Campbell Biology - Photosynthesis PDF

10 Photosynthesis Key Concepts 10.1 Photosynthesis converts light ▲ Figure 10.1 How does sunlight help build the trunk, branches, and leaves of this...

10 Photosynthesis Key Concepts 10.1 Photosynthesis converts light ▲ Figure 10.1 How does sunlight help build the trunk, branches, and leaves of this broadleaf tree? energy to the chemical energy of food 10.2 The light reactions convert solar energy to the chemical energy The Process That Feeds the Biosphere L of ATP and NADPH ife on Earth is solar powered. The chloroplasts in plants and other photosyn- 10.3 The Calvin cycle uses the thetic organisms capture light energy that has traveled 150 million kilometers chemical energy of ATP and from the sun and convert it to chemical energy that is stored in sugar and other NADPH to reduce CO2 to sugar organic molecules. This conversion process is called photosynthesis. Let’s begin by placing photosynthesis in its ecological context. 10.4 Alternative mechanisms of Photosynthesis nourishes almost the entire living world directly or indirectly. carbon fixation have evolved in hot, arid climates An organism acquires the organic compounds it uses for energy and carbon skel- etons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are “self-feeders” (auto- means “self,” and trophos means “feeder”); they sustain themselves without eating anything derived from other living beings. Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment. They are the ultimate sources of organic compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the producers of the biosphere. Almost all plants are autotrophs; the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize or- ganic substances (Figure 10.1). Photosynthesis also occurs in algae, certain other ▲ Other organisms also benefit from photosynthesis. c h a p t e r 1 0   Photosynthesis    185 unicellular eukaryotes, and some prokaryotes (Figure 10.2). Heterotrophs obtain organic material by the second In this chapter, we will touch on these other groups in pass- major mode of nutrition. Unable to make their own food, they ing, but our emphasis will be on plants. Variations in auto- live on compounds produced by other organisms (hetero- trophic nutrition that occur in prokaryotes and algae will be means “other”). Heterotrophs are the biosphere’s consumers. described in Chapters 27 and 28. The most obvious “other-feeding” occurs when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic lit- ter such as carcasses, feces, and fallen leaves; these types of organisms are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. Almost all heterotrophs, including humans, are completely depen- dent, either directly or indirectly, on photoautotrophs for food—and also for oxygen, a by-product of photosynthesis. The Earth’s supply of fossil fuels was formed from remains of organisms that died hundreds of millions of years ago. In a sense, then, fossil fuels represent stores of the sun’s energy (a) Plants from the distant past. Because these resources are being used at a much higher rate than they are replenished, researchers are exploring methods of capitalizing on the photosynthetic process to provide alternative fuels (Figure 10.3). In this chapter, you'll learn how photosynthesis works. After discussing general principles of photosynthesis, we’ll consider the two stages of photosynthesis: the light reactions, which capture solar energy and transform it into chemical energy; and the Calvin cycle, which uses that chemical energy to make the organic molecules of food. Finally, we will con- (b) Multicellular alga sider some aspects of photosynthesis from an evolutionary perspective. 10 μm (c) Unicellular eukaryotes (d) Cyanobacteria 40 μm 1 μm ▲ Figure 10.3 Alternative fuels from algae. The power of sun- light can be tapped to generate a sustainable alternative to fossil fuels. (e) Purple sulfur bacteria Species of unicellular algae that are prolific producers of plant oils can be cultured in long, transparent tanks called photobioreactors, such as ▲ Figure 10.2 Photoautotrophs. These organisms use light energy the one shown here at Arizona State University. A simple chemical pro- to drive the synthesis of organic molecules from carbon dioxide and (in cess can yield “biodiesel,” which can be mixed with gasoline or used most cases) water. They feed themselves and the entire living world. alone to power vehicles. (a) On land, plants are the predominant producers of food. In aquatic w h a t I F ? The main product of fossil fuel combustion is CO2, and this environments, photoautotrophs include unicellular and (b) multicellular algae, such as this kelp; (c) some non-algal unicellular eukaryotes, such is the source of the increase in atmospheric CO2 concentration. Scientists as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other have proposed strategically situating containers of these algae near indus- photosynthetic prokaryotes, such as these purple sulfur bacteria, which trial plants or near highly congested city streets. Considering the process produce sulfur (the yellow globules within the cells) (c–e, LMs). of photosynthesis, how does this arrangement make sense? 186    U n i t T w o   The Cell CONCEPT 10.1 Leaf cross section Photosynthesis converts light Chloroplasts Vein energy to the chemical energy of food Mesophyll The remarkable ability of an organism to harness light energy and use it to drive the synthesis of or­ganic compounds emerges from structural organi- zation in the cell: Photosynthetic enzymes and other molecules are grouped together in a biological membrane, enabling the necessary series of chemical reactions to be Stomata carried out efficiently. The process of photosynthesis most CO2 O2 likely originated in a group of bacteria that had infolded re- gions of the plasma membrane containing clusters of such molecules. In existing photosynthetic bacteria, infolded Mesophyll cell photosynthetic membranes function similarly to the inter- nal membranes of the chloroplast, a eukaryotic organelle. According to what has come to be known as the endosym- biont theory, the original chloroplast was a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells. (You learned about this theory in Chapter 6, and it will be described more fully in Chapter 25.) Chloroplasts are pres- Chloroplast ent in a variety of photosynthesizing organisms (see some examples in Figure 10.2), but here we focus on chloroplasts 20 μm in plants. Chloroplasts: The Sites of Photosynthesis in Plants All green parts of a plant, including green stems and unrip- ened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (Figure 10.4). There are about half a million chloroplasts in a chunk of leaf with Outer a top surface area of 1 mm2. Chloroplasts are found mainly membrane in the cells of the mesophyll, the tissue in the interior of the Thylakoid leaf. Carbon