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Photosynthesis Chapter 8 Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing.

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Presentation on theme: "Photosynthesis Chapter 8 Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing."— Presentation transcript:

1 Photosynthesis Chapter 8 Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing

2 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by –Cyanobacteria –7 groups of algae –All land plants – chloroplasts

3 Chloroplast Thylakoid membrane – internal membrane –Contains chlorophyll and other photosynthetic pigments –Pigments clustered into photosystems Grana – stacks of flattened sacs of thylakoid membrane Stroma lamella – connect grana Stroma – semiliquid surrounding thylakoid membranes 3

4 4 Stages Light-dependent reactions –Require light 1.Capture energy from sunlight 2.Make ATP and reduce NADP + to NADPH Carbon fixation reactions or light- independent reactions –Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO 2

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6 6 Pigments Molecules that absorb light energy in the visible range Light is a form of energy Photon – particle of light –Acts as a discrete bundle of energy –Energy content of a photon is inversely proportional to the wavelength of the light Photoelectric effect – removal of an electron from a molecule by light

7 7 Absorption spectrum When a photon strikes a molecule, its energy is either –Lost as heat –Absorbed by the electrons of the molecule Boosts electrons into higher energy level Absorption spectrum – range and efficiency of photons molecule is capable of absorbing

8 Organisms have evolved a variety of different pigments Only two general types are used in green plant photosynthesis –Chlorophylls –Carotenoids In some organisms, other molecules also absorb light energy 8

9 Chlorophylls Chlorophyll a –Main pigment in plants and cyanobacteria –Only pigment that can act directly to convert light energy to chemical energy –Absorbs violet-blue and red light Chlorophyll b –Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb 9

10 Structure of chlorophyll porphyrin ring –Complex ring structure with alternating double and single bonds –Magnesium ion at the center of the ring Photons excite electrons in the ring Electrons are shuttled away from the ring 10

11 Action spectrum –Relative effectiveness of different wavelengths of light in promoting photosynthesis –Corresponds to the absorption spectrum for chlorophylls 11

12 Carotenoids –Carbon rings linked to chains with alternating single and double bonds –Can absorb photons with a wide range of energies –Also scavenge free radicals – antioxidant Protective role Phycobiloproteins –Important in low-light ocean areas 12

13 13 Photosystem Organization Antenna complex –Hundreds of accessory pigment molecules –Gather photons and feed the captured light energy to the reaction center Reaction center –1 or more chlorophyll a molecules –Passes excited electrons out of the photosystem

14 Antenna complex Also called light-harvesting complex Captures photons from sunlight and channels them to the reaction center chlorophylls In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins 14

15 Reaction center Transmembrane protein– pigment complex When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level Light-energized electron can be transferred to the primary electron acceptor, reducing it Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule 15

16 16 Light-Dependent Reactions 1.Primary photoevent –Photon of light is captured by a pigment molecule 2.Charge separation –Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3.Electron transport –Electrons move through carriers to reduce NADP + 4.Chemiosmosis –Produces ATP Capture of light energy

17 17 In sulfur bacteria, only one photosystem is used Generates ATP via electron transport Anoxygenic photosynthesis Excited electron passed to electron transport chain Generates a proton gradient for ATP synthesis Cyclic photophosphorylation

18 18 Chloroplasts have two connected photosystems Oxygenic photosynthesis Photosystem I (P 700 ) –Functions like sulfur bacteria Photosystem II (P 680 ) –Can generate an oxidation potential high enough to oxidize water Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

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20 20 The Two Photosystems in Plants Work Together –Photosystem II drives ATP production Electron is passed to the reaction center of PS I (Replaces electron lost through excitation) the energy released is used to synthesize ATP –Photosystem I drives NADPH production. Electron is passed to NADP +  NADPH

21 21 Noncyclic photophosphorylation Plants use photosystems II and I in series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with electrons obtained by splitting water Z diagram

22 Chemiosmosis Electrochemical gradient used to synthesize ATP Chloroplast has ATP synthase enzymes in the thylakoid membrane –Allows protons back into stroma Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions 22

23 Making more ATP –It takes more energy to fix Carbon (1 ½ ATP / NADPH) –PS I can short circuit to make extra ATP via Cyclic phosphorylation.

