Photosynthesis Unit 5 PDF

PHOTOSYNTHESIS Unit 5 Mrs. Christine C. Coriento  Unit Content ▪ Overview of Photosynthesis ▪ The Discovery of Photosynthetic Processes ▪ Pigments ▪ Photosystem Organization ▪...

PHOTOSYNTHESIS Unit 5 Mrs. Christine C. Coriento  Unit Content ▪ Overview of Photosynthesis ▪ The Discovery of Photosynthetic Processes ▪ Pigments ▪ Photosystem Organization ▪ The Light-Dependent Reactions ▪ Carbon Fixation: The Calvin Cycle LESSON 1 OVERVIEW OF PHOSYNTHESIS  OBJECTIVES 1. Explain the reaction for photosynthesis; and 2. Describe the structure of the chloroplast. Overview of Photosynthesis ▪ Life is powered by sunshine. ▪ The diversity of life is only possible because our planet is awash in energy streaming Earthward from the Sun. equals the power from about 1 million Hiroshima-sized atomic bombs/per day ▪ Photosynthesis captures about 1% of this huge supply of energy and uses it to provide the energy that drives all life. Photosynthesis combines CO2 and H2O, producing glucose and O2 ▪ Photosynthesis occurs in a wide variety of organisms, and it comes in different forms. ▪ Anoxygenic photosynthesis in four different bacterial groups: purple bacteria, green sulfur bacteria, green nonsulfur bacteria, and heliobacteria. ▪ Oxygenic photosynthesis is found in cyanobacteria, seven groups of algae, and essentially all land plants. ▪ These two types of photosynthesis share similarities in the types of pigments they use to trap light energy, but they differ in the arrangement and action of these pigments. Photosynthesis combines CO2 and H2O, producing glucose and O2 ▪ In the case of plants, photosynthesis takes place primarily in the leaves. ▪ The cells of plant leaves contain organelles called chloroplasts, which are required for photosynthesis. ▪ No other structure in a plant cell is able to carry out photosynthesis. Leaf Anatomy 3 Stages of Photosynthesis 1. capturing energy from sunlight; 2. using the energy to make ATP and to reduce the compound NADP+, an electron carrier, to NADPH; and 3. using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air. 3 Stages of Photosynthesis The first two stages require light and are commonly called the light-dependent reactions. The third stage, the formation of organic molecules from CO2, is called carbon fixation. This process takes place via a cyclic series of reactions. As long as ATP and NADPH are available, the carbon fixation reactions can occur either in the presence or in the absence of light, and so these reactions are also called the light-independent reactions. Overview of Photosynthesis Overview of Photosynthesis ▪ the reverse of the reaction for respiration ▪ In respiration, glucose is oxidized to CO2 using O2 as an electron acceptor. ▪ In photosynthesis, CO2 is reduced to glucose using electrons gained from the oxidation of water. ▪ The oxidation of H2O and the reduction of CO2 requires energy that is provided by light. In plants, photosynthesis takes place in chloroplasts. ▪ The internal membrane of chloroplasts, called the thylakoid membrane, is a continuous phospholipid bilayer organized into flattened sacs called thylakoid disks. ▪ These are stacked in columns called grana (singular, granum). ▪ This forms three compartments: - the thylakoid membrane itself, and the spaces inside and outside this membrane. ▪ The thylakoid membrane contains the enzymatic machinery to make ATP, and chlorophyll and other photosynthetic pigments that capture light energy.  The compartment outside the thylakoid membrane system is called the stroma, and is analogous to the matrix in mitochondria. The stroma is a semiliquid substance containing the enzymes necessary to incorporate CO2 into organic compounds using energy from ATP coupled with reduction by NADPH.  