Photosynthesis

1. Photosynthesis

2. All energy on earth comes from the sun. • We depend on: • Plants • Algae (underwater plants) • Cyanobacteria (photosynthetic bacteria-unicellular) • To provide this energy to us! • These organisms obtain energy directly from the sun and are called autotrophs • We are heterotrophs-we obtain energy by eating other organisms

3. A little bit about Cyanobacteria • The endosymbiotic theory proposes that an ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cell and gave rise to plant cells • Algal and plant cells contain chloroplasts (primarily in the mesophyll and guard cells)

5. Leaf Structure and Photosynthesis

6. The leaf is perfectly designed for maximum function • Ground tissue: function = photosynthesis palisade and spongy mesophyll have many Chloroplasts and air spaces enable efficient gas exchange. • Vascular tissue: function = support and transport • xylem and phloem • Dermal tissue separates the inside of the leaf from the air outside the plant. • Epidermal cells • waxy cuticle • Guard cells for stomata

7. Structure of a leaf

8. Photosynthesis primarily occurs in chloroplasts of leaves Lilac (Syringa)

9. Leaves are thin to maximize surface area exposed to sunlight. • It also limits the distance that gases need to travel to the chloroplasts

10. Plant structure • Obtaining raw materials • sunlight • leaves = solar collectors • CO2 • stomates = gas exchange • H2O • uptake from roots • nutrients • N, P, K, S, Mg, Fe… • uptake from roots

11. stomate • transpiration • gas exchange

12. Plant Structure: Leaves • Stomata = pores in epidermis used for gas exchange • CO2 into cell for photosynthesis • H2O out of leaf by evaporation to facilitate transpiration (process by which water pulled up plant) • Guard cells = epidermal cells that open and close stomata • Stomata open when guard cells swell with water • Stomata close when guard cells collapse together (shrivel) Stomata

13. Stomates

14. Stomates • Stomata are open in the daytime and closed at night • Light activated proton pumps in guard cell membranes cause K+ to move from neighbouring epidermal cells into guard cells-this results in H2O moving into guard cells (by osmosis) causing them to swell. • Increased turgor pressure causes them to open. • As the concentration of sucrose in guard cells decreases in the evening, water moves out of the cells and stomata close.

15. Stomata typical of monocots Stomata typical of dicots Potato (Solanum) Maize (Zea) Scanning electron microscope images

16. CO2 Chloroplasts absorbsunlight & CO2 cross sectionof leaf leaves chloroplastsin plant cell chloroplastscontainchlorophyll chloroplast makeenergy & sugar

17. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ outer membrane inner membrane stroma thylakoid granum chloroplast Plant structure ATP thylakoid • Chloroplasts • double membrane • stroma • fluid-filled interior • thylakoid sacs • grana stacks (sing. granum) • Thylakoid membrane contains • chlorophyll molecules • electron transport chain • ATP synthase • H+ gradient built up within thylakoid sac

18. Chloroplasts • Have 3 membranes • The third membrane is called the thylakoid. • The thylakoid is folded and looks like stacks of coins called granum • The stroma is the space surrounding the grana

19. Chloroplasts • Chlorophyll molecules are embedded in the thylakoid membrane • Act like a light “antenna” • These molecules can absorb sunlight energy. Image from Biology 11: College Preparation. Pg 73. Nelson, Toronto. 2003.

20. Chlorophyll • There are several types of chlorophyll • Chlorophyll a (blue-green) and chlorophyll b (yellow-green) • Both contain a porphyrin ring attached to a long H-C tail or phytol tail (hydrophobic to anchor into membrane) • Porphyrin rings are also found in the cytochromes of ETC

23. Chlorophyll • The ring has a Mg atom in the centre and alternating single/double bonds • Electrons in the porphyrin ring absorb light energy and begin photosynthesis • Chlorophyll a differs in that it has a methyl group (-CH3) at position –R while chlorophyll b contains an aldehyde group (-CHO) • Chlorophyll a is the primary light-absorbing pigment.

