Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes.
1. C H A P T E R 8 Photosynthesis KEY CONCEPTS 8.1 Photosynthesis converts light energy to the chemical energy of food 8.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH 8.3 The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar ▲ Figure 8.1 How does sunlight help build the trunk, branches, and leaves of this broadleaf tree? The Process That Feeds organisms, and for this reason, biologists refer to autotrophs as the Biosphere the producers of the biosphere. Almost all plants are autotrophs; the only nutrients they re- L ife on Earth is solar powered. The chloroplasts in plants quire are water and minerals from the soil and carbon dioxide and other photosynthetic organisms capture light from the air. Specifically, plants are photoautotrophs, organ- energy that has traveled 150 million km from the sun isms that use light as a source of energy to synthesize organic and convert it to chemical energy that is stored in sugar and substances (Figure 8.1). Photosynthesis also occurs in algae, other organic molecules. This conversion process is called certain other unicellular eukaryotes, and some prokaryotes. photosynthesis. Let’s begin by placing photosynthesis in its Heterotrophs are unable to make their own food; they live ecological context. on compounds produced by other organisms (hetero- means Photosynthesis nourishes almost the entire living world “other”). Heterotrophs are the biosphere’s consumers. This directly or indirectly. An organism acquires the organic com- “other-feeding” is most obvious when an animal eats plants or pounds it uses for energy and carbon skeletons by one of two other animals, but heterotrophic nutrition may be more subtle. major modes: autotrophic nutrition or heterotrophic nutrition. Some heterotrophs decompose and feed on the remains of dead Autotrophs are “self-feeders” (auto- means “self,” and trophos organisms and organic litter such as feces and fallen leaves; means “feeder”); they sustain themselves without eating any- these types of organisms are known as decomposers. Most thing derived from other living beings. Autotrophs produce fungi and many types of prokaryotes get their nourishment this their organic molecules from CO2 and other inorganic raw way. Almost all heterotrophs, including humans, are completely materials obtained from the environment. They are the ulti- dependent, either directly or indirectly, on photoautotrophs for mate sources of organic compounds for all nonautotrophic food—and also for oxygen, a by-product of photosynthesis. 161 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 161 C/M/Y/K URRY2751_02_C08_PR3.indd 161 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
2. In this chapter, you’ll learn how photosynthesis works. A photosynthesis: the light reactions, which capture solar energy variety of photosynthetic organisms are shown in Figure 8.2, and transform it into chemical energy; and the Calvin cycle, including both eukaryotes and prokaryotes. Our discussion which uses that chemical energy to make the organic mol- here will focus mainly on plants. (Variations in autotrophic ecules of food. Finally, we’ll consider some aspects of photo- nutrition that occur in prokaryotes and algae will be described synthesis from an evolutionary perspective. in Concepts 24.2 and 25.4.) After discussing the general principles of photosynthesis, we’ll consider the two stages of CONCEPT 8.1 Photosynthesis converts light energy to the chemical energy of food The remarkable ability of an organism to harness light energy and use it to drive the synthesis of organic compounds emerges from structural organization in the cell: Photosynthetic en- zymes and other molecules are grouped together in a biological membrane, enabling the necessary series of chemical reactions to be carried out efficiently. The process of photosynthesis (a) Plants most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules. In photosynthetic bacteria that exist today, infolded photosynthetic membranes function similarly to the internal membranes of the chloroplast, a eukaryotic organelle. Accord- ing to the endosymbiont theory, the original chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells. (You learned about this theory in Concept 4.5, (b) Multicellular alga and it will be described more fully in Concept 25.1.) Chloro- plasts are present in a variety of photosynthesizing organisms, but here we focus on chloroplasts in plants. Chloroplasts: The Sites of Photosynthesis 10 μm in Plants All green parts of a plant, including green stems and unrip- (c) Unicellular eukaryotes ened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (Figure 8.3). There are about half a million chloroplasts in a chunk of leaf with a top surface area of 1 mm2. Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic (d) Cyanobacteria 40 μm pores called stomata (singular, stoma; from the Greek, mean- ing “mouth”). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots 1 μm and other nonphotosynthetic parts of the plant. A typical mesophyll cell has about 30–40 chloroplasts, each (e) Purple sulfur bacteria measuring about 2–4 μm by 4–7 μm. A chloroplast has an ▲ Figure 8.2 Photoautotrophs. These organisms use light energy envelope of two membranes surrounding a dense fluid called to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed themselves and the entire living world. the stroma. Suspended within the stroma is a third membrane (a) On land, plants are the predominant producers of food. In aquatic system, made up of sacs called thylakoids, which segregates environments, photoautotrophs include unicellular and (b) multicellular the stroma from the thylakoid space inside these sacs. In some algae, such as this kelp; (c) some non-algal unicellular eukaryotes, such places, thylakoid sacs are stacked in columns called grana as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which (singular, granum). Chlorophyll, the green pigment that produce sulfur (the yellow globules within the cells) (c–e, LMs). gives leaves their color, resides in the thylakoid membranes 162 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 162 C/M/Y/K URRY2751_02_C08_PR3.indd 162 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