OBJECTIVES In plants, photosynthesis takes place in chloroplasts. In the thylakoid membrane, photosynthetic pigments are organized into photosystems that absorb light, which excites an electron that can be passed to an electron carrier. Each pigment molecule within the photosystem is capable of capturing photons, which are packets of energy. When light of a proper wavelength strikes a pigment molecule in the photosystem, the resulting excitation passes from one pigment molecule to another. In plants, photosynthesis takes place in chloroplasts. ▪ The energy arrives at a key chlorophyll molecule in contact with a membrane-bound protein that can accept an electron. ▪ The energy is transferred as an excited electron to that protein, which passes it on to a series of other membrane proteins that put the energy to work making ATP and NADPH. ▪ These compounds are then used to build organic molecules. ▪ The photosystem thus acts as a large antenna, gathering the light energy harvested by many individual pigment molecules. Overview of Photosynthesis Travel Deep Inside a Leaf https://www.youtube.com/watch?v=pwymX2 LxnQs LESSON 2 THE DISCOVERY OF PHOTOSYNTHETIC PROCESSES  OBJECTIVES 1. Describe experiments that support our understanding of photosynthesis; and 2. Differentiate between the light-dependent and light independent reactions. Plants do not increase mass from soil and water alone. ▪ From the time of the Greeks, plants were thought to obtain their food from the soil, literally sucking it up with their roots. ▪ A Belgian doctor, Jan Baptista van Helmont (1580–1644) thought of a simple way to test this idea. ▪ He incorrectly concluded, however, that the water he had been adding mainly accounted for the plant’s increased biomass. Plants do not increase mass from soil and water alone. ▪ A hundred years passed before the story became clearer. ▪ The key clue was provided by the English scientist Joseph Priestly (1733–1804). On the 17th of August, 1771, Priestly put a living sprig of mint into air in which a wax candle had burnt out. On the 27th of the same month, Priestly found that another candle could be burned in this same air. Priestly found that while a mouse could not breathe candle-exhausted air, air “restored” by vegetation was not “at all inconvenient to a mouse.” The key clue was that living vegetation adds something to the air. Plants do not increase mass from soil and water alone. ▪ Twenty-five years later, the Dutch physician Jan Ingenhousz (1730–1799) solved the puzzle. ▪ He demonstrated that air was restored only in the presence of sunlight and only by a plant’s green leaves, not by its roots. ▪ He proposed that the green parts of the plant carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen. ▪ He suggested that the oxygen was released as O2 gas into the air, while the carbon atom combined with water to form carbohydrates. ▪ Other research refined his conclusions, and by the end of the 19th century, the overall reaction for photosynthesis could be written as: CO2 + H2O + light energy → (CH2O) + O2 Photosynthesis includes both light-dependent and light-independent reactions. ▪ At the beginning of the 20th century, the English plant physiologist F. F. Blackman (1866–1947) came to the surprising conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly. ▪ Blackman measured the effects of different light intensities,CO2 concentrations, and temperatures on photosynthesis. ▪ As long as light intensity was relatively low, he found photosynthesis could be accelerated by increasing the amount of light, but not by increasing the temperature or CO2 concentration ▪ At high light intensities, however, an increase in temperature or CO2 concentration greatly accelerated photosynthesis. Photosynthesis includes both light-dependent OBJECTIVES and light-independent reactions. ▪ Blackman concluded that photosynthesis consists of an initial set of what he called “light” reactions, that are largely independent of temperature but depend on light, and a second set of “dark” reactions (more properly called light-independent reactions), that seemed to be independent of light but limited by CO2. ▪ He found that increased temperature increased the rate of the light-independent reactions, but only up to about 35°C. ▪ Higher temperatures caused the rate to decrease rapidly. ▪ Because many plant enzymes begin to be denatured at 35°C, he concluded that enzymes must carry out the light-independent reactions. Discovery of the Light-independent Reactions O2 comes from H2O, not from CO2 In the 1930s, C. B. van Niel (1897–1985), working at the Hopkins Marine Station at Stanford, discovered that purple sulfur bacteria do not release oxygen during photosynthesis; instead, they convert hydrogen sulfide (H2S) into globules of pure elemental