared with five-membered ring Roman numeral 1 above, N at the upper right vertex shared with five-membered ring Roman numeral 1 above and with the top center six-membered ring, N at its upper right vertex shared with five-membered ring Roman numeral 1 above and with the top center six-membered ring that has an arrow pointing to M g that forms the upper right side, a highlighted double bond at its upper left side, a highlighted single bond at its lower left side, a highlighted double bond at its bottom side shared with five-membered ring Roman numeral 4 below, and highlighted N at its lower right vertex shared with five-membered ring Roman numeral 1 above and with the bottom center six-membered ring that has an arrow to M g that forms its lower right side. The left center six-membered ring has a lower right vertex that forms the upper left vertex of five-membered ring Roman numeral 4. The 17-carbon zigzag chain beginning with O on the right is labeled phytol side chain. Beginning with O, it is C H 2 bonded to C H double bonded to C bonded to C H 3 above and bonded to a chain of 3 C H 2 bonded to C H bonded to C H 3 above bonded to a chain of 3 C H 2 bonded to C H bonded to C H 3 above bonded to a chain of 3 C H 2 bonded to C H bonded to C H 3 above and bonded to C H 2 bonded to C H 3. Part b shows beta carotene as a six-carbon ring that has a highlighted double bond on the right side, a lower right vertex bonded to C H 3, a top vertex bonded to 2 C H 3, and an upper right vertex bonded to a red highlighted eighteen-carbon zigzag chain of C H double bonded to C H bonded to C bonded to C H 3 above and double bonded to C H bonded to C H double bonded to C H bonded to C bonded to C H 3 above and double bonded to C H bonded to C H double bonded to C H bonded to C H double bonded to C bonded to C H 3 below and bonded to C H double bonded to C H bonded to C H double bonded to C bonded to C H 3 below and bonded to C H double bonded to C H bonded to the lower left vertex of a six-carbon ring with a highlighted double bond on its left side, an upper left vertex bonded to C H 3, and a bottom vertex bonded to 2 C H 3. Part c shows lutein (xanthophyll), which is similar to beta-carotene except that the six-membered ring at the left side has a lower left vertex bonded to O H, the bond connecting the eighteenth carbon of the zigzag chain to the lower right vertex of the right-hand six-membered ring is not highlighted, and the right-hand six-membered ring has no highlighted bonds, a double bond at the upper left side, an upper left vertex bonded to C H 3 only, an upper right vertex bonded to O H, and a bottom vertex bonded to 2 C H 3. Part d shows phycoerythrobilin, which has a chain of four five-membered rings. Red highlighted bonds beginning with a double bond connected to the upper right vertex of the first ring trace the right side of that ring, the bond connecting it to the lower right vertex of the second ring, the part of the second ring from the lower left vertex clockwise to the lower right vertex, the bond connecting this vertex to the lower right vertex of the third ring, and the part of the third ring from the lower left vertex clockwise to the double bond at the lower right side. The top bond and substituent bonded to the upper right vertex are highlighted in the fourth ring. The detailed structure is as follows. It has a five-membered ring with N H at the bottom vertex, a lower left vertex double bonded to O, an upper left vertex bonded to C H 3, a highlighted upper right vertex double bonded to C H connected by a nonhighlighted bond to C H 3, a highlighted right side, and a lower right side connected to a highlighted double bond connected to a highlighted single bond connected to the lower left vertex of a second five-membered ring. The second ring has N H at its bottom vertex, an upper left vertex bonded to C H 3, an upper right vertex bonded to C H 2 C H 2 C O O minus, highlighted double bonds on its upper left and right sides, a highlighted top bond, and a lower right vertex connected by a highlighted single bond to a highlighted double bond connected to the lower left vertex of the third ring. This ring has N at the bottom vertex, an upper left vertex bonded to C H 2 C H 2 C O O minus, an upper right vertex bonded to C H 3, red highlighted bonds at its top and lower right sides, highlighted single bonds at its upper left and right sides, and a lower right vertex bonded to C H 2 bonded to the lower left vertex of the fourth ring. Text accompanying the bond to the lower left vertex of the fourth ring reads, unsaturated bond in phycocyanobilin. The fourth ring has N H at the bottom vertex, a lower right vertex double bonded to O, an upper left vertex bonded to C H 3, a highlighted double bond at the top side, and an upper right vertex connected by a highlighted bond to highlighted C bonded to H and connected by a highlighted double bond to C H 2. A bracket enclosing the substituent bonded to the upper right vertex of the fourth ring notes that this substituent is replaced by C H 2 bonded to C H 3 in phycocyanobilin.

