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10 Photosynthesis KEY CONCEPTS 10.1 Photosynthesis feeds the biosphere p. 188 10.2 Photosynthesis converts light energy to the chemical energy of food p. 189 10.3 The light reactions convert solar energy to the chemical energy of ATP and NADPH p. 192 10.4 The Calvin cycle uses the chemical...

10 Photosynthesis KEY CONCEPTS 10.1 Photosynthesis feeds the biosphere p. 188 10.2 Photosynthesis converts light energy to the chemical energy of food p. 189 10.3 The light reactions convert solar energy to the chemical energy of ATP and NADPH p. 192 10.4 The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar p. 201 10.5 Alternative mechanisms of carbon fixation have evolved in hot, arid climates p. 203 10.6 Photosynthesis is essential for life on Earth: a review p. 206 Study Tip Make a visual study guide: Draw a cell with a large chloroplast, labeling the parts of the chloroplast. As you go through the chapter, add key reactions for each stage of Chloroplast photosynthesis, linking the stages together. Label the carbon molecule(s) with the most energy and the carbon molecule(s) with the least energy. Figure 10.1 Each leaf of this tree is harvesting energy from sunlight and using it to convert CO2 and H2O into chemical energy stored in sugar and other organic molecules. The tree uses these sugars for energy and to build its own trunk, branches, and leaves. Remarkably, enough sugars are left over to feed other organisms (like moth larvae, shown below) that cannot carry out this extraordinary transformation. How do photosynthetic cells use light to change carbon dioxide and water into organic molecules and oxygen? Light energy Photosynthesis in chloroplasts generates used in Go to Mastering Biology For Students (in eText and Study Area) • Get Ready for Chapter 10 • BioFlix® Animation: Photosynthesis • Figure 10.13 Walkthrough: How Linear Electron Flow During the Light Reactions Generates ATP and NADPH For Instructors to Assign (in Item Library) • Activity: The Light Reactions • BioFlix® Tutorial: Photosynthesis Ready-to-Go Teaching Module (in Instructor Resources) • The Light Reactions (Concept 10.3) CO2 + H2O Plant cell Organic + O 2 molecules Chloroplast used in generates Cellular respiration generates ATP Heat 187 10.1 Photosynthesis feeds the biosphere CONCEPT CHECK 10.1 1. Why are heterotrophs dependent on autotrophs? 2. WHAT IF? Fossil fuels are being depleted faster than they are replenished. Therefore, researchers are developing ways to produce “biodiesel” from the products of photosynthetic algae. They have proposed placing containers of these algae near industrial plants or highly congested city streets. Considering the process of photosynthesis, how does this arrangement make sense? For suggested answers, see Appendix A. 188 UNIT TWO (a) Plants (b) Multicellular alga (c) Unicellular protists (d) Cyanobacteria 20 om 1 om The conversion process that transforms the energy of sunlight into chemical energy stored in sugars and other organic molecules is called photosynthesis. Let’s begin by placing photosynthesis in its ecological context. 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: 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 CO2 from the air. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize organic substances (Figure 10.1). Photosynthesis also occurs in algae, certain other protists, and some prokaryotes (Figure 10.2). In this chapter, we will touch on these other groups in passing, but our emphasis will be on plants. Variations in autotrophic nutrition that occur in prokaryotes and algae will be described in Concept 27.3. Heterotrophs are unable to make their own food; they live on compounds produced by other organisms (heteromeans “other”). Heterotrophs are the biosphere’s consumers. The most obvious “other-feeding” occurs when an animal eats plants or other organisms, but heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of other organisms and organic litter such as feces, and fallen leaves; these types of heterotrophs are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way. 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 from the distant past. Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on photoautotrophs for food— and also for oxygen, a by-product of photosynthesis. . Figure 10.2 Photoautotrophs. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed themselves and the entire living world. On land, (a) plants are the predominant producers of food. In aquatic environments, photoautotrophs include unicellular and (b) multicellular algae, such as this kelp; (c) some non-algal unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (the yellow globules within the cells) (c–e, LMs). 10 om CONCEPT The Cell (e) Purple sulfur bacteria Mastering Biology BioFlix® Animation: The Flow of Carbon Atoms in Producers, Consumers, and Decomposers Leaf cross section CONCEPT 10.2 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 enzymes and other molecules are grouped together as specialized molecular complexes in a biological membrane, enabling the necessary series of chemical reactions to be carried out efficiently. The process of photosynthesis most likely originated in a group of bacteria that had infolded regions of the plasma membrane containing clusters of such molecules. In existing photosynthetic bacteria, infolded photosynthetic membranes function similarly to the internal membranes of the chloroplast, the eukaryotic organelle that absorbs energy from sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide (CO2) and water (H2O). According to what has come to be known as 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 6.5, and it will be described more fully in Concept 25.3.) Chloroplasts are present in a variety of photosynthesizing organisms (see some examples in Figure 10.2); in this chapter we focus on chloroplasts in plants, while mentioning prokaryotes from time to time. Chloroplasts: The Sites of Photosynthesis in Plants All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (Figure 10.3). 