Photosynthesis

1. Photosynthesis Chapter 10

2. Photosynthesis • Light energy stored as chemical energy for future use • Original source of energy for other organisms • Except for a few species of bacteria, all life depends on the energy-storing reactions of photosynthesis

3. Discoveries Leading to the Understanding of Photosynthesis Until 17th century, scholars believed that plants derived the bulk of their substance from soil humus.

4. Discoveries Leading to the Understanding of Photosynthesis • Joannes van Helmont • Disproved idea that plants get bulk of substance from soil humus • Planted 5 lb. willow in 200 lbs. of dried soil • Over 5 year time span, only watered plant with rainwater • At end of 5 years • Plant grew from 5 lbs. to 169 lbs. • Soil only lost 2 oz. during the 5 years • Reasoned plant substance must have come from water

5. Discoveries Leading to the Understanding of Photosynthesis • Joseph Priestly • 1772 • Reported sprig of mint could restore air that had been made impure by a burning candle • Plant changed air so mouse could live in it • Experiment not always successful • Sometimes didn’t provide adequate light for plant

6. Discoveries Leading to the Understanding of Photosynthesis • Jean Senebier • 1780 • pointed out that “fixed air,” carbon dioxide was required for photosynthesis • Antoine Lavoisier • Stated that green plants use carbon dioxide and produce oxygen

7. Discoveries Leading to the Understanding of Photosynthesis • Jan Ingen-Housz • 1796 • Found that carbon went into the nutrition of the plant • Nicolas de Saussure • 1804 • Observed that water was involved in the photosynthetic process

8. Discoveries Leading to the Understanding of Photosynthesis • Julius von Sachs • Between 1862 and 1864 observed • Starch grains are present in chloroplasts of higher plants • If leaves containing starch are kept in darkness for some time, starch disappears • If same leaves are exposed to light, starch reappears in chloroplasts • First person to connect appearance of starch (carbohydrate) with both fixation of carbon in the chloroplasts and the presence of light

9. Discoveries Leading to the Understanding of Photosynthesis • Cornelis van Niel • 1930s • Compared photosynthesis in different groups of photosynthetic bacteria • Green and sulfur bacteria use H2S instead of H2O to reduce CO2 • Found that sulfur was liberated instead of O2 • Since sulfur could only come from H2S, van Niel reasoned that O2 liberated by higher plants comes from H2O not CO2

10. Discoveries Leading to the Understanding of Photosynthesis • Cornelis van Niel • His general equation for photosynthesis 6CO2 + 12H2A  C6H12O6 + 6H2O + 12A light Carbon dioxide Hydrogen donor carbohydrate water A H2A could be H2O, H2S, H2 or any molecule capable of donating an electron. Reaction requires energy input. When H2A gives up electrons, it is oxidized to A.

11. Specific Photosynthetic Reactions • T.W. Engelmann • Between 1883 and 1885 • Demonstrated which colors of light are used in photosynthesis • Found that red and blue light were trapped by algal photosynthetic organelles

12. Specific Photosynthetic Reactions • J. Reinke • Studied effect of changing the intensity of light on photosynthesis • Observed rate of photosynthesis increased proportionally to increase in light intensity at low-to-moderate light intensities • At greater light intensities, rate of photosynthesis was not affected by changing light intensities • Indicated reaction was already proceeding at maximum rate

13. Specific Photosynthetic Reactions • F.F. Blackman • 1905 • Reasoned photosynthesis could be divided into two general parts • Photochemical reactions (light reactions) • Temperature-sensitive reactions (previously called dark reactions)

14. Specific Photosynthetic Reactions • Photochemical reactions • Light reactions • Insensitive to temperature changes • Temperature-sensitive reactions • Previously called dark reactions • Enzymatic reactions • Do not depend directly on light • Chloroplast proteins, thioredoxins, regulate activities of some dark reactions

15. Chloroplast Research • Robin Hill • 1932 • Demonstrated chloroplasts isolated from cell could still trap light energy and liberate oxygen • Daniel Amon • 1954 • Proved isolated chloroplasts could convert light energy to chemical energy and use this energy to reduce CO2