The heterocyclic five-ring system that surrounds the ${Mg}^{2 +}$ $Mg Superscript 2 plus$ has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 20-6); the chlorophylls have unusually high molar extinction coefficients (see Box 3-1) and are therefore particularly well-suited for absorbing visible light during photosynthesis.

A graph shows the absorption of visible light by photopigments by plotting absorption against wavelength. — FIGURE 20-6 Absorption of visible light by photopigments. Plants are green because their pigments absorb light from the red and blue regions of the spectrum, leaving primarily green light to be reflected. Compare the absorption spectra of the pigments with the spectrum of sunlight reaching the earth’s surface; the combination of chlorophylls (a and b) and accessory pigments enables plants to harvest most of the energy available in sunlight. The relative amounts of chlorophylls and accessory pigments are characteristic of a particular plant species. Variation in the proportions of these pigments is responsible for the range of colors of photosynthetic organisms, from the deep blue-green of spruce needles, to the greener green of maple leaves, to the red, brown, or purple color of some species of multicellular algae and the leaves of some foliage plants favored by gardeners.

The horizontal axis is labeled wavelength (n m) and ranges from 300 to 800, labeled in increments of 100. The vertical axis is labeled absorption. Seven curves are plotted on the graph as follows. Sunlight reaching the earth: A black curve begins at the lower left and rises in a bumpy curve to peak at 490 on the horizontal axis near the top of the vertical axis, then drops slightly to 525 on the horizontal axis at three-quarters of the height of the height of the vertical axis, then decreases slowly to 640 on the horizontal axis and slightly lower on the vertical axis before declining more rapidly in a bumpy curve to end at 760 on the horizontal axis and one-quarter of the height of the vertical axis. Chlorophyll italicized a end italics: A purple curve begins at 380 on the horizontal axis and just below one-quarter of the height of the vertical axis, rises with a jag to peak at 420 on the horizontal axis and one-half of the height of the vertical axis, decreases rapidly to 440 on the horizontal axis near the bottom of the vertical axis, rises slightly from 580 to 620 on the horizontal axis and one-eighth of the height of the vertical axis, dips slightly, then rises sharply to peak at 670 on the horizontal axis at almost one-half the height of the vertical axis before dropping to end at 700 on the horizontal axis just above the horizontal axis. Beta carotene: An orange curve begins at 370 on the horizontal axis just below and slightly to the right of the purple curve for chlorophyll italicized a end italics and rises to peak at 430 on the horizontal axis at just over half the height of the vertical axis, bumps down slightly before rising to 500 on the horizontal axis at just below its previous peak, then decreases sharply to end at 525 on the horizontal axis near the bottom of the vertical axis. Chlorophyll italicized b end italics: A green curve begins at 400 on the horizontal axis near the bottom of the vertical axis, rises with a jag to peak at 440 on the horizontal axis at three-quarters of the height of the vertical axis, drops rapidly to 470 on the horizontal axis near the bottom of the vertical axis, curves slightly down to run almost horizontally along the horizontal axis, then rises sharply beginning at 620 on the horizontal axis to peak at 650 on the horizontal axis and just under one-quarter of the height of the vertical axis, then decreases smoothly to end at 680 on the horizontal axis near the bottom of the vertical axis. Lutein: A black curve begins just before the curve for chlorophyll italicized b end italics, peaks at 430 on the horizontal axis at one-quarter of the horizontal axis, dips slightly before peaking at 460 slightly lower than its previous peak, then decrease to end at 510 on the horizontal axis near the bottom of the vertical axis. Phycoerythrin: A red curve begins at 440 on the horizontal axis near the bottom of the vertical axis, increase to peak at 500 on the horizontal axis at about one-half the height of the vertical axis, decreases slightly before rising to peak at 540 on the horizontal axis and almost three-quarters the height of the vertical axis, then decreases slightly before peaking at 570 on the horizontal axis and three-quarters of the horizontal axis and then decreasing rapidly to end at 600 near the bottom of the horizontal axis. Phycocyanin: A blue curve begins at 510 on the horizontal axis and increases smoothly to peak at 620 on the horizontal axis at three-quarters of the height of the vertical axis, then decreases smoothly to end at 690 on the horizontal axis near the bottom of the vertical axis. All data are approximate.