3. of the chloroplast. (The internal photosynthetic membranes of some Leaf cross section prokaryotes are also called thylakoid Chloroplasts Vein membranes; see Figure 24.11b.) It is the light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast. Now that we have looked Mesophyll at the sites of photosynthesis in plants, we are ready to look more closely at the process of photosynthesis. Tracking Atoms Through Photosynthesis: Scientific Inquiry Stomata CO2 O2 Scientists have tried for centuries to piece together the process by which plants make food. Although some of the steps are still not completely understood, the overall photosynthetic equation has been known Mesophyll cell since the 1800s: In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex series of chemical reactions in photosynthesis with this chemical equation: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O Chloroplast We use glucose (C6H12O6) here to simplify the relationship between photosynthesis and respiration, but the direct prod- 20 μm uct of photosynthesis is actually a three-carbon sugar that can be used to make glucose. Water appears on both sides of the equation because 12 molecules are consumed and 6 molecules are newly formed during photosynthesis. We can simplify the equation by indicating only the net consumption of water: 6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2 Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the Outer one that occurs during cellular respiration. Both of these meta- membrane bolic processes occur in plant cells. However, as you will soon Thylakoid Thylakoid Intermembrane learn, chloroplasts do not synthesize sugars by simply revers- space space ing the steps of respiration. Inner membrane Now let’s divide the photosynthetic equation by 6 to put it Granum in its simplest possible form: Stroma CO2 + H2O → [CH2O] + O2 Here, the brackets indicate that CH2O is not an actual sugar but represents the general formula for a carbohydrate. In other words, we are imagining the synthesis of a sugar molecule one carbon at a time. Let’s now use this simplified formula to see how researchers tracked the elements C, H, and O from the Chloroplast 1 μm reactants of photosynthesis to the products. ▲ Figure 8.3 Zooming in on the location of photosynthesis The Splitting of Water in a plant. Leaves are the major organs of photosynthesis in plants. These images take you into a leaf, then into a cell, and finally into a One of the first clues to the mechanism of photosynthesis chloroplast, the organelle where photosynthesis occurs (middle, LM; came from the discovery that the O2 given off by plants is bottom, TEM). C H A P T E R 8 Photosynthesis 163 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 163 C/M/Y/K URRY2751_02_C08_PR3.indd 163 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
4. derived from H2O and not from CO2. The chloroplast splits Photosynthesis as a Redox Process water into hydrogen and oxygen. Before this discovery, the Let’s briefly compare photosynthesis with cellular respiration. prevailing hypothesis was that photosynthesis split carbon Both processes involve redox reactions. During cellular respi- dioxide (CO2 → C + O2) and then added water to the carbon ration, energy is released from sugar when electrons associated (C + H2O → [CH2O]). This hypothesis predicted that the O2 with hydrogen are transported by carriers to oxygen, forming released during photosynthesis came from CO2. This idea was water as a by-product (see Concept 7.1). The electrons lose challenged in the 1930s by C. B. van Niel, of Stanford Univer- potential energy as they “fall” down the electron transport sity. Van Niel was investigating photosynthesis in bacteria that chain toward electronegative oxygen, and the mitochondrion make their carbohydrate from CO2 but do not release O2. He harnesses that energy to synthesize ATP (see Figure 7.14). concluded that, at least in these bacteria, CO2 is not split into Photosynthesis reverses the direction of electron flow. Water carbon and oxygen. One group of bacteria used hydrogen sul- is split, and electrons are transferred along with hydrogen ions fide (H2S) rather than water for photosynthesis, forming yellow from the water to carbon dioxide, reducing it to sugar. globules of sulfur as a waste product (these globules are visible in Figure 8.2e). Here is the chemical equation for photosynthe- becomes reduced sis in these sulfur bacteria: Energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2 CO2 + 2 H2S → [CH2O] + H2O + 2 S becomes oxidized Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar. He then generalized that idea, Because the electrons increase in potential energy as they move proposing that all photosynthetic organisms require a hydro- from water to sugar, this process requires energy—in other gen source but that the source varies: words, is endergonic. This energy boost is provided by light. Sulfur bacteria: CO2 + 2 H2S → [CH2O] + H2O + 2 S Plants: CO2 + 2 H2O → [CH2O] + H2O + O2 The Two Stages of Photosynthesis: General: CO2 + 2 H2X → [CH2O] + H2O + 2 X A Preview The equation for photosynthesis is a deceptively simple sum- Thus, van Niel hypothesized that plants split H2O as a source of mary of a very complex process. Actually, photosynthesis is electrons from hydrogen atoms, releasing