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The Process That Feeds the Biosphere Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis.

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Presentation on theme: "The Process That Feeds the Biosphere Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis."— Presentation transcript:

1 The Process That Feeds the Biosphere Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world

2 (a) Plants (c) Unicellular protist 10 µm 1.5 µm 40 µm (d) Cyanobacteria (e) Purple sulfur bacteria (b) Multicellular alga

3 Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H 2 O and CO 2

4 Heterotrophs obtain their organic material from other organisms Heterotrophs are the consumers of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2

5 Photosynthesis converts light energy to the chemical energy of food Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these cells allows for the chemical reactions of photosynthesis

6 Chloroplasts: The Sites of Photosynthesis in Plants Leaves (organ) are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO 2 enters and O 2 exits the leaf through microscopic pores called stomata

7 Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has 30–40 chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana (granum) Chloroplasts also contain stroma, a dense fluid

8 Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Chloroplast Mesophyll cell Outer membrane Intermembrane space 5 µm Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

9 Tracking Atoms Through Photosynthesis Photosynthesis can be summarized as the following equation: 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

10 Reactants: 6 CO 2 Products: 12 H 2 O 6 O 2 6 H 2 O C 6 H 12 O 6 Photosynthesis as a Redox Process Photosynthesis is a redox process in which H 2 O is oxidized and C O 2 is reduced

11 The Two Stages of Photosynthesis Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids): – Split H 2 O – Release O 2 – Reduce NADP + to NADPH – Generate ATP from ADP by photophosphorylation

12 The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules

13 Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar)

14 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

15 The Nature of Sunlight Light is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength is the distance between crests of waves Wavelength determines the type of electromagnetic energy

16 The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see Light also behaves as though it consists of discrete particles, called photons

17 UV Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm 380 450 500 550 600 650 700 750 nm Longer wavelength Lower energyHigher energy Shorter wavelength

18 Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light

19 Reflected light Absorbed light Light Chloroplast Transmitted light Granum

20 A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

21 Galvanometer Slit moves to pass light of selected wavelength White light Green light Blue light The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Refracting prism Photoelectric tube Chlorophyll solution TECHNIQUE 1 23 4

22 An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

23 Wavelength of light (nm) (b) Action spectrum (a) Absorption spectra (c) Engelmann’s experiment Aerobic bacteria RESULTS Rate of photosynthesis (measured by O 2 release) Absorption of light by chloroplast pigments Filament of alga Chloro- phyll a Chlorophyll b Carotenoids 500 400 600700 600 500 400

24 The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess O 2 He used the growth of aerobic bacteria clustered along the alga as a measure of O 2 production

25 Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

26 Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center in chlorophyll a CH 3 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown CHO in chlorophyll b

27 Excitation of Chlorophyll by Light When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

28 (a) Excitation of isolated chlorophyll molecule Heat Excited state (b) Fluorescence Photon Ground state Photon (fluorescence) Energy of electron e–e– Chlorophyll molecule

29 A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

30 A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

31 THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e–e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a molecules Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes

32 There are two types of photosystems in the thylakoid membrane Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

33 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

34 A photon hits a pigment and its energy is passed among pigment molecules until it excites P680…this excited electron from P680 is transferred to the primary electron acceptor P680 + (P680 that is missing an electron) is a very strong oxidizing agent H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680 +, thus reducing it to P680 O 2 is released as a by-product of this reaction

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

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

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

38 Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Photosystem II (PS II)

39 Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle Some organisms such as purple sulfur bacteria have PS I but not PS II. Cyclic electron flow is thought to have evolved before linear electron flow. Cyclic electron flow may protect cells from light-induced damage

40 ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H +

41 A Comparison of Chemiosmosis in Chloroplasts and Mitochondria 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 Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

42 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

43 Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

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

45 Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/2 1 2 3

46 The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

47 Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) or PGAL For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2 The Calvin cycle has three phases: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor (RuBP)

48 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

49 Alternative mechanisms of carbon fixation have evolved in hot, arid climates 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

50 Photorespiration In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar

51 1. Rice 2. Wheat 3. Oranges 4. Grapes 5. Coffee plant 6. Tea Plants 7. Peanut 8. Lemon 9. Potatoes

52 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 Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle 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

53 C 4 Plants C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle

54 C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

55 1. Crab Grass 2. Corn 3. Amaranth 4. Sorgham 5. Millet 6. Sugarcane 7. Nut grass 8. Barnyard grass 9. fourwinged salt bush.

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

57 CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2

58 pH 7 pH 4 pH 8 ATP

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60 Animations and Videos Photosynthesis Bozeman - Plant Structures Sections Through a Leaf Plant Structure and Growth Light Reaction in Photosynthesis Cyclic and Noncyclic Photophosphorylation Carbon Fixation – 1 Carbon Fixation - 2

61 Animations and Videos Photosynthetic Electron Transport and ATP SynthesisPhotosynthetic Electron Transport and ATP Synthesis Khan – Photosynthesis Streaming Chloroplasts Thylakoid Membrane Chapter Quiz Questions – 1 Chapter Quiz Questions - 2

62 Which of the following is a correct explanation for how carbons in a triose-phosphate can be said to be in a more reduced state than carbons in carbon dioxide? (See image on next slide.) Carbons in triose-phosphates have accepted more electrons, and have taken on a full negative charge, relative to carbons in carbon dioxide. Carbons in triose-phosphate have more bonds with hydrogen and fewer bonds with oxygen relative to carbons in carbon dioxide. Converting carbon in carbon dioxide into carbon in triose- phosphate is an endergonic process. Carbons in carbon dioxide donate their excess electrons to NADP  as they are converted to carbons in triose-phosphate. The binding with a phosphate by the triose-phosphate makes it higher in energy relative to carbons in carbon dioxide.