24 Card Quiz A What high energy molecule is the final product of photosynthesis?  Oxygen  ATP  NADPH  Glucose

25 Card Quiz A Where is the chlorophyll located in a plant?  Cristae  Outer membrane of the chloroplast  Thylakoid membrane  Stroma

26 Card Quiz A A particle of light is a ____.  Wave  Photon  Proton  Newton

27 Card Quiz A What happens at the reaction center of a photosystem?  Light is absorbed  An electron is energized  NADP+ is reduced  ATP is formed

28 Card Quiz A Which of the following is made during the light reactions.  ADP and NADP+  Glucose and ATP  ATP and NADPH  NADPH and Carbon Dioxide

29 Card Quiz Answers  Green  Red  Blue  Green

30 30 Carbon Fixation – Calvin Cycle To build carbohydrates cells use Energy –ATP from light-dependent reactions –Cyclic and noncyclic photophosphorylation –Drives endergonic reaction Reduction potential –NADPH from photosystem I –Source of protons and energetic electrons

31 31 Calvin cycle Named after Melvin Calvin (1911–1997) Also called C 3 photosynthesis Key step is attachment of CO 2 to RuBP to form PGA Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

32 32 3 phases 1.Carbon fixation –RuBP + CO 2 → PGA 2.Reduction –PGA is reduced to G3P 3.Regeneration of RuBP –PGA is used to regenerate RuBP 3 turns incorporate enough carbon to produce a new G3P 6 turns incorporate enough carbon for 1 glucose

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34 34 Output of Calvin cycle Glucose is not a direct product of the Calvin cycle G3P is a 3 carbon sugar –Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose –Used to make starch Insoluble glucose polymer Stored for later use

35 35 Energy cycle Photosynthesis uses the products of respiration as starting substrates Respiration uses the products of photosynthesis as starting substrates Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

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37 37 Photorespiration Rubisco has 2 enzymatic activities –Carboxylation Addition of CO 2 to RuBP Favored under normal conditions –Photorespiration Oxidation of RuBP by the addition of O 2 Favored when stoma are closed in hot conditions Creates low-CO 2 and high-O 2 CO 2 and O 2 compete for the active site on RuBP

38 38 Types of photosynthesis C 3 –Plants that fix carbon using only C 3 photosynthesis (the Calvin cycle) C 4 and CAM –Add CO 2 to PEP to form 4 carbon molecule –Use PEP carboxylase –Greater affinity for CO 2, no oxidase activity –C 4 – spatial solution –CAM – temporal solution

39 39 C 4 plants Corn, sugarcane, sorghum, and a number of other grasses Initially fix carbon using PEP carboxylase in mesophyll cells Produces oxaloacetate, converted to malate, transported to bundle-sheath cells Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO 2 Carbon fixation then by rubisco and the Calvin cycle

40 C 4 pathway, although it overcomes the problems of photorespiration, does have a cost To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone C 4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C 3 pathway alone 40

41 41 CAM plants Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups Stomata open during the night and close during the day –Reverse of that in most plants Fix CO 2 using PEP carboxylase during the night and store in vacuole

42 When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO 2 High levels of CO 2 drive the Calvin cycle and minimize photorespiration 42

43 Compare C 4 and CAM Both use both C 3 and C 4 pathways C 4 – two pathways occur in different cells CAM – C 4 pathway at night and the C 3 pathway during the day 43

44 Card Quiz B Which structure is responsible for gas exchange ? Matrix Stroma Stoma Thylakoid

45 Question 4 Making glucose via photosynthesis costs ____ATPs and ___NADPHs. However, 1 molecule of glucose is enough to make ___ ATPs. 18, 12, 36 12, 18, 36 12, 36, 18 36, 18, 12

46 Question 9 What is the function of Rubisco? Absorption of photon energy Reduction of NADP+ Chemiosmosis Carbon fixation

47 Card Quiz B Where could a botanist expect to find C 4 plants?  Canada  Costa Rica  Tundra  Mount Everest

48 Card Quiz B The end product of photosynthesis is the starting material of cellular respiration. a.This is true b.This is false

49 Card Quiz B Which of the following would prevent the formation of phosphoglycerate?  Denaturation of the Rubisco enzyme  Non-functional b 6 -f complex  Anaerobic conditions  All of the above


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