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CHAPTER 10 PHOTOSYNTHESIS 1. I. OVERVIEW A.Photosynthesis 1. Photosynthesis is the process that converts light energy (sunlight) into chemical energy.

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Presentation on theme: "CHAPTER 10 PHOTOSYNTHESIS 1. I. OVERVIEW A.Photosynthesis 1. Photosynthesis is the process that converts light energy (sunlight) into chemical energy."— Presentation transcript:


2 I. OVERVIEW A.Photosynthesis 1. Photosynthesis is the process that converts light energy (sunlight) into chemical energy stored in sugars and other organic molecules 2. Directly or indirectly, photosynthesis nourishes almost the entire living world 2

3 B. Two Types of Nutrition 1. Autotrophic Producers Synthesize organic molecules from inorganic raw materials (CO 2 ) Require an energy source Ex: plants, algae, some protists, and some prokaryotes Photoautotrophs—use light energy source -Ex: plants Chemoautotrophs—use oxidation of inorganic subs (sulfur or ammonia) as energy source -rare form of autotrophic nutrition found in some bacteria 3

4 4 Examples of Autotrophs

5 2. Heterotrophic Consumers Acquire organic molecules from compounds produced by other organisms Includes decomposers (fungi and bacteria) and animals that eat plants or other animals Depend on photoautotrophs for food and oxygen 5

6 II. CONCEPT 10.1: CHLOROPLASTS: SITES OF PHOTOSYNTHESIS A. Introduction 1. Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria 2. The structural organization of these cells allows for the chemical reactions of photosynthesis B. Chloroplasts 1. All green plant parts have chloroplasts, but leaves are the major organs of PS 2. Their green color is from chlorophyll, the green pigment within chloroplasts 3. Are surrounded by 2 membranes 6

7 4. Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast 5. CO 2 enters and O 2 exits the leaf through microscopic pores called stomata 6. Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf 7. A typical mesophyll cell has 30–40 chloroplasts 8. The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana 9. Chloroplasts also contain stroma, a dense fluid 10. Photosynthetic prokaryotes lack chloroplast but have chlorophyll built into plasma membrane or into the membranes of vesicles. 7

8 5 µm Mesophyll cell Stomata CO 2 O2O2 Chloroplast Mesophyll Vein Leaf cross section

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11 C.Photosynthesis: An Overview 1. Equation of PS Reverse of CR 6 CO 2 + 12 H 2 O* + light energy C 6 H 12 O 6 + 6 O 2 * + 6H 2 O Simplest photosynthetic equation: CO 2 + H 2 O [CH 2 O] + O 2 2. Splitting of Water C. B. van Niel hypothesized that plants split H 2 O as a source of H + and release O 2 as a by-product. 3. Photosynthesis is a redox process in which H 2 O is oxidized and C O 2 is reduced 11

12 Fate of Atoms During Photosynthesis 12

13 D. A Preview of Photosynthesis 1. Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) 2. The light reactions (in the thylakoids): Split H 2 O Release O 2 Reduce NADP + to NADPH Generate ATP from ADP by photophosphorylation 3. The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH 4. The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules 13

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18 III. Concept 10.2: The light reactions Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH A.The Nature of Sunlight 1.Light is a form of electromagnetic energy, also called electromagnetic radiation 2.Like other electromagnetic energy, light travels in rhythmic waves 3.Wavelength is the distance between crests of waves 4.Wavelength determines the type of electromagnetic energy 18

19 5.The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation 6.Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see 7.Light also behaves as though it consists of discrete particles, called photons B.Photosynthetic Pigments: Light Receptors 1.Pigments are substances that absorb visible light 2.Different pigments absorb different wavelengths 3.Wavelengths that are not absorbed are reflected or transmitted 4.Leaves appear green because chlorophyll reflects and transmits green light 19

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21 5.A spectrophotometer measures a pigment’s ability to absorb various wavelengths 6.Chlorophyll a is the main photosynthetic pigment and absorbs best in the red and blue wavelengths, and least in the green. 7.Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis 8.Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll 21

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23 C.Excitation of Chlorophyll by Light 1.When a pigment absorbs light, it goes from a ground state (lowest-energy state) to an excited state (orbit of higher potential energy), which is unstable 2.When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence 3.If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat 23

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25 D.Photosystem 1.A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light- harvesting complexes 2.The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center 3.A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a 4.Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions 25

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27 5.There are two types of photosystems in the thylakoid membrane 6.Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm 7.The reaction-center chlorophyll a of PS II is called P680 8.Photosystem I (PS I) is best at absorbing a wavelength of 700 nm 9.The reaction-center chlorophyll a of PS I is called P700 27

28 E.Linear Electron Flow 1.During the light reactions, there are two possible routes for electron flow: cyclic and linear 2.Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy 3.A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 4.An excited electron from P680 is transferred to the primary electron acceptor 28