sulfur that accumulate inside them. The process van Niel observed was: CO2 + 2H2S + light energy → (CH2O) + H2O + 2S O2 comes from H2O, not from CO2 The striking parallel between this equation and Ingenhousz’s equation led van Niel to propose that the generalized process of photosynthesis can be shown as: CO2 + 2H2A + light energy → (CH2O) + H2O + 2A The substance H2A serves as an electron donor. In photosynthesis performed by green plants, H2A is water, whereas in purple sulfur bacteria, H2A is hydrogen sulfide. The product, A, comes from the splitting of H2A. Therefore, the O2 produced during green plant photosynthesis results from splitting water, not carbon dioxide. O2 comes from H2O, not from CO2 When isotopes came into common use in the early 1950s, van Niel’s revolutionary proposal was tested. Investigators examined photosynthesis in green plants supplied with water containing heavy oxygen (18O); they found that the 18O label ended up in oxygen gas rather than in carbohydrate, just as van Niel had predicted: CO2 + 2H218O + light energy → (CH2O) + H2O + 18O 2  O2 comes from H2O, not from CO2 In algae and green plants, the carbohydrate typically produced by photosynthesis is glucose. The complete balanced equation for photosynthesis in these organisms thus becomes: 6CO2 + 12H2O + light energy → C6H12O6 + 6H2O + 6O2  ATP and NADPH from light-dependent reactions reduce CO2 to make sugars. In his pioneering work on the light-dependent reactions, van Niel proposed that the H+ ions and electrons generated by the splitting of water were used to convert CO2 into organic matter in a process he called carbon fixation. In the 1950s, Robin Hill (1899–1991) demonstrated that van Niel was right: Light energy could be harvested and used in a reduction reaction.  ATP and NADPH from light-dependent reactions reduce CO2 to make sugars. Chloroplasts isolated from leaf cells were able to reduce a dye and release oxygen in response to light. Later experiments showed that the electrons released from water were transferred to NADP+ and that illuminated chloroplasts deprived of CO2 accumulate ATP. If CO2 is introduced, neither ATP nor NADPH accumulate, and the CO2 is assimilated into organic molecules.  ATP and NADPH from light-dependent reactions reduce CO2 to make sugars. These experiments are important for three reasons: 1. They firmly demonstrate that photosynthesis in plants occurs within chloroplasts. 2. They show that the light-dependent reactions use light energy to reduce NADP+ and to manufacture ATP. 3. They confirm that the ATP and NADPH from this early stage of photosynthesis are then used in the subsequent reactions to reduce carbon dioxide, forming simple sugars. LESSON 3 PIGMENTS  OBJECTIVES 1. Discuss how pigments are important to photosynthesis; and 2. Relate the absorption spectrum of a pigment to its color.  Pigments ▪ For plants to make use of the energy of sunlight, some biochemical structure must be present in chloroplasts and the thylakoids that can absorb this energy. ▪ Molecules that absorb light energy in the visible range are termed pigments. ▪ We are most familiar with them as dyes that impart a certain color to clothing or other materials. Light is a form of energy. ▪ The wave nature of light produces an electromagnetic spectrum that differentiates light based on its wavelength. ▪ Visible light is only a small part of the entire spectrum. ▪ Visible light can be divided into its separate colors by the use of a prism, which separates light based on wavelength. ▪ A particle of light, termed a photon, acts like a discrete bundle of energy. Light is a form of energy. ▪ The energy content of a photon is inversely proportional to the wavelength of the light: ▪ Short-wavelength light contains photons of higher energy than long- wavelength light. ▪ X-rays, which contain a great deal of energy, have very short wavelengths— much shorter than those of visible light. The Electromagnetic Spectrum Light is a form of energy. ▪ A beam of light is able to remove electrons from certain molecules, creating an electrical current. ▪ This phenomenon is called the photoelectric effect, and it occurs when photons transfer energy to electrons. ▪ The strength of the photoelectric effect depends on the wavelength of light—that