16. Chloroplast Structure • Double-membrane envelope • Two types of internal membranes • Grana (singular, granum) • Stroma lamella – interconnect grana • Stroma • Made up of grana and stroma lamella

17. Division of Labor in Chloroplasts • Research has shown that • Intact chloroplasts carry out complete process of photosynthesis • Broken plastids • Carry out only part of photosynthetic reactions • Will liberate oxygen

18. Division of Labor in Chloroplasts • Division of labor • Green thylakoids • Capture light • Liberate O2 from H2O • Form ATP from ADP and phosphate • Reduce NADP+ to NADPH • Colorless stroma • Contain water-soluble enzymes • Captures CO2 • Uses energy from ATP and NADPH in sugar synthesis

19. Characteristics of Light • Two models describing nature of light • Interpret light as electromagnetic waves • Light acts as if it were composed of discrete packets of energy called photons

20. Characteristics of Light • Light is small portion of electromagnetic energy spectrum that comes from sun • Longest waves • Cannot see • Infrared and radio waves • Longer than visible red wavelength • Shortest waves • Cannot see • Ultraviolet waves, X-rays, gamma rays • Shorter than violet

21. Characteristics of Light • White light (visible light) • Separate into component colors to form visible spectrum • Visible wavelengths range from • Red (640 – 740 nm) • Violet (400 – 425 nm)

22. Photons • Packet of energy making up light • Contains amount of energy inversely proportional to wavelength of light characteristic for that photon • Blue light has more energy per photon than does red light

23. Photons • Only one photon is absorbed by one pigment molecule at a time • Energy of photon is absorbed by an electron of pigment molecule • Gives electron more energy

24. Absorption of Light Energy by Plant Pigments • Spectrophotometer • Instrument used to measure amount of specific wavelength of light absorbed by a pigment • Absorption spectrum • Graph of data obtained • Chlorophyll • Reflects green light • Absorbs blue and red wavelengths • Wavelengths used in photosynthesis

25. Absorption of Light Energy by Plant Pigments • Chlorophyll • Two major types of chlorophyll in vascular plants • Chlorophylls a and b • In solution absorb much of red, blue, indigo, and violet light • In thin green leaf • Absorption spectrum similar to but not identical to that of chlorophyll in solution

26. Absorption of Light Energy by Chlorophyll • Chlorophyll molecule absorbs or traps photon • Energy of photon causes electron from one of chlorophyll’s atoms to move to higher energy state • Unstable condition • Electron moves back to original energy level

27. Absorption of Light Energy by Chlorophyll • Absorbed energy transferred to adjacent pigment molecule • Process called resonance • Energy eventually transferred to chlorophyll a reception center • Series of steps drives electrons from water to reduce NADP+ • Formation of NADPH represents conversion of light energy to chemical energy • NADPH reduces CO2 in enzymatic reactions leading to sugar formation

28. Two Photosystems • Robert Emerson • 1950s • Made observations that led to realization that there are two light reactions and two pigment systems • Photosystem I • Photosystem II

29. Two Photosystems * light-harvesting complex – functional pigment units that act as light traps

30. Adenosine Triphosphate Synthesis • Photophosporylation • Light-driven production of ATP in chloroplasts • Two types • Cyclic photophosphorylation • Noncyclic photophosphorylation

31. Adenosine Triphosphate Synthesis • Cyclic Photophosphorylation • Electrons flow from light-excited chlorophyll molecules to electron acceptors and cyclically back to chlorophyll • No O2 liberated • No NADP+ is reduced • Produces H+ gradient that leads to energy conservation in ATP production • Only photosystem I involved

32. Adenosine Triphosphate Synthesis • Noncyclic photophosphorylation • Electrons from excited chlorophyll molecules are trapped in NADP+ to form NADPH • Electrons do not cycle back to chlorophyll • Photosystems I and II are involved • ATP and NADPH are formed • Energy drives CO2 reduction reactions of photosynthesis