dioxide enters the leaf, and oxygen exits, by way Thylakoid Intermembrane space space of microscopic pores called stomata (singular, stoma; from Stroma Granum Inner the Greek, meaning “mouth”). Water absorbed by the roots membrane is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. A typical mesophyll cell has about 30–40 chloroplasts, each measuring about 2–4 μm by 4–7 μm. A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma. Suspended within the stroma is a third membrane system, made up of sacs called thylakoids, which segregates the stroma from the thylakoid space inside these 1 μm sacs. In some places, thylakoid sacs are stacked in columns called grana (singular, granum). Chlorophyll, the green pig- ▲ Figure 10.4 Zooming in on the location of photosynthesis in a plant. Leaves are the major organs of photosynthesis in plants. ment that gives leaves their color, resides in the thylakoid These pictures take you into a leaf, then into a cell, and finally into a membranes of the chloroplast. (The internal photosynthetic chloroplast, the organelle where photosynthesis occurs (middle, LM; membranes of some prokaryotes are also called thylakoid bottom, TEM). c h a p t e r 1 0   Photosynthesis    187 membranes; see Figure 27.8b.) It is the light energy absorbed prevailing hypothesis was that photosynthesis split carbon by chlorophyll that drives the synthesis of organic molecules dioxide (CO2 S C + O2) and then added water to the carbon in the chloroplast. Now that we have looked at the sites of (C + H2O S [CH2O]). This hypothesis predicted that the O2 photosynthesis in plants, we are ready to look more closely released during photosynthesis came from CO2. This idea at the process of photosynthesis. was challenged in the 1930s by C. B. van Niel, of Stanford University. Van Niel was investigating photosynthesis in Tracking Atoms Through Photosynthesis: bacteria that make their carbohydrate from CO2 but do not Scientific Inquiry release O2. He concluded that, at least in these bacteria, CO2 is not split into carbon and oxygen. One group of bacteria Scientists have tried for centuries to piece together the used hydrogen sulfide (H2S) rather than water for photosyn- process by which plants make food. Although some of the thesis, forming yellow globules of sulfur as a waste product steps are still not completely understood, the overall photo- (these globules are visible in Figure 10.2e). Here is the chem- synthetic equation has been known since the 1800s: In the ical equation for photosynthesis in these sulfur bacteria: presence of light, the green parts of plants produce organic CO2 + 2 H2S S [CH2O] + H2O + 2 S compounds and oxygen from carbon dioxide and water. Using molecular formulas, we can summarize the complex Van Niel reasoned that the bacteria split H2S and used the series of chemical reactions in photosynthesis with this hydrogen atoms to make sugar. He then generalized that chemical equation: idea, proposing that all photosynthetic organisms require a 6 CO2 + 12 H2O + Light energy S C6H12O6 + 6 O2 + 6 H2O hydrogen source but that the source varies: We use glucose (C6H12O6) here to simplify the relation- Sulfur bacteria: CO2 + 2 H2S S [CH2O] + H2O + 2 S ship between photosynthesis and respiration, but the direct Plants: CO2 + 2 H2O S [CH2O] + H2O + O2 product of photosynthesis is actually a three-carbon sugar General: CO2 + 2 H2X S [CH2O] + H2O + 2 X that can be used to make glucose. Water appears on both Thus, van Niel hypothesized that plants split H2O as a sides of the equation because 12 molecules are consumed source of electrons from hydrogen atoms, releasing O2 as a and 6 molecules are newly formed during photosynthesis. by-product. We can simplify the equation by indicating only the net con- Nearly 20 years later, scientists confirmed van Niel’s sumption of water: hypothesis by using oxygen-18 (18O), a heavy isotope, as a 6 CO2 + 6 H2O + Light energy S C6H12O6 + 6 O2 tracer to follow the fate of oxygen atoms during photosyn- thesis. The experiments showed that the O2 from plants was Writing the equation in this form, we can see that the labeled with 18O only if water was the source of the tracer overall chemical change during photosynthesis is the re- (experiment 1). If the 18O was introduced to the plant in the verse of the one that occurs during cellular respiration (see form of CO2, the label did not turn up in the released O2 Concept 9.1). Both of these metabolic processes occur in (experiment 2). In the following summary, red denotes la- plant cells. However, as you will soon learn, chloroplasts beled atoms of oxygen (18O): do not synthesize sugars by simply reversing the steps of respiration. Experiment 1: CO2 + 2 H2O S [CH2O] + H2O + O2 Now let’s divide the photosynthetic equation by 6 to put Experiment 2: CO2 + 2 H2O S [CH2O] + H2O + O2 it in its simplest possible form: A significant result of the shuffling of atoms during pho- CO2 + H2O S [CH2O] + O2 tosynthesis is the extraction of hydrogen from water and its Here, the brackets indicate that CH2O is not an actual sugar incorporation into sugar. The waste product of photosyn- but represents the general formula for a carbohydrate (see thesis, O2, is released to the atmosphere. Figure 10.5 shows Concept 5.2). In other words, we are imagining the synthe- the fates of all atoms in photosynthesis. sis of a sugar molecule one carbon at a time. Six repetitions would theoretically produce a glucose molecule (C6H12O6). Reactants: 6 CO2 12 H2O Let’s now see how researchers tracked the elements C, H, and O from the reactants of photosynthesis to the products. The Splitting of Water Products: C6H12O6 6 H2O 6 O2 One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is ▲ Figure 10.5 Tracking atoms through photosynthesis. The derived from H2O and not from CO2. The chloroplast splits atoms from CO2 are shown in magenta, and the atoms from H2O are water into hydrogen and oxygen. Before this discovery, the shown in blue. 188    U n i t T w o   The Cell Photosynthesis as a Redox Process The light reactions are the steps of photosynthesis that Let’s briefly compare photosynthesis with cellular respi- convert solar energy to chemical energy. Water is split, ration. Both processes involve redox reactions. During providing a source of electrons and protons (hydrogen ions, cellular respiration, energy is released from sugar when H+) and giving off O2 as a by-product. Light absorbed by electrons associated with hydrogen are transported by car- chlorophyll drives a transfer of the electrons and hydrogen riers to oxygen, forming water as a by-product. The elec- ions from water to an acceptor called NADP+ (nicotinamide trons lose potential energy as they “fall” down the electron adenine dinucleotide phosphate), where they are temporar- transport chain toward electronegative oxygen, and the ily stored. The electron acceptor NADP+ is first