is, short wavelengths are generally much more effective than long ones in producing the photoelectric effect because they have more energy. Light is a form of energy. ▪ A beam of light is able to remove electrons from certain molecules, creating an electrical current. ▪ This phenomenon is called the photoelectric effect, and it occurs when photons transfer energy to electrons. ▪ The strength of the photoelectric effect depends on the wavelength of light—that is, short wavelengths are generally much more effective than long ones in producing the photoelectric effect because they have more energy. ▪ In photosynthesis, chloroplasts are acting as photoelectric devices: They absorb sunlight and transfer the excited electrons to a carrier. Each pigment has a characteristic absorption spectrum. ▪ When a photon strikes a molecule with sufficient energy, the molecule will absorb the photon, which will raise an electron to a higher energy level. ▪ Whether the photon’s energy is absorbed depends on how much energy it carries (defined by its wavelength), and also on the chemical nature of the molecule it hits. ▪ A specific atom, therefore, can absorb only certain photons of light— namely, those that correspond to the atom’s available energy levels. ▪ As a result, each molecule has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. Each pigment has a characteristic absorption spectrum. ▪ Pigments are good absorbers of light in the visible range. ▪ Organisms have evolved a variety of different pigments, but only two general types are used in green plant photosynthesis: chlorophylls and carotenoids. ▪ In some organisms, other molecules also absorb light energy. Chlorophyll absorption spectra Chlorophylls absorb photons within narrow energy ranges. Two kinds of chlorophyll in plants, chlorophyll a and chlorophyll b, preferentially absorb violet-blue and red light. Neither of these pigments absorbs photons with wavelengths between about 500 and 600 nm; light of these wavelengths is reflected. Chlorophyll absorption spectra Chlorophyll a is the main photosynthetic pigment in plants and cyanobacteria and is the only pigment that can act directly to convert light energy to chemical energy. Chlorophyll b, acting as an accessory pigment, or secondary light absorbing pigment, complements and adds to the light absorption of chlorophyll a. Chlorophyll b has an absorption spectrum shifted toward the green wavelengths. Therefore, chlorophyll b can absorb photons that chlorophyll a cannot, greatly increasing the proportion of the photons in sunlight that plants can harvest. Structure of chlorophylls ▪ Chlorophylls absorb photons by means of an excitation process analogous to the photoelectric effect. ▪ These pigments contain a complex ring structure, called a porphyrin ring, with alternating single and double bonds. ▪ At the center of the ring is a magnesium atom. Chlorophyll absorption spectra Photons excite electrons in the porphyrin ring, which are then channeled away through the alternating carbon single- and double bond system. Electrons not associated with a single atom or bond are said to be delocalized. Different side groups attached to the outside of the ring alter the absorption properties of different types of chlorophyll. The precise absorption spectrum is also influenced by the association of chlorophyll with different proteins. Carotenoids and other accessory pigments ▪ Carotenoids consist of carbon rings linked to chains with alternating single and double bonds. ▪ They can absorb photons with a wide range of energies, although they are not always highly efficient in transferring this energy. ▪ Carotenoids assist in photosynthesis by capturing energy from light composed of wavelengths that are not efficiently absorbed by chlorophylls. Carotenoids and other accessory pigments ▪ Phycobiliproteins are accessory pigments found in cyanobacteria and some algae. ▪ These pigments contain a system of alternating double bonds similar to those found in other pigments and molecules that transfer electrons. ▪ These are probably ecologically important to cyanobacteria, helping them to exist in low- light situations in oceans. ▪ In this habitat, green light remains because red and blue light has been absorbed by green algae closer