Chloroplasts always contain both chlorophyll a and chlorophyll b (Fig. 20-5a). Although both are green, their absorption spectra are sufficiently different (Fig. 20-6) that they complement each other’s range of light absorption in the visible region. Most plants contain about twice as much chlorophyll a as chlorophyll b. The chlorophyll in cyanobacteria differs only slightly from those of plants.

In addition to chlorophylls, thylakoid membranes of plants contain secondary light-absorbing pigments, or accessory pigments, called carotenoids. Carotenoids may be yellow, red, or purple. The two most prominent in plant leaves are β-carotene, a red-orange isoprenoid, and the yellow carotenoid lutein (Fig. 20-5b, c). Cyanobacteria and red algae use the accessory pigments phycocyanobilin and phycoerythrobilin (Fig. 20-5d). Accessory pigments absorb light at wavelengths not absorbed by the chlorophylls (Fig. 20-6) and thus are supplementary light receptors. They also protect downstream components from a highly reactive form of oxygen (singlet oxygen) that is formed when intense light exceeds the system’s capacity to accept electrons.

Experimental determination of the effectiveness of light of different colors in promoting photosynthesis yields an action spectrum (Fig. 20-7), often useful in identifying the pigment primarily responsible for a biological effect of light. By capturing light in a region of the spectrum not used by other organisms, a photosynthetic organism can claim a unique ecological niche.

A figure shows the two ways to determine the action spectrum for photosynthesis by showing experimental results obtained by Engelmann in 1881 in part a and a modern experiment in part b. — FIGURE 20-7 Two ways to determine the action spectrum for photosynthesis. (a) Results of a classic experiment performed by T. W. Engelmann in 1882 to determine the wavelength of light that is most effective in supporting photosynthesis. Engelmann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with light from a prism, so that one part of the algal filament received mainly blue light, another part yellow, another red. To determine which cells carried out photosynthesis most actively, Engelmann also placed on the microscope slide bacteria known to migrate toward regions of high $O_{2}$ $upper O Subscript 2$ concentration. After a period of illumination, the distribution of bacteria showed highest $O_{2}$ $upper O Subscript 2$ levels (produced by photosynthesis) in the regions illuminated with violet and red light. (b) Results of a similar experiment that used modern techniques (an oxygen electrode) for the measurement of $O_{2}$ $upper O Subscript 2$ production. An action spectrum, as shown here, describes the relative rate of photosynthesis for illumination with a constant number of photons of different wavelengths. An action spectrum is useful because, by comparison with absorption spectra (such as those in Fig. 20-6), it suggests which pigments can channel energy into photosynthesis.