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. CO2 enters the leaf, and O2 exits, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning “mouth”). Water absorbed by the roots 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 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 sacs. In some places, thylakoid sacs are stacked in columns called grana (singular, granum). Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast. (The internal photosynthetic membranes of some prokaryotes are also called thylakoid membranes; see Figure 27.8b.) It is the light energy absorbed by chlorophyll that Mesophyll cell Chloroplasts Vein Mesophyll Stomata CO2 O2 Mesophyll cell Chloroplast 20 om Outer membrane Intermembrane space Thylakoid Granum Stroma Thylakoid space Inner membrane Chloroplast 1 om m Figure 10.3 Zooming in on the location of photosynthesis 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 chloroplast, the organelle where photosynthesis occurs (middle, LM; bottom, TEM). Mastering Biology Video: Chloroplasts in Motion drives the synthesis of organic molecules in the chloroplast. Now that we have looked at the sites of photosynthesis in plants, we are ready to look more closely at the process itself. Tracking Atoms Through Photosynthesis 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 since the 1800s: In the presence of light, the green parts of plants produce organic compounds and O2 from CO2 CHAPTER 10 Photosynthesis 189 and H2O. We can summarize the complex series of chemical reactions in photosynthesis with this chemical equation: proposing that all photosynthetic organisms require a hydrogen source but that the source varies: 6 CO2 + 12 H2O + Light energy S C6H12O6 + 6 O2 + 6 H2O Sulfur bacteria: CO2 + 2 H2S S [CH2O] + H2O + 2 S We use glucose (C6H12O6) here to simplify the relationship between photosynthesis and respiration, but the direct product 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: Plants: CO2 + 2 H2O S [CH2O] + H2O + O2 6 CO2 + 6 H2O + Light energy S C6H12O6 + 6 O2 Writing the equation in this form, we can see that the overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration (see Concept 9.1). It is important to realize that both of these metabolic processes occur in plant cells. However, as you will soon learn, chloroplasts do not synthesize sugars by simply reversing the steps of respiration. Now let’s divide the photosynthetic equation by 6 to put it in its simplest possible form: CO2 + H2O S 3CH2O4 + O2 Here, the brackets indicate that CH2O is not an actual sugar but represents the general formula for a carbohydrate (see Concept 5.2). In other words, we are imagining the synthesis of a sugar molecule one carbon at a time (with six repetitions theoretically adding up to a glucose molecule: C6H12O6). 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: Scientific Inquiry One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants is derived from H2O and not from CO2. The chloroplast splits water into hydrogen and oxygen atoms. Before this discovery, the prevailing hypothesis was that photosynthesis split carbon dioxide (CO2 S C + O2) and then added water to the carbon (C + H2O S [CH2O]). That hypothesis predicted that the O2 released during photosynthesis came from CO2, an idea challenged in the 1930s by C. B. van Niel, of Stanford University. Van Niel was investigating photosynthesis in bacteria that make their carbohydrate from CO2 but do not release O2. He concluded that, at least in these bacteria, CO2 is not split into carbon and oxygen. One group of bacteria used hydrogen sulfide (H2S) rather than water for photosynthesis, forming yellow globules of sulfur as a waste product (these globules are visible in Figure 10.2e). Here is the chemical equation for photosynthesis in these sulfur bacteria: General: CO2 + 2 H2X S [CH2O] + H2O + 2 X Thus, van Niel hypothesized that plants split H2O as a source of electrons from hydrogen atoms, releasing O2 as a by-product. Nearly 20 years later, scientists confirmed van Niel’s hypothesis by using oxygen-18 (18O), a heavy isotope, as a tracer to follow the path of oxygen atoms during photosynthesis. The experiments showed that the O2 produced by plants was labeled with 18O only if water was the source of the tracer (experiment 1). If the 18O was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2). In the following summary, green denotes labeled atoms of oxygen (18O): Experiment 1: CO2 + 2 H2O S 3CH2O4 + H2O + O2 Experiment 2: CO2 + 2 H2O S 3CH2O4 + H2O + O2 A significant result of the shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, O2, is released to the atmosphere. Figure 10.4 shows the fates of all atoms in photosynthesis. Photosynthesis as a Redox Process Let’s briefly compare photosynthesis with cellular respiration. Both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product (see Concept 9.1). The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP (see Figure 9.14). Photosynthesis reverses the direction of