O2 as a by-product. not a single process, but two processes, each with multiple Nearly 20 years later, scientists confirmed van Niel’s hy- steps. These two stages of photosynthesis are known as the pothesis by using oxygen-18 (18O), a heavy isotope, as a tracer light reactions (the photo part of photosynthesis) and the to follow the fate of oxygen atoms during photosynthesis. The Calvin cycle (the synthesis part) (Figure 8.5). experiments showed that the O2 from plants was labeled with 18 The light reactions are the steps of photosynthesis that con- O only if water was the source of the tracer (experiment 1). vert solar energy to chemical energy. Water is split, providing a If the 18O was introduced to the plant in the form of CO2, the source of electrons and protons (hydrogen ions, H+) and giving label did not turn up in the released O2 (experiment 2). In the off O2 as a by-product. Light absorbed by chlorophyll drives a following summary, red denotes labeled atoms of oxygen (18O): transfer of the electrons and hydrogen ions from water to an Experiment 1: CO2 + 2 H2O → [CH2O] + H2O + O2 acceptor called NADP1 (nicotinamide adenine dinucleotide Experiment 2: CO2 + 2 H2O → [CH2O] + H2O + O2 phosphate), where they are temporarily stored. The electron A significant result of the shuffling of atoms during pho- acceptor NADP+ is first cousin to NAD+, which functions as tosynthesis is the extraction of hydrogen from water and its an electron carrier in cellular respiration; the two molecules incorporation into sugar. The waste product of photosynthesis, differ only by the presence of an extra phosphate group in the O2, is released to the atmosphere. Figure 8.4 shows the fates of NADP+ molecule. The light reactions use solar energy to re- all atoms in photosynthesis. duce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemi- osmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is Reactants: 6 CO2 12 H2O initially converted to chemical energy in the form of two com- pounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron Products: C6H12O6 6 H2O 6 O2 acceptor, reducing it, while ATP is the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle. ▲ Figure 8.4 Tracking atoms through photosynthesis. The atoms from CO2 are shown in magenta, and the atoms from H2O are The Calvin cycle is named for Melvin Calvin, who, along shown in blue. with his colleagues, began to elucidate its steps in the late 164 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. 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5. ▶ Figure 8.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes (green) Light H2O CO2 are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma (gray). The light reactions use solar energy to make ATP and NADPH, which supply chemical energy and reducing power, respectively, to the Calvin cycle. The Calvin cycle incorporates CO2 into organic NADP + molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are ADP some multiple of CH2O.) + LIGHT Pi CALVIN ANIMATION Visit the Study Area in REACTIONS CYCLE MasteringBiology for the BioFlix® 3-D Animation on ATP Thylakoid Stroma Photosynthesis. NADPH Chloroplast O2 [CH2O] (sugar) 1940s. The cycle begins by incorporating CO2 from the air CONCEPT CHECK 8.1 into organic molecules already present in the chloroplast. 1. How do the reactant molecules of photosynthesis reach the This initial incorporation of carbon into organic compounds chloroplasts in leaves? is known as carbon fixation. The Calvin cycle then reduces 2. How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? the fixed carbon to carbohydrate by the addition of electrons. 3. WHAT IF? The Calvin cycle requires ATP and NADPH, The reducing power is provided by NADPH, which acquired products of the light reactions. If a classmate asserted that its cargo of electrons in the light reactions. To convert CO2 to the light reactions don’t depend on the Calvin cycle and, carbohydrate, the Calvin cycle also requires chemical energy with continual light, could just keep on producing ATP and in the form of ATP, which is also generated by the light reac- NADPH, how would you respond? tions. Thus, it is the Calvin cycle that makes sugar, but it can For suggested answers, see Appendix A. do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light- independent reactions, because none of the steps requires light CONCEPT 8.2 directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide The light reactions convert solar the NADPH and ATP that the Calvin cycle requires. In es- energy to the chemical energy sence, the chloroplast uses light energy to make sugar by coor- of ATP and NADPH dinating the two stages of photosynthesis. As Figure 8.5 indicates, the thylakoids of the chloroplast are Chloroplasts are chemical factories powered by the sun. Their the sites of the light reactions, while the Calvin cycle occurs thylakoids transform light energy into the chemical energy of in the stroma. On the outside of the thylakoids, molecules of ATP and NADPH. To understand this conversion better, we NADP+ and ADP pick up electrons and phosphate, respec- need to know about some important properties of light. tively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle. The two The Nature of Sunlight stages of photosynthesis are treated in this figure as metabolic Light is a form of energy known as electromagnetic energy, modules that take in ingredients and crank out products. In also called electromagnetic radiation. Electromagnetic energy the next two sections, we’ll look more closely at how the two travels in rhythmic waves analogous to those created by drop- stages work, beginning with the light reactions. ping a pebble into a pond. Electromagnetic waves, however, C H A P T E R 8 Photosynthesis 165 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 165 C/M/Y/K URRY2751_02_C08_PR3.indd 165 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