24. Overview

25. Overall Reaction What is the equation for photosynthesis? Similar to what? carbon dioxide + water  glucose (simple sugar) + oxygen CO2 (g) + H2O (l) + light energy [CH2O]+ O2(g) 6CO2 + 12H2O + light energy  C6H12O6 + 6O2 + 6H2O How are they different?

26. 3.1 Homework • P.145 • Q 1-7

27. 3.2Light energy and photosynthetic pigments

28. Overview • There are 3 distinct stages to photosynthesis; • Capturing light energy • Using this energy to make ATP and reduced NADP+ (energy shuttling coenzyme-similar to NAD+) • Using the free energy of ATP and the reducing power of NADPH + H+ to synthesize organic compounds such as glucose, from CO2

29. Light (dependent)Reactions(Stages 1 and 2) • Happen ONLY in sunlight on the thylakoid membrane • Light is absorbed by chlorophyll molecules • The energy generates molecules of ATP

30. Light Independent Reactions-Calvin Cycle-Carbon Fixation(formerly the “dark reactions”) • Happen in sunlight, and in the dark. • Hence “independent of light” • ATP generated by sunlight drives the Calvin Cycle. • Monosaccharides (eg. glucose) are manufactured in the cycle. • Monosaccharides are used to “build” polysaccharides (eg. Starch).

32. Summary Image from: Biology 11: College Preparation. Pg 74. Nelson, Toronto. 2003.

33. Photosynthesis • Light reactions • light-dependent reactions • energy conversion reactions • convert solar energy to chemical energy • ATP & NADPH • Calvin cycle • light-independent reactions • sugar building reactions • uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 It’s not theDark Reactions!

34. Absorption of Light by Chlorophyll

36. Pigments of photosynthesis • Chlorophylls & other pigments • embedded in thylakoid membrane • arranged in a “photosystem” • collection of molecules • structure-function relationship

37. A Look at Light • The spectrum of color V I B G Y O R

38. Light: absorption spectra • Photosynthesis gets energy by absorbing wavelengths of light • chlorophyll a • absorbs best in red & blue wavelengths & least in green • accessory pigments with different structures absorb light of different wavelengths • chlorophyll b, carotenoids, xanthophylls Why areplants green?

39. What wavelengths of light do you think plants use the least in photosynthesis?

40. Chlorophyll A and B absorb light mostly in the red and blue regions of the spectrum • Carotene and xanthophyll absorb light from other regions and pass the energy to chlorophyll

41. Light absorption by chlorophyll

42. Photosynthetic pigments are arranged as “photosystems”

43. Photosystems of photosynthesis • 2 photosystems in thylakoid membrane • collections of chlorophyll molecules • act as light-gathering molecules • Photosystem II • chlorophyll a • P680 = absorbs 680nm wavelength red light • Photosystem I • chlorophyll b • P700 = absorbs 700nm wavelength red light reactioncenter antennapigments

45. Measuring light absorption • A spectrometer is used to measure the amount of absorption at each wavelength • An action spectrum shows the rate of photosynthesis at different light intensities

46. Brown seaweeds • subjected to low light intensities • Chlorophyll C instead of chlorophyll B • fucoxanthin -very wide absorption spectrum -absorbs light in parts of the spectrum where chlorophyll is less efficient

47. Red seaweeds • Have chlorophyll D • Phycoerythrin

48. Green seaweeds • Found in shallow water • Do not have fucoxanthin • Does not have phycoerythrin

49. Accessory Pigments • Chlorophyll is not the only light-absorbing molecule in chloroplasts • Accessory pigments like orange carotenoids (beta-carotene) and yellow xanthophylls help absorb other light energy which may damage chlorophyll

50. Changing colours • During spring and summer leaves appear green because of the high concentration of chlorophyll • As temperatures cool, chlorophyll is broken down and we see the oranges and yellows and even reds (anthocyanin)