electron flow. Water is split, and its electrons are transferred along with hydrogen ions (H+ ) from the water to carbon dioxide, reducing it to sugar. becomes reduced Energy 1 6 CO2 1 6 H2O C6H12O6 1 6 O2 becomes oxidized . Figure 10.4 Tracking atoms through photosynthesis. The atoms from CO2 are shown in magenta, and the atoms from H2O are shown in blue. Reactants: 6 CO2 12 H2O CO2 + 2 H2S S [CH2O] + H2O + 2 S Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar. He then generalized that idea, 190 UNIT TWO The Cell Products: C6H12O6 6 H2O 6 O2 Because the electrons increase in potential energy as they move from water to sugar, this process requires energy—in other words, it is endergonic. This energy boost that occurs during photosynthesis is provided by light. The Two Stages of Photosynthesis: A Preview The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each of which has multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part) (Figure 10.5). The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H + ) and giving off O2 as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP1 (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. (The electron acceptor NADP + is first cousin to NAD +, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP + molecule.) The light reactions use solar energy to reduce NADP + to NADPH by adding a pair of electrons along with an H +. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of c Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes (green) 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 molecules, which are converted to sugar. (Recall that most simple sugars have formulas that are some multiple of CH2O.) To visualize these processes in their cellular context, see Figure 6.32. two compounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron 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. The Calvin cycle is named for Melvin Calvin, who, along with his colleagues James Bassham and Andrew Benson, began to elucidate its steps in the late 1940s. The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can 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 directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide the NADPH and ATP that the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis. Light H2O CO2 NADP + LIGHT REACTIONS ADP + Pi CALVIN CYCLE ATP Thylakoid Mastering Biology BioFlix® Animation: Photosynthesis Stroma NADPH Chloroplast O2 [CH2O] (sugar) CHAPTER 10 Photosynthesis 191 As Figure 10.5 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. On the outside of the thylakoids, during the light reactions, molecules of NADP + and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that take in ingredients and crank out products. In the next two sections, we’ll look more closely at how the two stages work, beginning with the light reactions. . Figure 10.6 The electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors by bending light of different wavelengths at different angles. (Droplets of water in the atmosphere can act as prisms, causing a rainbow to form.) Visible light drives photosynthesis. 10–5 nm 10–3 nm 103 nm 1 nm Gamma X-rays rays UV 106 nm Infrared 1m (109 nm) Microwaves 103 m Radio waves CONCEPT CHECK 10.2 Visible light 1. MAKE CONNECTIONS How do the CO2 molecules used in photosynthesis reach and enter the chloroplasts inside leaf cells? (See Concept 7.2.) 2. Explain how the use of an oxygen isotope helped elucidate the chemistry of photosynthesis. 3. WHAT IF? The Calvin cycle requires ATP and NADPH, products of the light reactions. If a classmate asserted that the light reactions don’t depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH, how would you respond? For suggested answers, see Appendix A. CONCEPT 10.3 The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy into the chemical energy of ATP and NADPH. To better understand the conversion of light to chemical energy, we need to know about some important properties of light. The Nature of Sunlight Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond. Electromagnetic waves, however, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as water. The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves). This entire range of radiation is known as the electromagnetic spectrum (Figure 10.6). The segment most important to life is the narrow band from about 380 nm to 740 nm in wavelength. This radiation is known as 192 UNIT TWO The Cell 380 450 500 550 600 Shorter wavelength Higher energy 650 700 740 nm Longer wavelength Lower energy visible light because it can be detected as various colors by the human eye. The model of light as waves explains many of light’s properties, but in certain respects light behaves as though it consists of discrete particles, called photons. Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of energy. The amount of energy is inversely related to the wavelength of the light: The shorter the wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light (see Figure 10.6). Although the sun radiates the full spectrum of electromagnetic energy, the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of other radiation. The part of the spectrum we can see—visible light—is also the radiation that drives photosynthesis. Photosynthetic Pigments: The Light Receptors When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light . Figure 10.7 Why leaves are green: interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green light. This is why leaves appear green. Light Reflected light Chloroplast . Figure 10.8 Research Method Determining an Absorption Spectrum Application An absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scientists decipher the role of each pigment in a plant. Technique A spectrophotometer measures the relative amounts of light of different wavelengths absorbed and transmitted by a pigment solution. 