6. are disturbances of electric and magnetic fields rather than illuminated with white light, the color we see is the color most disturbances of a material medium such as water. reflected or transmitted by the pigment. (If a pigment absorbs The distance between the crests of electromagnetic waves all wavelengths, it appears black.) We see green when we look is called the wavelength. Wavelengths range from less than at a leaf because chlorophyll absorbs violet-blue and red light a nanometer (for gamma rays) to more than a kilometer while transmitting and reflecting green light (Figure 8.7). The (for radio waves). This entire range of radiation is known ability of a pigment to absorb various wavelengths of light can as the electromagnetic spectrum (Figure 8.6). The seg- be measured with an instrument called a spectrophotometer. ment most important to life is the narrow band from about This machine directs beams of light of different wavelengths 380 nm to 750 nm in wavelength. This radiation is known as through a solution of the pigment and measures the fraction visible light because it can be detected as various colors by of the light transmitted at each wavelength. A graph plotting the human eye. a pigment’s light absorption versus wavelength is called an The model of light as waves explains many of light’s proper- absorption spectrum (Figure 8.8). ties, but in certain respects light behaves as though it consists The absorption spectra of chloroplast pigments provide of discrete particles, called photons. Photons are not tangible clues to the relative effectiveness of different wavelengths objects, but they act like objects in that each of them has a for driving photosynthesis, since light can perform work in fixed quantity of energy. The amount of energy is inversely chloroplasts only if it is absorbed. Figure 8.9a shows the ab- related to the wavelength of the light: The shorter the wave- sorption spectra of three types of pigments in chloroplasts: length, the greater the energy of each photon of that light. chlorophyll a, the key light-capturing pigment that partici- Thus, a photon of violet light packs nearly twice as much en- pates directly in the light reactions; the accessory pigment ergy as a photon of red light (see Figure 8.6). chlorophyll b; and a separate group of accessory pigments Although the sun radiates the full spectrum of electro- called carotenoids. The spectrum of chlorophyll a suggests magnetic energy, the atmosphere acts like a selective window, that violet-blue and red light work best for photosynthesis, allowing visible light to pass through while screening out a since they are absorbed, while green is the least effective color. substantial fraction of other radiation. The part of the spec- This is confirmed by an action spectrum for photosynthesis trum we can see—visible light—is also the radiation that drives (Figure 8.9b), which profiles the relative effectiveness of differ- photosynthesis. ent wavelengths of radiation in driving the process. An action spectrum is prepared by illuminating chloroplasts with light Photosynthetic Pigments: of different colors and then plotting wavelength against some The Light Receptors measure of photosynthetic rate, such as CO2 consumption or When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pig- ments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is Light Reflected 1m light 10–5 nm 10–3 nm 1 nm 103 nm 106 nm (109 nm) 103 m Chloroplast Gamma X-rays Micro- Radio UV Infrared waves rays waves Visible light Absorbed Granum light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy Transmitted light ▲ Figure 8.6 The electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light ▲ Figure 8.7 Why leaves are green: interaction of light with into its component colors by bending light of different wavelengths chloroplasts. The chlorophyll molecules of chloroplasts absorb violet- at different angles. (Droplets of water in the atmosphere can act as blue and red light (the colors most effective in driving photosynthesis) prisms, forming a rainbow.) Visible light drives photosynthesis. and reflect or transmit green light. This is why leaves appear green. 166 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 166 C/M/Y/K URRY2751_02_C08_PR3.indd 166 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
7. ▼ Figure 8.8 Research Method ▼ Figure 8.9 Inquiry Determining an Absorption Spectrum Which wavelengths of light are most effective in driving photosynthesis? Application An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of vis- Experiment Absorption and action spectra, along with a classic ible light. Absorption spectra of various chloroplast pigments help experiment by Theodor W. Engelmann, reveal which wavelengths scientists decipher the role of each pigment in a plant. of light are photosynthetically important. Technique A spectrophotometer measures the relative amounts of Results light of different wavelengths absorbed and transmitted by a pig- Chloro- Absorption of light by chloroplast pigments ment solution. phyll a Chlorophyll b 1 White light is separated into colors (wavelengths) by a prism. 2 One by one, the different colors of light are passed through the Carotenoids sample (chlorophyll in this example). Green light and blue light are shown here. 3 The transmitted light strikes a photoelectric tube, which con- verts the light energy to electricity. 