Photosynthesis Chapter 10 PDF

Chapter 10 Photosynthesis © 2017 Pearson Education, Inc.  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...

Chapter 10 Photosynthesis © 2017 Pearson Education, Inc.  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 Figure 10.1a © 2021 Pearson Education, Inc.  Figure 10.1b © 2021 Pearson Education, Inc. CONCEPT 10.1 Photosynthesis feeds the biosphere Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts Photosynthesis nourishes almost the entire living world directly or indirectly © 2021 Pearson Education, Inc. The Process That Feeds the Biosphere Autotrophs are “self-feeders” that sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere; they produce organic molecules from CO2 and other inorganic molecules © 2021 Pearson Education, Inc.  Almost all plants are photoautotrophs, that is, they use the energy of sunlight to make organic molecules Photosynthesis also occurs in algae, certain other protists, and some prokaryotes © 2021 Pearson Education, Inc.  Photoautotrophs Figure 10.2 © 2021 Pearson Education, Inc.  Heterotrophs obtain organic material from other organisms; they are the consumers of the biosphere Some consume other living things; others, called decomposers, eat dead organic material or feces Fossil fuels were formed from the remains of organisms that died hundreds of millions of years ago, representing ancient stores of the sun’s energy Almost all heterotrophs depend on photoautotrophs, either directly or indirectly, for food and O2 © 2021 Pearson Education, Inc. CONCEPT 10.2: Photosynthesis converts light energy to the chemical energy of food Plants and other photosynthetic organisms contain organelles called chloroplasts Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these organelles allows for the chemical reactions of photosynthesis © 2021 Pearson Education, Inc. Chloroplasts: The Sites of Photosynthesis in Plants Most photosynthesis in plants occurs in the leaves Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf CO2 enters and O2 exits the leaf through microscopic pores called stomata Veins transport water from the roots and export sugar to nonphotosynthetic parts of the plant © 2021 Pearson Education, Inc.  A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma Thylakoids are connected sacs in the chloroplast that compose a third membrane system Thylakoids may be stacked in columns called grana Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes © 2021 Pearson Education, Inc.  Zooming in on the location of photosynthesis in a plant Figure 10.3 © 2021 Pearson Education, Inc. Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O The overall chemical change during photosynthesis is the reverse of cellular respiration © 2021 Pearson Education, Inc. The Splitting of Water Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing O2 as a by-product © 2021 Pearson Education, Inc. Tracking atoms through photosynthesis Figure 10.4 © 2021 Pearson Education, Inc.  All photosynthetic organisms require a hydrogen source, but the source varies among organisms – For example, sulfur bacteria use H2S instead of water, forming yellow globules of sulfur as a waste product © 2021 Pearson Education, Inc. Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced Photosynthesis is an endergonic process; the energy boost is provided by light © 2021 Pearson Education, Inc.  Photosynthesis as a redox process Figure 10.UN01 © 2021 Pearson Education, Inc. The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) – Split H2O, providing electrons and protons (H+) – Release O2 as a by-product – Reduce the electron acceptor NADP+ to NADPH – Generate ATP from ADP by photophosphorylation © 2021 Pearson Education, Inc.  The Calvin cycle (in the stroma) makes sugar from CO2, using the ATP and NADPH generated during the light reactions The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules It then reduces fixed carbon to carbohydrate by transferring electrons from NADPH © 2021 Pearson Education, Inc.  Chloroplasts use light energy to make sugar by coordinating the two stages of photosynthesis The light reactions occur in the thylakoids and release NADPH and ATP to the stroma for use in the Calvin cycle © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Figure 10.5 CONCEPT 10.3: The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH © 2021 Pearson Education, Inc. The Nature of Sunlight Light is electromagnetic energy, also called electromagnetic radiation Electromagnetic energy travels in rhythmic waves Wavelength is a measure of the distance between crests of electromagnetic waves It can range from less than a nanometer (gamma rays) to more than a kilometer (radio waves) © 2021 Pearson Education, Inc.  The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light (wavelengths 380 nm to 740 nm) drives photosynthesis and produces the colors seen by the human eye © 2021 Pearson Education, Inc.  The electromagnetic spectrum © 2021 Pearson Education, Inc. Figure 10.6  Light also behaves as though it consists of discrete particles, called photons Each photon has a fixed quantity of energy which is inversely related to the wavelength of light; shorter wavelengths have more energy per photon of light © 2021 Pearson Education, Inc. Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths, and the wavelengths that are absorbed disappear Wavelengths that are not absorbed are reflected or transmitted – For example, most leaves appear green because chlorophyll absorbs violet-blue and red light while reflecting and transmitting green light © 2021 Pearson Education, Inc.  Why leaves are green: interaction of light with chloroplasts © 2021 Pearson Education, Inc. Figure 10.7  A spectrophotometer measures a pigment’s ability to absorb various wavelengths It sends light through pigments and measures the fraction of light transmitted at each wavelength An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength © 2021 Pearson Education, Inc.  Determining an absorption spectrum © 2021 Pearson Education, Inc. Figure 10.8  Three types of pigments in chloroplasts include: – Chlorophyll a, the key light-capturing pigment that participates directly in light reactions – Chlorophyll b, an accessory pigment – Carotenoids, a separate group of accessory pigments © 2021 Pearson Education, Inc.  The absorption spectrum of chlorophyll a indicates that violet-blue and red light will work best for photosynthesis, while green is the least effective The action spectrum for photosynthesis, a profile of the relative effectiveness of different wavelengths, confirms the effectiveness of violet- blue and red light © 2021 Pearson Education, Inc.  The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann He exposed different segments of a filamentous alga to different wavelengths of light and used the growth of aerobic bacteria as a measure of O2 production © 2021 Pearson Education, Inc.  Which wavelengths of light are most effective in driving photosynthesis? Figure 10.9 © 2021 Pearson Education, Inc.  Which wavelengths of light are most effective in driving photosynthesis? Aerobic bacteria Filament of alga 400 500 600 700 (c) Engelmann’s experiment Data from T. W. Engelmann, Bacterium photometricum. Ein Beitrag zur vergleichenden Physiologie des Licht-und Farbensinnes, Archiv. für Physiologie 30:95–124 (1883). Figure 10.9c © 2017 Pearson Education, Inc.  The action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Figure 10.10  CH3 in chlorophyll a CHO in chlorophyll b CH3 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown Figure 10.11 © 2017 Pearson Education, Inc.  In the last decade, two other forms of chlorophyll were discovered—chlorophyll d and chlorophyll f— that absorb higher wavelengths of light The cyanobacterium, Chroococcidiopsis thermalis, uses chlorophyll f in place of chlorophyll a in shaded conditions © 2021 Pearson Education, Inc.  Other accessory pigments called carotenoids, are yellow or orange because they absorb violet and blue-green light Carotenoids broaden the spectrum for photosynthesis Some are also photoprotective, that is, they absorb excessive light that would otherwise damage chlorophyll or react with oxygen © 2021 Pearson Education, Inc. Excitation of Chlorophyll by Light When a pigment molecule absorbs light, one of its electrons goes from a ground state to an excited state, which is unstable In isolation, excited electrons fall back to the ground state, releasing excess energy as heat or light, an afterglow called fluorescence © 2021 Pearson Education, Inc.  Excitation of isolated chlorophyll © 2021 Pearson Education, Inc. Figure 10.11 A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes The reaction-center complex is an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor © 2021 Pearson Education, Inc.  The light-harvesting complex consists of various pigment molecules bound to proteins Light-harvesting complexes transfer the energy of photons to the chlorophyll a molecules in the reaction-center complex These chlorophyll a molecules are special because they can transfer an excited electron to a different molecule © 2021 Pearson Education, Inc.  A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions © 2021 Pearson Education, Inc.  The structure and function of a photosystem Figure 10.12 © 2021 Pearson Education, Inc.  There are two types of photosystems in the thylakoid membrane, numbered in order of their discovery – Photosystem II (PS II) is called P680 because its reaction-center chlorophyll a is best at absorbing light with a wavelength of 680 nm – Photosystem I (PS I) is called P700 because its reaction-center chlorophyll a is best at absorbing light with a wavelength of 700 nm © 2021 Pearson Education, Inc. Linear Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and linear Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy © 2021 Pearson Education, Inc.  There are eight steps in linear electron flow: 1. A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites P680 2. An excited electron from P680 is transferred to the primary electron acceptor; we refer to the oxidized form as P680+ © 2021 Pearson Education, Inc.  3. An enzyme catalyzes the split of H2O into two electrons, two hydrogen ions (H+) and an oxygen atom  The electrons are transferred to the P680+ pair, reducing it back to P680  The H+ are released into the thylakoid space  The oxygen atom combines with another oxygen atom generated by the splitting of a different H2O and forms O2 © 2021 Pearson Education, Inc.  4. Electrons are passed in a series of redox reactions from the primary electron acceptor of PS II down an electron transport chain to PS I  The electron transport chain includes