FIGURE 20-7 Two ways to determine the action spectrum for photosynthesis. (a) Results of a classic experiment performed by T. W. Engelmann in 1882 to determine the wavelength of light that is most effective in supporting photosynthesis. Engelmann placed cells of a filamentous photosynthetic alga on a microscope slide and illuminated them with light from a prism, so that one part of the algal filament received mainly blue light, another part yellow, another red. To determine which cells carried out photosynthesis most actively, Engelmann also placed on the microscope slide bacteria known to migrate toward regions of high $O_{2}$ $upper O Subscript 2$ concentration. After a period of illumination, the distribution of bacteria showed highest $O_{2}$ $upper O Subscript 2$ levels (produced by photosynthesis) in the regions illuminated with violet and red light. (b) Results of a similar experiment that used modern techniques (an oxygen electrode) for the measurement of $O_{2}$ $upper O Subscript 2$ production. An action spectrum, as shown here, describes the relative rate of photosynthesis for illumination with a constant number of photons of different wavelengths. An action spectrum is useful because, by comparison with absorption spectra (such as those in Fig. 20-6), it suggests which pigments can channel energy into photosynthesis.

Part a shows a horizontal line of five green rectangles against a background that changes color across the spectrum of visible light from left to right with violet at the far left, then blue, then green, then yellow, and finally red on the right. Many small dots are visible above and below the rectangles. A triangular clump of dots extends up across the two left rectangles, peaking at about the point where they meet in the blue region. A similar triangle extends downward at the same horizontal location. Thin layers of dots run along the middle region of rectangles, then similar triangles extend above and below the last two rectangles peaking in the region where orange becomes red. Part b shows a graph that plots relative rate of photosynthesis against wavelength. The horizontal axis is labeled wavelength (n m) and ranges from below 400 to above 700, labeled in increments of 100. The vertical axis is labeled relative rate of photosynthesis and ranges from 0 to 100, labeled in increments of 20. A curve begins at (380, 82), rises to (410, 102), drops to (470, 90), rises to (480, 92), drops to (540, 35), rises to (640, 82), drops to (645, 80), rises to (660, 99), then drops to (690, 0). All data are approximate.

Chlorophylls Funnel Absorbed Energy to Reaction Centers by Exciton Transfer

The light-absorbing pigments of thylakoid or bacterial membranes are arranged in functional arrays called photosystems. In spinach chloroplasts, for example, each photosystem contains about 200 chlorophyll and 50 carotenoid molecules. All the pigment molecules in a photosystem can absorb photons, but only one pair of chlorophyll molecules associated with the photochemical reaction center is specialized to transduce light into chemical energy. The other pigment molecules in a photosystem serve as antenna molecules. They absorb light energy and transmit it rapidly and efficiently to the reaction center (Fig. 20-8). Some chlorophylls are part of a core complex around the reaction center. Others form light-harvesting complexes (LHCs) around the periphery of the core complex. Chlorophyll and other pigments are always associated with specific binding proteins, which fix the chromophores in relation to each other, to other protein complexes, and to the membrane. For example, each monomer of the trimeric light-harvesting complex LHCII (Fig. 20-9) contains seven molecules of chlorophyll a, five of chlorophyll b, and two of lutein.

A figure shows organization of photosystems in the thylakoid membrane. — FIGURE 20-8 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding each reaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (red arrow).

A schematic drawing of a chloroplast is cut away so that the stroma, grana, and stromal thylakoid are visible. The top of one granum has been cut along its length so that it is possible to see into the discs. A closeup shows a half-circle with the rounded portion extending away from the viewer and a cylinder in the front center labeled reaction center. This cylinder has a cylindrical opening in the center and consists of three pieces: a light red top piece, a light blue center piece, and a purple bottom piece. Text below reads, reaction center, photochemical reaction here converts the energy of a photon into a separation of charge, initiating electron flow. Four vertical rectangles extend across the membrane on each side of the reaction center. Lines of green squares radiate out from the reaction center, showing that more of these rectangles are present. The green squares are labeled, antenna chlorophylls, bound to protein. Several yellow squares are also present and there are labeled, carotenoids, other accessory pigments. Phospholipid bilayer membrane is visible to the left and right. A bracket pointing to the text describing antenna chlorophylls, carotenoids, and other accessory pigments reads, These molecules absorb light energy, transferring it between molecules until it reaches the reaction center. A wavy yellowish-range arrow points from light above to a green square near the outer boundary of the half-circle structure. From this green square, a red line curves up and then down to an adjacent square, from which another red line curves to meet a third square, from which another red line curves up to meet a fourth square, from which another red line curves up across the light red top of the reaction center to end in the light blue region of the reaction center.