cousin to mitochondrion harnesses that energy to synthesize ATP NAD+, which functions as an electron carrier in cellular (see Figure 9.15). Photosynthesis reverses the direction of respiration; the two molecules differ only by the presence of electron flow. Water is split, and electrons are transferred an extra phosphate group in the NADP+ molecule. The light along with hydrogen ions from the water to carbon dioxide, reactions use solar energy to reduce NADP+ to NADPH by reducing it to sugar. adding a pair of electrons along with an H+. The light reac- becomes reduced tions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called Energy + 6 CO2 + 6 H2O C6H12O6 + 6 O2 photophosphorylation. Thus, light energy is initially con- becomes oxidized verted to chemical energy in the form of two compounds: Because the electrons increase in potential energy as they NADPH and ATP. NADPH, a source of electrons, acts as move from water to sugar, this process requires energy—in “reducing power” that can be passed along to an electron other words is endergonic. This energy boost that occurs acceptor, reducing it, while ATP is the versatile energy cur- during photosynthesis is provided by light. rency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle. The Two Stages of Photosynthesis: A Preview The Calvin cycle is named for Melvin Calvin, who, along The equation for photosynthesis is a deceptively simple with his colleagues James Bassham and Andrew Benson, summary of a very complex process. Actually, photosynthe- began to elucidate its steps in the late 1940s. The cycle begins sis is not a single process, but two processes, each with mul- by incorporating CO2 from the air into organic molecules tiple steps. These two stages of photosynthesis are known as already present in the chloroplast. This initial incorporation the light reactions (the photo part of photosynthesis) and of carbon into organic compounds is known as carbon the Calvin cycle (the synthesis part) (Figure 10.6). fixation. The Calvin cycle then reduces the fixed carbon ▶ Figure 10.6 An overview of photosyn- thesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes (green) are the sites of Light H2O CO2 the light reactions, whereas the Calvin cycle oc- curs 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 molecules, which NADP + are converted to sugar. (Recall that most simple sugars have formulas that are some multiple ADP of CH2O.) + LIGHT Pi CALVIN REACTIONS CYCLE ATP Thylakoid Stroma NADPH A N I M AT I O N Visit the Study Area in Chloroplast MasteringBiology for the BioFlix® 3-D O2 [CH2O] Animation on Photosynthesis. BioFlix Tutorials (sugar) can also be assigned in MasteringBiology. c h a p t e r 1 0   Photosynthesis    189 to carbohydrate by the addition of electrons. The reducing The Nature of Sunlight power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO2 to carbohy- Light is a form of energy known as electromagnetic energy, drate, the Calvin cycle also requires chemical energy in the also called electromagnetic radiation. Electromagnetic en- form of ATP, which is also generated by the light reactions. ergy travels in rhythmic waves analogous to those created by Thus, it is the Calvin cycle that makes sugar, but it can do dropping a pebble into a pond. Electromagnetic waves, how- so only with the help of the NADPH and ATP produced by ever, are disturbances of electric and magnetic fields rather the light reactions. The metabolic steps of the Calvin cycle than disturbances of a material medium such as water. are sometimes referred to as the dark reactions, or light- The distance between the crests of electromagnetic waves independent reactions, because none of the steps requires is called the wavelength. Wavelengths range from less than light directly. Nevertheless, the Calvin cycle in most plants a nanometer (for gamma rays) to more than a kilometer (for occurs during daylight, for only then can the light reactions radio waves). This entire range of radiation is known as the provide the NADPH and ATP that the Calvin cycle requires. electromagnetic spectrum (Figure 10.7). The segment most In essence, the chloroplast uses light energy to make sugar by important to life is the narrow band from about 380 nm to coordinating the two stages of photosynthesis. 750 nm in wavelength. This radiation is known as visible light As Figure 10.6 indicates, the thylakoids of the chloro- because it can be detected as various colors by the human eye. plast are the sites of the light reactions, while the Calvin The model of light as waves explains many of light’s cycle occurs in the stroma. On the outside of the thyla- properties, but in certain respects light behaves as though koids, molecules of NADP+ and ADP pick up electrons and it consists of discrete particles, called photons. Photons are phosphate, respectively, and NADPH and ATP are then not tangible objects, but they act like objects in that each of released to the stroma, where they play crucial roles in the them has a fixed quantity of energy. The amount of energy Calvin cycle. The two stages of photosynthesis are treated is inversely related to the wavelength of the light: the shorter in this figure as metabolic modules that take in ingredients the wavelength, the greater the energy of each photon of and crank out products. In the next two sections, we’ll look that light. Thus, a photon of violet light packs nearly twice more closely at how the two stages work, beginning with as much energy as a photon of red light (see Figure 10.7). the light reactions. Although the sun radiates the full spectrum of electro- magnetic energy, the atmosphere acts like a selective win- dow, allowing visible light to pass through while screening Concept Check 10.1 out a substantial fraction of other radiation. The part of the 1. How do the reactant molecules of photosynthesis reach spectrum we can see—visible light—is also the radiation that the chloroplasts in leaves? drives photosynthesis. 2. How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? 3. w h a t I F ? The Calvin cycle requires ATP and NADPH, products of the light reactions. If a classmate asserted 1m that the light reactions don’t depend on the Calvin cycle 10–5 nm 10–3 nm 1 nm 103 nm 106 nm (109 nm) 103 m and, with continual light, could just keep on producing ATP and NADPH, how would you respond? Gamma X-rays Micro- Radio UV Infrared waves rays waves For suggested answers, see Appendix A. CONCEPT 10.2 Visible light The light reactions convert solar energy to the chemical energy of ATP and NADPH 380 450 500 Shorter wavelength 550 600 650 700 750 nm Longer wavelength Chloroplasts are chemical factories powered by the sun. Their Higher energy Lower energy thylakoids transform light energy into the chemical energy of ATP and NADPH, which will be used to synthesize glucose ▲ Figure 10.7 The electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light and other molecules that can be used as energy sources. To into its component colors by bending light of different wavelengths better understand the conversion of light to chemical energy, at different angles. (Droplets of water in the atmosphere can act as we need to know about some important properties of light. prisms, causing a rainbow to form.) Visible light drives photosynthesis. 