Unlocking the Secrets of Photosynthesis

1. Plants and Photosynthesis

2. Photosynthesis • Organisms • Autotrophs: “Self Feeders” • Photo-: Light • Chemo-: Oxidize inorganics (Ex: Sulfur, Ammonia), unique to bacteria • Heterotrophs: “Other Feeders”

3. History • Jean-Baptiste van Helmont (1600’s) • grew willow tree • Weighed soil before and after • Added only water • Tree gained 75 kg • No change in mass of soil • Concluded: mass in plants comes from water

4. History • Joseph Priestly (1770’s) • Showed that plants can live in places animals can’t • Plants could make the air better so animals could live there again • Antoine Lavoisier (1770’s) • Showed that O2 is removed during burning

5. History • Other scientists throughout the 1700’s showed: • O2 is required for burning and for animals to live • plants give off O2 only in sunlight • plants only take in CO2 in sunlight • By 1800, scientists knew plants require 3 things for growth: • CO2, H20, and Sunlight

6. Site of Photosynthesis • Parts of a Leaf • Cuticle: prevents water loss • Upper and Lower Epidermis • Guard cells/stomata: control what enters and exits the leaf • Mesophyll Layer • Palisades: Columnar cells • Spongy: round cells surrounded by air spaces

7. Site of Photosynthesis Upper Epidermis MesophyllCells LowerEpidermis Vein Stoma

8. Site of Photosynthesis • Chloroplast • ~ half a million chloroplasts per square millimeter of leaf surface • A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long

9. Site of Photosynthesis • Chloroplast • Inner and outer membranes • Thylakoids – sac-like membranes that hold pigments • Grana – stack of thylakoids • Stroma – fluid outside the thylakoids, but still within the chloroplast

10. Site of Photosynthesis Thylakoids Stroma Granum Inner & OuterMembranes

11. Photosynthesis Conversion of Light E into Chem E • Light E • Travels in waves (photons) • Wavelength (): crest to crest (measured in nm) •  inversely related to frequency • Higher frequency = more E • Different  = different properties

12. Nature of Light Micro-waves RadioWaves Gamma Rays X-Rays UV Infrared Visible Light 750 400 450 500 550 600 650 700 • Visible spectrum is ~380–750 nm Wavelength (nanometers)

13. Nature of Light • Pigments absorb certain  and reflect or transmit others

14. Nature of Light • Spectrophotometers measure amount of Light pigments absorb or reflect

15. Nature of Light • Pigments • Absorb and reflect light • Specific pigment = specific light • Chlorophylls • a and b – both absorb blues and reds • a is 1 pigment for photosynthesis – focuses solar E onto a pair of e-s

16. Nature of Light • Accessory pigments – funnel the E they collect to a central Chlorophyll A • Carotenoids • Carotenes – reflect oranges • Xanthophylls – reflect yellows • Phycocyanins – reflect blues • Some accessory pigments provide photoprotection against excess light • Carotenoids in human eyes serve same function

17. Absorption/Action Spectra 100 80 750 750 400 400 450 450 500 500 550 550 600 600 650 650 700 700 60 % Light Absorption 40 20 0 Wavelength (nanometers) Visible Light Collectively Chlorophyll Carotenoids Phycocyanin

18. Engelmann’s Experiment • Simple experiment in 1883 • Compare to action spectrum

19. Photosynthesis • Can be divided into • Light-dependent rxn • Makes E storing compounds NADPH and ATP to fuel L-i rxn • Occurs in thylakoids • Light-independent rxn • Uses NADPH and ATP to produce glucose, a more stable form of E • Occurs in stroma

20. Photosynthesis

21. Light-dependent rxn

22. Light-dependent rxn • Light is absorbed in photosystem II, an “antenna complex” of hundreds of pigments that funnel E to a reaction center • Rxn Center: central chlorophyll a molecule next to a protein, the 1° e- acceptor

23. Light-dependent rxn • The e-s from the broken bonds slide down the ETC, slowly losing E • The e-s are recharged by sunlight in photosystem I and are passed along more carrier proteins to NADP+, reducing it to NADPH