400 500 600 700 4 The electric current is measured by a galvanometer. The meter Wavelength of light (nm) indicates the fraction of light transmitted through the sample, (a) Absorption spectra. The three curves show the wavelengths of light from which we can determine the amount of light absorbed. best absorbed by three types of chloroplast pigments. White Refracting Chlorophyll Photoelectric (measured by O2 release) light prism solution tube Rate of photosynthesis Galvanometer 2 3 1 4 0 100 The high transmittance Slit moves to Green (low absorption) pass light light reading indicates that 400 500 600 700 of selected chlorophyll absorbs wavelength. very little green light. (b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. 0 100 Aerobic bacteria Filament The low transmittance of alga Blue (high absorption) light reading indicates that chlorophyll absorbs most blue light. 400 500 600 700 Results See Figure 8.9a for absorption spectra of three types of (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann chloroplast pigments. illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were O2 release. The action spectrum for photosynthesis was first releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga demonstrated by Theodor W. Engelmann, a German botanist, illuminated with violet-blue or red light. in 1883. Before equipment for measuring O2 levels had even Conclusion Light in the violet-blue and red portions of the spec- been invented, Engelmann performed a clever experiment trum is most effective in driving photosynthesis. in which he used bacteria to measure rates of photosynthesis Data from T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichen- in filamentous algae (Figure 8.9c). His results are a striking den Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883). match to the modern action spectrum shown in Figure 8.9b. A related Experimental Inquiry Tutorial can be assigned in Notice by comparing Figure 8.9a and 8.9b that the action MasteringBiology. spectrum for photosynthesis is much broader than the ab- INTERPR ET TH E DATA What wavelengths of light drive the sorption spectrum of chlorophyll a. The absorption spectrum highest rate of photosynthesis? of chlorophyll a alone underestimates the effectiveness of C H A P T E R 8 Photosynthesis 167 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 167 C/M/Y/K URRY2751_02_C08_PR3.indd 167 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
8. CH3 in chlorophyll a excessive light energy that would otherwise damage chloro- CHO in chlorophyll b phyll or interact with oxygen, forming reactive oxidative mol- CH2 CH3 ecules that are dangerous to the cell. Interestingly, carotenoids CH H similar to the photoprotective ones in chloroplasts have a pho- C C C H3C C C C C CH2 CH3 Porphyrin ring: toprotective role in the human eye. (Carrots, known for aiding C N N C light-absorbing night vision, are rich in carotenoids.) “head” of molecule; H C Mg C H note magnesium H3C C N N C atom at center Excitation of Chlorophyll by Light C C C C CH3 H C C C What exactly happens when chlorophyll and other pigments CH2 H H C C absorb light? The colors corresponding to the absorbed wave- CH2 C O O lengths disappear from the spectrum of the transmitted and C O O reflected light, but energy cannot disappear. When a molecule O CH3 absorbs a photon of light, one of the molecule’s electrons CH2 is elevated to an electron shell where it has more potential energy (see Figure 2.5). When the electron is in its normal shell, the pigment molecule is said to be in its ground state. Absorption of a photon boosts an electron to a higher-energy electron shell, and the pigment molecule is then said to be in Hydrocarbon tail: interacts with hydrophobic an excited state (Figure 8.11a). The only photons absorbed are regions of proteins inside those whose energy is exactly equal to the energy difference thylakoid membranes of between the ground state and an excited state, and this energy chloroplasts; H atoms not shown difference varies from one kind of molecule to another. Thus, a particular compound absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique ▲ Figure 8.10 Structure of chlorophyll molecules in absorption spectrum. chloroplasts of plants. Chlorophyll a and chlorophyll b differ only Once absorption of a photon raises an electron from the in one of the functional groups bonded to the porphyrin ring. (Also see the space-filling model of chlorophyll in Figure 1.3.) ground state to an excited state, the electron cannot stay there long. The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their certain wavelengths in driving photosynthesis. This is partly excited electrons drop back down to the ground-state electron because accessory pigments with different absorption spectra shell in a billionth of a second, releasing their excess energy also present in chloroplasts—including chlorophyll b and carotenoids—broaden the spectrum of colors that can be used for photosynthesis. Figure 8.10 shows Excited e– state the structure of chlorophyll a compared with that of chlorophyll b. A slight struc- tural difference between them is enough Energy of electron Heat to cause the two pigments to absorb at slightly different wavelengths in the red and blue parts of the spectrum (see Figure 8.9a). As a result, chlorophyll a appears blue green and chlorophyll b olive Photon (fluorescence) green under visible light. Photon Other accessory pigments include Ground Chlorophyll state carotenoids, hydrocarbons that are molecule various shades of yellow and orange be- cause they absorb violet and blue-green light (see Figure 8.9a). Carotenoids may (a) Excitation of isolated chlorophyll molecule (b) Fluorescence broaden the spectrum of colors that can ▲ Figure 8.11 Excitation of isolated chlorophyll by light. (a) Absorption of a photon drive photosynthesis. However, a more causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon important function of at least some ca- boosts an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to the ground-state orbital, and rotenoids seems to be photoprotection: its excess energy is given off as heat and fluorescence (light). (b) A chlorophyll solution excited with These compounds absorb and dissipate ultraviolet light fluoresces with a red-orange glow. 168 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 168 C/M/Y/K URRY2751_02_C08_PR3.indd 168 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