190    U n i t T w o   The Cell Photosynthetic Pigments: The Light Receptors ▼ Figure 10.9 Research Method When light meets matter, it may be reflected, transmit- Determining an Absorption Spectrum ted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of dif- Application An absorption spectrum is a visual representation of ferent wavelengths, and the wavelengths that are absorbed how well a particular pigment absorbs different wavelengths of visible disappear. If a pigment is illuminated with white light, the light. Absorption spectra of various chloroplast pigments help scientists decipher the role of each pigment in a plant. color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it ap- Technique A spectrophotometer measures the relative amounts of pears black.) We see green when we look at a leaf because light of different wavelengths absorbed and transmitted by a pigment solution. chlorophyll absorbs violet-blue and red light while trans- mitting and reflecting green light (Figure 10.8). The ability 1 White light is separated into colors (wavelengths) by a prism. of a pigment to absorb various wavelengths of light can be 2 One by one, the different colors of light are passed through the measured with an instrument called a spectrophotometer. sample (chlorophyll in this example). Green light and blue light are This machine directs beams of light of different wavelengths shown here. through a solution of the pigment and measures the fraction 3 The transmitted light strikes a photoelectric tube, which converts of the light transmitted at each wavelength. A graph plotting the light energy to electricity. a pigment’s light absorption versus wavelength is called an 4 The electric current is measured by a galvanometer. The meter in- absorption spectrum (Figure 10.9). dicates the fraction of light transmitted through the sample, from The absorption spectra of chloroplast pigments provide which we can determine the amount of light absorbed. clues to the relative effectiveness of different wavelengths Refracting Chlorophyll Photoelectric White for driving photosynthesis, since light can perform work light prism solution tube in chloroplasts only if it is absorbed. Figure 10.10a shows Galvanometer the absorption spectra of three types of pigments in chlo- 2 3 roplasts: chlorophyll a, the key light-capturing pigment 1 4 0 100 that participates directly in the light reactions; the accessory pigment chlorophyll b; and a separate group of accessory pigments called carotenoids. The spectrum of chlorophyll Slit moves to Green The high transmittance (low absorption) pass light light reading indicates that of selected chlorophyll absorbs wavelength. very little green light. Light Reflected 0 100 light Chloroplast The low transmittance Blue (high absorption) light reading indicates that chlorophyll absorbs most blue light. Results See Figure 10.10a for absorption spectra of three types of chloroplast pigments. Absorbed Granum light a suggests that violet-blue and red light work best for pho- tosynthesis, since they are absorbed, while green is the least Transmitted effective color. This is confirmed by an action spectrum for light photosynthesis (Figure 10.10b), which profiles the relative effectiveness of different wavelengths of radiation in driving ▲ Figure 10.8 Why leaves are green: interaction of light with the process. An action spectrum is prepared by illuminating chloroplasts. The chlorophyll molecules of chloroplasts absorb violet- blue and red light (the colors most effective in driving photosynthesis) chloroplasts with light of different colors and then plotting and reflect or transmit green light. This is why leaves appear green. wavelength against some measure of photosynthetic rate, c h a p t e r 1 0   Photosynthesis    191  such as CO2 consumption or O2 release. The action spec- ▼ Figure 10.10 Inquiry trum for photosynthesis was first demonstrated by Theodor Which wavelengths of light are most effective in W. Engelmann, a German botanist, in 1883. Before equip- driving photosynthesis? ment for measuring O2 levels had even been invented, En- gelmann performed a clever experiment in which he used Experiment Absorption and action spectra, along with a classic bacteria to measure rates of photosynthesis in filamentous experiment by Theodor W. Engelmann, reveal which wavelengths of light are photosynthetically important. algae (Figure 10.10c). His results are a striking match to the modern action spectrum shown in Figure 10.10b. Results Chloro- Notice by comparing Figures 10.10a and 10.10b that the Absorption of light by chloroplast pigments phyll a Chlorophyll b action spectrum for photosynthesis is much broader than the absorption spectrum of chlorophyll a. The absorption spectrum of chlorophyll a alone underestimates the effec- Carotenoids tiveness of certain wavelengths in driving photosynthesis. This is partly because accessory pigments with different ab- sorption spectra also present in chloroplasts—including 400 500 600 700 chlorophyll b and carotenoids—broaden the spectrum of Wavelength of light (nm) colors that can be used for photosynthesis. Figure 10.11 (a) Absorption spectra. The three curves show the wavelengths of light shows the structure of chlorophyll a compared with that of best absorbed by three types of chloroplast pigments. chlorophyll b. A slight structural difference between them is enough to cause the two pigments to absorb at slightly dif- (measured by O2 release) ferent wavelengths in the red and blue parts of the spectrum Rate of photosynthesis (see Figure 10.10a). As a result, chlorophyll a appears blue green and chlorophyll b olive green under visible light. CH3 in chlorophyll a 400 500 600 700 CHO in chlorophyll b CH2 (b) Action spectrum. This graph plots the rate of photosynthesis CH H CH3 versus wavelength. The resulting action spectrum resembles the C C C absorption spectrum for chlorophyll a but does not match exactly C C Porphyrin ring: H3C C C CH2 CH3 (see part a). This is partly due to the absorption of light by accessory light-absorbing C N N C pigments such as chlorophyll b and carotenoids. “head” of molecule; H C Mg C H note magnesium Aerobic bacteria H3C C N N C atom at center C C C C CH3 Filament C H C C of alga H CH2 H C C CH2 O C O C O O 400 500 600 700 O CH3 CH2 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann 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 releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga Hydrocarbon tail: illuminated with violet-blue or red light. interacts with hydrophobic regions of proteins inside Conclusion Light in the violet-blue and red portions of the spectrum thylakoid membranes of is most effective in driving photosynthesis. chloroplasts; H atoms not shown Source: T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichenden Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883). An Experimental Inquiry Tutorial can be assigned in MasteringBiology. ▲ Figure 10.11 Structure of chlorophyll molecules in chloro- I n t e r p r e t t h e Data What wavelengths of light drive the highest plasts of plants. Chlorophyll a and chlorophyll b differ only in one rates of photosynthesis? of the functional groups bonded to the porphyrin ring. (Also see the space-filling model of chlorophyll in Figure 1.3.) 