to the surface. LESSON 4 PHOTOSYSTEM ORGANIZATION  OBJECTIVES 1. Describe the nature of photosystems; and 2. Contrast the function of reaction center and antenna chlorophyll molecules. Production of O2 requires many chlorophyll molecules ▪ When photosynthetic saturation is achieved, further increases in intensity cause no increase in output. ▪ This saturation occurs far below the level expected for the number of individual chlorophyll molecules present. Production of O2 requires many chlorophyll molecules ▪ Light is absorbed not by independent pigment molecules, but rather by clusters of chlorophyll and accessory pigment molecules (photosystems). ▪ Light is absorbed by any one of hundreds of pigment molecules in a photosystem, and each pigment molecule transfers its excitation energy to a single molecule with a lower energy level than the others. A generalized photosystem contains an antenna complex and a reaction center ▪ Each photosystem is a network of chlorophyll a molecules, accessory pigments, and associated proteins held within a protein matrix on the surface of the photosynthetic membrane. ▪ Like a magnifying glass focusing light on a precise point, a photosystem channels the excitation energy gathered by any one of its pigment molecules to a specific molecule, the reaction center chlorophyll. ▪ This molecule then passes the energy out of the photosystem as excited electrons that are put to work driving the synthesis of ATP and organic molecules. A generalized photosystem contains an antenna complex and a reaction center ▪ A photosystem thus consists of two closely linked components: (1) an antenna complex of hundreds of pigment molecules that gather photons and feed the captured light energy to the reaction center; and (2) a reaction center consisting of one or more chlorophyll a molecules in a matrix of protein, that passes excited electrons out of the photosystem. ▪ When light of the proper wavelength strikes any pigment molecule within a photosystem, the light is absorbed by that pigment molecule. ▪ The excitation energy is then transferred from one molecule to another within the cluster of pigment molecules until it encounters the reaction center chlorophyll a. ▪ When excitation energy reaches the reaction center chlorophyll, electron transfer is initiated. ▪ By energizing an electron of the reaction center chlorophyll, light creates a strong electron donor where none existed before. ▪ The chlorophyll transfers the energized electron to the primary acceptor (a molecule of quinone), reducing the quinone and converting it to a strong electron donor. ▪ A nearby weak electron donor then passes a low-energy electron to the chlorophyll, restoring it to its original condition. ▪ The quinone transfers its electrons to another acceptor, and the process is repeated. ▪ In plant chloroplasts, water serves as this weak electron donor. ▪ When water is oxidized in this way, oxygen is released along with two protons (H+). LESSON 5 THE LIGHT DEPENDENT REACTIONS  OBJECTIVES 1. Compare the function of the two photosystems in green plants; 2. Explain how the light reactions generate ATP and NADPH. ▪ The internal thylakoid membrane is highly organized and contains the structures involved in the light-dependent reactions. ▪ For this reason, the reactions are also referred to as the thylakoid reactions. ▪ ▪ The thylakoid reactions take place in four stages: 1. Primary photoevent 2. Charge separation 3. Electron transport 4. Chemiosmosis 1. Primary photoevent. A photon of light is captured by a pigment. This primary photoevent excites an electron within the pigment. 2. Charge separation. This excitation energy is transferred to the reaction center, which transfers an energetic electron to an acceptor molecule, initiating electron transport. 3. Electron transport. The excited electrons are shuttled along a series of electron carrier molecules embedded within the photosynthetic membrane. Several of them react by transporting protons across the membrane, generating a proton gradient. Eventually the electrons are used to reduce a final acceptor, NADPH. 4. Chemiosmosis. The protons that accumulate on one side of the membrane now flow back across the membrane through ATP synthase where chemiosmotic synthesis of ATP takes place, just as it does in aerobic respiration.  