1 White light is separated into colors (wavelengths) by a prism. 2 One by one, the different colors of light are passed through the sample (chlorophyll in this example). Green light and blue light are shown here. 3 The transmitted light strikes a photoelectric tube, which converts the light energy to electricity. Absorbed light Granum Transmitted light 4 The electric current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed. Refracting prism White light Mastering Biology Animation: Light Energy and Pigments Chlorophyll solution 2 1 Slit moves to pass light of selected wavelength. Galvanometer 3 4 (Figure 10.7). The ability of a pigment to absorb various wave- lengths of light can be measured with an instrument called a spectrophotometer. This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted at each wavelength. A graph plotting a pigment’s light absorption versus wavelength is called an absorption spectrum (Figure 10.8). The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis since light can perform work in chloroplasts only if it is absorbed. Figure 10.9a shows the absorption spectra of three types of pigments in chloroplasts: chlorophyll a, the key light-capturing pigment 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 a suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis (Figure 10.9b), which profiles the relative effectiveness of different wavelengths of radiation in driving the process. An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting wavelength against some measure of photosynthetic rate, such as CO2 consumption or O2 release. The action spectrum for photosynthesis was first demonstrated by Theodor W. Engelmann, a German botanist, in 1883. Photoelectric tube Green light 0 The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 Blue light 100 100 The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Results See Figure 10.9a for absorption spectra of three types of chloroplast pigments. Before equipment for measuring O2 levels had even been invented, Engelmann performed a clever experiment in which he used O2-requiring bacteria to measure rates of photosynthesis in filamentous algae (Figure 10.9c). His results are a striking match to the modern action spectrum shown in Figure 10.9b. CHAPTER 10 Photosynthesis 193 . Figure 10.9 . Figure 10.10 Structure of chlorophyll a and b. Inquiry Which wavelengths of light are most effective in driving photosynthesis? Chlorophyll a and b differ in only one functional group: CH3 Experiment Absorption and action spectra, along with a classic experiment by Theodor W. Engelmann, reveal which wavelengths of light are photosynthetically important. Absorption of light by chloroplast pigments Chlorophyll a Mg Chlorophyll b N N Carotenoids O O 400 500 600 700 Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. O O Rate of photosynthesis (measured by O2 release) Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts Mastering Biology Animation: Space-Filling Model of Chlorophyll 400 500 600 700 Wavelength of light (nm) (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. Aerobic bacteria Filament of alga 400 500 600 Wavelength of light (nm) 700 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann shined light through a prism onto an alga, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which congregate 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 illuminated with violet-blue or red light. Conclusion The action spectra, confirmed by Engelmann’s experiment, show which portions of the spectrum are most effective in driving photosynthesis. Data from T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichenden Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883). Instructors: A related Experimental Inquiry Tutorial can be assigned in Mastering Biology. INTERPRET THE DATA According to the graph, which wavelengths of light drive the highest rates of photosynthesis? 194 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center N N Results CH3 in chlorophyll a CHO in chlorophyll b UNIT TWO The Cell Notice by comparing Figures 10.9a and 10.9b that the action spectrum for photosynthesis is much broader than the absorption spectrum of chlorophyll a. The absorption spectrum of chlorophyll a alone underestimates the effectiveness of certain wavelengths in driving photosynthesis. This is partly because accessory pigments with different absorption spectra— including chlorophyll b and carotenoids—broaden the spectrum of colors that can be used for photosynthesis. Figure 10.10 shows the structure of chlorophyll a compared with that of chlorophyll b. A slight structural difference between them is enough to cause the two pigments to absorb at slightly different wavelengths in the red and blue parts of the spectrum (see Figure 10.9a). As a result, chlorophyll a appears blue green and chlorophyll b olive green under visible light. In the last decade, researchers have identified two other forms of chlorophyll—chlorophyll d and chlorophyll f—that absorb higher wavelengths of light. In 2018, researchers grew a species of cyanobacterium called Chroococcidiopsis thermalis c Colony of the cyanobacteria Chroococcidiopsis thermalis. In this micrograph of cells becoming acclimated to higher wavelengths of light, cells that haven’t yet acclimated and are still using chlorophyll a for photosynthesis appear purplish-pink, and cells that have acclimated and are using chlorophyll f appear yellow. under only far-red light with a wavelength of 750 nm. They concluded that chlorophyll f was functioning in place of chlorophyll a, enabling this cyanobacterium to flourish in very shaded conditions. Higher-wavelength light has less energy, so this observation extends the lower limit of energy (the “red limit”) needed for photosynthesis to occur. Other accessory pigments include carotenoids, hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light (see Figure 10.9a). Carotenoids may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell. Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the human eye. (Carrots, known for aiding night vision, are rich in carotenoids.) These and related molecules are, of course, found naturally in many vegetables and fruits. They are also often advertised in health food products as “phytochemicals” (from the Greek phyton, plant), some of which have antioxidant properties. Plants can synthesize all the antioxidants they require, but humans and other animals must obtain some of them from their diets. Excitation of Chlorophyll by Light What exactly happens when chlorophyll and other pigments absorb light? The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy cannot disappear. When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated to an orbital where it has more potential energy (see Figure 2.6b). When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption WHAT IF? If a leaf containing the same concentration of chlorophyll as in the solution was exposed to the same ultraviolet light, no fluorescence would be seen. Propose an explanation for the difference in fluorescence emission between the solution and the leaf. A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes Chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in isolation (see Figure 10.11). In their native environment of the thylakoid membrane, chlorophyll e– Energy of electron c Figure 10.11 Excitation of isolated chlorophyll by light. (a) Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state. The photon 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 its excess energy is given off as heat and fluorescence (light). (b) A chlorophyll solution excited with ultraviolet light fluoresces with a red-orange glow. of a photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and this energy 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 absorption spectrum. Once absorption of a photon raises an electron to an excited state, the electron cannot stay there long (Figure 10.11a). The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy 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 are coolest because their paint reflects all wavelengths of visible 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, an afterglow called fluorescence. An illuminated solution of chlorophyll isolated from chloroplasts will fluoresce in the red part of the spectrum and also give off heat (Figure 10.11b). This is best seen by illuminating with ultraviolet light, which chlorophyll can also absorb (see Figures 10.6 and 10.9a). Viewed under visible light, the fluorescence would be difficult to see against the green of the solution. Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence CHAPTER 10 Photosynthesis 195 molecules are organized along with other small organic molecules and proteins into complexes called photosystems. A photosystem is composed of a reaction-center complex surrounded by several light-harvesting complexes (Figure 10.12). The reaction-center complex is an . Figure 10.12 The structure and function of a photosystem. Thylakoid Photosystem Photon Thylakoid membrane Light-harvesting Reactioncomplexes center complex STROMA Primary electron acceptor e– Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light. When a photon strikes a pigment molecule in a light-harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction-center complex. Here, an excited electron from the special pair of chlorophyll a molecules is transferred to the primary electron acceptor. STROMA Thylakoid membrane Chlorophyll (green) Protein subunits (purple) THYLAKOID SPACE (b) Structure of a photosystem. This computer model, based on X-ray crystallography, shows two photosystem complexes side by side. Chlorophyll molecules (bright green ball-and-stick models within the membrane; the tails are not shown) are interspersed with protein subunits (purple ribbons; notice the many c helices spanning the membrane). For simplicity, a photosystem will be shown as a single complex in the rest of the chapter. 