A figure shows the light-harvesting compound L H C Roman numeral 2 of the pea as a combination of helices and wireframe skeletons. — FIGURE 20-9 The light-harvesting complex LHCII of the pea. The functional unit is a trimer, with 36 chlorophyll and 6 lutein molecules. Shown here is a monomer, viewed in the plane of the membrane, with its three transmembrane α-helical segments, seven chlorophyll a molecules (light green), five chlorophyll b molecules (dark green), and two molecules of lutein (yellow), which form an internal cross-brace. [Data from PDB ID 2BHW, J. Standfuss et al., *EMBO J.* 24:919, 2005.]

A helix runs slightly diagonally along the left side to end at a green wireframe structure with a sphere behind it. Two helices run almost vertically in the center, beginning together and moving apart so there is a space between them at the top. Small helices extend to the left and right of the top of these two central helices. Wireframe ring structures with red spheres in the center are between the left-hand helix and the central helices. A similar structure is at the lower right side and is labeled chlorophyll italicized b end italics. A similar blue structure to its upper left is labeled chlorophyll italicized a end italics. More of these blue structures are present across the front of the two vertical helices and immediately to their left, one is near the top where the left-hand vertical structure joins with the helix that angles to the upper left, and two are visible beneath the angled helix to the upper right. Yellow wireframe structure cross at the center of the two vertical strands and extend to the upper and lower right and left.

The chlorophyll molecules in light-harvesting complexes and other chlorophyll-binding proteins have light-absorption properties that are subtly different from those of free chlorophyll. When isolated chlorophyll molecules are excited by light, the absorbed energy is quickly released as fluorescence and heat; but when chlorophyll in intact leaves is excited by visible light (Fig. 20-10, step ), very little fluorescence is observed. Instead, the excited antenna chlorophyll transfers energy directly to a neighboring chlorophyll molecule, which becomes excited as the first molecule returns to its ground state (step ). This transfer of energy, exciton transfer, extends to a third, fourth, or subsequent neighbor, until one of a “special pair” of chlorophyll a molecules at the photochemical reaction center is excited (step ). The special pair of chlorophyll molecules, often designated ${(Chl)}_{2}$ $left-parenthesis Chl right-parenthesis Subscript 2$ , are held close enough to each other to share bonding orbitals, and to react as a single compound when excited. In this excited chlorophyll pair, an electron is promoted to a higher-energy orbital. This electron then passes to a nearby electron acceptor that is part of the photosynthetic electron-transfer chain, leaving the reaction-center chlorophyll pair with a missing electron (an “electron hole,” denoted by $+$ $plus$ in Fig. 20-10) (step ). The electron acceptor acquires a negative charge in this transaction. The electron lost by the reaction-center chlorophyll pair is replaced by an electron from a neighboring electron-donor molecule (step ), which thereby becomes positively charged. In this way, excitation by light causes electric charge separation and initiates an oxidation-reduction chain.

A figure shows a generalized scheme of how energy from an absorbed photon is used to separate charges at the reaction center in five steps. — FIGURE 20-10 Exciton and electron transfer. This generalized scheme shows conversion of the energy of an absorbed photon into separation of charges at the reaction center. Note that step may repeat between successive antenna molecules until the exciton reaches the special pair of chlorophylls in the reaction center. An asterisk (*) denotes the excited state of a molecule.