192    U n i t T w o   The Cell  Other accessory pigments include carotenoids, hydrocar- between the ground state and an excited state, and this en- bons that are various shades of yellow and orange because ergy difference varies from one kind of molecule to another. they absorb violet and blue-green light (see Figure 10.10a). Thus, a particular compound absorbs only photons cor- Carotenoids may broaden the spectrum of colors that can responding to specific wavelengths, which is why each pig- drive photosynthesis. However, a more important function of ment has a unique absorption spectrum. at least some carotenoids seems to be photoprotection: These Once absorption of a photon raises an electron to an ex- compounds absorb and dissipate excessive light energy that cited state, the electron cannot stay there long. The excited would otherwise damage chlorophyll or interact with oxygen, state, like all high-energy states, is unstable. Generally, when forming reactive oxidative molecules that are dangerous to isolated pigment molecules absorb light, their excited elec- the cell. Interestingly, carotenoids similar to the photopro- trons drop back down to the ground-state orbital in a bil- tective ones in chloroplasts have a photoprotective role in lionth of a second, releasing their excess energy as heat. This the human eye. (Remember being told to eat your carrots for conversion of light energy to heat is what makes the top of improved night vision?) These and related molecules are, of an automobile so hot on a sunny day. (White cars are coolest course, found naturally in many vegetables and fruits. They because their paint reflects all wavelengths of visible light.) In are also often advertised in health food products as “phyto- isolation, some pigments, including chlorophyll, emit light as chemicals” (from the Greek phyton, plant), some of which well as heat after absorbing photons. As excited electrons fall have antioxidant properties. Plants can synthesize all the an- back to the ground state, photons are given off, an afterglow tioxidants they require, but humans and other animals must called fluorescence. An illuminated solution of chlorophyll obtain some of them from their diets. isolated from chloroplasts will fluoresce in the red part of the spectrum and also give off heat (Figure 10.12). This is best Excitation of Chlorophyll by Light seen by illuminating with ultraviolet light, which chlorophyll can also absorb (see Figures 10.7 and 10.10a). Viewed under What exactly happens when chlorophyll and other pigments visible light, the fluorescence would be harder to see against absorb light? The colors corresponding to the absorbed the green of the solution. wavelengths disappear from the spectrum of the transmit- ted and reflected light, but energy cannot disappear. When A Photosystem: A Reaction-Center Complex a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more poten- Associated with Light-Harvesting Complexes tial energy (see Figure 2.6b). When the electron is in its nor- Chlorophyll molecules excited by the absorption of light mal orbital, the pigment molecule is said to be in its ground energy produce very different results in an intact chloro- state. Absorption of a photon boosts an electron to an plast than they do in isolation (see Figure 10.12). In their orbital of higher energy, and the pigment molecule is then native environment of the thylakoid membrane, chlorophyll said to be in an excited state. The only photons absorbed are molecules are organized along with other small organic mol- those whose energy is exactly equal to the energy difference ecules and proteins into complexes called photosystems. ▶ Figure 10.12 Excitation of isolated chlorophyll by light. (a) Absorption of a Excited photon causes a transition of the chlorophyll e– state molecule from its ground state to its excited state. The photon boosts an electron to an orbital where it has more potential energy. If Energy of electron the illuminated molecule exists in isolation, Heat its excited electron immediately drops back down to the ground-state orbital, and its excess energy is given off as heat and fluo- rescence (light). (b) A chlorophyll solution excited with ultraviolet light fluoresces with a red-orange glow. Photon (fluorescence) w h a t I F ? If a leaf containing a similar con- Photon centration of chlorophyll as the solution was Ground Chlorophyll state exposed to the same ultraviolet light, no fluo- molecule rescence would be seen. Propose an explana- tion for the difference in fluorescence emission between the solution and the leaf. (a) Excitation of isolated chlorophyll molecule (b) Fluorescence c h a p t e r 1 0   Photosynthesis    193  A photosystem is composed of a reaction-center complex surrounded by several light-harvesting com- Thylakoid plexes (Figure 10.13). The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules. Each light-harvesting complex consists of various pigment molecules (which may include Photosystem STROMA chlorophyll a, chlorophyll b, and multiple carotenoids) Photon bound to proteins. The number and variety of pigment Light-harvesting Reaction- Primary molecules enable a photosystem to harvest light over a complexes center complex electron acceptor larger surface area and a larger portion of the spectrum than could any single pigment molecule alone. Together, these light-harvesting complexes act as an antenna for the reaction-center complex. When a pigment molecule Thylakoid membrane absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting e– complex, 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 electron acceptor. The pair of chlorophyll a molecules in the reaction-center complex are special be- cause their molecular environment—their location and the Transfer Special pair of Pigment of energy chlorophyll a molecules other molecules with which they are associated—enables molecules them to use the energy from light not only to boost one of THYLAKOID SPACE their electrons to a higher energy level, but also to transfer (INTERIOR OF THYLAKOID) it to a different