196 UNIT TWO The Cell organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor. Each light-harvesting complex consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins. The number and variety of pigment molecules enable a photosystem to harvest light over a 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 absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex, like a human “wave” at a sports arena, until it is passed to the pair of chlorophyll a molecules in the reaction-center complex. This pair of chlorophyll a molecules is special because their molecular environment—their location and the other molecules with which they are associated—enables them to use the energy from light not only to boost one of their electrons to a higher energy level, but also to transfer it to a different molecule—the primary electron acceptor, which is a molecule capable of accepting electrons and becoming reduced. The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox reaction. In the flask shown in Figure 10.11b, isolated chlorophyll 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 chloroplast, 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—a reaction-center complex surrounded by light-harvesting complexes— functions in the chloroplast as a unit. It converts light energy to chemical energy, which will ultimately be used for the synthesis of sugar. The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions of photosynthesis: photosystem II (PS II) and photosystem I (PS I). (They were named in order of their discovery, but photosystem II functions first in the light reactions.) 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 proteins. The reaction-center chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a at the reaction-center complex of photosystem I is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum). These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their light-absorbing properties. Now let’s see how the two types of photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions. its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. The process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state. 2 This electron is transferred from the excited P680 to the pri- Linear Electron Flow mary electron acceptor. We can refer to the resulting form of P680, missing the negative charge of an electron, as P680+. Light drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called linear electron flow, and it occurs during the light reactions of photosynthesis, as shown in Figure 10.13. The numbered steps in the text correspond to the numbered steps in the figure. 3 An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H+ ), and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. (P680+ is the strongest biological oxidizing agent known; its electron “hole” must be filled. This greatly facilitates the transfer of electrons from the split water molecule.) The H+ are released into the thylakoid space (interior of the thylakoid). The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2. 1 A photon of light strikes one of the pigment molecules in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. As this electron falls back to H2O . Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH. The gold arrows trace the flow of light-driven electrons from water to NADPH. The black arrows trace the transfer of energy from pigment molecule to pigment molecule. To see these proteins in their cellular context, see Figure 6.32b. CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS Mastering Biology Figure Walkthrough ATP NADPH E tra lect n ro ch spo n ai rt n [CH2O] (sugar) O2 Primary electron acceptor 2 H+ + 1/2 O 2 1 Light H2O e– 2 Ele ctro Primary electron acceptor 4 n tr ans Pq por t ch Cytochrome complex 3 ain Fd 7 – e e– 8 NADP+ reductase e– NADP+ + H+ NADPH Pc e– e– P700 5 Light P680 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) CHAPTER 10 Photosynthesis 197 tron acceptor of PS II to PS I via an electron transport chain, the components of which are similar to those of the electron transport chain that functions in cellular respiration. The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc). Each component carries out redox reactions as electrons flow down the electron transport chain, releasing free energy that is used to pump protons (H + ) into the thylakoid space, contributing to a proton gradient across the thylakoid membrane. . Figure 10.14 A mechanical analogy for linear electron flow during the light reactions. e– e– e– Mill makes ATP e– 5 The potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis, to be discussed shortly. Photon harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there. The photoexcited electron is then transferred to PS I’s primary electron acceptor, creating an electron “hole” in the P700—which we now can call P700+. In other words, P700+ can now act as an electron acceptor, accepting an electron that reaches the bottom of the electron transport chain from PS II. reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd). (This chain does not create a proton gradient and thus does not produce ATP.) electrons from Fd to NADP +. Two electrons are required for its reduction to NADPH. Electrons in NADPH are at a higher energy level than they are in water (where they started), so they are more readily available for the reactions of the Calvin cycle. This process also removes an H + from the stroma. Primary acceptor Fd Pq NADP+ NADPH Pc Photosystem I 198 UNIT TWO The Cell NADP+ + H+ reductase Cytochrome complex Photosystem I In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses photosystem I but not photosystem II. You can see in Figure 10.15 that cyclic flow is a short circuit: The electrons cycle back from ferredoxin (Fd) to the cytochrome complex, then via a plastocyanin molecule (Pc) to a P700 chlorophyll in the PS I reaction-center complex. There is 8 The enzyme NADP reductase catalyzes the transfer of ATP Photosystem II Cyclic Electron Flow + Photosystem II ATP The energy changes of electrons during their linear flow through the light reactions are shown in a mechanical analogy in Figure 10.14. Although the scheme shown in Figures 10.13 and 10.14 may seem complicated, do not lose track of the big picture: The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, respectively, to the carbohydratesynthesizing reactions of the Calvin cycle. 