Some bacteria use a single photosystem ▪ In these bacteria, a single photosystem is used that generates ATP via electron transport. ▪ This process returns the electrons back to the reaction center. ▪ For this reason, it is called cyclic photophosphorylation. ▪ These systems do not produce oxygen and so are also anoxygenic.  Some bacteria use a single photosystem ▪ In the purple nonsulfur bacteria, peak absorption occurs at a wavelength of 870 nm (near infrared, not visible to the human eye), and thus the reaction center pigment is called P870. ▪ Absorption of a photon by chlorophyll P870 does not raise an electron to a high enough level to be passed to NADP, so they must generate reducing power in a different way.  Some bacteria use a single photosystem ▪ When the P870 reaction center absorbs a photon, the excited electron is passed to an electron transport chain that passes the electrons back to the reaction center, generating a proton gradient for ATP synthesis. ▪ The proteins in the purple bacterial photosystem appear to be homologous to the proteins in the modern photosystem II. Chloroplasts have two connected photosystems ▪ This overcomes the limitations of cyclic photophosphorylation by providing an alternative source of electrons from the oxidation of water. ▪ The oxidation of water also generates O2, and thus is called oxygenic photosynthesis. ▪ The noncyclic transfer of electrons also produces NADPH, which can be used in the biosynthesis of carbohydrates. Chloroplasts have two connected photosystems ▪ One photosystem, called photosystem I, has an absorption peak of 700 nm, so its reaction center pigment is called P700. ▪ This photosystem can pass electrons to NADPH similarly to the photosystem found in the sulfur bacteria. ▪ The other photosystem, called photosystem II, has an absorption peak of 680 nm, so its reaction center pigment is called P680. ▪ This photosystem can generate an oxidation potential high enough to oxidize water. ▪ Working together, the two photosystems carry out a noncyclic transfer of electrons that generate both ATP and NADPH. Chloroplasts have two connected photosystems ▪ The photosystems were named I and II in the order of their discovery, and not in the order in which they operate in the light dependent reactions. ▪ Photosystem I transfers electrons ultimately to NADP+, producing NADPH. ▪ The electrons lost from photosystem I are replaced by electrons from photosystem II. ▪ Photosystem II with its high oxidation potential can oxidize water to replace the electrons transferred to photosystem I. ▪ Thus there is an overall flow of electrons from water to NADPH. Chloroplasts have two connected photosystems ▪ These two photosystems are connected by a complex of electron carriers called the cytochrome b6-f complex. ▪ This complex can use the energy from the passage of electrons to move protons across the thylakoid membrane to generate the proton gradient used by an ATP synthase enzyme. ▪ The enhancement effect, can be explained by a mechanism involving two photosystems acting in series; one photosystem absorbs preferentially in the red, the other in the far- red.  The rate of photosynthesis when red and far-red light are provided together is greater than the sum of the rates when each wavelength is provided individually. This result baffled researchers in the 1950s. Today, it provides key evidence that photosynthesis is carried out by two photochemical systems that act in series. One absorbs maximally in the far red, the other in the red portion of the spectrum. Chloroplasts have two connected photosystems ▪ Plants use photosystems II and I in series, first one and then the other, to produce both ATP and NADPH. ▪ This two-stage process is called noncyclic photophosphorylation because the path of the electrons is not a circle—the electrons ejected from the photosystems do not return to them, but rather end up in NADPH. ▪ The photosystems are replenished with electrons obtained by splitting water. Chloroplasts have two connected photosystems ▪ Photosystem I passes electrons to ferredoxin on the stromal side of the membrane (outside the thylakoid). ▪ The reduced ferredoxin carries an electron with very high potential. ▪ Two of them, from two