molecule—the primary electron acceptor. (a) How a photosystem harvests light. When a photon strikes a pig- The solar-powered transfer of an electron from the ment molecule in a light-harvesting complex, the energy is passed reaction-center chlorophyll a pair to the primary electron from molecule to molecule until it reaches the reaction-center com- acceptor is the first step of the light reactions. As soon as plex. Here, an excited electron from the special pair of chlorophyll a molecules is transferred to the primary electron acceptor. the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox Chlorophyll STROMA reaction. In the flask shown in Figure 10.12b, isolated chlo- Thylakoid membrane rophyll fluoresces because there is no electron acceptor, so electrons of photoexcited chlorophyll drop right back to the ground state. In the structured environment of a chloro- plast, however, an electron acceptor is readily available, and the potential energy represented by the excited electron is not dissipated as light and heat. Thus, each photosystem— © 2004 AAAS a reaction-center complex surrounded by light-harvesting complexes—functions in the chloroplast as a unit. It con- Protein THYLAKOID verts light energy to chemical energy, which will ultimately subunits SPACE be used for the synthesis of sugar. (b) Structure of a photosystem. This computer model, based on The thylakoid membrane is populated by two types of X-ray crystallography, shows two photosystem complexes side by photosystems that cooperate in the light reactions of pho- side, oriented opposite to each other. Chlorophyll molecules (small tosynthesis. They are called photosystem II (PS II) and green ball-and-stick models) are interspersed with protein subunits (cylinders and ribbons). For simplicity, this photosystem will be photosystem I (PS I). (They were named in order of their shown as a single complex in the rest of the chapter. discovery, but photosystem II functions first in the light reac- ▲ Figure 10.13 The structure and function of a photosystem. tions.) Each has a characteristic reaction-center complex—a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associated with specific pro- light having a wavelength of 680 nm (in the red part of the teins. The reaction-center chlorophyll a of photosystem II spectrum). The chlorophyll a at the reaction-center complex is known as P680 because this pigment is best at absorbing of photosystem I is called P700 because it most effectively 194    U n i t T w o   The Cell absorbs light of wavelength 700 nm (in the far-red part of the 1 A photon of light strikes one of the pigment molecules in a spectrum). These two pigments, P680 and P700, are nearly light-harvesting complex of PS II, boosting one of its elec- identical chlorophyll a molecules. However, their association trons to a higher energy level. As this electron falls back to with different proteins in the thylakoid membrane affects the its ground state, an electron in a nearby pigment molecule electron distribution in the two pigments and accounts for is simultaneously raised to an excited state. The process the slight differences in their light-absorbing properties. Now continues, with the energy being relayed to other pigment let’s see how the two photosystems work together in using molecules until it reaches the P680 pair of chlorophyll a light energy to generate ATP and NADPH, the two main molecules in the PS II reaction-center complex. It excites an products of the light reactions. electron in this pair of chlorophylls to a higher energy state. 2 This electron is transferred from the excited P680 to the Linear Electron Flow primary electron acceptor. We can refer to the resulting Light drives the synthesis of ATP and NADPH by ener- form of P680, missing an electron, as P680+. gizing the two photosystems embedded in the thylakoid 3 An enzyme catalyzes the splitting of a water molecule membranes of chloroplasts. The key to this energy trans­ into two electrons, two hydrogen ions (H+), and an oxy- formation is a flow of electrons through the photosystems gen atom. The electrons are supplied one by one to the and other molecular components built into the thylakoid P680+ pair, each electron replacing one transferred to the membrane. This is called linear electron flow, and it occurs primary electron acceptor. (P680+ is the strongest bio- during the light reactions of photosynthesis, as shown in logical oxidizing agent known; its electron “hole” must Figure 10.14. The numbered steps in the text correspond to be filled. This greatly facilitates the transfer of electrons the numbered steps in the figure. from the split water molecule.) The H+ are released into ▼ Figure 10.14 How linear electron flow during the light reac- H2O CO2 tions generates ATP and NADPH. The gold arrows trace the flow Light of light-driven electrons from water to NADPH. The black arrows trace the transfer of energy from pigment molecule to pigment molecule. NADP+ ADP LIGHT CALVIN CYCLE REACTIONS 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 1/2 O 2 NADPH Pc e– e– P700 1 Light P680 5 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) c h a p t e r 1 0   Photosynthesis    195  the thylakoid space. The oxygen atom immediately com- bines with an oxygen atom generated by the splitting of e– another water molecule, forming O2. 4 Each photoexcited electron passes from the primary elec- e– e– tron acceptor of PS II to PS I via an electron transport Mill chain, the components of which are similar to those of the makes NADPH e– ATP electron transport chain that functions in cellular respira- e– e– tion. The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cy- n Photo tochrome complex, and a protein called plastocyanin (Pc). 5 The exergonic “fall” of electrons to a lower energy level provides energy for the synthesis of ATP. As electrons e– pass through the cytochrome complex, H+ are pumped ATP Photon into the thylakoid space, contributing to the proton gra- dient that is subsequently used in chemiosmosis. Photosystem II Photosystem I 6 Meanwhile, light energy has been transferred via light- ▲ Figure 10.15 A mechanical analogy for linear electron flow harvesting complex pigments to the PS I reaction-center during the light reactions. complex, exciting an electron of the P700 pair of chlo- rophyll a molecules located there. The photoexcited electron is then transferred to PS I’s primary electron ac- The energy changes of electrons during their linear flow ceptor, creating an electron “hole” in the P700—which we through the light reactions are shown in a mechanical anal- now can call P700+. In other words, P700+ can now act as ogy in Figure 10.15. Although the scheme shown in Figures an electron acceptor, accepting an electron that reaches 10.14 and 10.15 may seem complicated, do not lose track of the bottom of the electron transport chain from PS II. the big picture: The light reactions use solar power to gener- 7 Photoexcited electrons are passed in a series of redox re- ate ATP and NADPH, which provide chemical energy and actions from the primary electron acceptor of PS I down reducing power, respectively, to the carbohydrate-synthesiz- a second electron transport chain through the protein ing reactions of the Calvin cycle. ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.) Cyclic Electron Flow + 8 The enzyme NADP reductase catalyzes the transfer of In certain cases, photoexcited electrons can take an alterna- electrons from Fd to NADP+. Two electrons are required tive path called cyclic electron flow, which uses photosystem for its reduction to NADPH. This molecule is at a higher I but not photosystem II. You can see in Figure 10.16 that energy level than water, so its electrons are more readily cyclic flow is a short circuit: The electrons cycle back from available for the reactions of the Calvin cycle. This pro- ferredoxin (Fd) to the cytochrome complex and from there cess also removes an H+ from the stroma. continue on to a P700 chlorophyll in the PS I reaction-center ◀ Figure 10.16 Cyclic electron flow. Primary Photoexcited electrons from PS I are occa- acceptor sionally shunted back from ferredoxin (Fd) to Primary Fd acceptor chlorophyll via the cytochrome complex and Fd plastocyanin (Pc). This electron shunt supple- NADP+ ments the supply of ATP (via chemiosmosis) Pq NADP+ + H+ but produces no NADPH. The “shadow” of reductase linear electron flow is included in the diagram Cytochrome NADPH for comparison with the cyclic route. The two complex Fd molecules in this diagram are actually one and the same—the final electron carrier in the Pc electron transport chain of PS I—although it is depicted twice to clearly show its role in two parts of the process. ? Look at Figure 10.15, and explain how you Photosystem I would alter it to show a mechanical analogy for Photosystem II ATP cyclic electron flow. 196    U n i t T w o   The Cell complex. There is no production of NADPH and no release A Comparison of Chemiosmosis in of oxygen that results from this process. On the other hand, Chloroplasts and Mitochondria cyclic flow does generate ATP. Rather than having both PSII and PSI, several of the cur- Chloroplasts and mitochondria generate ATP by the same rently existing groups of photosynthetic bacteria are known basic mechanism: chemiosmosis. An electron transport to have a single photosystem related to either PSII or PSI. chain pumps protons (H+) across a membrane as electrons For these species, which include the purple sulfur bacteria are passed through a series of carriers that are progres- (see Figure 10.2e) and the green sulfur bacteria, cyclic elec- sively more electronegative. Thus, electron transport chains tron flow is the one and only means of generating ATP dur- transform redox energy to a proton-motive force, potential ing the process of photosynthesis. Evolutionary biologists energy stored in the form of an H+ gradient across a mem- hypothesize that these bacterial groups are descendants of brane. An ATP synthase complex in the same membrane ancestral bacteria in which photosynthesis first evolved, in a couples the diffusion of hydrogen ions down their gradient form similar to cyclic electron flow. to the phosphorylation of ADP, forming ATP. Cyclic electron flow can also occur in photosynthetic spe- Some of the electron carriers, including the iron-containing cies that possess both photosystems; this includes some pro- proteins called cytochromes, are very similar in chloroplasts karyotes, such as the cyanobacteria shown in Figure 10.2d, and mitochondria. The ATP synthase complexes of the two as well as the eukaryotic photosynthetic species that have organelles are also quite similar. But there are noteworthy dif- been tested thus far. Although the process is probably in part ferences between photophosphorylation in chloroplasts and an “evolutionary leftover,” research suggests it plays at least oxidative phosphorylation in mitochondria. In chloroplasts, one beneficial role for these organisms. Mutant plants that the high-energy electrons dropped down the transport chain are not able to carry out cyclic electron flow are capable of come from water, while in mitochondria, they are extracted growing well in low light, but do not grow well where light is from organic molecules (which are thus oxidized). Chloro- intense. This is evidence for the idea that cyclic electron flow plasts do not need molecules from food to make ATP; their may be photoprotective. Later you’ll learn more about cyclic photosystems capture light energy and use it to drive the elec- electron flow as it relates to a particular adaptation of photo- trons from water to the top of the transport chain. In other synthesis (C4 plants; see Concept 10.4). words, mitochondria use chemiosmosis to transfer chemical Whether ATP synthesis is driven by linear or cyclic energy from food molecules to ATP, whereas chloroplasts electron flow, the actual mechanism is the same. Before we transform light energy into chemical energy in ATP. move on to consider the Calvin cycle, let’s review chemi- Although the spatial organization of chemiosmosis dif- osmosis, the process that uses membranes to couple redox fers slightly between chloroplasts and mitochondria, it is reactions to ATP production. easy to see similarities in the two (Figure 10.17). The inner ▶ Figure 10.17 Comparison of chemiosmosis Mitochondrion Chloroplast in mitochondria and chloroplasts. In both kinds of organelles, electron transport chains pump pro- tons (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. Inter- H+ Diffusion membrane Thylakoid space space 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+] c h a p t e r 1 0   Photosynthesis    197 membrane of the mitochondrion pumps protons from the organelles, while the mitochondrial matrix is analogous to mitochondrial matrix out to the intermembrane space, the stroma of the chloroplast. which then serves as a reservoir of hydrogen ions. The thyla- In the mitochondrion, protons diffuse down their con- koid membrane of the chloroplast pumps protons from the centration gradient from the intermembrane space through stroma into the thylakoid space (interior of the thylakoid), ATP synthase to the matrix, driving ATP synthesis. In the which functions as the H+ reservoir. If you imagine the chloroplast, ATP is synthesized as the hydrogen ions dif- cristae of mitochondria pinching off from the inner mem- fuse from the thylakoid space back to the stroma through brane, this may help you see how the thylakoid space and ATP synthase complexes, whose catalytic knobs are on the the intermembrane space are comparable spaces in the two stroma side of the membrane (Figure 10.18). Thus, ATP forms in the stroma, where it is used to help drive sugar syn- thesis during