7 Photoexcited electrons are passed in a series of redox Fd NADPH e– e– 6 Meanwhile, light energy has been transferred via light- Primary acceptor e– n Photo 4 Each photoexcited electron passes from the primary elec- b Figure 10.15 Cyclic electron flow. Photoexcited electrons from PS I are occasionally shunted back from ferredoxin (Fd) to chlorophyll via the cytochrome complex and plastocyanin (Pc). This electron shunt supplements the supply of ATP (via chemiosmosis) but produces no NADPH. The “shadow” of linear electron flow is included in the diagram for comparison with the cyclic route. The two Fd molecules in this diagram are actually one and the same—the final electron carrier in the electron transport chain of PS I—although it is depicted twice to clearly show its role in two parts of the process. VISUAL SKILLS Look at Figure 10.14 and explain how you would alter it to show a mechanical analogy for cyclic electron flow. no production of NADPH and no release of oxygen that results from this process. On the other hand, cyclic flow does generate ATP. Rather than having both PS II and PS I, several of the currently existing groups of photosynthetic bacteria are known to have a single photosystem related to either PS II or PS I. For these species, which include the purple sulfur bacteria (see Figure 10.2e) and the green sulfur bacteria, cyclic electron flow is the one and only means of generating ATP during the process of photosynthesis. Evolutionary biologists hypothesize that these bacterial groups are descendants of ancestral bacteria in which photosynthesis first evolved, in a form similar to cyclic electron flow. Cyclic electron flow can also occur in photosynthetic species that possess both photosystems; this includes some prokaryotes, such as the cyanobacteria shown in Figure 10.2d, as well as the eukaryotic photosynthetic species that have been tested thus far. Although the process is probably in part an “evolutionary leftover,” research suggests it plays at least one beneficial role for these organisms. Plants with mutations that render them unable to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is intense. This is evidence for the idea that cyclic electron flow may be photoprotective. Later you’ll learn more about cyclic electron flow as it relates to a particular adaptation of photosynthesis (C4 plants; see Concept 10.5). Whether ATP synthesis is driven by linear or cyclic electron flow, the actual mechanism is the same. Before we move on to consider the Calvin cycle, let’s review chemiosmosis, the process that uses membranes to couple redox reactions to ATP production. c Figure 10.16 Comparison of 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 synthesis (a) outside an isolated mitochondrion (assume H+ can freely cross the outer membrane; see Figure 9.14) and (b) in the stroma of a chloroplast. Explain. MITOCHONDRION STRUCTURE Higher [H+] Lower [H+] Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis (see Figure 9.14). An electron transport chain pumps protons (H+ ) across a membrane as electrons are passed through a series of carriers that have progressively more affinity for electrons. Thus, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. An ATP synthase complex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP. Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in mitochondria and chloroplasts (see Figure 6.32b and c). The ATP synthase complexes of the two organelles are also quite similar. But there are noteworthy differences between photophosphorylation in chloroplasts and oxidative phosphorylation in mitochondria. Both work by way of chemiosmosis, but in chloroplasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria, they are extracted from organic molecules (which are thus oxidized). Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain. In other words, mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts use it to transform light energy into chemical energy in ATP. Although the spatial organization of chemiosmosis differs slightly between chloroplasts and mitochondria, it is easy to see similarities in the two (Figure 10.16). Electron transport Mitochondrion Chloroplast Diffusion of H+ through ATP synthase Intermembrane space Inner membrane Matrix Key A Comparison of Chemiosmosis in Chloroplasts and Mitochondria H+ Electron transport chain (ETC) Thylakoid space Thylakoid membrane ATP synthase Pumping of H+ by ETC H+ ADP + P CHLOROPLAST STRUCTURE Stroma i ATP H+ CHAPTER 10 Photosynthesis 199 In the mitochondrion, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase to the matrix, driving ATP synthesis. In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes (Figure 10.17), whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle. The proton (H+ ) gradient, or pH gradient, across the thylakoid membrane is substantial. When chloroplasts in an chain proteins in the inner membrane of the mitochondrion pump protons from the mitochondrial matrix out to the intermembrane space, which then serves as a reservoir of hydrogen ions. Similarly, electron transport chain proteins in the thylakoid membrane of the chloroplast pump protons from the stroma into the thylakoid space, which functions as the H+ reservoir. If you imagine the cristae of mitochondria pinching off from the inner membrane, this may help you see how the thylakoid space and the intermembrane space are comparable spaces in the two organelles, while the