24. Photosynthesis

25. Light-dependent rxn • Photosystem II – P680 (absorption peak is 680nm) • Photosystem I – P700 (absorption peak is 700nm) • P700 normally ships its e-s down the ETC to NADP+ (noncyclic e- flow) • Sometimes P700 will ship the e-s back to the Cytochrome complex (cyclic e- flow) • This pumps more H+ across the thylakoid membrane

26. Light-dependent rxn

27. Light-dependent rxn • Chemiosmosis • The lost E from the ETC “pumps” H+ ions from the stroma into the thylakoid, creating a high [H+] in the thylakoid • Diffusion allows the H+ to flow through ATP Synthase to the stroma • This flow of ions through the enzyme generates enough E to phosphorylate ADP to ATP

28. Light-dependent rxn • Chemi- osmosis

31. H+ H+ H+ H+ H+ H+ H+ H+ sun O2 Light-dependent H+ H20 H+

32. ATP H+ H+ H+ H+ H+ H+ H+ H+ H+ Light-dependent sun sun O2 ADP H+ H+ H20

33. Light-dependent rxn summary • H2O is broken up by sunlight • O2 is released as waste • e-s flow down ETC, pump H+ ions, and finally make NADPH • H+ ions diffuse across thylakoid membrane and help form ATP • ATP and NADPH move on to the light-independent rxn

34. Light-independent rxn • Called the Calvin-Benson Cycle for the men who discovered it • Also called the C3 cycle because CO2 is fixed into a 3 carbon molecule (3PG) • Calvin Cycle has three phases • Carbon fixation • Reduction • Regeneration of RuBP

35. L-i rxn – C fixation • 6 CO2 are “fixed” to 6 ribulose bisphosphate (RuBP), a 5C sugar • Process is sped up by Rubisco (RuBP carboxylase) • These new intermediates (6 6C compounds) quickly break into 12 3C molecules called 3-phosphoglycerate (3PG)

36. L-i rxn – C fixation

37. L-i rxn – Reduction • 12 ATPs phosphorylate the 12 3PGs to form 12 1,3 bisphosphoglycerates • A pair of e-s from NADPH reduces each 1,3 bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P) • The electrons reduce a carboxyl group to a carbonyl group

38. L-i rxn – Reduction

39. L-i rxn – Reduction • Two G3Ps can now be removed from the cycle to make glucose or be used for as any other carb the plant cell needs

40. L-i rxn – Regeneration • Only having 10 G3Ps is good because now we have the same number of C as we started – just in a different form • 6 ATPs are used to change the ten 3C G3Ps into six 5C RuBPs • And the cycle starts all over again

42. Light-independent rxn summary • Carbon Fixation • CO2 binds with RuBP and forms 3PG • Synthesis of G3P • ATP and NADPH are used • Regeneration of RuBP • Removal of 2 G3Ps to make glucose

43. Photorespiration • Stomata not only allow gas exchange, but transpiration also • Hot, dry day – stomata close • Problem: CO2, O2  • Rubisco can bind either CO2 ORO2 to RuBP • When O2 binds, no useful cellular E is produced

44. Photorespiration • When rubisco adds O2 to RuBP, RuBP splits into a 3-C piece and a 2-C piece • The 2-C fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes • Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle • Up to 50% of the C fixed by Calvin cycle can be drained away on a hot, dry day

45. C4 Plants • Mesophyll cells use PEP carboxylase to fix CO2 to phosphoenolpyruvate, forming oxaloacetate (4C) • PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot - on hot, dry days with the stomata closed

46. C4 Plants • Oxaloacetate then dumps the extra CO2 into the Calvin cycle in bundle-sheath cells • Rubisco can then work with a high concentration of CO2, thus minimizing photorespiration • C4 plants thrive in hot regions with intense sunlight • Examples: sugar, corn

47. C4 Plants

48. CAM Plants • Crassulacean Acid Metabolism • CO2 is fixed at night, but NO photosynthesis takes place at night • During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids

49. CAM Plants • Allows plants to keep their stomata closed during the hot, dry hours of day and open in the cooler hours of night • Less water is lost in the process • Less photorespiration occurs • Ex: succulent plants, cacti, pineapples, and several other plant families

50. CAM Plants