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30 5.P680 + (P680 that is missing an electron) is a very strong oxidizing agent 6.H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680 +, thus reducing it to P680 7.O 2 is released as a by- product of this reaction 30

31 8. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I 9. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane 10. Diffusion of H + (protons) across the membrane drives ATP synthesis 31

32 11.In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor 12.P700 + (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain 32

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34 13.Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) 14.The electrons are then transferred to NADP + and reduce it to NADPH 15.The electrons of NADPH are available for the reactions of the Calvin cycle 34

35 Linear Electron Flow 35

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37 F. Cyclic Electron Flow 1.Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH 2.Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle 3.Cyclic electron flow is thought to have evolved before linear electron flow and may protect cells from light- induced damage 37

38 Cyclic Electron Flow 38

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40 G. Comparison of Chemiosmosis in Chloroplast and Mitochondria 1. Ways Similar: ETC located in membranes ATP synthase in same membrane as ETC ATP synthase complexes and some electron carriers and cytochromes are very similar 2. Ways Different: Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP 40

41 Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows 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 as they diffuse back into the stroma 41

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43 3. ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place 4. In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H 2 O to NADPH 43

44 H. Summary of the Light Reaction 1. Linear Electron Flow produces: NADPH—electron donor used to reduce CO 2 to sugar in the Calvin Cycle ATP—energy source for the Calvin Cycle O 2 —released as by-product 2. Cyclic Electron Flow produces: ATP 44

45 IV. Concept 10.3 Calvin Cycle A. The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle B. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH C. Requires ATP and NADPH from the light reaction D. Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) E. For synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2 45

46 F. Three Phases of Calvin Cycle for each CO 2 1. Carbon Fixation Each CO 2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP) This reaction is catalyzed by RuBP carboxylase, or rubisco which is probably the most abundant protein on Earth and is the most abundant protein in chloroplasts The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO 2 46

47 2. Reduction Couples ATP hydrolysis with reduction (pair of electrons donated by NADPH) of 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P) Requires 6 ATP and 6 NADPH 1 G3P exits the cycle to be used by the plant cell 3. Regeneration of CO 2 Acceptor (RuBP) 5 G3P are used to regenerate 3 RuBP Requires 3 ATP 47

48 G. Summary of the Calvin Cycle 1. To produce one G3P molecule requires: 9 ATP 6 NADPH 3 CO 2 2. G3P is the raw material used to synthesize glucose and other carbohydrates. 3. Two G3P molecules are needed to produce 1 glucose molecule. 4. It takes 18 ATP, 12 NADPH and 6 CO 2 to produce 1 glucose molecule. 48

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53 V. Concept 10.4 Alternative Mechanisms of Carbon Fixation Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis The closing of stomata reduces access to CO 2 and causes O 2 to build up These conditions favor a seemingly wasteful process called photorespiration 53

54 A. Photorespiration: An Evolutionary Relic? 1. In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound 2. In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle 3. Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar 4. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 5. Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle 54

55 6. In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle 7. The two most important photosynthetic adaptations are C 4 photosynthesis and CAM B. C 4 Plants 1. C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells 2. This step requires the enzyme PEP carboxylase 3. PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low 55

56 4. These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle 5. Adaptive process which enhances carbon fixation under conditions that favor photorespiration (hot, arid) 6. Occurs in corn, sugarcane, agricultural grasses 56

57 C 4 Leaf Anatomy 57

58 C. CAM Plants 1. Some plants, including succulents, use crassulacean acid metabolism (CAM ) to fix carbon 2. CAM plants open their stomata at night, incorporating CO 2 into organic acids 3. Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle 58

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60 D. The Importance of Photosynthesis: A Review 1. The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds 2. Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells 3. Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits 4. In addition to food production, photosynthesis produces the O 2 in our atmosphere 5. 50% for cellular respiration 6. Sucrose—transport form of carbohydrate 60

61 Overview of Photosynthesis 61

62 E. Comparison of CR and PS Cellular RespirationPhotosynthesis Exergonic redox processEndergonic redox process Energy released from oxidation of sugar Energy required to reduce CO 2 Electrons from sugar’s H + lose potential energy in ETC as transported to O 2 forming water Light is energy source that boost potential energy of e- as they move from water to sugar Produces ATPProduces sugar 62

63 You should now be able to: 1. Describe the structure of a chloroplast 2. Describe the relationship between an action spectrum and an absorption spectrum 3. Trace the movement of electrons in linear electron flow 4. Trace the movement of electrons in cyclic electron flow 5. Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts 5.Describe the role of ATP and NADPH in the Calvin cycle 6.Describe the major consequences of photorespiration 7.Describe two important photosynthetic adaptations that minimize photorespiration 63

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