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64 Which of the following is a correct explanation for how carbons in a triose-phosphate can be said to be in a more reduced state than carbons in carbon dioxide is? Carbons in triose-phosphates have accepted more electrons, and have taken on a full negative charge, relative to carbons in carbon dioxide. Carbons in triose-phosphate have more bonds with hydrogen and fewer bonds with oxygen relative to carbons in carbon dioxide. Converting carbon in carbon dioxide into carbon in triose- phosphate is an endergonic process. Carbons in carbon dioxide donate their excess electrons to NADP  as they are converted to carbons in triose-phosphate. The binding with a phosphate by the triose-phosphate makes it higher in energy relative to carbons in carbon dioxide.

65 Which of the following INCORRECTLY matches a process with its typical location? Oxygen gas is produced—the soluble space surrounded by the thylakoid membranes Activated chlorophyll donates an electron—in the thylakoid membranes NADPH is oxidized to NADP  —the stroma of the chloroplast ATP is produced—the space between the two chloroplast envelope membranes RUBISCO catalyzes carbon fixation—the stroma of the chloroplast

66 Which of the following INCORRECTLY matches a process with its typical location? Oxygen gas is produced—the soluble space surrounded by the thylakoid membranes Activated chlorophyll donates an electron— in the thylakoid membranes NADPH is oxidized to NADP  —the stroma of the chloroplast ATP is produced—the space between the two chloroplast envelope membranes RUBISCO catalyzes carbon fixation—the stroma of the chloroplast

67 Of the following events from the light reactions of photosynthesis, which would be expected to occur first? Light-induced reduction of the primary electron acceptor in the reaction center of PS II takes place. While being split, electrons are taken out of water. donation of electrons from reduced Pq to the cytochrome complex acceptance of electrons by Pc from the cytochrome complex Pq gets electrons from the reduced primary electron acceptor of PS II.

68 Of the following events from the light reactions of photosynthesis, which would be expected to occur first? Light-induced reduction of the primary electron acceptor in the reaction center of PS II takes place. While being split, electrons are taken out of water. donation of electrons from reduced Pq to the cytochrome complex acceptance of electrons by Pc from the cytochrome complex Pq gets electrons from the reduced primary electron acceptor of PS II.

69 When donating its activated electron, the chlorophyll in photosystem II (P680) is said to be a very powerful oxidizing agent. This is best shown by its ability to make use of a proton electrochemical gradient to drive the formation of ATP. force the oxidation of oxygen in water to oxygen gas. donate an electron to plastoquinone (Pq). absorb light energy to power redox reactions. force the reduction of NADP + to NADPH.

70 When donating its activated electron, the chlorophyll in photosystem II (P680) is said to be a very powerful oxidizing agent. This is best shown by its ability to make use of a proton electrochemical gradient to drive the formation of ATP. force the oxidation of oxygen in water to oxygen gas. donate an electron to plastoquinone (Pq). absorb light energy to power redox reactions. force the reduction of NADP  to NADPH.

71 One good reason for carrying out the production of oxygen gas (O 2 ) in the space surrounded by the thylakoid membranes, and not in the stroma of the chloroplasts, is that this makes it easier for O 2 to exit the chloroplast. that the hydrogen ions released can contribute to the H  electrochemical gradient being generated. to reduce the concentration of O 2 in the stroma so that organic matter located there is not oxidized by it. that the concentration of water in this space is high, making it easier to form O 2 from the water. that carrying out this process in the stroma would tend to dry out this compartment and denature the enzymes of the Calvin cycle located there.

72 One good reason for carrying out the production of oxygen gas (O 2 ) in the space surrounded by the thylakoid membranes, and not in the stroma of the chloroplasts, is: that this makes it easier for O 2 to exit the chloroplast. that the hydrogen ions released can contribute to the H  electrochemical gradient being generated. to reduce the concentration of O 2 in the stroma so that organic matter located there is not oxidized by it. the concentration of water in this space is high, making it easier to form O 2 from the water. carrying out this process in the stroma would tend to dry out this compartment and denature the enzymes of the Calvin cycle located there.

73 Which makes an INCORRECT comparison between the membrane and surrounding compartments indicated in mitochondria and chloroplasts by the boxes (see figure)? The darker compartment will often be more positively charged and more acidic. The flow of electrons between items in the membrane results in protons being pumped from the darker to the lighter compartments. The lighter compartment is where much of the carbon metabolism is done. This membrane has an ATP synthase in it. The lighter compartments are both similar to the cytosolic compartment of bacteria.