FIGURE 20-10 Exciton and electron transfer. This generalized scheme shows conversion of the energy of an absorbed photon into separation of charges at the reaction center. Note that step may repeat between successive antenna molecules until the exciton reaches the special pair of chlorophylls in the reaction center. An asterisk (*) denotes the excited state of a molecule.

Part of a thylakoid membrane is shown with a reaction center in the front center within a horizontal piece of membrane that contains vertical green rectangles. An arrow points down from one of these green rectangles to a green rectangle below shown next to a second green rectangle. These two green rectangles are labeled antenna molecules. Each antenna molecule has two short horizontal lines aligned horizontally near the top and a horizontal line at the bottom with a red dot in the center. A yellowish-orange arrow labeled light points to the center of the left-hand antenna molecule. A cylinder to the right is labeled reaction-center chlorophyll special pair. This cylinder has a cylindrical opening inside and has three parts: a light red top, a light blue center, and a purple bottom. Each part has two horizontally aligned short horizontal lines near the top and a horizontal line near the bottom with a red dot in its center. Step 1: Light excites an antenna molecule (chlorophyll or accessory pigment), raising an electron to a higher energy level. An arrow points down to show a similar pair of antenna molecules next to a reaction center. The left-hand antenna molecule now has a horizontal line near the top with a red dot int eh center and an asterisk to the left. It also has two horizontally aligned short horizontal lines near the bottom. A red arrow points from this antenna molecule to the right-hand antenna molecule. Step 2: The excited antenna molecule passes energy to a neighboring chlorophyll molecule (exciton transfer), exciting it. Two similar antenna molecules are shown next to a reaction center. An arrow points down from the right-hand antenna molecule above to the right-hand antenna molecule in this step. The left-hand antenna molecule now has two horizontally aligned short horizontal lines near the top and a horizontal line near the bottom with a red dot in the middle. The right-hand antenna molecule has a horizontal line near the top with a red circle in the center and an asterisk to the left. It also has two horizontally aligned short horizontal lines near the bottom. An arrow points from the right-hand antenna molecule to the blue portion of the reaction center to the right. Step 3: This energy is transferred to a chlorophyll of the reaction-center special pair, exciting it. The two antenna molecules have returned to their original state. An arrow points down from the reaction center above to the one in this step. The reaction center now has an asterisk next to the blue part, which now has a horizontal bar near its top with a red dot in the center and two horizontally aligned short horizontal lines below. The top light red part of the reaction center is labeled, electron acceptor. Step 4: The excited reaction-center chlorophyll passes an electron to an electron acceptor. The two antenna molecules are present next to the reaction center. An arrow points down from the reaction center above to the one in this step. The central blue portion of the reaction center has horizontally aligned short horizontal lines near the top and bottom. The upper light red portion of the reaction center now has two red dots on the horizontal line near its bottom. The purple bottom portion of the reaction center is now labeled electron donor. Step 5: The electron hole in the reaction center is filled by an electron from an electron donor. An arrow points down from the reaction center above to the one in this step. The blue center part of the reaction center now has a horizontal line with a single red dot at its bottom. The purple bottom part of the reaction center now has two horizontally aligned short horizontal lines near its top and bottom.

SUMMARY 20.1 Light Absorption

Photosynthesis takes place in plant chloroplasts, structures enclosed in double membranes and filled with an elaborate system of thylakoid membranes containing the photosynthetic machinery.
Chlorophyll molecules and other light-absorbing pigments are associated with proteins in light-harvesting complexes arrayed around photochemical reaction centers. The proteins are embedded in thylakoid membranes.
The many chlorophyll molecules that surround the reaction center serve as antennas for light. When they absorb light, they pass its energy (exciton) to the reaction center. There the energy is used to create a charge separation that initiates electron flow through a series of oxidation-reduction reactions.