mitochondrial matrix is analogous to the stroma of the chloroplast. . Figure 10.17 The light reactions: organization of the thylakoid membrane. H2O Chloroplast CO2 Light NADP+ ADP Thylakoid Mastering Biology Animation: The Light Reactions CALVIN CYCLE LIGHT REACTIONS Stroma ATP NADPH 1 Energy from light excites an electron (e–) from chlorophyll, which is transferred to the primary electron acceptor in photosystem II. Thylakoid membrane Photosystem II 4 H+ Light Cytochrome complex [CH2O] (sugar) O2 STROMA Low [H+] NADP+ reductase Photosystem I Light NADP+ + H+ Fd Pq H2O e– Pc e– 12 O2 + 2 H+ 2 In the thylakoid space, water is split into: e–, NADPH which replace chlorophyll's •2 electron twice, •2 H+, which contribute to the high [H+] in the thylakoid space, and •an O atom, which joins another O, forming O2 gas. 5 Low-energy e– THYLAKOID SPACE High [H+] 4 H+ 3 Gold arrows track the flow of electrons. As electrons travel down the electron transport chain, 4 H+ are pumped into the thylakoid space, increasing [H+], decreasing the pH. ATP synthase ADP + Pi CALVIN CYCLE H+ ATP STROMA Low [H+] VISUAL SKILLS Which three steps in the figure contribute to the [H + ] gradient across the thylakoid membrane? 200 UNIT TWO The Cell from water end up in NADPH (highenergy e–). NADPH formation removes H+ from the stroma, decreasing its [H+], increasing the pH. 4 In chemiosmosis, the diffusion of H+ from the thylakoid space back to the stroma down the [H+] gradient powers ATP synthase, producing ATP. experimental setting are illuminated, the pH in the thylakoid space drops to about 5 (the H+ concentration increases), and the pH in the stroma increases to about 8 (the H+ concentration decreases). This gradient of three pH units corresponds to a thousandfold difference in H+ concentration. If the lights are then turned off, the pH gradient is abolished, but it can quickly be restored by turning the lights back on. Experiments such as this provided strong evidence in support of the chemiosmotic model. The currently accepted model for the organization of the light-reaction “machinery” within the thylakoid membrane is based on several research studies. Each of the molecules and molecular complexes in Figure 10.17 is present in numerous copies in each thylakoid. Notice that NADPH, like ATP, is produced on the side of the membrane facing the stroma, where the Calvin cycle reactions take place. Let’s step back and see the big picture by summarizing the light reactions. Electron flow pushes electrons from water, where they are at a state of low potential energy, ultimately to NADPH, where they are stored at a state of high potential energy. The light-driven electron flow also generates ATP. Thus, the equipment of the thylakoid membrane converts light energy to chemical energy stored in ATP and NADPH. O2 is produced as a by-product. Let’s now see how the enzymes of the Calvin cycle use the products of the light reactions to synthesize sugar from CO2. Mastering Biology BioFlix® Animation: The Light Reactions CONCEPT CHECK 10.3 1. What color of light is least effective in driving photosynthesis? Explain. 2. In the light reactions, what is the initial electron donor? Where do the electrons finally end up? 3. WHAT IF? In an experiment, isolated chloroplasts placed in an illuminated solution with the appropriate chemicals can carry out ATP synthesis. Predict what would happen to the rate of synthesis if a compound is added to the solution that makes membranes freely permeable to hydrogen ions. For suggested answers, see Appendix A. CONCEPT 10.4 The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle takes place in the stroma; it is similar to the citric acid cycle in that a starting material is regenerated after some molecules enter and others exit the cycle. However, the citric acid cycle is catabolic, oxidizing acetyl CoA and using the energy to synthesize ATP (see Figure 9.11), while the Calvin cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make the sugar. As we mentioned in Concept 10.2, the carbohydrate produced directly from the Calvin cycle is not glucose. It is actually a three-carbon sugar called glyceraldehyde 3-phosphate (G3P). For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO2—one per turn of the cycle. (Recall that the term carbon fixation refers to the initial incorporation of CO2 into organic material.) As we trace the steps of the Calvin cycle, keep in mind that we are following three molecules of CO2 through the reactions. Figure 10.18 divides the Calvin cycle into three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor. Phase 1: Carbon fixation. The Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (which is abbreviated RuBP). The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or rubisco (see Figure 6.32c). (This 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 is short-lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO2 fixed). Phase 2: Reduction. Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate. Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming glyceraldehyde 3-phosphate (G3P). Specifically, the electrons from NADPH reduce a carboxyl group on 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 splitting of glucose (see Figure 9.8). Notice in Figure 10.18 that for every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to complete the cycle. The cycle began with 15 carbons’ worth of carbohydrate in the form of three molecules of the five-carbon sugar RuBP. Now there are 18 carbons’ worth of carbohydrate in the form of six mole