74 Which makes an INCORRECT comparison between the membrane and surrounding compartments indicated in mitochondria and chloroplasts by the boxes (see figure)? The darker compartment will often be more positively charged and more acidic. The flow of electrons between items in the membrane results in protons being pumped from the darker to the lighter compartments. The lighter compartment is where much of the carbon metabolism is done. This membrane has an ATP synthase in it. The lighter compartments are both similar to the cytosolic compartment of bacteria.

75 The net reactions for some aerobic respiratory processes in mitochondria, and for some reactions of photosynthesis in the chloroplast are given below. Pyruvate import and the Citric acid cycle: Pyruvic acid + FAD + 4NAD + + ADP + P i ÷ 3CO 2 + FADH 2 + 4NADH + 4H + + ATP The Calvin cycle: 3CO 2 + 9ATP + 5H 2 O + 6NADPH  Glyceraldehyde 3-phosphate + 9ADP + 8P i + 6NADP + + 3H + The following are descriptions of proposed similarities between these two sets of reactions. Which is FALSE? Both alter the redox state of carbons. Both take place in a soluble space that is homologous to a bacterial cytoplasmic space. Both involve a three carbon organic acid, either as a substrate or as a product. Both couple very exergonic reactions to drive forward endergonic reactions of smaller net magnitude. Both involve various types of nucleic acids in the exchange of hydrogens.

76 The net reactions for some aerobic respiratory processes in mitochondria, and for some reactions of photosynthesis in the chloroplast are given below. Pyruvate import and the Citric acid cycle: Pyruvic acid + FAD + 4NAD + + ADP + P i ÷ 3CO 2 + FADH 2 + 4NADH + 4H + + ATP The Calvin cycle: 3CO 2 + 9ATP + 5H 2 O + 6NADPH  Glyceraldehyde 3-phosphate + 9ADP + 8P i + 6NADP + + 3H + The following are descriptions of proposed similarities between these two sets of reactions. Which is FALSE? Both alter the redox state of carbons. Both take place in a soluble space that is homologous to a bacterial cytoplasmic space. Both involve a three carbon organic acid, either as a substrate or as a product. Both couple very exergonic reactions to drive forward endergonic reactions of smaller net magnitude. Both involve various types of nucleic acids in the exchange of hydrogens.

77 The enzyme rubisco catalyzes the fixation of carbon (see reaction on next slide). Considering all the carbons involved, is the production of 3-PGA a net oxidation, reduction, or neither, and why? Oxidation. Adding a carbon dioxide makes the products more oxidized. Reduction. Adding the hydrogens from the water results in a more reduced condition. Reduction. The carbon in the carbon dioxide has been slightly reduced. Neither. There is no change in the total C–O and C–H bonds between the products and reactants. Oxidation. The RuBP acts as oxidizing agent in this reaction.

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79 The enzyme rubisco catalyzes the fixation of carbon (see reaction on next slide). Considering all the carbons involved, is the production of 3-PGA a net oxidation, reduction, or neither, and why? Oxidation. Adding a carbon dioxide makes the products more oxidized. Reduction. Adding the hydrogens from the water results in a more reduced condition. Reduction. The carbon in the carbon dioxide has been slightly reduced. Neither. There is no change in the total C–O and C–H bonds between the products and reactants. Oxidation. The RuBP acts as oxidizing agent in this reaction.

80 One way in which photosynthesis as done in a typical C 4 plant differs from that in a C 3 plant, is that the C 4 plant does not produce any oxygen gas at all. actively pumps oxygen gas away from the cells that contain rubisco. avoids the use of rubisco entirely; instead, it uses PEP carboxylase to catalyze all carbon fixation. keeps its stomata more open, so that more CO 2 can enter the plant. carries out the Calvin cycle only in the chloroplasts of bundle-sheath cells.

81 One way in which photosynthesis as done in a typical C 4 plant differs from that in a C 3 plant, is that the C 4 plant does not produce any oxygen gas at all. actively pumps oxygen gas away from the cells that contain rubisco. avoids the use of rubisco entirely; instead, it uses PEP carboxylase to catalyze all carbon fixation. keeps its stomata more open, so that more CO 2 can enter the plant. carries out the Calvin cycle only in the chloroplasts of bundle-sheath cells.

82 In CAM plants, CO 2 is temporarily fixed in phloem cells and later permanently fixed in the bundle-sheath cells. mainly obtained from oxidative respiratory processes. temporarily fixed at night and later permanently fixed during the day. fixed into organic matter just by the action of the enzyme rubisco. brought up to the leaves through air spaces in the stem so that the stomata of the leaves can be kept shut to prevent water loss.

83 In CAM plants, CO 2 is temporarily fixed in phloem cells and later permanently fixed in the bundle-sheath cells. mainly obtained from oxidative respiratory processes. temporarily fixed at night and later permanently fixed during the day. fixed into organic matter just by the action of the enzyme rubisco. brought up to the leaves through air spaces in the stem so that the stomata of the leaves can be kept shut to prevent water loss.


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