the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc)  Energy released by electron transfer is used to pump H+ into the thylakoid space, creating a proton gradient across the thylakoid membrane © 2021 Pearson Education, Inc.  5. Potential energy stored in the proton gradient drives production of ATP by chemiosmosis 6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor  P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain © 2021 Pearson Education, Inc.  7. Electrons are passed from the primary electron acceptor of PS I down a second electron transport chain to the protein ferredoxin (Fd)  There is no proton gradient or ATP produced by this electron transport chain © 2021 Pearson Education, Inc.  8. The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+  Two electrons are needed to reduce NADP+ to NADPH  The electrons of NADPH are at a higher energy level than they were in H2O, so are more readily available for the reactions of the Calvin cycle  The formation of NADPH also removes an H+ from the stroma © 2021 Pearson Education, Inc.  Figure 10.UN02 © 2021 Pearson Education, Inc.  How linear electron flow during the light reactions generates ATP and NADPH Figure 10.13 © 2021 Pearson Education, Inc.  Light reactions use solar power to generate ATP and NADPH, providing the chemical energy and reducing power needed by the Calvin cycle to make sugar The energy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy © 2021 Pearson Education, Inc.  A mechanical analogy for linear electron flow during the light reactions Figure 10.14 © 2021 Pearson Education, Inc. Cyclic Electron Flow In cyclic electron flow, photoexcited electrons cycle back from Fd to the cytochrome complex instead of being transferred to NADP+ Electrons are passed to a P700 chlorophyll in the PS I reaction center via the plastocyanin molecule (Pc) Cyclic electron flow uses only photosystem I It produces ATP, but no NADPH or oxygen results from this process © 2021 Pearson Education, Inc.  Cyclic electron flow- compare/contrast to linear flow Figure 10.15 © 2021 Pearson Education, Inc.  Several groups of photosynthetic bacteria have only a single photosystem related to either PS II or PS I For these organisms, cyclic electron flow is the only means of generating ATP during photosynthesis Photosynthesis may have first evolved in the ancestors of these bacteria in a form similar to cyclic electron flow © 2021 Pearson Education, Inc.  Cyclic electron flow is probably, in part an “evolutionary leftover” in organisms with both photosystems Cyclic electron flow may have some photoprotective capability; plants that do not have it grow well in low light, but cannot grow well in intense light © 2021 Pearson Education, Inc. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria both generate ATP by chemiosmosis – Electron transport chains pump protons (H +) across a membrane as electrons are passed through carriers with progressively higher electron affinity – ATP synthase couples the diffusion of H + down their gradient to the phosphorylation of ADP to ATP © 2021 Pearson Education, Inc.  Some of the electron carriers, including iron- containing proteins called cytochromes, are very similar in mitochondria and chloroplasts The ATP synthase complexes are also very similar © 2021 Pearson Education, Inc.  Photophosphorylation differs from oxidative phosphorylation in a few key ways – In chloroplasts, high energy electrons drop down the transport chain from water, while in mitochondria, they are extracted from organic molecules – Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP © 2021 Pearson Education, Inc.  Although the spatial organization of chemiosmosis differs slightly, there are similarities – In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix – In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis on the stroma side of the membrane as they diffuse back into the stroma © 2021 Pearson Education, Inc.  Comparison of chemiosmosis in mitochondria and chloroplasts Figure 10.16 © 2021 Pearson Education, Inc.  Both ATP and NADPH are produced on the stroma side of the thylakoid membrane, making them available for sugar synthesis in the Calvin cycle © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Figure 10.UN03  The light reactions: organization of the thylakoid membrane Figure 10.17 © 2021 Pearson Education, Inc. CONCEPT 10.4: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The Calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH © 2021 Pearson Education, Inc.  Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2, one for each turn of the cycle The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor © 2021 Pearson Education, Inc.  Phase 1: Carbon fixation The binding of CO2 to a five-carbon sugar named ribulose bisphosphate (RuBP) is catalyzed by RuBP carboxylase-oxygenase, or rubisco The six-carbon intermediate molecule is immediately split into two molecules of 3- phosphoglycerate (for each CO2 fixed) © 2021 Pearson Education, Inc.  Phase 2: Reduction Each molecule of 3-phosphoglycerate is altered through phosphorylation by six ATP and reduction by six NADPH to ultimately produce a G3P sugar For every three CO2 molecules that enter the cycle, six molecules of G3P are formed Only one of these can be counted as a net gain of carbohydrate © 2021 Pearson Education, Inc.  