9. as heat. This conversion of light energy to heat is what makes the top of an automobile so hot on a sunny day. (White cars Thylakoid are coolest because their paint reflects all wavelengths of vis- ible light.) In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing photons. As excited electrons fall back to the ground state, photons are given off, Photosystem STROMA an afterglow called fluorescence. An illuminated solution of Photon chlorophyll isolated from chloroplasts will fluoresce in the red- Light-harvesting Reaction- Primary orange part of the spectrum and also give off heat. This is best complexes center complex electron acceptor seen by illuminating with ultraviolet light, which chlorophyll can also absorb (Figure 8.11b). Viewed under visible light, the fluorescence would be hard to see against the green of the solution. Thylakoid membrane A Photosystem: A Reaction-Center Complex e– Associated with Light-Harvesting Complexes Chlorophyll molecules excited by the absorption of light en- ergy produce very different results in an intact chloroplast than they do in isolation. In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems. Transfer Special pair of Pigment of energy chlorophyll a molecules A photosystem is composed of a reaction-center molecules complex surrounded by several light-harvesting complexes THYLAKOID SPACE (Figure 8.12). The reaction-center complex is an organized (INTERIOR OF THYLAKOID) association of proteins holding a special pair of chlorophyll a (a) How a photosystem harvests light. When a photon strikes a pig- molecules. Each light-harvesting complex consists of various ment molecule in a light-harvesting complex, the energy is passed pigment molecules (which may include chlorophyll a, chlo- from molecule to molecule until it reaches the reaction-center com- rophyll b, and multiple carotenoids) bound to proteins. The plex. Here, an excited electron from the special pair of chlorophyll a molecules is transferred to the primary electron acceptor. number and variety of pigment molecules enable a photosys- tem to harvest light over a larger surface area and a larger por- Chlorophyll (green) STROMA tion of the spectrum than could any single pigment molecule Thylakoid membrane alone. Together, these light-harvesting complexes act as an an- tenna for the reaction-center complex. When a pigment mol- ecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting com- plex, somewhat like a human “wave” at a sports arena, until it is passed into the reaction-center complex. The reaction- center complex also contains a molecule capable of accepting electrons and becoming reduced; this is called the primary Protein electron acceptor. The pair of chlorophyll a molecules in the subunits THYLAKOID (purple) SPACE reaction-center complex are special because their molecular environment—their location and the other molecules with (b) Structure of a photosystem. This computer model, based on X-ray crystallography, shows two photosystem complexes side by which they are associated—enables them to use the energy side. Chlorophyll molecules (bright green ball-and-stick models from light not only to boost one of their electrons to a higher within the membrane; the tails are not shown) are interspersed energy level, but also to transfer it to a different molecule—the with protein subunits (purple ribbons; notice the many α helices spanning the membrane). For simplicity, a photosystem will be primary electron acceptor. shown as a single complex in the rest of the chapter. The solar-powered transfer of an electron from the ▲ Figure 8.12 The structure and function of a photosystem. reaction-center chlorophyll a pair to the primary electron ac- ceptor is one of the first steps of the light reactions. As soon as the chlorophyll electron is excited to a higher energy level, of photoexcited chlorophyll drop right back to the ground the primary electron acceptor captures it; this is a redox reac- state. In the structured environment of a chloroplast, however, tion. In the flask shown in Figure 8.11b, isolated chlorophyll an electron acceptor is readily available, and the potential en- fluoresces because there is no electron acceptor, so electrons ergy represented by the excited electron is not dissipated as C H A P T E R 8 Photosynthesis 169 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 169 C/M/Y/K URRY2751_02_C08_PR3.indd 169 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
10. light and heat. Thus, each photosystem—a reaction-center electron distribution in the two pigments and accounts for the complex surrounded by light-harvesting complexes—functions slight differences in their light-absorbing properties. Now let’s in the chloroplast as a unit. It converts light energy to chemical see how the two photosystems work together in using light en- energy, which will ultimately be used for the synthesis of sugar. ergy to generate ATP and NADPH, the two main products of The thylakoid membrane is populated by two types of the light reactions. photosystems that cooperate in the light reactions of pho- tosynthesis. They are called photosystem II (PS II) and Linear Electron Flow photosystem I (PS I). (They were named in order of their Light drives the synthesis of ATP and NADPH by energizing discovery, but photosystem II functions first in the light reac- the two photosystems embedded in the thylakoid membranes tions.) Each has a