1. Lesson 8: Photosynthesis March 17, 2015

2. 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic (produces oxygen) photosynthesis is carried out by – Cyanobacteria – All land plants including algae – chloroplasts Anoxygenic (no production of oxygen) photosynthesis is carried out by – Purple bacteria, green-sulfur bacteria

3. Chloroplast Structure Thylakoid membrane – internal membrane of chloroplast – Contains chlorophyll and other photosynthetic pigments – Pigments clustered into photosystems Grana – stacks of flattened sacs of thylakoid membrane Stroma lamella – connect grana Stroma – semi-liquid surrounding thylakoid membranes – Contains enzymes needed to assemble organic molecules from CO 2 3

4. 4

5. Chloroplast Each pigment molecule is capable of capturing photons from light – Results in the passage of energy from one pigment to the next (photosystems) Energy arrives at a chlorophyll molecule that contacts a membrane bound protein – Transfers electron energy through series of membrane bound proteins (similar to ETC) – Results in the production of ATP and NADPH This energy is used to build organic molecules 5

6. 6 Photosynthesis Occurs in Stages 1.Light-dependent reactions – Require light (photons) 1.Capture energy from sunlight 2.Make ATP and reduce NADP + to NADPH 2.Light-independent reactions or Carbon Fixation REaction – Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO 2

7. 7 Grana Light-Independent Reactions Reactions

8. 8 Pigments Molecules that absorb (and reflect) light energy in the visible range – Colors that we see are colors not absorbed Light is a form of energy Photon – particle of light energy – Energy content of a photon is inversely proportional to the wavelength of the light Shorter the wavelength, higher the energy Visible light is a small fraction of the electromagnetic spectrum Photoelectric effect – removal of an electron from a molecule by light

9. 9

10. 10 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 depends on energy of photon and chemical nature of molecule it strikes Absorption spectrum – range and efficiency of photons a molecule is capable of absorbing Pigments are good absorbers of light in the visible range

11. Organisms have evolved a variety of different pigments Only two general types are used in green plant photosynthesis – Chlorophylls Chlorphyll A Chlorphyll B – Carotenoids In some organisms, other molecules also absorb light energy 11

12. 12 Absorption spectra for Chlorophyll and Carotenoid

13. Chlorophylls Two types of chlorophylls in plants – 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 13

14. 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 14

15. Action spectrum – Relative effectiveness of different wavelengths of light in promoting photosynthesis – Corresponds to the absorption spectrum for chlorophylls 15

16. Carotenoids – Carbon rings linked to chains with alternating single and double bonds – Can absorb photons with a wide range of energies. Not efficient in transferring energy – Also scavenge free radicals – antioxidant Protective role – β-carotene when split forms Vitamin A Oxidation of Vitamin A produces retinal pigment used in vision – Cause for leaves color change? 16

17. Photosynthesis Flash Animation Play till first break, Structure of Chloroplasts 17

18. 18 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

19. 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 19

20. 20

21. 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 21

22. 22

23. 23 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

24. 24 Chloroplasts Contain 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 non-cyclic transfer of electrons that is used to generate both ATP and NADPH

25. 25 Photosystem I transfers electrons ultimately to NADP +, producing NADPH Electrons lost from photosystem I are replaced by electrons from photosystem II Photosystem II oxidizes water to replace the electrons transferred to photosystem I 2 photosystems connected by cytochrome/ b 6 - f complex

26. 26 Non-Cyclic 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

27. 27

28. 28 Photosystem II Resembles the reaction center of purple bacteria Core of 10 transmembrane protein subunits with electron transfer components and two P 680 chlorophyll molecules Reaction center differs from purple bacteria in that it also contains four manganese atoms – Essential for the oxidation of water b 6 -f complex – Proton pump embedded in thylakoid membrane

29. 29 Photosystem I Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P 700 chlorophyll molecules Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron Passes electrons to NADP + to form NADPH

30. 30

31. Photosynthesis Flash Animation Play till Section 4, The Calvin Cycle. 31

32. Chemiosmosis Electrochemical gradient can be 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 32

33. 33 Carbon Fixation – Calvin Cycle To build carbohydrates cells need Energy – ATP from light-dependent reactions – Cyclic and non-cyclic photophosphorylation – Drives endergonic reaction Reduction Potential – NADPH from photosystem I – Source of protons and energetic electrons

34. 34 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

35. 35 Three Phases of the Calvin Cycle 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

36. 36

37. Photosynthesis Flash Animation Play the remainder 37

38. 38 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

39. 39 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

40. 40

41. 41 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

42. 42

43. 43 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

44. 44 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

45. 45

46. 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 46

47. 47 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

48. 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 48

49. 49

50. 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 50