Phase 3: Regeneration of the CO2 acceptor (RuBP) The remaining five molecules of G3P are rearranged in a complex series of reactions yielding three molecules of RuBP Three additional molecules of ATP are used to facilitate the regeneration of RuBP © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Figure 10.UN04  The Calvin cycle Figure 10.18 © 2021 Pearson Education, Inc.  For the net synthesis of one G3P molecule, the Calvin cycle consumes nine molecules of ATP and six molecules of NADPH The light reactions regenerate the ATP and NADPH The G3P is the starting molecule for metabolic pathways that synthesize other organic molecules, including glucose, sucrose, and other carbohydrates © 2021 Pearson Education, Inc. CONCEPT 10.5: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a major challenge of terrestrial life for plants, particularly in arid climates Plants have metabolic adaptations to help conserve water; but these adaptations often involve trade-offs © 2021 Pearson Education, Inc.  One important trade-off is the balance between photosynthesis and water conservation – On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis – The closing of stomata reduces access to CO 2 and causes O2 to build up – These conditions favor an apparently wasteful process called photorespiration © 2021 Pearson Education, Inc. Photorespiration: An Evolutionary Relic? Most plants are C3 plants, in which the initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate) In photorespiration, rubisco binds with O2 instead of CO2, producing a two-carbon compound © 2021 Pearson Education, Inc.  Photorespiration is costly because it consumes O2 and organic fuel without producing any ATP or sugar It is thought to be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 Photorespiration may provide some protection from the damaging products of the light reactions that build up when the Calvin cycle slows due to low CO2 © 2021 Pearson Education, Inc.  In many plants, photorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle In some plant species, alternate modes of carbon fixation have evolved to minimize photorespiration and optimize the Calvin cycle © 2021 Pearson Education, Inc. C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound as the first product of the Calvin cycle C4 has evolved several times and is used by several thousand species in at least 19 different families Important agricultural examples include sugarcane and corn © 2021 Pearson Education, Inc.  In hot, dry weather, C4 plants partially close their stomata, conserving water but reducing CO2 Photosynthesis begins in mesophyll cells, but is completed in bundle-sheath cells, cells arranged in tightly packed sheaths around the leaf veins © 2021 Pearson Education, Inc.  Sugar production in C4 plants occurs in a three-step process 1. The production of the four-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells  PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low © 2021 Pearson Education, Inc.  2. The four-carbon compounds are exported to bundle-sheath cells through plasmodesmata 3. Within the bundle-sheath cells, CO2 is released from the four-carbon compound and then used in the Calvin cycle  Pyruvate is transported to the mesophyll cells where one ATP is used to convert it back to PEP  This ATP is generated using cyclic electron flow © 2021 Pearson Education, Inc.  C4 leaf anatomy and the C4 pathway Figure 10.19 © 2021 Pearson Education, Inc.  CO2 levels have drastically increased since the Industrial Revolution began in the 1800s, and continue to rise today due to human activities Increasing CO2 and temperature may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species The effects of such changes are unpredictable and a cause for concern © 2021 Pearson Education, Inc.  Suitable agricultural land is decreasing due to the effects of climate change, while the world population and demand for food continue to increase C4 photosynthesis uses less water and resources than C3 photosynthesis Scientists have genetically modified rice, a C3 plant, to carry out C4 photosynthesis for an estimated 30– 50% increase in yield for given water and resources © 2021 Pearson Education, Inc. CAM Plants Some plants, including succulents, conserve water by using crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, and incorporate CO2 into organic acids that are stored in the vacuoles Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle © 2021 Pearson Education, Inc.  The CAM pathway is similar to the C4 pathway in that they both incorporate CO2 into organic intermediates before it enters the Calvin cycle The C4 pathway structurally separates the initial steps of carbon fixation from the Calvin cycle In the CAM pathway, these steps occur in the same cell, but are separated in time © 2021 Pearson Education, Inc.  C4 and CAM photosynthesis compared Figure 10.20 © 2021 Pearson Education, Inc. CONCEPT 10.6: Photosynthesis is essential for life on Earth: a review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar in the form of starch in chloroplasts and other structures such as roots, tubers, seeds, and fruits © 2021 Pearson Education, Inc.  A review of photosynthesis Figure 10.21 © 2021 Pearson Education, Inc.  Figure 10.22 © 2021 Pearson Education, Inc.  Summary of Key Concepts: Calvin Cycle Figure 10.UN07 © 2021 Pearson Education, Inc.