1. Chapter 8 Photosynthesis Dr. Joseph Silver

2. this chapter deals with 1. what is photosynthesis 2
this chapter deals with 1. what is photosynthesis 2. discovery of photosynthesis 3. pigments involved in photosynthesis 4. the light reaction 5. the dark reaction 6. photorespiration (C3 & C4 plants)

5. photosynthesis depends on cells capturing the energy in some wavelengths of visible light

6. it is estimated that only about 1% of the suns energy is captured by plants

7. there are two types of photosynthesis 1
there are two types of photosynthesis 1. anoxygenic (does not produce oxygen) 2. oxygeneic (produces oxygen)

8. anoxygenic photosynthesis takes place in purple, green sulfur, green nonsulfur, & heliobacteria in a previous chapter we learned about the evolution of metabolism anoxygenic photosynthesis was mentioned as a very primitive form of photosynthesis where energy (energized electrons) is gotten from molecules without oxygen such as H2S or FeCl3

9. oxygenic photosynthesis takes place in cyanobacteria, 7 groups of algae, all land plants this is modern photosynthesis responsible for oxygen in our atmosphere water is used as an electron donor

11. we will concentrate on oxygenic photosynthesis which takes place in a chloroplast

12. chloroplasts - have an outer and inner membrane - the inner membrane has many folds - thylakoid surface contains photosynthetic pigments - a stalk of thylakoids forms a grana - the grana are surrounded by a fluid stroma - the stroma contains glucose forming enzymes - chloroplast DNA is in the stroma

14. there are 3 steps to photosynthesis 1. capture light energy 2
there are 3 steps to photosynthesis 1. capture light energy 2. make ATP & NADPH steps 1 & 2 require light 3. use ATP & NADPH to make glucose step 3 (carbon fixation) does not need light

16. photosynthesis 6C H2O + light(energy)  C6H12O6 + 6H2O + 6O2 you should see that this equation is the reverse of cellular respiration you need to know this equation

17. outer membrane inner membrane intermembrane space thylakoid stroma pigments photosystems plastoglobule

20. Greeks – growth from soil Jan Helmont(1600s) – growth from soil & water Joseph Priestly(1700s) – plants add “life” to air Jan Ingenhousz(1700s) – O2 off & carbon to carbohydrate F.F.Blackman(1900s) – only one part needs light Robin Hill(1950s) – use of radioactive labels to trace chemicals through a biological pathway found that electrons from water used to make sugar

21. visible light is only a small part of the electromagnetic spectrum

24. a particle of light = a photon a chemical which absorbs light energy = a pigment the shorter the wavelength of light the more energy the light contains

25. pigments absorb light which cause excited electrons to be released which are picked up by electron carriers which will be used to construct ATP & sugar

28. plant cells contain 3 types of pigments 1. chlorophyll a 2
plant cells contain 3 types of pigments 1. chlorophyll a 2. chlorophyll b 3. carotenoids

30. the main pigment = chlorophyll a 2nd is chlorophyll b 3rd = carotenoids (also antioxidants)

31. plant pigments absorb energy in the violet, blue and red areas they do not absorb in the green area they look green because green is nor absorbed green is reflected away from the chloroplasts

32. a photon of light excites electrons in the chlorophyll molecule which is passed among the double and triple bonds of chlorophyll and passed from molecule to molecule until it reaches the reaction center

34. SO lets do the light reaction light dependent reaction

36. antenna complex = microdomain photosystem II and photosystem I a whole bunch of pigment molecules in a protein matrix designed to do a job 1. which is to capture the energy in light 2. to transfer energy from molecule to molecule 3. until it reaches the reaction center chlorophyll 4. where energized electrons are released 5. into the thylakoid lumen 6. while producing a proton gradient and NADPH

38. Photosystem II 1. photons of light energize chlorophyll (680) 2
Photosystem II 1. photons of light energize chlorophyll (680) 2. cause energy to pass from pigment to pigment 3. at reaction center energized electron released 4. to an electron transport complex 5. H+ released to from proton gradient 6. ATP synthase use H+ to make ATP to use in Calvin cycle to make sugar 7. pair of electrons carried to photosystem I 8. water oxidized to release H+, e-, & O which regenerate chlorophyll 9. and oxygen is released

39. Photosystem I 1. energized electrons from photosystem II(700) 2
Photosystem I 1. energized electrons from photosystem II(700) 2. energizes chlorophylls which pass 3. electrons to reaction center 4. which releases energized electrons 5. to a second electron transport chain 6. which energizes NADP to NADPH 7. which will be used in Calvin cycle to make sugar

40. just as in mitochondria the energized NADs go to the dark cycle and the proton gradient is used to make ATP which goes to the dark cycle where energy is used to make sugar from CO2

43. the Calvin cycle the dark reaction carbon fixation cycle C3 photosynthesis the light-independent reaction where the ATP and NADPH are used to take CO2 and make sugar which will go to a mitochondrion and be used to make ATP ??????