characteristic reaction-center complex—a of chloroplasts. The key to this energy transformation is a flow particular kind of primary electron acceptor next to a special of electrons through the photosystems and other molecular pair of chlorophyll a molecules associated with specific pro- components built into the thylakoid membrane. This is called teins. The reaction-center chlorophyll a of photosystem II linear electron flow, and it occurs during the light reactions is known as P680 because this pigment is best at absorbing of photosynthesis, as shown in Figure 8.13. The numbered light having a wavelength of 680 nm (in the red part of the steps in the text correspond to those in the figure. spectrum). The chlorophyll a at the reaction-center complex of photosystem I is called P700 because it most effectively 1 A photon of light strikes one of the pigment molecules in absorbs light of wavelength 700 nm (in the far-red part of the a light-harvesting complex of PS II, boosting one of its spectrum). These two pigments, P680 and P700, are nearly electrons to a higher energy level. As this electron falls identical chlorophyll a molecules. However, their association back to its ground state, an electron in a nearby pigment with different proteins in the thylakoid membrane affects the molecule is simultaneously raised to an excited state. The H2O CO2 Light NADP+ ADP ▼ Figure 8.13 How linear electron flow during the light CALVIN reactions generates ATP and NADPH. The gold arrows trace the LIGHT REACTIONS CYCLE current of light-driven electrons from water to NADPH. ATP NADPH E O2 [CH2O] (sugar) tra lect n ro ch spo n Primary ai rt n Ele acceptor Primary ctro 4 7 acceptor n tr ans Fd por e– Pq t ch 2 ain e– 8 e– e– NADP+ H2O Cytochrome 2 H+ NADP+ + H+ + complex 3 reductase / O2 12 NADPH Pc e– e– P700 1 Light P680 5 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) 170 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 170 C/M/Y/K URRY2751_02_C08_PR3.indd 170 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
11. process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlo- e– rophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a e– e– higher energy state. Mill 2 This electron is transferred from the excited P680 to the makes NADPH e– ATP primary electron acceptor. We can refer to the resulting e– e– form of P680, missing an electron, as P680+. 3 An enzyme catalyzes the splitting of a water molecule n Photo into two electrons, two hydrogen ions (H+), and an oxy- gen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the e– primary electron acceptor. (P680+ is the strongest biolog- ATP Photon ical oxidizing agent known; its electron “hole” must be filled. This greatly facilitates the transfer of electrons from Photosystem II Photosystem I the split water molecule.) The H+ are released into the thylakoid space. The oxygen atom immediately combines ▲ Figure 8.14 A mechanical analogy for linear electron flow during the light reactions. with an oxygen atom generated by the splitting of another water molecule, forming O2. 4 Each photoexcited electron passes from the primary elec- The energy changes of electrons during their linear flow tron acceptor of PS II to PS I via an electron transport through the light reactions are shown in a mechanical analogy chain, the components of which are similar to those of the in Figure 8.14. Although the scheme shown in Figures 8.13 electron transport chain that functions in cellular respira- and 8.14 may seem complicated, do not lose track of the big tion. The electron transport chain between PS II and PS I is picture: The light reactions use solar power to generate ATP made up of the electron carrier plastoquinone (Pq), a cyto- and NADPH, which provide chemical energy and reducing chrome complex, and a protein called plastocyanin (Pc). power, respectively, to the carbohydrate-synthesizing reactions 5 The exergonic “fall” of electrons to a lower energy level of the Calvin cycle. Before we move on to the Calvin cycle, provides energy for the synthesis of ATP. As electrons let’s review chemiosmosis, the process that uses membranes pass through the cytochrome complex, H+ are pumped to couple redox reactions to ATP production. into the thylakoid space, contributing to the proton gradi- ent that is then used in chemiosmosis, to be discussed A Comparison of Chemiosmosis shortly. in Chloroplasts and Mitochondria 6 Meanwhile, light energy has been transferred via light- Chloroplasts and mitochondria generate ATP by the same harvesting complex pigments to the PS I reaction-center basic mechanism: chemiosmosis (see Figure 7.14). An electron complex, exciting an electron of the P700 pair of chloro- transport chain assembled in a membrane pumps protons phyll a molecules located there. The photoexcited electron (H+) across the membrane as electrons are passed through a is then transferred to PS I’s primary electron acceptor, cre- series of carriers that are progressively more electronegative. ating an electron “hole” in the P700—which we now can Thus, electron transport chains transform redox energy to a call P700+. In other words, P700+ can now act as an proton-motive force, potential energy stored in the form of electron acceptor, accepting an electron that reaches the an H+ gradient across a membrane. An ATP synthase com- bottom of the electron transport chain from PS II. plex in the same membrane couples the diffusion of hydrogen 7 Photoexcited electrons are passed in a series of redox re- ions down their gradient to the phosphorylation of ADP, actions from the primary electron acceptor of PS I down forming ATP. Some of the electron carriers, including the a second electron transport chain through the protein iron-containing proteins called cytochromes, are very similar ferredoxin (Fd). (This chain does not create a proton in chloroplasts and mitochondria. The ATP synthase com- gradient and thus does not produce ATP.) plexes of the two organelles are also quite similar. But there 8 The enzyme NADP+ reductase catalyzes the transfer of are noteworthy differences between photophosphorylation in electrons from Fd to NADP+. Two electrons are required chloroplasts and oxidative phosphorylation in mitochondria. for its reduction to NADPH. This molecule is at a higher Both work by way of chemiosmosis, but in chloroplasts, the energy level than water, so its electrons are more readily high-energy electrons dropped down the transport chain come available for the reactions of the Calvin cycle. This pro- from water, whereas in mitochondria, they are extracted from cess also removes an H+ from the stroma. organic molecules (which are thus oxidized). Chloroplasts do C H A P T E R 8 Photosynthesis 171 DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 171 C/M/Y/K URRY2751_02_C08_PR3.indd 171 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
12. ▶ Figure 8.15 Comparison of Mitochondrion Chloroplast chemiosmosis in mitochondria and chloroplasts. In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a region of low H+ concentration (light gray in this diagram) to one of high H+ concentration (dark gray). The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP. MAKE CONNECTIONS Describe how you would change the pH in order to artificially cause ATP Inter- synthesis (a) outside an isolated mitochondrion H+ Diffusion membrane Thylakoid (assume H+ can freely cross the outer membrane) space space and (b) in the stroma of a chloroplast. Explain. Electron Inner Thylakoid transport MITOCHONDRION membrane membrane CHLOROPLAST chain STRUCTURE STRUCTURE ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+] not need molecules from food to make ATP; their photosys- to a thousandfold difference in H+ concentration. If the lights tems capture light energy and use it to drive the electrons from are turned off, the pH gradient is abolished, but it can quickly water to the top of the transport chain. In other words, mito- be restored by turning the lights back on. Experiments such chondria use chemiosmosis to transfer chemical energy from as this provided strong evidence in support of the chemios- food molecules to ATP, whereas chloroplasts use it to trans- motic model. form light energy into chemical energy in ATP. Based on studies in several laboratories, Figure 8.16 shows Although the spatial organization of chemiosmosis differs a current model for the organization of the light-reaction slightly between chloroplasts and mitochondria, it is easy to “machinery” within the thylakoid membrane. Each of the see similarities in the two (Figure 8.15). The inner membrane molecules and molecular complexes in the figure is present of the mitochondrion pumps protons from the mitochondrial in numerous copies in each thylakoid. Notice that NADPH, matrix out to the intermembrane space, which then serves as like ATP, is produced on the side of the membrane facing the a reservoir of hydrogen ions. The thylakoid membrane of the stroma, where the Calvin cycle reactions take place. chloroplast pumps protons from the stroma into the thylakoid Let’s summarize the light reactions. Electron flow pushes space (interior of the thylakoid), which functions as the H+ electrons from water, where they are at a low state of potential reservoir. If you imagine the cristae of mitochondria pinching energy, ultimately to NADPH, where they are stored at a high off from the inner membrane, this may help you see how the state of potential energy. The light-driven electron flow also thylakoid space and the intermembrane space are comparable generates ATP. Thus, the equipment of the thylakoid mem- spaces in the two organelles, while the mitochondrial matrix brane converts light energy to chemical energy stored in ATP is analogous to the stroma of the chloroplast. In the mito- and NADPH. (Oxygen is a by-product.) Let’s now see how the chondrion, protons diffuse down their concentration gradient Calvin cycle uses the products of the light reactions to synthe- from the intermembrane space through ATP synthase to the size sugar from CO2. matrix, driving ATP synthesis. In the chloroplast, ATP is syn- thesized as the hydrogen ions diffuse from the thylakoid space CONCEPT CHECK 8.2 back to the stroma through ATP synthase complexes, whose 1. What color of light is least effective in driving photosynthesis? catalytic knobs are on the stroma side of the membrane. Thus, Explain. ATP forms in the stroma, where it is used to help drive sugar 2. In the light reactions, what is the initial electron donor? At the synthesis during the Calvin cycle. end of the light reactions, where are the electrons? The proton (H+) gradient, or pH gradient, across the 3. WHAT IF? In an experiment, isolated chloroplasts placed in an illuminated solution with the appropriate chemicals thylakoid membrane is substantial. When chloroplasts in an can carry out ATP synthesis. Predict what will happen to the experimental setting are illuminated, the pH in the thylakoid rate of synthesis if a compound is added to the solution that space drops to about 5 (the H+ concentration increases), and makes membranes freely permeable to hydrogen ions. the pH in the stroma increases to about 8 (the H+ concentra- For suggested answers, see Appendix A. tion decreases). This gradient of three pH units corresponds 172 U N I T O N E CHEMISTRY AND CELLS DESIGN SERVICES OF # 153397 Cust: Pearson / BC Au: Urry Pg. No. 172 C/M/Y/K URRY2751_02_C08_PR3.indd 172 Title: Campbell Biology in Focus, 2e Short / Normal / Long S4CARLISLE Publishing Services 01/07/15 11:08 pm
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