33. Enzymes of Light-Independent Reactions • All enzymes participating directly in photosynthesis occur in chloroplasts • Many are water-soluble • Many found in stroma • Ribulose biphosphate carboxylase/oxygenase (rubisco) • Catalyzes first step in carbon cycle of photosynthesis

34. Enzymes of Light-Independent Reactions rubisco Carbon dioxide + ribulose biphosphate  2 phosphoglyceric acid *(RuBP) • RuBP  5-C sugar present in plastid stroma, spontaneous reaction

35. Photosynthetic Carbon Reduction Cycle • Methods used to isolate carbon compounds formed during enzymatic reactions • Used radioactive carbon (14C) in CO2 to trace each intermediate product • Two-dimensional paper chromatography

36. Photosynthetic Carbon Reduction Cycle • Melvin Calvin • 1950s • Used radioactive C (14C) in CO2 to trace intermediate products of carbon reduction cycle • Nobel Prize

37. C3 Pathway • First product PGA contains 3 Cs • Calvin cycle (in honor of discoverer, Melvin Calvin) • Key points • CO2 enters cycle and combines with RuBP produced in stroma • 2 molecules of PGA are produced • Energy stored in NADPH and ATP transferred into stored energy in phosphoglyceraldehyde (PGAL)

38. C3 Pathway • PGAL may be enzymatically converted to 3-C sugar phosphate, dihydroxyacetone phosphate • Two molecules of dihydroxyacetone phosphate combine to form a sugar phosphate, fructose 1,6 - biphosphate

39. C3 Pathway • Some fructose 1,6 – biphosphate transformed into other carbohydrates, including starch (reactions not part of C3 cycle) • RuBP is regenerated • Free to accept more CO2

40. Photorespiration • Differs from aerobic respiration • Yields no energized energy carriers • Does not occur in the dark • Involves interaction with chloroplasts, peroxisomes, mitochondria

41. Photorespiration

42. Environmental Stress and Photorespiration • Succulents • Developed methods of storing and conserving water • Highly developed parenchyma tissue • Large vacuoles • Reduced intercellular spaces • Absorb and store water when moisture is available

43. Environmental Stress and Photorespiration • Succulents • Stoma closed during the day and open at night • Advantage • Reduces water loss during day • Disadvantage • Reduces CO2 uptake in daylight when photosynthesis can occur • Exhibit type of carbon metabolism called crassulacean acid metabolism (CAM)

44. Major Features of CAM • Stomata open at night • Leaves rapidly absorb CO2 • Enzyme phosphoenolpyruvate (PEP) carboxylase initiates fixation of CO2 • Malate, 4-C compound is usually produced • Total amount of organic acids rapidly increases in leaf-cell vacuoles at night • Leaf acidity rapidly decreases during following day • Organic acids are decarboxylated and CO2 released into leaf mesophyll

45. Major Features of CAM • Stomata closed during the day • Prevents or greatly reduces CO2 absorption and water loss • C3 cycle of photosynthesis usually takes place and converts the internally released CO2 into carbohydrate

46. C4 Pathway • Discovered in 1965 • H.P. Kortschak, C.E. Hartt, G.O. Burr • Extensively studied by M.D. Hatch and C.R. Slack • Pathway also known as Hatch-Slack cycle • Differs from C3 or Calvin cycle • Ensures an efficient absorption of CO2 and results in low CO2 compensation point

47. C4 Pathway • Compensation point • Concentration of CO2 remaining in closed chamber at the point when CO2 produced by respiration balances or compensates for CO2 absorbed during photosynthesis • Varies among different plants

48. C4 Pathway • Example of compensation point • Place bean plant and corn plant in chamber in light • Bean plant will die before corn plant • Corn plant has very low CO2 compensation point • Both plants eventually die of starvation

49. Factors Affecting Productivity • Only about 0.3% to 0.5% of light energy that strikes leaf is stored in photosynthesis • Yield could be increased by factor of 10 under ideal conditions

50. Factors Affecting Productivity • Breed productivity into plants • Norman Borlaug • Nobel Prize 1970 • Developed high-yielding wheat strains • Disadvantages • Strains require high levels of fertilizer • Expensive • Create pollution • Potential for genetic problems