44. the dark reaction converts inorganic carbon to organic carbon

45. the dark reaction the Calvin Cycle is similar to the Krebs cycle in that a molecule enters and goes through a cycle and regenerates the starting compound

46. The Calvin Cycle the light-independent reaction has 3 parts 1
The Calvin Cycle the light-independent reaction has 3 parts 1. carbon fixation 2. chemical reduction 3. regeneration of RuBP

47. in the stroma of a chloroplast RuBP(ribulose 1,5-bisphosphate) [5 carbons] reacts with CO2 (1 carbon) to form 3-phosphoglycerate (PGA) {3 carbons}

48. you need to go through the cycle twice to make 1 molecule of glucose

50. 6CO2 + 6rUBP  3 unstable 6 carbon molecules which quickly split to form 12 molecules of 3 carbon PGA ( 3-phosphoglycerate) then 6ATP provide the energy to convert PGA to 12 molecules of 1,3-biphosphoglycerate and 6NADPH provide the energy to convert 1,3 BPG to 12 G3P(glyceraldehyde 3-phosphate) 2 G3P leave the cycle and can be made into sugars

51. the remaining 15 molecules of G3P use 6ATP to convert G3P back to the starting three 5 carbon molecules RuBP(ribulose 1,5-biphosphate

53. although it is called the dark reaction it still needs light without light many of the molecules and enzymes needed for the production of G3P are light activated or produced as a byproduct of reactions needing light so without light the dark reaction cannot proceed

54. at night plants do not make sugar for energy most plants breakdown starch or other stored carbohydrate to power life processes

55. the result of photosynthesis II photosynthesis I and carotenoids are to produce ATP and NADPH to provide the energy to use CO2 to make sugars and other organic molecules

56. what are the plants going to do with the sugar send it to the cytoplasm then to a mitochondria where it will be used to produce ATP to power the cell

58. photosynthesis = sugar production then sugar enters the glycolysis pathway and pyruvate enters Krebs cycle and ATP is made

59. there is a problem with photosynthesis known as photorespiration

60. at an average summer growing temperature the enzyme used in carbon fixation also works on breaking down RuBP by using oxygen and causing a molecule of CO2 to be released which undoes the carbon fixation step

61. on an average day carbon fixation = 80% (good) photorespiration = 20% (bad)

62. as temperature goes up and water needs to be conserved the leaves stomata (fig 8.20) close causing oxygen to remain in the cells and not allowing CO2 to enter the leaf this causes photorespiration to increase

63. C3 plants (those using Calvin cycle) on hot summer days lose up to 50% of photosynthetic activity to photorespiration

64. some plants are known as C4 plants or CAM plants these plants thrive in hot climates and have a slightly different photosynthetic pathway which allows for increased efficiency

67. CO2 is captured in a mesophyll cell by PEP which becomes oxaloacetate which is converted to malate which passes through to an adjacent bundle sheath cell where malate releases CO2 and becomes pyruvate

68. the pyruvate passes back into the mesophyll cell and ATP is used to convert pyruvate to PEP

69. the CO2 cannot leave the bundle sheath cell this causes a high concentration of CO2 and allows for C3 Calvin cycle to proceed with little photorespiration

70. to escape the heat and dry conditions C4 works at night and C3 in daytime

71. to avoid photorespiration C4 plants must use an additional 12 ATP to regenerate PEP BUT this is better than losing 50% of carbon fixation to photorespiration

72. plants that do C4 are corn, sugar cane, sorghum, many grasses

75. CAM plants such as pineapple, many cactuses, and other dessert plants or plants growing where it is hot and arid have separated the accumulation of CO2 to nights and use of CO2 to daytime

76. the stomata are open at night and closed during the day