molecules of reduced ferredoxin, are then donated to a molecule of NADP+ to form NADPH. ▪ The reaction is catalyzed by the membrane-bound enzyme NADP reductase. ATP is generated by chemiosmosis Protons are pumped from the stroma into the thylakoid compartment by the b6-f complex. The splitting of water also produces added protons that contribute to the gradient. The thylakoid membrane is impermeable to protons, so this creates an electrochemical gradient that can be used to synthesize ATP. LESSON 5 CARBON FIXATION: THE CALVIN CYCLE  OBJECTIVES 1. Describe carbon fixation; and 1. Demonstrate how six CO2 molecules can be used to make one glucose.  The Calvin Cycle ▪ Carbohydrates contain many C—H bonds and are highly reduced compared with CO2. ▪ To build carbohydrates, cells use energy and a source of electrons produced by the light- dependent reactions of the thylakoids: 1. Energy. ATP (provided by cyclic and noncyclic photophosphorylation) drives the endergonic reactions. 2. Reduction potential. NADPH (provided by photosystem I) provides a source of protons and the energetic electrons needed to bind them to carbon atoms. ▪ Much of the light energy captured in photosynthesis ends up invested in the energy- rich C—H bonds of sugars. Calvin cycle reactions convert inorganic carbon into organic molecules ▪ Because early research showed temperature dependence, photosynthesis was predicted to involve enzyme-catalyzed reactions. ▪ These reactions form a cycle of enzyme- catalyzed steps much like the citric acid cycle of respiration. ▪ Unlike the citric acid cycle, however, carbon fixation is geared toward producing new compounds, so the nature of the cycles is quite different. Calvin cycle reactions convert inorganic carbon into organic molecules ▪ The cycle of reactions that allow carbon fixation is called the Calvin cycle, after its discoverer, Melvin Calvin (1911– 1997). ▪ Because the first intermediate of the cycle, phosphoglycerate, contains three carbon atoms, this process is also called C3 photosynthesis. Calvin cycle reactions convert inorganic carbon into organic molecules ▪ The key step in this process—the event that makes the reduction of CO2 possible—is the attachment of CO2 to a highly specialized organic molecule. ▪ Photosynthetic cells produce this molecule by reassembling the bonds of two intermediates in glycolysis—fructose 6-phosphate and glyceraldehyde 3- phosphate (G3P)—to form the energy- rich 5-carbon sugar ribulose 1,5- bisphosphate (RuBP). Calvin cycle reactions convert inorganic carbon into organic molecules ▪ CO2 reacts with RuBP to form a transient 6- carbon intermediate that immediately splits into two molecules of the 3-carbon 3- phosphoglycerate (PGA). ▪ This overall reaction is called the carbon fixation reaction because inorganic carbon (CO2) has been incorporated into an organic form: the acid PGA. ▪ The enzyme that carries out this reaction, ribulose bisphosphate carboxylase/ oxygenase (usually abbreviated rubisco) is a large, 16-subunit enzyme found in the chloroplast stroma.  Carbon is transferred through cycle intermediates, eventually producing glucose. ▪ In a series of reactions, six molecules of CO2 are bound to six RuBP by rubisco to produce 12 molecules of PGA (containing 12 × 3 = 36 carbon atoms in all, 6 from CO2 and 30 from RuBP). ▪ The 36 carbon atoms then undergo a cycle of reactions that regenerates the six molecules of RuBP used in the initial step (containing 6 × 5 = 30 carbon atoms). ▪ This leaves two molecules of glyceraldehyde 3-phosphate (G3P) (each with three carbon atoms) as the net gain. ▪ These two molecules of G3P can then be used to make one molecule of glucose.  The net equation of the Calvin cycle is: 6CO2 + 18 ATP + 12 NADPH + water → 2 glyceraldehyde 3-phosphate + 16 Pi + 18 ADP + 12 NADP+ With six full turns of the cycle, six molecules of carbon dioxide enter, two molecules of G3P are produced, and six molecules of RuBP are regenerated. Thus six turns of the cycle produce two G3P that can be used to make a single glucose molecule. The six turns of the cycle also incorporated six CO molecules, providing enough carbon to synthesize 2 glucose, although the six carbon atoms do not all end up in this molecule of glucose. Photosynthesis and Cellular Respiration