the Calvin cycle. The proton (H+) gradient, or pH gradient, across the H2O CO2 thylakoid membrane is substantial. When chloroplasts in an Light NADP+ ADP LIGHT CALVIN CYCLE REACTIONS ATP NADPH O2 [CH2O] (sugar) STROMA (low H+ concentration) NADP+ Cytochrome reductase Photosystem II Photosystem I complex Light 3 4 H+ Light NADP+ + H+ Fd Pq NADPH e– Pc e– 2 H2O THYLAKOID SPACE 1 12 O2 (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP Pi H+ ▲ Figure 10.18 The light reactions and the side of the membrane facing the thylakoid thylakoid space, as in Figure 10.17. The diffu- chemiosmosis: Current model of the or- space; 2 as plastoquinone (Pq) transfers elec- sion of H+ from the thylakoid space back to the ganization of the thylakoid membrane. trons to the cytochrome complex, four protons stroma (along the H+ concentration gradient) The gold arrows track the linear electron flow are translocated across the membrane into powers the ATP synthase. These light-driven outlined in Figure 10.14. At least three steps in the thylakoid space; and 3 a hydrogen ion is reactions store chemical energy in NADPH the light reactions contribute to the H+ gradient removed from the stroma when it is taken up and ATP, which shuttle the energy to the by increasing H+ concentration in the thylakoid by NADP+. Notice that in step 2, hydrogen ions carbohydrate-producing Calvin cycle. space: 1 Water is split by photosystem II on are being pumped from the stroma into the 198    U n i t T w o   The Cell experimental setting are illuminated, the pH in the thylakoid and consuming energy. Carbon enters the Calvin cycle in space drops to about 5 (the H+ concentration increases), the form of CO2 and leaves in the form of sugar. The cycle and the pH in the stroma increases to about 8 (the H+ con- spends ATP as an energy source and consumes NADPH as centration decreases). This gradient of three pH units cor- reducing power for adding high-energy electrons to make responds to a thousandfold difference in H+ concentration. the sugar. If the lights are then turned off, the pH gradient is abolished, As we mentioned previously (in Concept 10.1), the car- but it can quickly be restored by turning the lights back on. bohydrate produced directly from the Calvin cycle is actu- Experiments such as this provided strong evidence in sup- ally not glucose, but a three-carbon sugar; the name of this port of the chemiosmotic model. sugar is glyceraldehyde 3-phosphate (G3P). For the net The currently-accepted model for the organization of the synthesis of one molecule of G3P, the cycle must take place light-reaction “machinery” within the thylakoid membrane three times, fixing three molecules of CO2—one per turn of is based on several research studies. Each of the molecules the cycle. (Recall that the term carbon fixation refers to the and molecular complexes in the figure is present in numer- initial incorporation of CO2 into organic material.) As we ous copies in each thylakoid. Notice that NADPH, like ATP, trace the steps of the cycle, it's important to keep in mind is produced on the side of the membrane facing the stroma, that we are following three molecules of CO2 through the where the Calvin cycle reactions take place. reactions. Figure 10.19 divides the Calvin cycle into three Let’s summarize the light reactions. Electron flow pushes phases: carbon fixation, reduction, and regeneration of the electrons from water, where they are at a low state of poten- CO2 acceptor. tial energy, ultimately to NADPH, where they are stored at a high state of potential energy. The light-driven electron flow Phase 1: Carbon fixation. The Calvin cycle incorpo- also generates ATP. Thus, the equipment of the thylakoid rates each CO2 molecule, one at a time, by attaching membrane converts light energy to chemical energy stored it to a five-carbon sugar named ribulose bisphosphate in ATP and NADPH. (Oxygen is a by-product.) Let’s now (abbreviated RuBP). The enzyme that catalyzes this first see how the Calvin cycle uses the products of the light reac- step is RuBP carboxylase-oxygenase, or rubisco. (This tions to synthesize sugar from CO2. is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.) The product of the reaction is a six-carbon intermediate that Concept Check 10.2 is short-lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 1. What color of light is least effective in driving photosyn- thesis? Explain. 3-phosphoglycerate (for each CO2 fixed). 2. In the light reactions, what is the initial electron donor? Phase 2: Reduction. Each molecule of 3-phosphoglycerate Where do the electrons finally end up? receives an additional phosphate group from ATP, be- 3. w h a t I F ? In an experiment, isolated chloroplasts coming 1,3-bisphosphoglycerate. Next, a pair of electrons placed in an illuminated solution with the appropriate chemicals can carry out ATP synthesis. Predict what donated from NADPH reduces 1,3-bisphosphoglycerate, would happen to the rate of synthesis if a compound is which also loses a phosphate group in the process, be- added to the solution that makes membranes freely per- coming glyceraldehyde 3-phosphate (G3P). Specifically, meable to hydrogen ions. the electrons from NADPH reduce a carboyxl group on For suggested answers, see Appendix A. 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar—the same three-carbon sugar formed in glycolysis by the split- CONCEPT 10.3 ting of glucose (see Figure 9.9). Notice in Figure 10.19 that for every three molecules of CO2 that enter the cycle, The Calvin cycle uses the chemical there are six molecules of G3P formed. But only one energy of ATP and NADPH to reduce CO2 molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to sugar to complete the cycle. The cycle began with 15 carbons’ The Calvin cycle is similar to the citric acid cycle in that a worth of carbohydrate in the form of three molecules of starting material is regenerated after some molecules enter the five-carbon sugar RuBP. Now there are 18 carbons’ the cycle and others exit the cycle. However, the citric worth of carbohydrate in the form of six molecules of acid cycle is catabolic, oxidizing acetyl CoA and using the G3P. One molecule exits the cycle to be used by the plant energy to synthesize ATP. In contrast, the Calvin cycle is cell, but the other five molecules must be recycled to re- anabolic, building carbohydrates from smaller molecules generate the three molecules of RuBP. c h a p t e r 1 0   Photosynthesis    199  Input H2O CO2 3 Light CO2, entering one per cycle NADP+ ADP CALVIN LIGHT REACTIONS CYCLE Phase 1: Carbon fixation ATP Rubisco NADPH 3 P P Short-lived O2 [CH2O] (sugar) intermediate 3 P P 6 P Ribulose bisphosphate 3-Phosphoglycerate 6 ATP (RuBP) 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of 6 NADP+ the CO2 acceptor 6 Pi (RuBP) 5 P G3P 6 P Glyceraldehyde 3-phosphate Phase 2: (G3P) Reduction ▲ Figure 10.19 Th